Monitoring Listeria in the food production environment. I. Dectection of Listeria in processing plants and isolation methodology

Food Research International 25 (1992) 45-56

REVIEW PAPER

Monitoring Listeria in the food production environment. I. Detection of Listeria in processing plants and isolation methodology P. J. Slade* Department of Environmental Biology, University of Guelph, Guelph, Ontario, Canada NIG 2 WI

Surveys for Listeria spp. have shown the ubiquity of the organisms in the food production environment. Listeria are found most commonly in cool, damp environments, on both food contact and non-contact surfaces, particularly conveyors, floors and drains. Listeria very often persist in meat and dairy plants despite vigorous sanitation regimes. Comparisons of methods for isolation of Listeria spp. show that no one protocol is suitable for detecting all Listeria-positive samples. The USDA technique is more suitable for most applications, but the FDA method may be acceptable under certain circumstances. Use of Fraser broth and ELBA identification may be useful adjuncts to conventional culture techniques. Oxford agar, MOX and PALCAM are currently favoured agar plating media, although tandem use of more than one is indicated for optimal detection. The effect of method of choice on recovery of sublethally stressed Listeria, and use of indicators for the presence of L. monocytogenes in the food production environment are discussed. Keywords: Listeria spp., L. monocytogenes, detection,

methodology

comparisons,

‘shutting the stable door after the horse has bolted’. It is more desirable to track and control incursions by microorganisms of concern at any stage of production/processing. Application of advanced tools of molecular biology, developed primarily for use in epidemiological investigations, may now permit rapid detection and subsequent control of L. monocytogenes in the food supply (Schuchat et al., 1991). This review, in three parts, aims to address recent developments in isolation, identification and typing of Listeria in the food production environment. The first part considers detection of Listeriu in processing plant environments, and outstanding issues in Listeria isolation methodology, particularly the results of comparing different protocols.

INTRODUCTION Progress in laboratory

detection and subtyping of Listeriu monocytogenes, the bacterial agent responsible for the disease listeriosis, has enhanced our ability to compare human and environmental isolates of the organism (Schuchat et al., 1991). Epidemiological tracing of the strains causing outbreaks of the disease serves a useful function in determining the common or point source of infection. However, with reference to the increased utilization of total quality concepts in food production, such as hazard analysis critical control point (HACCP) and preventive quality assurance, epidemiological tracing is conceptually much akin to *To whom correspondence Soup Company, Campbell USA.

food processing plants,

indicators.

should be addressed at: Campbell Place, Camden, N.J. 08103-1799,

Listeria in the food production environment Brackett

Food Research International 0963-9969/92/$05.00 0 1992 Canadian Institute of Food Science Technology

(1988) high!ighted the ubiquity of L. in the general environment and

monocytogenes 45

46

P. J. Slade

how this relates to contamination of food and water. It has been suggested that soil and decaying vegetation serve as the natural reservoir of Listeriu (Welshimer, 1960) where the organisms exist as free-living forms but with the potential to inhabit the gut of animals causing disease, under certain circumstances (Welshimer & Donker-Voet, 197 1). The gastrointestinal tract is now considered to be the human reservoir of the organism (Schuchat et al., 1991). Furthermore, the association of L. monocytogenes with several large foodborne outbreaks suggests that contaminated food may be the primary source of the organism in human infections (Farber & Peterkin, 1991). Surveys of processing plants Cox et al. (1989) studied the occurrence of Listeria spp. in 17 food factories (representing six different product groups) and 35 Dutch households. In food factories, listerias were found in drains, floors, standing water, residues, and food-contact surfaces in descending order of frequency. In two dry culinary food units no contaminated samples were found. This indicates that dry conditions and restriction of food residues contribute to the control of the organisms. Seven (20%) of 35 household kitchens were found to be contaminated with listerias. Six from seven dishcloths were positive for Listeria, as were swabs from two dustbins and one refrigerator. Fluid milk and frozen dairy product plants have a higher incidence of Listeria than cheese or dried product plants (Nelson, 1990). The incidence of Listeria correlates well with wet locations, particularly conveyers, floors and drains. Other contaminated sites found in this study included a CIP rinse tank, case washer water and glycol solution (Nelson, 1990). A very low number of Listeriu-positive samples were reported in analysis of 410 samples from dry dairy product processing plant environments (Gabis et al., 1989). Processors were free to choose the number of samples and specific sites, so these results may reflect the participants’ biases. In California, a total of 577 environmental samples from 156 milk processing plants were tested for Listeria spp. (Charlton et al., 1990). Listeria spp. were found in 46 (29.5”/0) of 156 plants, and 75 (12.6%) of 577 individual samples were positive. Just under 20% of plants found contaminated with Listeria harboured L. monocytogenes, and just over 50% of individual Listeria-positive sam-

ples contained L. monocytogenes. Listeria were recovered more frequently from fluid and frozen milk product plants, particularly from packaging/filling room locations but less often from raw milk receiving areas. Samples taken from drains (and conveyors) were the best indicators of hygiene conditions at any particular location. Klausner and Donnelly (1991) examined a total of 361 environmental samples, particularly from floors and other non-product contact surfaces, from 34 Vermont dairy processing plants. The incidence of L. monocytogenes was low (1.4%) compared to that of L. innocua (16.1%). Fluid milk plants had the highest incidence with case washers highest overall. An additional area of concern was sanitizing floor mats and footbaths. Wet areas were significantly more contaminated than dry areas. Walker et al. (1991) found 111 (12.0%) of 922 environmental samples from frozen milk product plants in California were positive for Listeria spp. (L. innocua and L. monocytogenes, only). In the 39 plants sampled, L. monocytogenes was the only species recovered from 5 (12.8%) and L. innocua the only species from 13 (33.3%). Both species were isolated from 9 (23.1%) plants. Listeriu were not isolated from 12 (308%) plants. Highest recovery rates were from selected sites in batch flavouring, freezing, ingredient blending and package filling areas, and from corresponding floor drains. However, not all positive selected site samples correlated with floor drain results, so erroneous conclusions about level of contamination in a plant may be reached if only drains are sampled. The rate of recovery of Listeria from plants with above-average sanitation and excellent or moderate environment contamination control programs (ECCP) was 6.8%, whereas the recovery rate from plants with below-average sanitation and slight or no ECCP was 27.5%. Environmental testing detected Listeria spp. in 21 of 52 dairy factories in Australia (Venables, 1989). L. monocytogenes was isolated in all but one plant and, in, five other species were also detected. In one factory L. seeligeri was the only species found. Among 763 environmental samples, 142 (19’/,) were Listeriapositive, with L. monocytogenes predominant in 93% of cases. Listeriu were most frequently found on conveyors and in cool, damp areas, and sometimes were persistent for several months even after vigorous cleaning. In meat plants Listeria were found in 20% of food contact surfaces tested in 41 plants, and in 37% of floor drain samples (Anon, 1988). Genigeorgis

