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Prevalence of Listeria spp. in ready to eat foods (RTE) from Algiers (Algeria)
Food Control 23 (2012) 397e399
Contents lists available at SciVerse ScienceDirect
Food Control journal homepage: www.elsevier.com/locate/foodcont
Prevalence of Listeria spp. in ready to eat foods (RTE) from Algiers (Algeria) Leila Bouayad*, Taha-Mossadak Hamdi Animal Health and Production Laboratory, High National Veterinary School, BP 161 El-Harrach, Algiers, Algeria
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 February 2011 Received in revised form 27 July 2011 Accepted 2 August 2011
The aim of this study was to establish the occurrence of Listeria spp., especially Listeria monocytogenes in ready to eat RTE food marketed in Algiers (Algeria). A total of 227 samples were collected from different producers and retailers. All samples were analyzed using a conventional cultivation method AFNOR V08-055. Out of 227 samples tested, 21 (9.3%) tested positive for Listeria spp. among them, 6 (2.6%) tested positive for L. monocytogenes. L. innocua was the most common Listeria species found being detected in 11 samples (4.8%), although both Listeria ivanovii and Listeria welshimeri were detected in 3 (1.3%) and 1 (0.4%) food samples respectively. The study of the antimicrobial sensitivity of Listeria monocytogenes strains showed no resistance. The study has enabled us to detect these contaminants in a wide range of RTE foods, to suggest that contamination likely occurs after heat treatment, and to assess the danger represented by this category of food for populations at risk. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: RTE Listeria spp. Listeria monocytogenes Prevalence Antimicrobial sensitivity
1. Introduction Listeria monocytogenes is the causative agent of listeriosis, an often fatal disease linked to the consumption of contaminated foods. It has long been known that the pathogen is transmitted by food and has been implicated in several large outbreaks of foodborne listeriosis in North America and Europe (Farber & Peterkin, 1991). Clinical symptoms of listeriosis include meningitis, septicaemia and neonatal infections (Rocourt, 1994). The disease is particularly dangerous for pregnant women, newborns and immunocompromised individuals (Farber & Peterkin, 1991). A variety of food, mainly ready to eat products have been found to be contaminated with L. monocytogenes. The Prevalence of L. monocytogenes in raw and cooked meats was about 35% and 18%, respectively in Spain (Vitas & Garcia-Jalon, 2004). In United States, 1% of cooked meats were contaminated with L. monocytogenes (Gombas, Chen, Clavero, & Scott, 2003). Little is known about the distribution of this pathogen in food supply in Algeria and no data exist on the prevalence of L. monocytogenes in RTE foods, only the contamination in raw milk has been reported (Hamdi, Naim, Martin, & Jacquet, 2007). The main objectives were an initial evaluation of the Listeria spp. prevalence in some RTE foods products, marketed in retail stores or * Corresponding author. Tel./fax: þ213 21 82 14 33. E-mail address: [email protected] (L. Bouayad). 0956-7135/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodcont.2011.08.006
distributed in different schools or company cafeterias in Algiers, and the study of the antimicrobial sensitivity of isolated L. monocytogenes strains. 2. Materials and methods Samples of RTE foods (227 samples in total) were collected in sterile plastic bags in different municipalities of Algiers over a period from April 2007 to April 2008, in collaboration with each municipal health office. Food samples were stored at 4 C and processed within 4 h of collection. Among the samples 33.5% were from retail stores, 26% from school or company cafeterias, 21.5% from restaurants and 19% from manufacturers of dairy or meat products. Isolation and identification of Listeria spp. was carried out using a cultivation methodology (AFNOR V08-055; French Agency for Standardization). Briefly, a 25 g sample of each food was weighed into a sterile stomacher bag, then 225 mL of half Fraser broth (Merck) was added. The sample was homogenized using a stomacher for 2 min. Homogenates were incubated for 24 h at 30 C. Positive enrichment cultures (broth darkening) were streaked onto Palcam agar (Merck) and incubated at 37 C for 24 h. Putative Listeria spp. isolates were Gram stained, stabbed into a motility medium for observing the characteristic umbrella motility and tested for oxidase activity. Isolates were speciated using the API Listeria strips carried out according to the manufacturer recommendations (BioMérieux, France).
