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Problems in the investigation of an apparent outbreak of coagulase-negative staphylococcal septicaemia following cardiac surgery
Journal
of Hospital
Infection
(1986) 8, 224-232
Problems in the investigation of an apparent outbreak of coagulase-negative staphylococcal septicaemia following cardiac surgery Elizabeth
T. Houang *, R. R. Marplest, I. WeirI, A. J. Mourantgy, Maureen J. de Saxet and B. Singleton11
Departments of Microbiology, $Capdiology and $Cardiothoracic Unit, The London Hospital, and TDivision of Hospital Infection, Central Public Health Laboratory. Accepted for publication
16 October 1985
Summary:
Two hundred and twenty-six hospital staff and patients were investigated for the carriage of gentamicin-resistant coagulase-negative staphylococci (CNS) during an apparent outbreak of infection after cardiac surgery. Of the four index strains from infected wounds, three were indistinguishable. The carriage of similar organisms was widespread, particularly among ITU staff (72%) and patients. Ninety-one of the 296 gentamicinresistant isolates were further investigated, and of these 33 were indistinguishable from index strains even with the use of specialized techniques. Our experience indicates that in outbreaks of infection caused by gentamicinresistant CNS, resources should be focused on the interruption of transmission and prevention of introduction of these organisms to susceptible patients.
Introduction Isolated cases of prosthetic valve endocarditis caused by coagulase-negative staphylococci (CNS) are common but outbreaks of infection in a single unit are unusual (Blouse et al., 1978; Marples et al., 1978; Parker, 1981). Methods of investigation have developed in recent years with the improvements in biotyping (Baird-Parker, 1965; Holt, 1969; Marples, 1980), antibiotic-resistance typing (Richardson & Marples, 1982), methods to study the phages carried by these organisms (de Saxe & Notley, 1978) and the plasmid profiles of potentially epidemic strains. We investigated an apparent outbreak of infections in four patients who developed serious wound infection or septicaemia within 4 weeks of heart valve replacement. The investigation was extended to include potential sources of CNS resistant to multiple antibiotics among patients and staff. *Present address: Chelsea Hospital for Women, London SW3. T[Present address: Royal Cornwall Hospital, Truro. /I Present address: 10 Beacon Hill Road, Newark, Notts. Ol95-6701/86/060224+09
$03,00/O
0 1986 The Hospital
224
Infection
Society
Outbreak
This study highlights outbreak.
of infection
the problems
Materials
with CNS
of an epidemiological
225
study of such an
and methods
All patients of the cardiothoracic unit were admitted for pre-operative assessment to one ward. Immediately after operation they were transferred to a seven-bedded intensive care unit (ICU) for at least 24 h before transfer to a postoperative ward. Both the pre- and postoperative wards also housed patients from other specialties. About 300 patients underwent cardiac surgery every year. Antibiotic prophylaxis consisted of flucloxacillin 500 mg qds and gentamicin 80 mg tds intravenously from the day of operation for 3-5 days. In the first 3 weeks of February 1981, four patients developed infective complications following cardiac surgery. Two patients died, and two recovered after debridement, treatment with antibiotics and povidone iodine irrigation of the wound. Their case histories are summarized in Table I. The cardiothoracic theatre was closed for 4 weeks while the outbreak was investigated. Formal cleaning of the wards and ICU was carried out. Since re-opening the operating theatre no further infection has occurred. Microbiological investigation The strains of CNS isolated from each patient had similar antibiotic resistance patterns but it was not clear whether the strains were related. An outbreak was suspected, and carriage of similar strains was sought among staff and patients by using gentamicin resistance as a marker, as it was thought initially that this was not a common character in the normal flora. Eight groups of staff and patients were studied: (1) medical staff of the unit; (2) nursing and physiotherapy staff of the ICU; (3) technical auxiliary staff of the ICU without direct patient contact; (4) nurses from pre- ;and postoperative wards; (5) theatre nurses and (6) all patients in the pre- and postoperative wards, which in addition to cardiac patients, also housed other medical patients. Two control groups were included: 19 radiographers who had had some patient contact and 20 patients from emergency dental outpatients. The latter formed a group with no known contact with the hospital environment and which had not had previous antibiotic treatment. Theatre and ICU procedures for skin disinfection, sterilization, and aseptic techniques were reviewed. No error was detected but alcoholic chlorhexidine (‘Hibisol’, ICI) was added to the standard handwashing regime for nursing staff. Sampling During the first week that the theatre was closed, nose and axillary swabs and a handprint plate were taken from all personnel in the first seven groups
3 February 1981 mitral replacement 4 h later re-exploration bleeding
1981 aortic
51
47
63
Male
Female
Male
and treatment
jindings
Wound discharge and pyrexia 3 March 1981 excision infected sternum PVI irrigt rif + clindt
2 weeks
12 February 1981 triple coronary artery vein grafting, re-exploration 3 h later for pericardial clot
*CNS; coagulase-negative staphylococci. tPV1 i&g; irrigation with 5% povidone iodine in normal saline. Srif + clind; rifampicin and clindamycin.