Monitoring Listeria in the food production environment. I. et al. (1989) examined the presence of Listeria spp. in a chicken processing plant. Listeria were found in 18.8% of feather picker drip water, 12.5% of chiller water overflow and 37.5% of recycling water for cleaning gutters, but not in scalding tank water overflow or incoming chilling water samples. The prevalence of L. monocytogenes on the hands and gloves of persons hanging birds after chilling, cutting carcasses, and packaging parts was 20.0, 45.5 and 59.0X,, respectively. Hudson and Mead (1989) found that chicken carcasses acquired listerias mainly via contaminated surfaces and equipment, particularly the automatic carcass opener, the neck-skin trimmer, the evisceration-line drain and the conveyor to the packaging area. The frequent occurrence of listerias on finished carcasses (50% of those examined) was thought directly attributable to contamination of processing equipment and the inevitable problem of cross-contamination. In a turkey processing plant, Genigeorgis et al. (1990) indicated that Listeria spp. were present in 13.4, 6.7 and 33.3% of feather picker drip water, chiller water overflow, and recycling water for cleaning gutters, respectively, but not in scald tank overflow samples. On gloves and hands of persons hanging birds after chilling, cutting carcasses and packaging parts the incidence of Listeria spp. was 16.7, 33.3 and 40.00/o, respectively. Wenger et al. (1990) examined sites in the facility which produced turkey franks microbiologically linked to a case of human listeriosis (Barnes et al., 1989). L. monocytogenes was isolated from only two of 41 samples obtained from the plant environment. The peeler room conveyor belt was positive for the implicated strain of L. monocytogenes. This exercise demonstrated that systematic monitoring of production facilities for L. monocytogenes may help identify sources of the hazard, thus facilitating elimination of the organism from these, and similar, ready-to-eat meat products. Listeria isolation methodology

Gray and Killinger (1966) recognized that ‘perhaps as many different media have been recommended as there are individuals who have made serious efforts to isolate L. monocytogenes’. Their review of early methodology for isolation of L. monocytogenes from food, clinical and environmental sources was followed by a summary of the most notable work in this area (Ralovich, 1975). More recently, these early efforts and the latest developments in methodology have been reconsid-

47

ered (Klinger, 1988; Cassiday & Brackett, 1989; Ralovich, 1989; Palumbo, 199 1). Direct plating procedures do not reliably isolate Listeriu spp. and typically are used in conjunction with a prior enrichment (Farber & Peterkin, 1991). Several comparative studies have been conducted to evaluate various isolation procedures involving combinations of non-selective pre-enrichments and/ or selective enrichments followed by plating to non-selective and/or selective agars. Some recent comparisons of different protocols are listed in Table 1. Comparisons of different protocols

Slade and Collins-Thompson (1988) found that the two-stage enrichment of Fenlon (1985) was as effective as the direct selective enrichment technique of Lovett et al. (1987) (FDA method) in isolating Listeria spp. from raw milk. The advantage of the direct technique is the shorter time required for isolation. The number of isolations of L. monocytogenes after 24 or 48 h of incubation, with or without KOH treatment, were similar. Lovett et al. (1987) also found that with low levels of Listeria (normally found in raw milk), recoveries were not significantly different after 24 or 48 h. Their suggestion that KOH treatment improved recovery of Listeria at 24 h but not 48 h was not corroborated in the study of Slade and CollinsThompson (1988). Further isolations by incubating enrichment broth (EB; Lovett et al., 1987) for 7 days at 30°C were not obtained. Possibly, growth of Listeria during (prolonged) enrichment at 30°C might be suppressed by competitive flora, as intimated by Beckers et al. (1987). In a comparative study of methods by Farber et al. (1988) no one method alone was effective at detecting all Listeria-positive raw milk samples. This was also found by Slade and Collins-Thompson (1988). Inhibition by selective agents may not account solely for the varied instances of isolation by one method but not others. Growth appears more dependent upon the ability of listerias to successfully overcome the challenge from competing microorganisms during the selection process. Buchanan (1990) reasoned that growth of interfering organisms in a primary enrichment broth containing a low level of acriflavine most likely would also occur in secondary broth with an elevated level of this agent. Ideally, completely different selective agents should be used in each step of the isolation procedure to maximize the selection pro-

CE

0 See text for references to media/protocols. b Detection of Listeria spp. c Mattingly et al. (1988). d Klinger & Johnson (1988). e Dominguez-Rodriguez et al. (1984). f Slade & Collins-Thompson (1987). g Schiemann et al. (1990). h See text for explanation.

Fraser broth

+

+

+

i-

Meat (ground beef)

Fraser broth

+

Various (environmental)

Fraser broth +

+

+

Various (environmental)

+

+

+

+

Meat (minced beef), cheese

Buffered EB

+

+

+

Seafoods +

+

+

Raw milk, Fraser broth

+ +

+

+

+ L-PALCAM

+ +

+

+

+

Seafoods

CE

+

Ready-to-eat foods

+

-

+

+

+

MLA

Listeria-Tek

Listeria-Tek

PALCAM

Listeria-Tek

+

Various

+

+

Buffered MOPS Fraser broth

+

+

Cheese, meat (chicken)

+

AC PCP/PNAg

+

+

+

+

+

Raw milk, meat

TNABf PPYlPCPg

LSAMe

+

+

Listeria-Tekc Gene-Trakd

Other

+

+

OA

Cheese, raw meat

LPM (USDA)

MMA (FDA) +

EM No. 3e

Other

Selective agar

+

(FEDA) (%!A)

Selective enrichment

Commentsb

> LEBIMMA

All good EB/MOX best

FDA > USDA USDA = FDAh

USDA > FDA OA or LPM > MMA

USDA > ELISA/FDA USDA = FDAh

EBIELISA superior

OA or PALCAM

USDA > FDA

CE -, FDA

USDA > CE

MOPS superior OA > MMA or LPM ELISA comparable

PPY or PCP/PCP agar superior OA > LPM

Any enrichment LPM or LSAM > MMA

FDA probe > FDA > ELISA > Gene-Trak

of protocols for isolating Listeria from foods0

Raw milk, vegetables

Food Category

Table 1. Comparison

et al. (1991b)

et al. (1991a)

et al. (1991)

Yu & Fung (1991)

Warburton

Warburton

Nsrrung

Noah et al. (1991)

Lund et al. (1991)

Lovett et al. (1991)

Lewis & Carry (1991a)

Hayes et al. (1991)

Walker et al. (1990)

Schiemann et al. (1990)

Femandez-Garayzabal & Genigeorgis (1990)

Heisick et al. (1989)

Reference

gcb

a 5 h

Monitoring Listeria in the food production environment. I.

cess (Buchanan, 1990). In a comparison of four techniques to detect Listeria in food, Heisick et al. (1989) concluded that detectablility by the methods was deemed to be a function of the condition and levels of the initial listerial population, and of the types and numbers of commensal organisms. The effects of competition, antagonism and interactions with microflora from various environments on the growth of Listeria in selective media have not been thoroughly investigated. Tran et al. (1990) found that competition may be due to specific bacteria rather than bacterial numbers. Certain lactic acid bacteria (Harris et al., 1989) (particularly pediococci (Yousef et al., 1991) leuconostocs (Harding & Shaw, 1990) and Lactococcus (Streptococcus) lactis (Wenzel & Marth, 1990)), Enterococcus faecalis (Arihara et al., 1991) and corynebacteria (ValdesStauber et al., 1991) are known to be antagonistic to Listeria. Many also resist several agents currently incorporated into Listeria selective media. Perhaps these species should be targeted for elimination in future developments of selective media. Some strains found in foods are known to be unable to survive enrichment procedures and compete with Listeria (Dallas et al., 1991). Conversely, some psychrotrophic pseudomonads (Marshall & Schmidt, 1991) and flavobacteria (Farrag & Marth, 1991) appear to enhance growth of L. monocytogenes under certain conditions. The search for optimal conditions for isolating Listeria from food has been paramount in recent years. Doyle & Schoeni (1986) failed to grow any Listeria from SO samples of raw milk using their selective enrichment procedure (SEP) or by plating directly to McBride Listeria agar (MLA; McBride & Girard, 1960). Later, Doyle and Schoeni (1987) examined three methods for isolating L. monocytogenes from cheese and found that, at any time, only one method was usually effective. Isolations of L. monocytogenes from cheese were more numerous after a cold enrichment procedure than by the FDA method. Hayes et al. (1986) found that secondary enrichment in a two-stage technique enhanced the isolation of L. monocytogenes from milk when compared with plating directly to selective agars. Pini and Gilbert (1988) compared cold enrichment and a modified FDA technique and found that neither method alone detected all Listeria isolates from raw chickens or soft cheeses. Truscott and McNab (1988) compared enrichment in the broth of Donnelly and Baigent (1986) and their own Listeria test broth, each followed by plating to LiCl-phenyl-