398
L. Bouayad, T.-M. Hamdi / Food Control 23 (2012) 397e399
Antimicrobial sensitivity was assessed using the disk diffusion assay, according to the criteria defined by the CLSI (Clinical Laboratory Standard Institute) recommended by the WHO (World Health Organization). The isolated strains were seeded in Mueller-Hinton agar supplemented with 0.5% defibrinated sheep blood. The following antibiotics were used: gentamicin (10 mg), piperacillin (30 mg), tetracycline (30 mg) (BIORAD), trimethoprim/sulfamethoxazole (1.25/23.75 mg), amoxicilin/Clavulanic acid (20/10 mg) and chloramphenicol (30 mg) (OXOID).
Origin of samples
Number of L. m analyzed No (%) samples
Retail trade School and company cafeterias Restaurant Manufacturer
76 59
3 (3.9) 1 (1.3) 1 (1.3) 0 (0) 5 (6.5) 3 (5.1) 6 (10) 1 (1.7) 1 (1.7) 11 (18.6)
49 43
0 (0) 0 (0)
L. in No (%)
L. iv No (%)
3 (6.1) 1 (2) 1 (2.3) 0 (0)
L. w No (%)
0 (0) 0 (0)
Total of positives samples No (%)
4 (8.1) 1 (2.3)
Abbreviations: No, Number of samples; L. m, Listeria monocytogenes; L. in, Listeria innocua; L. iv, Listeria ivanovii; L. w, Listeria welshimeri.
3. Results Listeria spp. was obtained from 21 (9.3%) out of 227 food samples tested, distributed as follows: 2.6% to L. monocytogenes, 4.8% to L. innocua, 1.3% to L. ivanovii and 0.4% to L. welshimeri (Table 1). Samples of RTE foods were classified into four categories: 99 (43.6%) were dairy products, of which 64 (64.6%) had undergone heat treatment and 35 (35.4%) represented fermented products produced from raw milk. 23 (10.1%) unpacked sliced meat products, 94 (41.4%) cooked meat dishes (sandwiches and hot dishes). 4.8% of the RTE samples did not fall within the four main categories (salad, mayonnaise). In dairy products, L. monocytogenes was isolated only in soft cheese (dairy products which have undergone a heat treatment) and fermented milk product with à pH 5.1. In the unpacked sliced meat products and cooked meat dishes, only the sausage sandwiches and one cooked meat dish were found contaminated with L. monocytogenes. According the origins of the samples, the overall prevalence of Listeria spp. positive samples (L. monocytogenes positive samples in parentheses) was 18.5% (5.1%) in school and company cafeterias, 8.1% (0%) in restaurants, 6.5% (3.9%) in retail and 2.3% (0%) in manufacturers (Table 2). The study of the antimicrobial sensitivity of L. monocytogenes isolated strains showed no resistance. 4. Discussion 9.3% is the overall prevalence of Listeria spp. recorded during our study. Respective prevalences of 12% and 5.6% were reported by
Table 1 Prevalence of Listeria spp. in RTE food. Nature of samples
Table 2 Prevalence of Listeria spp. according their origin.
No Listeria L. m analyzed spp. No No (%) (%)
L. in No (%)
Milk products which have undergone heat treatment Soft cheese 39 2 (5.1) 1 Homemade cheese 5 0 (0) 0 Cheese 3 0 (0) 0 Semihard cheese 3 0 (0) 0 Cream cheese 14 0 (0) 0 Raw fermented dairy product Whey 35 1 (2.8) 1 Unpacked sliced meat product Cachir (local pâté) 8 0 (0) 0 Pâté 15 0 (0) 0 Cooked meat dishes Merguez sandwich 14 2 (14.2) 2 Meat dishes 80 1 (1.2) 6 Various 11 0 (0) 1 Total 227 21(9.3) 6 (2.6) 11
L. iv No (%)
L. w No (%)
(2.6) (0) (0) (0) (0)
0 0 0 0 0
0 0 0 0 0
(2.8)
0 (0)
(0) (0)
1 (12.5) 0 (0) 1 (6.6) 0 (0)
(14.2) (7.5) (9) (4.8)
0 1 0 3
(0) (0) (0) (0) (0)
(0) (1.2) (0) (1.3)
(0) (0) (0) (0) (0)
0 (0)
0 1 0 1
(0) (1.2) (0) (0.4)
Abbreviations: No, Number of samples; L. m, Listeria monocytogenes; L. in, Listeria innocua; L. iv, Listeria ivanovii; L. w, Listeria welshimeri.