of
Pyrexia, gross oedema Re-operation: myocardial necrosis and infection, rupture sinus of Valsalva
10 days
valve
9 February replacement
for
Inflamed and discharging wound, pyrexia 4 March 1981 debridement PVI irrigt, rif + clind$
Wound discharge, mobile sternum, pyrexia No infection found on reexploration
Nature
infection
and microbiological
1 week
valve
for
2 weeks
&
26 January 1981 mitral aortic valve replacement R coronary bypass 4 h later re-exploration bleeding
57
Male
history
Postoperative
of the clinical
Onset
Sex
I. Summary
Operation
Age (years)
Table
and
CNS from wound blood culture
and
CNS from pre-operative blood culture and myocardial specimen
CNS from wound blood cultures
and
index patients
CNS* from wound blood culture
Microbiological results
of the four
compli-
No further cation
compli-
Died 25 February 1981
No further cation
Died 26 February 1981 Post-mortem: endocarditic vegetation
Outcome
z
X 2 E m
m 9
Outbreak
of infection
with CNS
227
listed above. A swab sample from behind the ears was taken from the dental outpatients instead of an axillary swab. Blood agar plates (Oxoid) were used for all primary platings, handprint platings being made by pressing the fingers and heel of the palm of the dominant hand onto the agar surface. At this time air samples were taken in the wards and ICU using the Reuter Centrifugal Sampler, set to sample 180 1 of air as stated in the manufacturer’s instructions. Blood agar strips (Oxoid) were used as the collection medium. Swabs from the baths of the pre- and postoperative wards were also taken. Follow-up handprint samples were taken from ICU nurses, ward nurses and radiographers at intervals for up to 4 months. Isolation of strains Primary plates were incubated overnight at 37°C. Gentamicin-resistant strains were selected from sweep sensitivity plates; 10 l.tg gentamicin discs were placed on Sensitest agar (Oxoid) containing 5% lysed blood. After overnight incubation, these plates were examined for the presence of gentamicin-resistant colonies. Gram-positive, gentamicin-resistant cocci were subcultured for further sensitivity testing. These strains were also tested for DNAase production (Blair, Emerson & Tull, 1967). Strains that were DNAase-negative and gentamicin-resistant under these conditions were considered to be presumptive epidemic CNS strains and subcultured for further study. At the Division of Hospital Infection, the isolates were identified and bacteriophage-typed by standard methods (Marples et al., 1978). Briefly, biotyping included acid production from 10 carbohydrate substrates and seven other reactions. Susceptibility to 19 antibiotics was tested by a disc method and primary bacteriophage typing was carried out on all strains. On selected strains only, hydrolysis of ‘Tween 80’, arginine and sodium caseinate and production of CAMP-like factor were studied. Reverse typing, i.e. determination of the host range of phages carried by the organism, was performed by the method of de Saxe & Notley (1978). Results
Characteristics of the index strains The characteristics of the four index strains are listed in Table II. These strains were indistinguishable biochemically, even when additional biochemical tests were performed. Notably, all four failed to hydrolyse arginine, a characteristic recorded for some strains from serious infections (Marples & Richardson, 1981), although Staphylococcus epidermidis is characteristically arginine-positive. A tentative distinction between the isolates was made on antibiotic resistance and phage typing characters. Strain 1 showed inducible resistance to lincomycin [analogous to the dissociated erythromycin resistance of Staph. aureus (Garrod, 1957)], and was lysed by phage 48, while strains 2-4 were constitutively resistant to
E. T. Houang
228 Table Number patients
of
:
I I. Characters
et al. of the index strains
Biotype
Antibiotic resistance
Sll Sll
P M S Ne G E Li F PMSNeGELcF
*P, penicillin; M, methicillin; F, fusidic acid; Li, lincomycin tNot typable.
pattern*
Ne, neomycin; inducible
Phage susceptibility
Reverse pattern
48 NV
A B
S, sulphonamide; resistance; Lc, lincomycin
typing
G, gentamicin; E, erythromycin; constitutive resistance.
lincomycin (comparable to undissociated lincomycin resistance), and untypable by the standard bacteriophages. Study of the phages carried by these strains and of their plasmid profiles (Dr G. M. Fisher, personal communication) confirmed this distinction. Search for strains similar to the index strains In total, 226 hospital staff and patients were sampled; 692 samples were obtained-153 nose swabs, 104 axilla swabs, 415 handprints and 20 ear swabs. These generated 314 isolates of gentamicin-resistant, DNAasenegative staphylococci by the methods listed above. Carriage rates During the first week of theatre closure, 177 staff were screened. The results are given in Table III. Two thirds of ICU personnel, nurses and physiotherapists, carried gentamicin-resistant strains on one or more of the sites sampled. Similar but lower rates were found among patients and staff from the pre- and postoperative wards, although only one-third of medical staff and l&20% of radiographers and other hospital staff were colonized. No gentamicin resistance was found in strains from patients attending the dental outpatients.