49

ethanol-moxalactam (LPM) agar (Lee & McLain, 1986). They found that neither approach alone recovered all Listeria from ground meat samples. Lammerding and Doyle (1989) explored combinations of six enrichment techniques and five plating media to isolate L. monocytogenes from several dairy products, and found the USDA method (Lovett, 1988a) best, with LPM agar the optimal plating medium. Omission of selective ingredients from primary Listeria enrichment broth (LEB; McLain & Lee, 1988) and transfer of a larger-thannormal volume of this to secondary LEB was particularly useful for recovering stressed Listeria. In the FDA method, 7-day incubation of EB gave no better recoveries than 24 h incubation, and KOH treatment prior to plating provided no advantage for isolation of L. monocytogenes. In experiments with Listeria in single-strength orange juice, Parish and Higgins (1989) also found that KOH was detrimental to recovery of low levels of Listeria, but recovery was enhanced by 7-day enrichment. Hitchins (1989) found that the FDA and USDA methods were equally effective at isolating L. monocytogenes from ice cream, but that prolonged enrichment from 7 days and use of KOH in the FDA method had no effect on results. Subsequently, FDA revised their protocol to: (a) eliminate KOH treatment; (b) reduce the time of emichment from 7 days to 48 h; and (c) use LPM agar in addition to the modified McBride agar (MMA) of Lovett (1988b) (Federal Register, 1988). Tran et al. (1990) found LPM agar superior to MMA after FDA enrichment of L. monocytogenes in inoculated foods. Tiwari and Aldenwrath (1990) compared four selective agars, MMA, acriflavineceftazidime (AC) medium (Bannerman & Bille, 1988) LPM agar and Oxford agar (OA) also known as Listeria selective agar (LSA; Curtis et al., 1989) following enrichment of cheese, raw milk, processed meat and raw vegetable samples in FDA EB. They found that MMA was the least reliable, AC was not good for all L. monocytogenes strains and, although LPM agar and OA gave equivalent recoveries, the former was not as good as OA at recovering L. monocytogenes type 3a strains. Fernandez-Garayzabal and Genigeorgis (1990) included the selective media developed by Dominguez-Rodriguez et al. (1984) in their comparison of FDA and USDA methods. LPM agar was superior to MMA and secondary enrichment improved the recovery of Listeria from meat samples. Schiemann et al. (1990) developed a range of enrichment broths and plating agars which they

50

P. J. Slade

found superior to media used in FDA and USDA procedures, and others. No further trials of these formulations have, as yet, been reported. Two recent reports compare cold enrichment (CE) with both the FDA method (Lewis & Corry, 1991a) and the USDA method (Hayes et al., 1991). In the former, more positive samples were found using CE than by the FDA method, which was also the case in a study with raw chickens (Lewis & Corry, 1991b). The USDA method, on the other hand, was found to be significantly better than CE by Hayes et al. (1991). Lovett et al. (1991) found that the FDA procedure was less effective than the USDA method at isolating heated L. monocytogenes from seafoods, but the two methods were equally effective with unheated cells. In the FDA procedure, KOH treatment and 24 h culture provided no advantage for Listeria recovery! The greater selectivity of the USDA was thought to be useful for isolating procedu non-heat “7stressed Listeria when the level of background flora was high. Bailey et al. (1990a) found no significant difference in the ability of FDA and USDA enrichment broths to isolate L. monocytogenes from chicken and Brie cheese, although the FDA broth permitted greater growth of background flora from chicken. Warburton et al. (1991a) showed that the FDA method can be shortened from 7 to 2 days without substantially reducing the number of positive samples. With a limited number of samples, the USDA method proved to be slightly more effective than the FDA method in isolating L. monocytogenes. Oxford agar (OA) and LPM agar were better than MMA at isolating the organism. The inclusion of Fraser broth (Fraser & Sperber, 1988) to both FDA and USDA protocols was found to be a useful screening tool, albeit lacking in selectivity. This, and the use of OA and LPM agar are the modifications to standard Canadian methods investigated by Warburton et al. (1991b). Modified FDA and USDA methods showed no significant differences in ability to isolate L. monocytogenes from routine food samples. However, the modified FDA method outperformed the modified USDA method when spiked food samples were tested. Oxford agar (OA), LPM agar or modified Oxford agar (MOX; McLain & Lee, 1989) were considered the best plating media, although there were instances when L. monocytogenes was isolated on one medium but not the others. These authors recommended that the modified USDA method may be the method of choice for all food

and environmental samples, and that multiple plating to OA and at least one other medium (either LPM agar or MOX) be performed. Lund et al. (1991) found that recovery of Listeriu from raw milk was enhanced by enrichment in L-PALCAMY broth (van Netten et al., 1989) with plating to OA or PALCAM (van Netten et al., 1989). Fewest isolations were achieved using the combination of LEB (USDA) with MMA (FDA). Yu and Fung (199 1) evaluated the performance of 16 combinations of enrichment and plating agar in recovering L. monocytogenes from spiked meat slurries and pure cultures. All media performed well, but EB (FDA) combined with MOX (USDA) was the most useful. A comparison was made of four procedures to detect Listeriu spp. in two food categories (Heisick et al., 1989). In total 309 assays, 71 on cow’s milk and 238 on ten types of fresh vegetables, were performed. Notably, none of the four procedures detected all positive samples. In milk, 98-100% of positive samples were detected, and, in the vegetables, from 4586% were positive by various methods. The Listeriu-Tek enzyme-linked immunosorbent assay (ELISA) of Organon Teknika Corp., Durham, NC, USA (Mattingly et al., 1988) detected 68% of the 44 positive vegetable samples, the Gene-Trak Listeriu assay of Gene-Trak Systems, Framingham, MA, USA (Klinger & Johnson, 1988, Klinger et al., 1988) 45%, the FDA cultural procedure 75% and the FDA probe procedure (Hill, 1987) 86%. Recovery was higher with LPM agar as used in the FDA probe procedure than with MMA used in the FDA cultural procedure. The Listeria-Tek ELISA has also been compared with conventional cultural procedures by Walker et al. (1990), Noah et al. (1991) and Norrung et al. (1991). The best procedures for isolating L. monocytogenes from inoculated Camembert cheese and roast chickens were found by Walker et al. (1990) to be cultural procedures using enrichment in MOPS-buffered enrichment broth. Overall the Listeriu-Tek procedure produced results comparable to those of the best cultural procedure. Noah et al. (1991) suggested that EB (FDA) was the most suitable of four enrichments for 48-h ELISA testing of naturally contaminated seafoods. Nor-rung et al. (1991) showed that the USDA procedure was the most sensitive detection method for meat samples artificially contaminated with ~3 colony forming units (CFU) of L. monocytogenes per gram. Higher