Jalali and Abedi in Iran in 2008 and Wagner, Auer, Trittremmel, Hein, and Schoder in Austria in 2007. The prevalence of L. monocytogenes was in agreement with previous studies (Shen et al, 2006; Wilson, 1995), however it remains fairly low compared to that reported in 2003 by the FDA/ USDA (0.2%e12.9%). In heat treated dairy products, we recorded an L. monocytogenes contamination, 2 (3%) samples out 64 were positive, numerous studies showed that L. monocytogenes flora contaminates dairy product after heat treatment (Anonymous, 2003; Jouve, 1996). This contamination likely results from (i) Insufficient heat treatment of the milk to kill the organism (Farber & Peterkin, 1991), (ii) The resistance of Listeria to the decreased pH which occurs in cheese production (Rosset, 2001), (iii) Cross contamination through air, soil, equipment and processing units staff (Jouve, 1996). In whey samples, product widely consumed in Algeria, we have recorded the same contamination by L. monocytogenes and L. innocua. In Algeria Hamdi et al. (2007) have reported a prevalence of 9.1% per L. innocua. The contamination of this kind of product would be due to the resistance of Listeria to acidic pH (Tienungoon, Ratkowsky, McMeekin, & Ross, 2000). We observe that there are differences between the prevalences that we recorded and those reported in other studies, this could be due to different sampling procedures and sites and the method of detection of the bacteria may also contribute to these variations. In unpacked sliced meat products and cooked meat dishes we have isolated four different species of Listeria which L. monocytogenes was detected in 3 (2.6%) samples. In the USA, Gombas et al. (2003) have reported a prevalence of L. monocytogenes of 0.9% in deli meats. While, in (1991), the International Committee on Food Microbiology and Hygiene in Great Britain estimated it in cooked meats between 5% and 7%. All analyzed meat products have undergone a heat treatment, which can destroy most bacteria present in the raw material. Contamination of cooked meat products and dishes is likely related to cross contamination occurring during subsequent handling of the heat treated products. The contamination recorded in grilled sausages likely results from ground meat used in their preparation, grilling does not always ensure complete heat treatment (Pierre & Veit, 1996). In our study, the contamination rates of Listeria spp. recorded in cooked meat dishes from school and company cafeterias (18.6%) and restaurants (8%) were very high. This high prevalence probably reflects poor hygiene practices. The study of the antimicrobial sensitivity of L. monocytogenes isolated strains showed no resistance, thus confirming the observations of Charpentier and Courvalin (1999) who found that L. monocytogenes was very sensitive to various antibiotics. However, it should be noted that several studies have shown that L. monocytogenes had acquired resistance to various antibiotics (Walsh, Duffy, Sheridan, Blair, & Mc Dowell, 2001).
L. Bouayad, T.-M. Hamdi / Food Control 23 (2012) 397e399
5. Conclusion We have shown that a large part of RTE food sold in Algiers may be contaminated with Listeria spp., some of which can also be contaminated with L. monocytogenes. Analysis of a wide variety of RTE foods indicates that the heat treatments used in the preparation of RTE foods, are in most cases, sufficient to kill Listeria contaminants. Exceptions to this were grilled meats (sausages) and some dairy products, where heat treatment seems insufficient to kill contaminating Listeria. Our findings suggest that contamination with Listeria spp. likely occurs at the post processing stages and may reflect poor hygienic practices in the preparation of these foods at the retail level. The high prevalence of L. monocytogenes in both meat and prepared food suggests that Algerian regulatory authorities should consider introducing a regulatory framework to include L. monocytogenes aimed at improving the safety of RTE foods. References Anonymous. (2003). Microbiological quality of food. In Annual report of Directorate General of Health. Switzerland: Department of Consumer Protection in Geneva. Charpentier, E., & Courvalin, P. (1999). Antibiotic resistance of Listeria spp. Antimicrobial Agents and Chemotherapy, 43, 2103e2108. Farber, J. M., & Peterkin, P. I. (1991). Listeria monocytogenes a food-borne pathogen. Microbiology Review, 55, 476e511. FDA/USDA. (2003). Quantitative assessment of relative risk to public health from foodborne Listeria monocytogenes among selected categories of ready-to-eat foods. Gombas, D. E., Chen, Y., Clavero, R. S., & Scott, V. N. (2003). Survey of Listeria monocytogenes in ready-to-eat foods. Journal of Food Protection, 66, 559e569.