Table III. Frequency of isolation of gentamicin-resistant coagulase-negative staphylococci in individuals sampled in the jirst week of theatre closure Positive Group ICU staff Ward patients Ward nurses Doctors Radiographers Theatre nurses ICU auxiliaries Dental emergency Total
Total
number 29 48 26 19 19
Number 21 28 13 6 4 1
72 58
74
42
1: 20 177
Percentage
i9 21 17 10 0
E. T. Houang
et al.
229
Of the 62 carriers among ITU staff, ward staff and patients, the handprint was positive in 49 (79%), the nose in 26 (42%) and the axilla in 18 (29%). Axillary carriage was found only among the patients. Although several groups of staff were resampled with handprints over a period of 4 months, there was considerable variation in the number of individuals available for sampling, falling from 74 in the first week after closure of the theatre to only 30 after 4 weeks and 25 after 4 months. There was an apparent increase in the frequency of carriage of gentamicinresistant strains in ICU and ward nurses, while radiographers remained infrequent carriers of gentamicin-resistant staphylococci. The extent to which permanent nursing staff biased the results is not known, since a few individuals were resampled sufficiently often to show persistence of up to three distinguishable strains on the skin over the study period. Different strains were also found at different times. Gentamicin-resistant strains were also isolated from environmental samples, i.e. from the samples taken from the ICU and both wards, and from all the four baths sampled, suggesting that these strains were widely distributed in the environment. A total of 296 gentamicin-resistant isolates were available for identification and typing. Fifty-four per cent were identified as belonging to biotype 1 of Staph. epidermi&(II), 18% to Staph. hominis biotype (SV) and 22% to biotype 4 (SVI). Half the isolates were typable by the routine set of typing phages and fell into at least six distinct patterns. To assess whether either of the index strains, strain 1 being considered distinct from strains 2, 3 and 4, were present, the laboratory results of the isolates of Staph. epidermidis biotype 1 were retabulated for isolates approximating to strain 1 (46 in all), and those approximating to strains 2, 3 and 4 (41 in all). These were selected for further study (Table IV). Of the
Table
IV.
Results of sampling patients and staff for strains similar strains
to the index
Criteria Biotype Sl 1; lincomycin constitutive resistance; not typable by phages Total strains possibly index strains* Excluded
on biochemical
Excluded on antibiotic phage susceptibility Indistinguishable
*seetext.
related tests
Biotype Sl 1; phage 48
to 41
46
21
6
12
15
8
25
and
230
Outbreak
of infection
with
CNS
isolates resembling strain 1, 25 were finally considered to be related. Eleven were isolated from ICU nurses, seven from ward nurses, four from patients and two from medical staff. Of the 41 isolates resembling strains 2, 3 and 4, eight were considered indistinguishable. Six were recovered from the hands of three nurses, one from the nose of a nurse in the ICU and one from the wound of another patient. Discussion
The primary reason for suspecting an outbreak was the occurrence of four cases of postoperative infection of the wound or mediastinum during a short period after cardiac surgery, whilst no infections had presented in the preceding 12 months. The organisms apparently involved were CNS that showed similarities in biochemical and antibiotic-resistance characteristics, notably resistance to gentamicin. Outbreaks in which CNS have been implicated are not common. Blouse et al. (1978) reported an outbreak, related to the extracorporeal circulation, that involved 20 patients. A possible theatre-based outbreak was described by Hammond 8z Stiver (1978) where at least eight patients became infected over a 9-month period, subsequent to a 3-year period in which no infection occurred. Marples et al. (1978) described an outbreak in which the source appeared to be the ICU rather than the operating theatre. Other outbreaks have recently been reported (Archer, Deitrick & Johnstone, 1985). Investigation of a potential outbreak requires firstly, characterization of the index strains to confirm the existence of an outbreak and to delineate its scope, and simultaneously the collection of strains from potential sources. These may then be characterized and compared with index strains to exclude unrelated isolates. Potentially related strains may then need further characterization. Finally, the investigation should suggest routes of transmission and monitor the efficacy of new or newly applied regimens introduced to prevent further infection. In this study, three of the four index strains were indistinguishable even when specialized techniques were employed, but strains from one patient were demonstrably different. All four index strains were resistant to penicillin, gentamicin, erythromycin, lincomycin, fusidic acid and trimethoprim. In the search for potential sources, gentamicin resistance was used to select strains for further study. Other characters might have been more appropriate, but a decision was necessary to prevent overloading of the laboratory. Because of this selection there may have been some small loss of related strains and there was an unexpectedly large number of strains of Staph. epidermidis biotype 4 (SVI). Gentamicin-resistant isolates were widespread, but were most frequent from the ICU nurses and patients. Ward nurses and medical staff showed intermediate carriage rates, while radiographers and dental outpatients