Monitoring Listeria in the food production environment. I.

than this the ELISA and USDA methods were equally sensitive. However, detection of low numbers of Listeria by ELISA in raw food with competitive microflora at high levels was not so good. Rollier et al. (1991) compared EB + OA, PALCAMY broth + PALCAM agar, and LEB + ELISA + OA with double-layer agar (tryptic soy yeast extract agar overlaid with PALCAM agar) for detection of Listeria spp. in fermented sausage. The latter was most effective for enumeration, and the two-stage USDA enrichment broth technique was best for isolation. A method including a preenrichment step described by Varabioff (1990) gave greater recoveries of Listeria from meat samples than the USDA method or ELISA (Tecra, Bioenterprises, Sydney, NSW, Australia). No Listeria were found in dairy factory environmental or cheese samples by either the FDA method or the developed method. Aside from evaluations of entire protocols (i.e. those including enrichment steps), some comparative studies have focused on recovery of L. monocytogenes by direct plating of food homogenates to selective agar. Oxford agar (OA), PALCAM and MOX were either not available or not included in the following studies described. Buchanan et al. (1989) determined that modified Vogel Johnson agar (MVJA; Buchanan et al., 1987) performed as well as LPM agar when L. monocytogenes from meat, poultry and seafoods was recovered directly. MVJA was easier to use because of its ability to differentiate Listeria spp. from other microorganisms. Loessner et al. (1988) compared seven plating media for isolating Listeria spp. and found LPM agar most suitable even though it inhibited L. grayi and L. murrayi. They found L. monocytogenes easy to enumerate on MVJA, but that some strains were inhibited on this medium. Cassiday et al. (1989a) found LPM agar most suitable for isolating L. monocytogenes from ham, and the medium of Dominguez-Rodriguez et al. (1984) most suitable for oysters. Again, MVJA was reliable for differentiating L. monocytogenes from background contaminants, but was inhibitory to some strains. The same was found by Cassiday et al. (1989h) in a study of direct enumeration of L. monocytogenes from a number of foodstuffs. LPM agar was most suitable for isolating L. monocytogenes from Brie cheese and cabbage. Golden et al. (1990) in a comparison of 14 agar media, determined that direct plating procedures can successfully be utilized to recover L. monocytogenes from foods such as pasteurized milk and ice cream mix-

51

ture, which contain low populations of background microflora. Recovery of L. monocytogenes from Brie cheese and cabbage, which contain high populations of other microorganisms, was not satisfactory using direct plating procedures. Several newly-developed agar media have not, as yet, been evaluated in comprehensive, comparative studies. Lachica (1990) described LiCl-ceftazidime agar (LCA) which was as effective as LPM agar for enumeration of L. monocytogenes, more so for recovery of sublethally heat-injured cells. AlZoreky and Sandine (1990) have named a new medium after themselves (AS Listeria medium; ASLM) which contains acriflavine, ceftazidime and moxalactam. They found it to be just as effective as OA, but with the advantage of improved inhibition of micrococci, enterococci, and Gramnegative organisms. Recently, Cox et al. (1991a) described enhanced haemolysis agar (EHA) medium for selective isolation of L. monocytogenes from food enrichments. EHA medium compared favourably with OA (Cox et al., 1991b). PALCAM overlaid with agar containing sheep erythrocytes and supernatant fluid of a blood culture of Staphylococcus aureus allowed reliable visual reading of haemolysis (van Netten et al., 199 1). This modification gave up to 95% recovery of L. monocytogenes from raw food samples. Addition of egg yolk to PALCAM appears to benefit recovery of sublethally injured cells (in? Veld & de Boer, 1991). In a collaborative study comparing OA and PALCAM using reference samples, these authors reported significant differences between laboratories. Mean counts by direct plating on blood agar (control) were significantly higher than those on PALCAM or OA. This suggests that direct plating to the latter may not be reliable. Counts on PALCAM were higher than on OA after 48 h, but not significantly so. General conclusions from these comparisons are: (a) the USDA method is probably more suitable for most applications; (b) the FDA method may be acceptable under certain circumstances (e.g. when less selectivity is required to recover stressed cells and/or when less competition is expected; Lovett et al., 1991); (c) the inclusion of Fraser broth may be a useful screening test although it lacks selectivity; (d) the Listeria-Tek ELISA has shown promise as an alternative to conventional culture techniques; (e) OA, and probably PALCAM and MOX, are superior to LPM agar, but MMA is not an effective plating medium. (Although no one agar plating medium

52

P. J. Slade

has clearly emerged as superior, PALCAM medium appears to be preferred in Europe, whereas LPM agar and OA are the most widely used in North America; Farber & Peterkin, 1991); (f) no one protocol is suitable for detecting all Wisteria-positive samples. (For optimal detection of Listeria it may be necessary to run concurrently more than one protocol and almost certainly more than one selective plating agar; cf Salmonella isolation methodology.) Recovery of stressed L. monocytogenes Selective methods currently employed are not satisfactory for the recovery of injured Listeria (Smith & Archer, 1988). Bailey et al. (1990b) found that whereas both FDA and USDA enrichment broths were effective for recovery of nonstressed L. monocytogenes from chicken and Brie cheese, only the USDA broth consistently allowed recovery of heat-injured L. monocytogenes. The presence of glucose and lack of adequate buffering in the FDA formulation accounted for these differences. Crawford et al. (1989) determined that any L. monocytogenes surviving high-temperature short-time (HTST) pasteurization of milk will be injured and unable to multiply during either cold enrichment or FDA and USDA enrichment procedures. Thus L. monocytogenes detected by these methods in pasteurized milk products probably represent uninjured environmental contamination. Although the prevailing concensus is that properly conducted HTST pasteurization is effective at destroying L. monocytogenes in milk (Farber, 1989; Griffiths 1989; Mackey & Bratchell, 1989; Donnelly, 1990; Lovett et al., 1990; Bradshaw et al., 1991) there is still some disagreement on this issue (Farber & Peterkin, 1991). Two recent developments, both related to a greater or lesser extent to isolation methodology, may help explain some of the discrepancies. The first is the phenomenon known as the heat shock effect, and the other relates directly to the methods used for recovering heat-stressed cells (Farber & Peterkin, 1991). Use of strict anaerobic conditions when enumerating L. monocytogenes can lead to recovery of significantly more cells than are recovered in the presence of oxygen (Knabel et al., 1990). These workers found that culture of L. monocytogenes at 43°C followed by enumeration using strict anaerobic techniques resulted in decimal reduction times at 62.8”C (062.80cvalues) at least six-fold greater than those obtained using cells grown at 37°C prior to