399
Hamdi, T. M., Naim, M., Martin, P., & Jacquet, C. (2007). Identification and molecular characterization of Listeria monocytogenes isolated in raw milk in the region of Algiers (Algeria). International Journal of Food Microbiology, 116, 190e193. International Committee on Food Microbiology and Hygiene. (1991). L. monocytogenes: recommendations by the National Advisory Committee on microbiological criteria foods. International Journal of Food Microbiology, 14, 185e246. Jalali, M., & Abedi, D. (2008). Prevalence of Listeria species in food products in Isfahan, Iran. International Journal of Food Microbiology, 122, 336e340. Jouve, J. L. (1996). The microbiological quality of food. Master and criteria. In Polytechnica (pp. 372e444), Paris. Pierre, O., & Veit, P. (1996). Monitoring plan Listeria monocytogenes contamination in food distribution, results of the plans 1993-1994. Weekly Epidemiology Bulletin, 45, 195e197. Rocourt, J. (1994). Listeria monocytogenes. The state of the science. Dairy Food and Environmental Sanitation, 14, 70e82. Rosset, R. (2001). Microbial growth and cold study of the particular case of Listeria monocytogenes. Bulletin de l’Académie Nationale de Médecine, 185(2), 287e298. Shen, Y., Liu, Y., Zhang, Y., Cripe, J., Conway, W., Meng, J., et al. (2006). Isolation and characterization of Listeria monocytogenes isolates from ready-to-eat foods in Florida. Applied Environmental Microbiology, 72, 5073e5076. Tienungoon, S., Ratkowsky, D. A., McMeekin, T. A., & Ross, T. (2000). Growth limits of L. monocytogenes as a function of temperature, pH, NaCl, and lactic acid. Applied Environmental Microbiology, 66(11), 4979e4987. Vitas, A. I., & Garcia-Jalon, V. A. (2004). Occurrence of Listeria monocytogenes in fresh processed foods in Navarra (Espagne). International Journal of Food Microbiology, 90, 349e356. Wagner, M., Auer, B., Trittremmel, C., Hein, I., & Schoder, D. (2007). Survey on the Listeria contamination of ready-to-eat food products and menage environments in Vienna, Austria. Zoonoses Public Health, 54(1), 16e22. Walsh, D., Duffy, G., Sheridan, J. J., Blair, I. S., & Mc Dowell, D. A. (2001). Antibiotic resistance among Listeria, including Listeria monocytogenes in retail foods. Journal of Applied Microbiology, 90(14), 517e522. Wilson, I. G. (1995). Occurrence of Listeria species in ready to eat foods. Epidemiology Infection, 115(3), 519e526.