Outbreak
of infection
with CNS
231
carried significantly fewer gentamicin-resistant isolates. The association between carriage rate and place of work resembles that previously described (Marples et al., 1978; Marples, 1980), with the main apparent source being the ICU, an area subjected to considerable antibiotic exposure. Of the sites of gentamicin-resistant isolates was found most sampled, carriage frequently on hands (79%). The results perhaps suggest that much of the carriage was transient, and that hands acted as an important route of transmission. Epidemiological screening for strains with multiple resistance to antibiotics may not be as useful in demonstrating the source of an outbreak of infections caused by CNS as it can be with Staph. aweus or a member of the Enterobacteriaceae. In this study, relatively large numbers of isolates indistinguishable from the index strains were recovered from staff. More than 80 isolates could not easily be distinguished from the two index patterns; exclusion of carrier could not therefore be contemplated. Infection could have come from other patients, staff or de novo, but the balance of probability suggests that the ICU team may have been the source. The design of the study allows one to say only that strains indistinguishable from the index strains were circulating in the ICU and related areas of the hospital after the infections became apparent. Control of an outbreak involves interruption of the route of infection from a source through a transfer mechanism to the site of infection in the patient. Sources should be isolated or treated. In this case isolation was used to interrupt transfer; the operating theatre was closed for 4 weeks’ and, incidental to the outbreak, the ICU moved to a new site. A new regime of handwashing was also introduced although this did not influence the subsequent hand carriage rate. Awareness of infection risks by the nursing staff was increased by local education. The outbreak was thus brought under control. Prevention of infections following cardiac surgery may be best achieved by continuing aseptic management into the ICU. The eradication of multiply resistant organisms which form part of the skin flora of patients and staff in an environment with extensive exposure to antibiotics may prove an impossible task. We thank the consultants in the cardiothoracic unit for permission to report We are indebted to all the hospital staff who took part in the microbiological
their patients. screening.
References Archer, G. L., Deitrick, D. R. & Johnstone, J. L. (1985). Molecular epidemiology of transmissible gentamicin resistance among coagulase-negative staphylococci in a cardiac surgery unit. Journal of Infectious Diseases 151, 243-251. Baird-Parker, A. C. (1965). The classification of staphylococci and micrococci from world-wide sources. Journal of General Microbiology 38, 363-387. Blair, E. B., Emerson, J. S. & Tull, A. H. (1967). A new medium, salt mannitol plasma agar, for the isolation for Staphylococcus aureus. American Journal of Clinical Pathology 48, 3&39.
232
E. T. Houang
et al.
R. M. (1978). Epidemiologic Blouse, L. E., Lathrop, G. D., Kolonel, L. N. & B rockett, features and phage types associated with nosocomial infections caused by Staphylococcus epidermidis. Zentralblatt fiir Bakteriologie, Eiygiene, I Abteilung, Originale A 241, 119-135. Garrod, L. P. (1957). The erythromycin group of antibiotics. British Medical Journal 1, 57-63. Hammond, G. W. & Stiver, G. G. (1978). Combination antibiotic therapy in an outbreak of prosthetic endocarditis caused by Staphylococcus epidermidis. Canadian Medical Association Journal 118, 524530. Holt, R. S. (1969). The classification of staphylococci from colonized ventriculo-atria1 shunts. Journal of Clinical Pathology 22, 475482. Marples, R. R. (1980). Coagulase-negative staphylococci-their classification and problems. Congress Symposium 23, 57764. Royal Society qf Medicine International Marples, R. R., Hone, R., Notley, C. M., Richardson, J. F. & Crees-Morris, J. A. (1978). Investigation of coagulase-negative staphylococci from infections in surgical patients. Zentralblatt fiir Bakteriologie, Hygiene, 1 Abteilung, Originale A 241, 14&156. Marples, R. R. & Richardson, J. F. (1981). Characters of coagulase-negative staphylococci collected for a collaborative phage-typing study. Zentralblatt fii’r Bakteriologie 10, 175-l 80. Supplementary Parker, M. T. (1981). Infection and colonization by the “other” staphylococci. In The Staphylococci (Macdonald, A. & Smith, G., Eds), pp. 156-174. Aberdeen University Press, Aberdeen. Richardson, J. F. & Marples, R. R. (1982). Changing resistance to antimicrobial drugs and resistance typing in clinical significant strains of Staphylococcus epidermidis. Journal of Medical Microbiology 15, 47.5484. de Saxe, M. J. & Notley, C. M. (1978). Experiences with the typing of coagulase-negative staphylococci and micrococci. Zentralblatt fiir Bakteriologie, Hygiene, 1 Abteilung, Originale A 241, 46-59.