plating aerobically. Increased to!erance to elevated (near-pasteurization) temperatures by prior holding of L. monocytogenes at high temperatures (43352°C) the so-called ‘tempering’ or ‘sublethal heat shock’ effect, has been described by Fedio and Jackson (1989), Farber and Brown (1990), Linton et al. (1990) Lovett et al. (1990), Mackey et al. (1990), Smith and Marmer (1991), and Smith et al. (1991). It appears that if foods containing L. monocytogenes are temperature-abused for even short periods, the organisms will acquire an increased heat tolerance and will require higher inactivation temperatures or longer processing (Smith & Mat-met-, 1991) L. monocytogenes growing at low temperatures are more susceptible to heat-induced death (Smith et al., 1991). Substantial proportions of surviving populations of L. monocytogenes heated at 52, 54 and 56°C were found by Golden et al. (1988a) to be injured. These authors suggested that methods to detect L. monocytogenes in heat-processed foods should be formulated to allow resuscitation of sublethally injured cells. They also found that cells stored frozen at -18°C for 14 days were only decreased 3-6%, but 82% of the surviving population was injured. Direct plating procedures were assessed to be satisfactory for recovery of injured L. monocytogenes (Golden et al., 1988b). Certainly more work is needed in the whole area of heat and other stress-induced injury, particularly how it relates to the heat-shock phenomenon and recovery of optimal numbers of Listeria using contemporary isolation methodology. Indicators of L. monocytogenes production environment

in the food

Feresu and Jones (1988) stated that because the five genomic species of L. monocytogenes sensu lato have such similar phenotypes, the presence of one species in a particular environment should be taken as indicative of the presence of others. Just how important is the concept of ‘all for one and one for all’ with regard to the presence of any Listeria spp. in a food production environment has never been substantiated. The presence of other Listeria spp. as indicators for L. monocytogenes has not been comprehensively and systematically studied, although Seeliger (1988) noted that L. innocua is a good indicator for L. monocytogenes and the presence of either species is equally significant. In an evaluation of the Gene-Trak Listeria assay, Klinger and Johnson (1988) found several

Monitoring Listeria

in the food production environment.I.

retail samples of meats and cheese contaminated with various levels of L. monocytogenes and at least one other Listeria sp. This highlighted the usefulness of a screening test for all species. In justifying the genus level specificity of the Gene-Trak assay, King et al. (1990) stated that ‘since all Listeria inhabit the same environment the presence of one species would be a compelling argument for the possible presence of pathogenic strains’. However, it appears that very often single species of Listeria may predominate at given sites in processing plants (Walker et al., 1991). These authors suggested that competition of Listeria species for environmental niches in processing plants warrants further examination. Moreover, there is evidence to suggest that different selective techniques may be more predisposing to isolation of one or more Listeria spp. but not to others. For example, LPM agar has been found inhibitory to L. grayi and L. murrayi (Loessner et al., 1988). Current techniques for isolation of ‘Listeria spp.’ have been designed to facilitate recovery of L. monocytogenes sense stricto. It is uncertain that all Listeria spp. grow equally well in selective media formulated primarily to isolate L. monocytogenes. Present methodology probably does not give a true picture of the occurrence of all Listeria spp. in the environment. Furthermore, some newer indicator systems do not recognize all species. Use of the PALCAM medium of van Netten et al. ( 1989) for example, would exclude mannitol-positive L. grayi and L. murrayi from further investigation. The low reported incidence of L. grayi, L. murrayi and even L. invanovii and L. seeligeri may indicate that these species occupy environmental niches that are rarely tested or which may not be productive using methods currently employed. Frances et al. (1991) isolated L. seeligeri more often than any other species from freshwater sites. Whether or not the primary ecological niche of L. seeligeri is the aquatic environment on whether these findings represent contamination of fresh waters from other environmental sources is presently uncertain. Use of other microorganisms to indicate the possible presence of Listeria has been proposed. Frank et al. (1990) analyzed environmental swabs from 15 dairy plants for the presence of various indicators of hygienic practices and Listeria. Some 85% of Listeria-positive swabs also had counts of staphylococci in excess of 6.6 X 103 CFU. In vacuum-packaged, uncured turkey loaf inoculated with E. faecalis and L. monocytogenes and stored

53

at 3°C the survival of the enterococci was not significantly different to that of L. monocytogenes (Ingham & Tautorus, 1991). This suggested that enterococci could be used as indicators of L. monocytogenes contamination in processed meats (Ingham & Tautorus, 1991). It is evident that a better understanding of the microbial ecology of Listeria in the food production environment will assist in identification of potential sources of contamination in the food supply. This will enable the design of more effective control measures. Of quintessential importance is the establishment of sensitive methodology for isolation and enumeration of Listeria spp. (specifically L. monocytogenes) from production environments and foodstuffs. Resuscitation from sublethal injury, interactions of Listeria with the normal flora of processing plant environments and foods (particularly how these affect selective recovery of Listeria), and the development of screening devices, indicators or measurements by which to aid detection of Listeria are aspects still requiring considerable attention. The second and third parts of this review will respectively consider identification and typing of Listeria spp., by both conventional and alternative means, and describe how such sophisticated tools may be of benefit in ‘finetuning’ the monitoring of Listeria in the food production environment.

ACKNOWLEDGEMENTS The support of Dr D. L. Collins-Thompson is gratefully appreciated. Thanks to Jeanne Hogeterp for typing the manuscript.

REFERENCES Al-Zoreky, N. & Sandine, W. E. (1990). Highly selective medium for isolation of Listeria monocytogenes from food. Appl. Environ. Microbial., 56, 3 154-l. Anon (1988). Microbial control during production of readyto-eat meat products. Controlling the incidence of Listeria monocytogenes, 2nd edn. American Meat Institute, Washington, D.C., 1988. Arihara, K., Ogihara, S., Sakata, J., Itoh, M. & Kondo, Y. (1991). Antimicrobial activity of Enterococcus faecalis against Listeria monocytogenes. Lett. Appl. Microbial., 13, 190-2.

Bailey, J. S., Fletcher, D. L. & Cox, N. A. (1990a). Effect of enrichment media and sampling protocol on recovery of Listeria monocytogenes.

J. Food Prot., 53, 505-7.

Bailey, J. S., Fletcher, D. L. & Cox, N. A. (19906). Efficacy of enrichment media for recovery of heat-injured Listeria monocytogenes.

J. Food Pmt.. 53, 473-1.

P. J. Slade

54

Bannerman, E. S. & Bille, J. (1988). A new selective medium for isolating Listeria spp. from heavily contaminated material. Appl. Environ. Microbial., 54, 165-7. Barnes, R., Archer, P., Strack, J. & Istre, G. R. (1989). Listeriosis associated with consumption of turkey franks. MMWR,

38, 267-8.

Beckers, H. J., Soentoro, P. S. S. & Delfgou-van Asch, E. H. M. (1987). The occurrence of Listeriu monocytogenes in soft cheeses and raw milk and its resistance to heat. Int. J. Food Microbial.. 4, 249-56.

Brackett, R. E. (1988). Presence and persistence of Listeria monocytogenes in food and water. Food Technol., 42, 1624.

Bradshaw, J. G., Peeler, J. T. & Twedt, R. M. (1991). Thermal resistance of Listeriu spp. in milk. J. Food Prot., 54, 12-14.

Buchanan, R. L. (1990). Advances in cultural methods for the detection of Listeria monocytogenes. In Foodborne Listeriosis, ed. A. J. Miller, J. L. Smith & G. A. Somkuti. Elsevier, New York, pp. 85-95. Buchanan, R. L., Stahl, H. G. & Archer, D. L. (1987). Improved plating media for simplified, quantitative detection of Listeria monocytogenes in foods. Food Microbial., 4, 269-75.

Buchanan, R. L., Stahl, H. G., Beneivengo, M. M. & Del Corral, F. (1989). Comparison of lithium chloridephenylethanol-moxalactam and modified Vogel Johnson agar for detection of Listeriu spp. in retail-level meats, poultry and seafood. Appl. Environ. Microhiol., 55, 599-603.