Contents lists available at SciVerse ScienceDirect
Food Control journal homepage: www.elsevier.com/locate/foodcont
Prevalence of Listeria spp. in ready to eat foods (RTE) from Algiers (Algeria) Leila Bouayad*, Taha-Mossadak Hamdi Animal Health and Production Laboratory, High National Veterinary School, BP 161 El-Harrach, Algiers, Algeria
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 February 2011 Received in revised form 27 July 2011 Accepted 2 August 2011
The aim of this study was to establish the occurrence of Listeria spp., especially Listeria monocytogenes in ready to eat RTE food marketed in Algiers (Algeria). A total of 227 samples were collected from different producers and retailers. All samples were analyzed using a conventional cultivation method AFNOR V08-055. Out of 227 samples tested, 21 (9.3%) tested positive for Listeria spp. among them, 6 (2.6%) tested positive for L. monocytogenes. L. innocua was the most common Listeria species found being detected in 11 samples (4.8%), although both Listeria ivanovii and Listeria welshimeri were detected in 3 (1.3%) and 1 (0.4%) food samples respectively. The study of the antimicrobial sensitivity of Listeria monocytogenes strains showed no resistance. The study has enabled us to detect these contaminants in a wide range of RTE foods, to suggest that contamination likely occurs after heat treatment, and to assess the danger represented by this category of food for populations at risk. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: RTE Listeria spp. Listeria monocytogenes Prevalence Antimicrobial sensitivity
1. Introduction Listeria monocytogenes is the causative agent of listeriosis, an often fatal disease linked to the consumption of contaminated foods. It has long been known that the pathogen is transmitted by food and has been implicated in several large outbreaks of foodborne listeriosis in North America and Europe (Farber & Peterkin, 1991). Clinical symptoms of listeriosis include meningitis, septicaemia and neonatal infections (Rocourt, 1994). The disease is particularly dangerous for pregnant women, newborns and immunocompromised individuals (Farber & Peterkin, 1991). A variety of food, mainly ready to eat products have been found to be contaminated with L. monocytogenes. The Prevalence of L. monocytogenes in raw and cooked meats was about 35% and 18%, respectively in Spain (Vitas & Garcia-Jalon, 2004). In United States, 1% of cooked meats were contaminated with L. monocytogenes (Gombas, Chen, Clavero, & Scott, 2003). Little is known about the distribution of this pathogen in food supply in Algeria and no data exist on the prevalence of L. monocytogenes in RTE foods, only the contamination in raw milk has been reported (Hamdi, Naim, Martin, & Jacquet, 2007). The main objectives were an initial evaluation of the Listeria spp. prevalence in some RTE foods products, marketed in retail stores or * Corresponding author. Tel./fax: þ213 21 82 14 33. E-mail address: [email protected] (L. Bouayad). 0956-7135/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodcont.2011.08.006
distributed in different schools or company cafeterias in Algiers, and the study of the antimicrobial sensitivity of isolated L. monocytogenes strains. 2. Materials and methods Samples of RTE foods (227 samples in total) were collected in sterile plastic bags in different municipalities of Algiers over a period from April 2007 to April 2008, in collaboration with each municipal health office. Food samples were stored at 4 C and processed within 4 h of collection. Among the samples 33.5% were from retail stores, 26% from school or company cafeterias, 21.5% from restaurants and 19% from manufacturers of dairy or meat products. Isolation and identification of Listeria spp. was carried out using a cultivation methodology (AFNOR V08-055; French Agency for Standardization). Briefly, a 25 g sample of each food was weighed into a sterile stomacher bag, then 225 mL of half Fraser broth (Merck) was added. The sample was homogenized using a stomacher for 2 min. Homogenates were incubated for 24 h at 30 C. Positive enrichment cultures (broth darkening) were streaked onto Palcam agar (Merck) and incubated at 37 C for 24 h. Putative Listeria spp. isolates were Gram stained, stabbed into a motility medium for observing the characteristic umbrella motility and tested for oxidase activity. Isolates were speciated using the API Listeria strips carried out according to the manufacturer recommendations (BioMérieux, France).
398
L. Bouayad, T.-M. Hamdi / Food Control 23 (2012) 397e399
Antimicrobial sensitivity was assessed using the disk diffusion assay, according to the criteria defined by the CLSI (Clinical Laboratory Standard Institute) recommended by the WHO (World Health Organization). The isolated strains were seeded in Mueller-Hinton agar supplemented with 0.5% defibrinated sheep blood. The following antibiotics were used: gentamicin (10 mg), piperacillin (30 mg), tetracycline (30 mg) (BIORAD), trimethoprim/sulfamethoxazole (1.25/23.75 mg), amoxicilin/Clavulanic acid (20/10 mg) and chloramphenicol (30 mg) (OXOID).