of Hospital
Infection
(1986) 8, 224-232
Problems in the investigation of an apparent outbreak of coagulase-negative staphylococcal septicaemia following cardiac surgery Elizabeth
T. Houang *, R. R. Marplest, I. WeirI, A. J. Mourantgy, Maureen J. de Saxet and B. Singleton11
Departments of Microbiology, $Capdiology and $Cardiothoracic Unit, The London Hospital, and TDivision of Hospital Infection, Central Public Health Laboratory. Accepted for publication
16 October 1985
Summary:
Two hundred and twenty-six hospital staff and patients were investigated for the carriage of gentamicin-resistant coagulase-negative staphylococci (CNS) during an apparent outbreak of infection after cardiac surgery. Of the four index strains from infected wounds, three were indistinguishable. The carriage of similar organisms was widespread, particularly among ITU staff (72%) and patients. Ninety-one of the 296 gentamicinresistant isolates were further investigated, and of these 33 were indistinguishable from index strains even with the use of specialized techniques. Our experience indicates that in outbreaks of infection caused by gentamicinresistant CNS, resources should be focused on the interruption of transmission and prevention of introduction of these organisms to susceptible patients.
Introduction Isolated cases of prosthetic valve endocarditis caused by coagulase-negative staphylococci (CNS) are common but outbreaks of infection in a single unit are unusual (Blouse et al., 1978; Marples et al., 1978; Parker, 1981). Methods of investigation have developed in recent years with the improvements in biotyping (Baird-Parker, 1965; Holt, 1969; Marples, 1980), antibiotic-resistance typing (Richardson & Marples, 1982), methods to study the phages carried by these organisms (de Saxe & Notley, 1978) and the plasmid profiles of potentially epidemic strains. We investigated an apparent outbreak of infections in four patients who developed serious wound infection or septicaemia within 4 weeks of heart valve replacement. The investigation was extended to include potential sources of CNS resistant to multiple antibiotics among patients and staff. *Present address: Chelsea Hospital for Women, London SW3. T[Present address: Royal Cornwall Hospital, Truro. /I Present address: 10 Beacon Hill Road, Newark, Notts. Ol95-6701/86/060224+09
$03,00/O
0 1986 The Hospital
224
Infection
Society
Outbreak
This study highlights outbreak.
of infection
the problems
Materials
with CNS
of an epidemiological
225
study of such an
and methods
All patients of the cardiothoracic unit were admitted for pre-operative assessment to one ward. Immediately after operation they were transferred to a seven-bedded intensive care unit (ICU) for at least 24 h before transfer to a postoperative ward. Both the pre- and postoperative wards also housed patients from other specialties. About 300 patients underwent cardiac surgery every year. Antibiotic prophylaxis consisted of flucloxacillin 500 mg qds and gentamicin 80 mg tds intravenously from the day of operation for 3-5 days. In the first 3 weeks of February 1981, four patients developed infective complications following cardiac surgery. Two patients died, and two recovered after debridement, treatment with antibiotics and povidone iodine irrigation of the wound. Their case histories are summarized in Table I. The cardiothoracic theatre was closed for 4 weeks while the outbreak was investigated. Formal cleaning of the wards and ICU was carried out. Since re-opening the operating theatre no further infection has occurred. Microbiological investigation The strains of CNS isolated from each patient had similar antibiotic resistance patterns but it was not clear whether the strains were related. An outbreak was suspected, and carriage of similar strains was sought among staff and patients by using gentamicin resistance as a marker, as it was thought initially that this was not a common character in the normal flora. Eight groups of staff and patients were studied: (1) medical staff of the unit; (2) nursing and physiotherapy staff of the ICU; (3) technical auxiliary staff of the ICU without direct patient contact; (4) nurses from pre- ;and postoperative wards; (5) theatre nurses and (6) all patients in the pre- and postoperative wards, which in addition to cardiac patients, also housed other medical patients. Two control groups were included: 19 radiographers who had had some patient contact and 20 patients from emergency dental outpatients. The latter formed a group with no known contact with the hospital environment and which had not had previous antibiotic treatment. Theatre and ICU procedures for skin disinfection, sterilization, and aseptic techniques were reviewed. No error was detected but alcoholic chlorhexidine (‘Hibisol’, ICI) was added to the standard handwashing regime for nursing staff. Sampling During the first week that the theatre was closed, nose and axillary swabs and a handprint plate were taken from all personnel in the first seven groups
3 February 1981 mitral replacement 4 h later re-exploration bleeding
1981 aortic
51
47
63
Male
Female
Male
and treatment
jindings
Wound discharge and pyrexia 3 March 1981 excision infected sternum PVI irrigt rif + clindt
2 weeks
12 February 1981 triple coronary artery vein grafting, re-exploration 3 h later for pericardial clot
*CNS; coagulase-negative staphylococci. tPV1 i&g; irrigation with 5% povidone iodine in normal saline. Srif + clind; rifampicin and clindamycin.