Cassiday, P. K. & Brackett, R. E. (1989). Methods and media to isolate and enumerate Listeria monocytogenes: A review. J. Food Prot., 52, 207-14.

Cassiday, P. K., Brackett, R. E. & Beuchat, L. R. (1989~). Evaluation of ten selective direct plating media for enumeration of Listeria monocytogenes in hams and oysters. Food Microbial., 6, 113-25. Cassiday, P. K., Brackett, R. E. & Beuchat, L. R. (1989b). Evaluation of three newly developed direct plating media to enumerate Listeria monocytogenes in foods. Appl. Environ. Microbial., 55, 1645-8.

Charlton, B. R., Kinde, H. & Jensen, L. H. (1990). Environmental survey for Listeriu species in California milk processing plants, J. Food Prof., 53, 198-201. Cox, L. J., Kleiss, T., Cordier, J. L., Cordellana, C., Konkel, P., Pedrazzini, C., Beumer, R. & Siebenga, A. (1989). Listeriu spp. in food processing, non-food and domestic environments. Food Microbial.. 6, 49-61. Cox, L. J., Siebenga, A., Pedrazzini, C. & Moreton, J. (1991~). Enhanced haemolysis agar (EHA)-An improved selective and differential medium for isolation of Listeria monocytogenes. Food Microbial., 8, 3749. Cox, L. J., Siebenga, A., & Pedrazzini,

C. (1991b). Performance of enhanced haemolysis agar compared to Oxford medium for the isolation of Listeria monocytogenes from food enrichment broths. Food Microbial., 8, 51-62. Crawford, R. W., Beliveau, C. M., Peeler, J. T., Donnelly, C. W. & Bunning, V. K. (1989). Comparative recovery of uninjured and heat-injured Listeriu monocytogenes cells from bovine milk. Appl. Environ. Microbial., 55, 14904. Curtis, G. D. W., Mitchell, R. G., King, A. F. & Griffin, E. J. (1989). A selective differential medium for the isolation of Listeriu monocytogenes. Lett. Appl. Microbial., 8, 95-8. Dallas, H. L., Tran, T. T., Poindexter, C. E., Hitchins, A. D. & Romanell, L. J. (1991). Competition of food bacteria with Listeriu monocytogenes during enrichment culture. J. Food Safety, 11, 293-30 1. Dominguez-Rodriguez, L., Suarez-Fernandez, G., Fernandez-

Garayzabal, J. F. & Rodriguez-Ferri, E. (1984). New methodology for the isolation of Listeria monocytogenes from heavily contaminated environments. Appl. Environ. Microbial., 47, 1188-90. Donnelly, C. W. (1990). Resistance of Listeria monocytogenes to heat. In Foodborne Listeriosis, ed. A. J. Miller, J. L Smith & G. A. Somkuti. Elsevier, New York, pp. 189-93. Donnelly, C. W. & Baigent, G. J. (1986). Method for flow cytometric detection of Listeria monocytogenes in milk. Appl. Environ. Microbial., 51, 689-95.

Doyle, M. P. & Schoeni, J. L. (1986). Selective-enrichment procedure for isolation of Listeria monocytogenes from fecal and biologic specimens. Appl. Environ. Microbial., 51, 1127-9. Doyle, M. P. & Schoeni, J. L. (1987). Comparison of procedures for isolating Listeria monocytogenes in soft, surfaceripened cheese. J. Food Prot., 50, 4-6. Farber, J. M. (1989). Thermal resistance of Listeriu monocytogenes in foods. Int. J. Food Microbial., 8, 285-91. Farber, J. M. & Brown, B. E. (1990). Effect of prior heat shock on heat resistance of Listeriu monocytogenes in meat. Appl. Environ. Microbial., 56, 15847. Farber, J. M. & Peterkin, P. E. (1991). Listeriu monocytogenes, a food-borne pathogen. Microbial. Rev., 55, 476-511. Farber, J. M., Sanders, G. W. & Malcolm, S. A. (1988). The presence of Listeriu spp. in raw milk in Ontario. Can. J. Microbial., 34, 95-100.

Farrag,

S. A. & Marth, E. H. (1991). Behavior of Listeria in skim milk

monocytogenes in the presence of flavobacteria at 7 or 13°C. J. Food Prot., 54, 677-80.

Federal Register (1988). Bacteriological Analytical Manual, Chapter 29, Listeria isolation; revised method of analysis. Fed. Regist., 55, 44153-8.

Fedio, W. M. & Jackson, H. (1989). Effect of tempering on the heat resistance of Listeria monocytogenes. Lett. Appl. Microbial., 9, 157-60.

Fenlon, D. R. (1985). Wild birds and silage as reservoirs of Listeria in the agricultural environment. J. Appl. Bacterial., 59, 53743.

Feresu, S. B. & Jones, D. (1988). Taxonomic studies on Brochothrix, Erysipelothrix, Listeriu and atypical lactobacilli. J. Gen. Microbial., 134, 1165-83. Fernandez-Garayzabal, J. & Genigeorgis, C. (1990). Quantitative evaluation of three selective enrichment broths and agars used in recovering Listeriu microorganisms. J. Food Prot., 53, 105-10.

Frances, N., Hornby, H. & Hunter, P. R. (1991). The isolation of Listeria species from fresh-water sites in Cheshire and North Wales. Epidemiol. Infect., 107, 23558. Frank, J. F., Gillett, R. A. N. & Ware, G. 0. (1990). Association of Listeria spp. contamination in the dairy processing plant environment with the presence of staphylococci. J. Food Prot., 53, 928-32.

Fraser, J. A. & Sperber, W. H. (1988). Rapid detection of Listeria spp. in food and environment samples by esculin hydrolysis. J. Food Prot., 51, 762-5. Gabis, D. A., Flowers, R. S., Evanson, D. & Faust, R. E. (1989). A survey of 18 dry dairy product processing plant environments for Sulmonella. Listeria and Yersinia. J. Food Prot., 52, 1224.

Genigeorgis, C. A., Dutulescu, D. & Fernandez-Garayzabal, J. (1989). Prevalence of Listeria spp. in poultry meat at the supermarket and slaughterhouse level. J. Food Prot., 52, 618-24.