Origin of samples
Number of L. m analyzed No (%) samples
Retail trade School and company cafeterias Restaurant Manufacturer
76 59
3 (3.9) 1 (1.3) 1 (1.3) 0 (0) 5 (6.5) 3 (5.1) 6 (10) 1 (1.7) 1 (1.7) 11 (18.6)
49 43
0 (0) 0 (0)
L. in No (%)
L. iv No (%)
3 (6.1) 1 (2) 1 (2.3) 0 (0)
L. w No (%)
0 (0) 0 (0)
Total of positives samples No (%)
4 (8.1) 1 (2.3)
Abbreviations: No, Number of samples; L. m, Listeria monocytogenes; L. in, Listeria innocua; L. iv, Listeria ivanovii; L. w, Listeria welshimeri.
3. Results Listeria spp. was obtained from 21 (9.3%) out of 227 food samples tested, distributed as follows: 2.6% to L. monocytogenes, 4.8% to L. innocua, 1.3% to L. ivanovii and 0.4% to L. welshimeri (Table 1). Samples of RTE foods were classified into four categories: 99 (43.6%) were dairy products, of which 64 (64.6%) had undergone heat treatment and 35 (35.4%) represented fermented products produced from raw milk. 23 (10.1%) unpacked sliced meat products, 94 (41.4%) cooked meat dishes (sandwiches and hot dishes). 4.8% of the RTE samples did not fall within the four main categories (salad, mayonnaise). In dairy products, L. monocytogenes was isolated only in soft cheese (dairy products which have undergone a heat treatment) and fermented milk product with à pH 5.1. In the unpacked sliced meat products and cooked meat dishes, only the sausage sandwiches and one cooked meat dish were found contaminated with L. monocytogenes. According the origins of the samples, the overall prevalence of Listeria spp. positive samples (L. monocytogenes positive samples in parentheses) was 18.5% (5.1%) in school and company cafeterias, 8.1% (0%) in restaurants, 6.5% (3.9%) in retail and 2.3% (0%) in manufacturers (Table 2). The study of the antimicrobial sensitivity of L. monocytogenes isolated strains showed no resistance. 4. Discussion 9.3% is the overall prevalence of Listeria spp. recorded during our study. Respective prevalences of 12% and 5.6% were reported by
Table 1 Prevalence of Listeria spp. in RTE food. Nature of samples
Table 2 Prevalence of Listeria spp. according their origin.
No Listeria L. m analyzed spp. No No (%) (%)
L. in No (%)
Milk products which have undergone heat treatment Soft cheese 39 2 (5.1) 1 Homemade cheese 5 0 (0) 0 Cheese 3 0 (0) 0 Semihard cheese 3 0 (0) 0 Cream cheese 14 0 (0) 0 Raw fermented dairy product Whey 35 1 (2.8) 1 Unpacked sliced meat product Cachir (local pâté) 8 0 (0) 0 Pâté 15 0 (0) 0 Cooked meat dishes Merguez sandwich 14 2 (14.2) 2 Meat dishes 80 1 (1.2) 6 Various 11 0 (0) 1 Total 227 21(9.3) 6 (2.6) 11
L. iv No (%)
L. w No (%)
(2.6) (0) (0) (0) (0)
0 0 0 0 0
0 0 0 0 0
(2.8)
0 (0)
(0) (0)
1 (12.5) 0 (0) 1 (6.6) 0 (0)
(14.2) (7.5) (9) (4.8)
0 1 0 3
(0) (0) (0) (0) (0)
(0) (1.2) (0) (1.3)
(0) (0) (0) (0) (0)
0 (0)
0 1 0 1
(0) (1.2) (0) (0.4)
Abbreviations: No, Number of samples; L. m, Listeria monocytogenes; L. in, Listeria innocua; L. iv, Listeria ivanovii; L. w, Listeria welshimeri.