of
Pyrexia, gross oedema Re-operation: myocardial necrosis and infection, rupture sinus of Valsalva
10 days
valve
9 February replacement
for
Inflamed and discharging wound, pyrexia 4 March 1981 debridement PVI irrigt, rif + clind$
Wound discharge, mobile sternum, pyrexia No infection found on reexploration
Nature
infection
and microbiological
1 week
valve
for
2 weeks
&
26 January 1981 mitral aortic valve replacement R coronary bypass 4 h later re-exploration bleeding
57
Male
history
Postoperative
of the clinical
Onset
Sex
I. Summary
Operation
Age (years)
Table
and
CNS from wound blood culture
and
CNS from pre-operative blood culture and myocardial specimen
CNS from wound blood cultures
and
index patients
CNS* from wound blood culture
Microbiological results
of the four
compli-
No further cation
compli-
Died 25 February 1981
No further cation
Died 26 February 1981 Post-mortem: endocarditic vegetation
Outcome
z
X 2 E m
m 9
Outbreak
of infection
with CNS
227
listed above. A swab sample from behind the ears was taken from the dental outpatients instead of an axillary swab. Blood agar plates (Oxoid) were used for all primary platings, handprint platings being made by pressing the fingers and heel of the palm of the dominant hand onto the agar surface. At this time air samples were taken in the wards and ICU using the Reuter Centrifugal Sampler, set to sample 180 1 of air as stated in the manufacturer’s instructions. Blood agar strips (Oxoid) were used as the collection medium. Swabs from the baths of the pre- and postoperative wards were also taken. Follow-up handprint samples were taken from ICU nurses, ward nurses and radiographers at intervals for up to 4 months. Isolation of strains Primary plates were incubated overnight at 37°C. Gentamicin-resistant strains were selected from sweep sensitivity plates; 10 l.tg gentamicin discs were placed on Sensitest agar (Oxoid) containing 5% lysed blood. After overnight incubation, these plates were examined for the presence of gentamicin-resistant colonies. Gram-positive, gentamicin-resistant cocci were subcultured for further sensitivity testing. These strains were also tested for DNAase production (Blair, Emerson & Tull, 1967). Strains that were DNAase-negative and gentamicin-resistant under these conditions were considered to be presumptive epidemic CNS strains and subcultured for further study. At the Division of Hospital Infection, the isolates were identified and bacteriophage-typed by standard methods (Marples et al., 1978). Briefly, biotyping included acid production from 10 carbohydrate substrates and seven other reactions. Susceptibility to 19 antibiotics was tested by a disc method and primary bacteriophage typing was carried out on all strains. On selected strains only, hydrolysis of ‘Tween 80’, arginine and sodium caseinate and production of CAMP-like factor were studied. Reverse typing, i.e. determination of the host range of phages carried by the organism, was performed by the method of de Saxe & Notley (1978). Results
Characteristics of the index strains The characteristics of the four index strains are listed in Table II. These strains were indistinguishable biochemically, even when additional biochemical tests were performed. Notably, all four failed to hydrolyse arginine, a characteristic recorded for some strains from serious infections (Marples & Richardson, 1981), although Staphylococcus epidermidis is characteristically arginine-positive. A tentative distinction between the isolates was made on antibiotic resistance and phage typing characters. Strain 1 showed inducible resistance to lincomycin [analogous to the dissociated erythromycin resistance of Staph. aureus (Garrod, 1957)], and was lysed by phage 48, while strains 2-4 were constitutively resistant to
E. T. Houang
228 Table Number patients
of
:
I I. Characters
et al. of the index strains
Biotype
Antibiotic resistance
Sll Sll
P M S Ne G E Li F PMSNeGELcF
*P, penicillin; M, methicillin; F, fusidic acid; Li, lincomycin tNot typable.
pattern*
Ne, neomycin; inducible
Phage susceptibility
Reverse pattern
48 NV
A B
S, sulphonamide; resistance; Lc, lincomycin
typing
G, gentamicin; E, erythromycin; constitutive resistance.