Genigeorgis, C. A., Oanca, P. & Dutulescu, D. (1990). Prevalence of Listeria spp. in turkey meat at the supermarket and slaughterhouse level. J. Food Prot., 53, 282-8. Golden, D. A., Beuchat, L. R. & Brackett, R. E. (1988a). In-

Monitoring

Listeria

in the food

activation and injury of Listeria monocytogenes as affected by heating and freezing. Food Microbial., 5, 17-23. Golden, D. A., Beuchat, L. R. & Brackett, R. E. (39886). Evaluation of selective direct plating media for their suitability to recover uninjured, heat-injured, and freeze-injured Listeria monocytogenes from foods. Appl. Environ. Microbiol., 54, 1451-6. Golden, D. A., Brackett, R. E. & Beuchat, L. R. (1990). Efficacy of direct plating media for recovery of Listeria monocytogenes from foods. Int. J. Food Microbial., 10, 143355. Gray, M. L. & Killinger, A. H. (1966). Listeria monocytogenes and listeric infections. Bacterial. Rev., 30, 309-82. Griffiths, M. W. (1989). Listeria monocytogenes: Its importance in the dairy industry. J. Sci. Food Agric., 47, 133-58. Harding, C. D. & Shaw, B. G. (1990). Antimicrobial activity of Leuconostoc gelidum against closely related species and Listeria monocytogenes. J. Appl. Bacterial., 69, 648854. Harris, L. J., Daeschel, M. A., Stiles, M. E. & Klaenhammer, T. R. (1989) Antimicrobial activity of lactic acid bacteria against Listeria monocytogenes. J. Food Prot., 52, 384-7. Hayes, P. S., Feeley, J. C., Graves, L. M., Ajello, G. W. & Fleming, D. W. (1986). Isolation of Listeria monocytogenes from raw milk. Appt. Environ. Microbial., 51, 438840. Hayes, P. S., Graves, L. M., Ajello, G. W., Swaminathan, B., Weaver, R. E., Wenger, J. D., Schuchat, A., Broome, C. V. & the Listeria study group (1991). Comparison of cold enrichment and U.S. Department of Agriculture methods for isolating Listeria monocytogenes from naturally contaminated foods. Appl. Environ. Microbial., 57, 2109-13. Heisick, J. E., Harrell, F. M., Peterson, E. H., McLaughlin, S., Wagner, D. E., Wesley, I. V. & Bryner, J. (1989). Comparison of four procedures to detect Listeria spp. in foods. J. Food Prot., 52, 1541. Hill, W. E. (1987). Detection of pathogenic bacteria by DNA colony hybridization. In Food and Drug Administration Bucteriological Amdvtical Manual, 6th edn, Ch. 26 (Supplement). AOAC, Arlington, 1984. Hitchins, A. D. (1989). Quantitative comparison of two enrichment methods for isolating Listeria monocytogenes from inoculated ice cream. J. Food Prot., 52, 8988900. Hudson, W. R. & Mead, G. C. (1989). Listeria contamination at a poultry processing plant. Left. Appl. Microbial., 9, 21 l-14. Ingham, S. C. & Tautorus, C. L. (1991). Survival of Salmonella typhimurium, Listeriu monocytogenes and indicator bacteria on cooked uncured turkey loaf stored under vacuum at 3°C. J. Food Safety, 11, 285592. In? Veld, P. H. & de Boer, E. (1991). Recovery of Listeria monocytogenes on selective agar media in a collaborative study using reference samples. Int. J. Food Microbial., 13, 295-300. King, W., Raposa, S. M., Warshaw, J. E., Johnson, A. R., Lane, D., Klinger, J. D. & Halbert, D. N. (1990). A colorimetric assay for the detection of Listeria using nucleic acid probes. In Foodborne Listeriosis, ed. A. J. Miller, J. L. Smith & G. A. Somkuti. Elsevier, New York, pp. 117-24. Klausner, R. B. & Donnelly, C. W. (1991). Environmental sources of Listeria and Yersinia in Vermont dairy plants. J. Food Prot., 54, 607-l 1. Klinger, J. D. (1988). Isolation of Listeria: A review of procedures and future prospects. Injection, 16, S98-S105. Klinger, J. D. & Johnson, A. R. (1988). A rapid nucleic acid hybridization assay for Listeria in foods. Food Technol., 42, 66670. Klinger, J. D., Johnson, A., Croan, D., Flynn, P., Whippie, K., Kimball, M., Lawrie, J. & Curiale, M. (1988). Comparative studies of nucleic acid hybridization assay for Listeria in foods. J. Assoc. 08 Anal. Chem., 71, 669973.

production

environment.

I.

55

Knabel, S. J., Walker, H. W., Hartman, P. A. & Mendonca, A. F. (1990). Effects of growth temperature and strictly anaerobic recovery on the survival of Listeria monocytogenes during pasteurization. Appl. Environ. Microbial., 56, 370-6. Lachica, R. V. (1990). Selective plating medium for quantitive recovery of foodborne Listeria monocytogenes. Appl. Environ., 56, 16779. Lammerding, A. M. & Doyle, M. P. (1989). Evaluation of Listeria monocytoenrichment procedures for recovering genes from dairy products. Int. J. Food Microbial., 9, 249-68. Lee, W. H. & McLain, D. (1986). Improved Listeria monocytogenes selective agar. Appt. Environ. Microbial., 52. 1215-17. Lewis, S. J. & Corry, J. E. L. (1991a). Comparison of a cold enrichment and the FDA method for isolating Listeria monocytogenes and other Listeria spp. from ready-to-eat food on retail sale in the U.K. Int. J. Food Microbial., 12, 281-6. Lewis, S. J. & Corry, J. E. L. (1991b). Survey of the incidence of Listeria monocytogenes and other Listeria spp. in experimentally irradiated and in matched unirradiated raw chickens. Int. J. Food Microbial., 12, 257-62. Linton, R. H., Pierson, M. D. & Bishop, J. R. (1990). Increase in heat resistance of Listeria monocvtogenes Scott A by sublethal heat shock. J. Food Prot., 53, 92447. Loessner, M. J., Bell, R. H., Jay, J. M. & Shelef, L. A. (1988). Comparison of seven plating media for enumeration of Listeria spp. Appl. Environ. Microbial.. 54, 3003-7. Lovett, J. (1988a) Isolation and enumeration of Listeriu monocytogenes. Food Technol., 42, 17225. Lovett, J. (1988b). Isolation and identification of Listeriu monocytogenes in dairy products. J. Assoc. ofi: Anul. Chem., 71, 658860. Lovett, J.. Francis. D. W. & Hunt, J. M. (1987). Listeriu monocytogenes in raw milk: Detection, incidence and pathogenicity. J. Food Prot., 50, 188892. Lovett, J., Wesley, I. V., Vandermaaten, M. J., Bradshaw, J. G., Francis, D. W., Crawford, R. G., Donnelly, C. W. & Messer, J. W. (1990). High-temperature, short-time pasteurization inactivates Listeriu monocytogenes. J. Food. Prot., 53, 734-8. Lovett, J., Francis, D. W., Peeler, J. T. & Twedt, R. M. (1991). Quantitative comparison of two enrichment methods for from seafoods. J. Food isolating Listeriu monocytogenes Prot., 54, 7--l I. Lund, A. M., Zottola, E. A. & Pusch, D. J. (1991). Comparison of methods for isolation of Listeriu from raw milk. J. Food Prot., 54, 60226. Mackey, B. M. & Bratchell, N. (1989). A review: The heat reLett. Appl. Microhiol.. sistance of Listeriu monocytogenes. 9, 89994. Mackey, B. M., Pritchet, C., Norris, A. & Mead, G. C. (1990). Heat resistance of Listeriu: Strain differences and effects of meat type and curing salts. Lett. Appt. Microbial., 10, 251-5. Marshall, D. L. & Schmidt, R. H. (1991). Physiological evaluation of stimulated growth of Listeria monocytogenes by Pseudomonas species in milk. Can. J. Microbial., 37, 5949. Mattingly, J. A., Butman, B. T., Plank, M. C., Durham, R. J. & Robison, B. J. (1988). Rapid monoclonal antibody-based enzyme-linked immunosorbent assay for detection of Listeria in food products. J. Assoc. Ofl Anal. Chem., 71, 67998 I. McBride, M. E. & Guard, K. F. (1960). A selective method for the isolation of Listeria monocytogenes from mixed bacterial populations. J. Lub. Clin. Med., 55, 15337. McLain, D. & Lee, W. H. (1988). Development of the

P. J. Slade

56

USDA-FSIS

method

for isolation

genes from raw meat and poultry. Chem., 71, 6604.

of Listeriu monocytoJ. Assoc. Ofl

Anal.