Jalali and Abedi in Iran in 2008 and Wagner, Auer, Trittremmel, Hein, and Schoder in Austria in 2007. The prevalence of L. monocytogenes was in agreement with previous studies (Shen et al, 2006; Wilson, 1995), however it remains fairly low compared to that reported in 2003 by the FDA/ USDA (0.2%e12.9%). In heat treated dairy products, we recorded an L. monocytogenes contamination, 2 (3%) samples out 64 were positive, numerous studies showed that L. monocytogenes flora contaminates dairy product after heat treatment (Anonymous, 2003; Jouve, 1996). This contamination likely results from (i) Insufficient heat treatment of the milk to kill the organism (Farber & Peterkin, 1991), (ii) The resistance of Listeria to the decreased pH which occurs in cheese production (Rosset, 2001), (iii) Cross contamination through air, soil, equipment and processing units staff (Jouve, 1996). In whey samples, product widely consumed in Algeria, we have recorded the same contamination by L. monocytogenes and L. innocua. In Algeria Hamdi et al. (2007) have reported a prevalence of 9.1% per L. innocua. The contamination of this kind of product would be due to the resistance of Listeria to acidic pH (Tienungoon, Ratkowsky, McMeekin, & Ross, 2000). We observe that there are differences between the prevalences that we recorded and those reported in other studies, this could be due to different sampling procedures and sites and the method of detection of the bacteria may also contribute to these variations. In unpacked sliced meat products and cooked meat dishes we have isolated four different species of Listeria which L. monocytogenes was detected in 3 (2.6%) samples. In the USA, Gombas et al. (2003) have reported a prevalence of L. monocytogenes of 0.9% in deli meats. While, in (1991), the International Committee on Food Microbiology and Hygiene in Great Britain estimated it in cooked meats between 5% and 7%. All analyzed meat products have undergone a heat treatment, which can destroy most bacteria present in the raw material. Contamination of cooked meat products and dishes is likely related to cross contamination occurring during subsequent handling of the heat treated products. The contamination recorded in grilled sausages likely results from ground meat used in their preparation, grilling does not always ensure complete heat treatment (Pierre & Veit, 1996). In our study, the contamination rates of Listeria spp. recorded in cooked meat dishes from school and company cafeterias (18.6%) and restaurants (8%) were very high. This high prevalence probably reflects poor hygiene practices. The study of the antimicrobial sensitivity of L. monocytogenes isolated strains showed no resistance, thus confirming the observations of Charpentier and Courvalin (1999) who found that L. monocytogenes was very sensitive to various antibiotics. However, it should be noted that several studies have shown that L. monocytogenes had acquired resistance to various antibiotics (Walsh, Duffy, Sheridan, Blair, & Mc Dowell, 2001).
L. Bouayad, T.-M. Hamdi / Food Control 23 (2012) 397e399
5. Conclusion We have shown that a large part of RTE food sold in Algiers may be contaminated with Listeria spp., some of which can also be contaminated with L. monocytogenes. Analysis of a wide variety of RTE foods indicates that the heat treatments used in the preparation of RTE foods, are in most cases, sufficient to kill Listeria contaminants. Exceptions to this were grilled meats (sausages) and some dairy products, where heat treatment seems insufficient to kill contaminating Listeria. Our findings suggest that contamination with Listeria spp. likely occurs at the post processing stages and may reflect poor hygienic practices in the preparation of these foods at the retail level. The high prevalence of L. monocytogenes in both meat and prepared food suggests that Algerian regulatory authorities should consider introducing a regulatory framework to include L. monocytogenes aimed at improving the safety of RTE foods. References Anonymous. (2003). Microbiological quality of food. In Annual report of Directorate General of Health. Switzerland: Department of Consumer Protection in Geneva. Charpentier, E., & Courvalin, P. (1999). Antibiotic resistance of Listeria spp. Antimicrobial Agents and Chemotherapy, 43, 2103e2108. Farber, J. M., & Peterkin, P. I. (1991). Listeria monocytogenes a food-borne pathogen. Microbiology Review, 55, 476e511. FDA/USDA. (2003). Quantitative assessment of relative risk to public health from foodborne Listeria monocytogenes among selected categories of ready-to-eat foods. Gombas, D. E., Chen, Y., Clavero, R. S., & Scott, V. N. (2003). Survey of Listeria monocytogenes in ready-to-eat foods. Journal of Food Protection, 66, 559e569.
399
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