lincomycin (comparable to undissociated lincomycin resistance), and untypable by the standard bacteriophages. Study of the phages carried by these strains and of their plasmid profiles (Dr G. M. Fisher, personal communication) confirmed this distinction. Search for strains similar to the index strains In total, 226 hospital staff and patients were sampled; 692 samples were obtained-153 nose swabs, 104 axilla swabs, 415 handprints and 20 ear swabs. These generated 314 isolates of gentamicin-resistant, DNAasenegative staphylococci by the methods listed above. Carriage rates During the first week of theatre closure, 177 staff were screened. The results are given in Table III. Two thirds of ICU personnel, nurses and physiotherapists, carried gentamicin-resistant strains on one or more of the sites sampled. Similar but lower rates were found among patients and staff from the pre- and postoperative wards, although only one-third of medical staff and l&20% of radiographers and other hospital staff were colonized. No gentamicin resistance was found in strains from patients attending the dental outpatients.
Table III. Frequency of isolation of gentamicin-resistant coagulase-negative staphylococci in individuals sampled in the jirst week of theatre closure Positive Group ICU staff Ward patients Ward nurses Doctors Radiographers Theatre nurses ICU auxiliaries Dental emergency Total
Total
number 29 48 26 19 19
Number 21 28 13 6 4 1
72 58
74
42
1: 20 177
Percentage
i9 21 17 10 0
E. T. Houang
et al.
229
Of the 62 carriers among ITU staff, ward staff and patients, the handprint was positive in 49 (79%), the nose in 26 (42%) and the axilla in 18 (29%). Axillary carriage was found only among the patients. Although several groups of staff were resampled with handprints over a period of 4 months, there was considerable variation in the number of individuals available for sampling, falling from 74 in the first week after closure of the theatre to only 30 after 4 weeks and 25 after 4 months. There was an apparent increase in the frequency of carriage of gentamicinresistant strains in ICU and ward nurses, while radiographers remained infrequent carriers of gentamicin-resistant staphylococci. The extent to which permanent nursing staff biased the results is not known, since a few individuals were resampled sufficiently often to show persistence of up to three distinguishable strains on the skin over the study period. Different strains were also found at different times. Gentamicin-resistant strains were also isolated from environmental samples, i.e. from the samples taken from the ICU and both wards, and from all the four baths sampled, suggesting that these strains were widely distributed in the environment. A total of 296 gentamicin-resistant isolates were available for identification and typing. Fifty-four per cent were identified as belonging to biotype 1 of Staph. epidermi&(II), 18% to Staph. hominis biotype (SV) and 22% to biotype 4 (SVI). Half the isolates were typable by the routine set of typing phages and fell into at least six distinct patterns. To assess whether either of the index strains, strain 1 being considered distinct from strains 2, 3 and 4, were present, the laboratory results of the isolates of Staph. epidermidis biotype 1 were retabulated for isolates approximating to strain 1 (46 in all), and those approximating to strains 2, 3 and 4 (41 in all). These were selected for further study (Table IV). Of the
Table
IV.
Results of sampling patients and staff for strains similar strains
to the index
Criteria Biotype Sl 1; lincomycin constitutive resistance; not typable by phages Total strains possibly index strains* Excluded
on biochemical
Excluded on antibiotic phage susceptibility Indistinguishable
*seetext.
related tests
Biotype Sl 1; phage 48
to 41
46
21
6
12
15
8
25
and
230
Outbreak
of infection
with
CNS
isolates resembling strain 1, 25 were finally considered to be related. Eleven were isolated from ICU nurses, seven from ward nurses, four from patients and two from medical staff. Of the 41 isolates resembling strains 2, 3 and 4, eight were considered indistinguishable. Six were recovered from the hands of three nurses, one from the nose of a nurse in the ICU and one from the wound of another patient. Discussion
The primary reason for suspecting an outbreak was the occurrence of four cases of postoperative infection of the wound or mediastinum during a short period after cardiac surgery, whilst no infections had presented in the preceding 12 months. The organisms apparently involved were CNS that showed similarities in biochemical and antibiotic-resistance characteristics, notably resistance to gentamicin. Outbreaks in which CNS have been implicated are not common. Blouse et al. (1978) reported an outbreak, related to the extracorporeal circulation, that involved 20 patients. A possible theatre-based outbreak was described by Hammond 8z Stiver (1978) where at least eight patients became infected over a 9-month period, subsequent to a 3-year period in which no infection occurred. Marples et al. (1978) described an outbreak in which the source appeared to be the ICU rather than the operating theatre. Other outbreaks have recently been reported (Archer, Deitrick & Johnstone, 1985). Investigation of a potential outbreak requires firstly, characterization of the index strains to confirm the existence of an outbreak and to delineate its scope, and simultaneously the collection of strains from potential sources. These may then be characterized and compared with index strains to exclude unrelated isolates. Potentially related