McLain, D. & Lee, W. H. (1989). FSIS method for the isolation and identification of Listeria monocytogenes from processed meat and poultry products. Laboratory communication No. 57. Revised May 24, 1989, USDA FSIS, Beltsville, MD. Nelson, J. H. (1990). Where are Listeria likely to be found in dairy plants? Dairy, Food, Environ. Sanit., 10, 344-5. Noah, C. W., Perez, J. C., Ramos, N. C., McKee, C. R. & Gipson, M. V. (1991). Detection of Listeria spp. in naturally contaminated seafoods using four enrichment procedures. J. Food Prot., 54, 1747. Norrung, B., Solve, M., Ovesen, M. & Skovgaard, N. (1991). Evaluation of an ELISA test for detection of Listeria spp. J. Food Prot., 54, 752-S.

Palumbo, S. A. (1991). A review of methods for detection of the psychrotrophic foodborne pathogens Listeria monocytogenes and Aeromonas hydrophila. J. Food Safety, 11, 105-22. Parish, M. E. & Higgins, D. P. (1989). Extinction of Listeriu monocytogenes in single-strength orange juice: Comparison of methods for detection of in mixed populations. J. Food Sqfety, 9, 267-77.

Pini, P. N. & Gilbert, R. J. (1988). A comparison of two procedures for the isolation of Listeria monocytogenes from raw chickens and soft cheeses. Int. J. Food Microbial., 7, 331-7.

169-83.

Seehger, H. P. R. (1988). Listeriosis-History and actual developments. Infection, 16, S80-S84. Slade, P. J. & Collins-Thompson, D. L. (1988). Comparison of two-stage and direct selective enrichment techniques for isolating Listeriu spp. from raw milk. J. Food Sri., 53,

20-2.

Varabioff, Y. (1990). Incidence and recovery of Listeria from chicken with a pre-enrichment technique. J. Food Prot., 53, 555-7, 624.

Venables, L. J. (1989). Listeriu monocytogenes in dairy products-The Victorian experience. Food Austral., 41, 942-3. Walker, S. J., Archer, P. & Appleyard, J. (1990). Comparison of the Listeria-Tek ELISA kit with cultural procedures for the detection of Listeria species in foods. Food Microbial., Walker, R. L., Jensen, L. H., Kinde, H., Alexander, A. V. & Owens, L. S. (1991). Environmental survey for Listeria species in frozen milk product plants in California. J. Food Prot., 54, 178-82.

Warburton, D. W., Farber, J. M., Armstrong, A., Caldeira, R., Hunt, T., Messier, S., Plante, R., Tiwari, N. P. & Vinet, J. (1991a). A comparative study of the FDA and USDA methods for the detection of Listeria monocytogenes in foods. Int. J. Food Microbial., 13, 105-17. Warburton, D. W., Farber, J. M., Armstrong, A., Caldeira, R., Tiwari, N. P., Babiuk, T., LaCasse, P. & Read, S. (19916). A Canadian comparative study of modified versions of the ‘FDA’ and ‘USDA’ methods for the detection of Listeria monocytogenes. J. Food Prot., 54, 669-76. Welshimer, H. J. (1960). Survival of Listeriu monocytogenes in soil. J. Bucteriol., 80, 316-20. Welshimer, H. J. & Donker-Voet, J. (1971). Listeria monocytogenes in nature. Appl. Microbial., 21, 516-19. Wenger, J. D., Swaminathan, B., Hayes, P. S., Green, S. S., Pratt, M., Pinner, R. W., Schuchat, A. & Broome, C. V. (1990). Listeriu monocytogenes contamination in turkey franks: Evaluation of a production facility. J. Food Prot., 53, 1015-19.

1694-5, 1702.

injury in

Listeria monocytogenes. J. Ind. Microbial., 3, 105-10.

Smith, J. L. & Marmer, B. S. (1991). Temperature shift effects on injury and death in Listeria monocytogenes Scott A. J. Food Safety, 11, 73-80. Smith, J. L., Marmer, B. S. & Benedict, R. C. (1991). Influence of growth temperature on injury and death of Listeriu monocytogenes Scott A during a mild heat treatment. J. Food Prot., 54, 166-9.

Tiwari, N. P. & Aldenwrath, S. G. (1990). Occurrence of Listeria species in food and environmental samples in Alberta. Gun. Inst. Food Sci. Technol. J., 23, 109-I 3.

Microbial., 8, 299-3 16.

van Netten, P., van Gaal, B. & Mossel, D. A. A. (1991). Selection, differentiation and counting of haemolytic Listeria spp. on PALCAM medium. Left. Appl. Microbial., 12,

7, 33542.

Ralovich, B. (1975). Selective and enrichment media to isolate Listeria. In Problems of Listeriosis. Proc. Sixth International Symposium, ed. M. Woodbine, University Press, Leicester, pp. 287794. Ralovich, B. (1989). Data on the enrichment and selective cultivation of listeriae. Int. J. Food Microbial., 8, 205-17. Rallier, I., Croegaert, Th., Verplaetse, A. & van Hoof, J. (1991). Comparison of three plating media for enumeration and three media for isolation of Listeriu spp. in fermented sausages. Arch. Lebensmittelhyg., 42, 65-9. Schiemann, D. A., Shope, S. R. & Brown, M. J. (1990). Development of new enrichment broths and plating agars for isolation of hemolytic species of Listeriu. J. Food Safety, 10, 233-52. Schuchat, A., Swaminathan, B. & Broome, C. V. (1991). Epidemiology of human listeriosis. Clin. Microbial. Rev., 4,

Smith, J. L. & Archer, D. L. (1988). Heat-induced

Tran, T. T., Stephenson, P. & Hitchins, A. D. (1990). The effect of aerobic mesophilic microflora levels on the isolation of inoculated Listeria monocytogenes strain LM82 from selected foods. J. Food Safety, 10, 267-75. Truscott, R. B. & McNab, W. B. (1988). Comparison of media and procedures for the isolation of Listeriu monocytogenes from ground beef. J. Food Prot., 51, 6268. Valdes-Stauber, N., G&z, H. & Busse, M. (1991). Antagonistic effect of coryneform bacteria from red smear cheese against Listeria species. Int. J. Food Microbial., 13, 119-30. van Netten, P., Perales, I., van de Moosdijk, A., Curtis, G. D. W. & Mossel, D. A. A. (1989). Liquid and solid selective differential media for the detection and enumeration of L. monocytogenes and other Listeria spp. Int. J. Food

Wenzel, J. M. & Marth, E. H. (1990). Behavior of Listeria monocytogenes in the presence of Streptococcus lactis in a medium with internal pH control. J. Food Prot., 53, 918-23.

Yousef, A. E., Luchansky, J. B., Degnan, A. J. & Doyle, M. P. (1991). Behavior of Listeria monocytogenes in Wiener exudates in the presence of Pediococcus acidilactici H or pediocin AcH during storage at 4 or 25°C. Appl. Environ. Microbial., 57, 1461-7. Yu, L. S. L. & Fung, D. Y. C. (1991). Evaluation of FDA and USDA procedures for enumerating Listeria monocytogenes in ground beef. Food Microbial., 8, 69-74.