strains may then need further characterization. Finally, the investigation should suggest routes of transmission and monitor the efficacy of new or newly applied regimens introduced to prevent further infection. In this study, three of the four index strains were indistinguishable even when specialized techniques were employed, but strains from one patient were demonstrably different. All four index strains were resistant to penicillin, gentamicin, erythromycin, lincomycin, fusidic acid and trimethoprim. In the search for potential sources, gentamicin resistance was used to select strains for further study. Other characters might have been more appropriate, but a decision was necessary to prevent overloading of the laboratory. Because of this selection there may have been some small loss of related strains and there was an unexpectedly large number of strains of Staph. epidermidis biotype 4 (SVI). Gentamicin-resistant isolates were widespread, but were most frequent from the ICU nurses and patients. Ward nurses and medical staff showed intermediate carriage rates, while radiographers and dental outpatients
Outbreak
of infection
with CNS
231
carried significantly fewer gentamicin-resistant isolates. The association between carriage rate and place of work resembles that previously described (Marples et al., 1978; Marples, 1980), with the main apparent source being the ICU, an area subjected to considerable antibiotic exposure. Of the sites of gentamicin-resistant isolates was found most sampled, carriage frequently on hands (79%). The results perhaps suggest that much of the carriage was transient, and that hands acted as an important route of transmission. Epidemiological screening for strains with multiple resistance to antibiotics may not be as useful in demonstrating the source of an outbreak of infections caused by CNS as it can be with Staph. aweus or a member of the Enterobacteriaceae. In this study, relatively large numbers of isolates indistinguishable from the index strains were recovered from staff. More than 80 isolates could not easily be distinguished from the two index patterns; exclusion of carrier could not therefore be contemplated. Infection could have come from other patients, staff or de novo, but the balance of probability suggests that the ICU team may have been the source. The design of the study allows one to say only that strains indistinguishable from the index strains were circulating in the ICU and related areas of the hospital after the infections became apparent. Control of an outbreak involves interruption of the route of infection from a source through a transfer mechanism to the site of infection in the patient. Sources should be isolated or treated. In this case isolation was used to interrupt transfer; the operating theatre was closed for 4 weeks’ and, incidental to the outbreak, the ICU moved to a new site. A new regime of handwashing was also introduced although this did not influence the subsequent hand carriage rate. Awareness of infection risks by the nursing staff was increased by local education. The outbreak was thus brought under control. Prevention of infections following cardiac surgery may be best achieved by continuing aseptic management into the ICU. The eradication of multiply resistant organisms which form part of the skin flora of patients and staff in an environment with extensive exposure to antibiotics may prove an impossible task. We thank the consultants in the cardiothoracic unit for permission to report We are indebted to all the hospital staff who took part in the microbiological
their patients. screening.
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232
E. T. Houang
et al.
R. M. (1978). Epidemiologic Blouse, L. E., Lathrop, G. D., Kolonel, L. N. & B rockett, features and phage types associated with nosocomial infections caused by Staphylococcus epidermidis. Zentralblatt fiir Bakteriologie, Eiygiene, I Abteilung, Originale A 241, 119-135. Garrod, L. P. (1957). The erythromycin group of antibiotics. British Medical Journal 1, 57-63. Hammond, G. W. & Stiver, G. G. (1978). Combination antibiotic therapy in an outbreak of prosthetic endocarditis caused by Staphylococcus epidermidis. Canadian Medical Association Journal 118, 524530. Holt, R. S. (1969). The classification of staphylococci from colonized ventriculo-atria1 shunts. Journal of Clinical Pathology 22, 475482. Marples, R. R. (1980). Coagulase-negative staphylococci-their classification and problems. Congress Symposium 23, 57764. Royal Society qf Medicine International Marples, R. R., Hone, R., Notley, C. M., Richardson, J. F. & Crees-Morris, J. A. (1978). Investigation of coagulase-negative staphylococci from infections in surgical patients. Zentralblatt fiir Bakteriologie, Hygiene, 1 Abteilung, Originale A 241, 14&156. Marples, R. R. & Richardson, J. F. (1981). Characters of coagulase-negative staphylococci collected for a collaborative phage-typing study. Zentralblatt fii’r Bakteriologie 10, 175-l 80. Supplementary Parker, M. T. (1981). Infection and colonization by the “other” staphylococci. In The Staphylococci (Macdonald, A. & Smith, G., Eds), pp. 156-174. Aberdeen University Press, Aberdeen. Richardson, J. F. & Marples, R. R. (1982). Changing resistance to antimicrobial drugs and resistance typing in clinical significant strains of Staphylococcus epidermidis. Journal of Medical Microbiology 15, 47.5484. de Saxe, M. J. & Notley, C. M. (1978). Experiences with the typing of coagulase-negative staphylococci and micrococci. Zentralblatt fiir Bakteriologie, Hygiene, 1 Abteilung, Originale A 241, 46-59.