Analysis of an outbreak of non-phage-typeable methicillin-resistant Staphylococcus aureus by using a randomly amplified polymorphic DNA assay

A cluster of methicillin-resistant Staphylococcus aureus (MRSA) infections among patients on an intensive care unit (ICU) was detected by routine infection control surveillance. In the period from 5 January to 22 June 1995, 10 patients on the ICU and a further 6 patients (5 on one ward that had received colonized patients transferred from the ICU) were affected by MRSA strains with the same antibiotic susceptibility patterns. Seven (44%) of these 16 colonized patients developed MRSA bacteremia. MRSA isolates with the same characteristics were also found on the hands of one member of the ICU staff. The isolates were untypeable by phage typing, but 15 of 17 outbreak strains analyzed genetically had identical randomly amplified polymorphic DNA (RAPD) and pulsed-field gel electrophoresis (PFGE) profiles. A single strain of MRSA that was nontypeable by phage typing and that was isolated on the ICU on 1 January and six nontypeable and epidemiologically unrelated MRSA isolates all had RAPD profiles distinct from that of the outbreak strain. Implementation of strict infection control measures stopped the further spread of MRSA on the ICU, the affected general ward, and seven other wards that received MRSA carriers from the ICU. Although nontypeable by phage typing and not previously recognized as an epidemic strain, this strain of MRSA was readily transmissible and highly virulent. RAPD typing was found to be a simple, rapid, and effective method for the epidemiological investigation of this outbreak, and performance of typing by this method was simpler and less time-consuming than that of typing by PFGE. RAPD typing may have more general application for the study of S. aureus infections in hospitals.     

Characteristics of the staphylococci isolated from the udder of cows with mastitis

A total of 175 strains of Staphylococcus aureus and 67 strains of Staphylococcus epidermidis were studied, isolated from 486 samples of milk secretion taken aceptically from the individual quarters of the udder of cows affected with subclinical and purulent (clinical) mastitis. The staphylococci were referred to as the causative agent of mastitis in case they were the only microflora in the seedings of the investigated material. Tests were applied as given in Fig. 1 to characterize the strains. It was found that mastitis in cows could be due to both plasma coagulating staphylococci (Staphylococcus aureus) and coagulase-negative Staphylococcus epidermidis organisms. The two Staphylococcus species were isolated from cows with clinical and subclinical mastitis. The division between pathogenic and nonpathogenic Staphylococcus strains by the plasma-coagulating symptom proved impossible, and this made it necessary to use other tests for pathogenicity. It became evident that the thing Staph. aureus and Staph. epidermidis had in common when isolated from cows with mastitis was the production of a gold-like pigment and delta hemolysin. Similarly to Staph. aureus isolated animals, the bovine Staph. epidermidis organisms did not possess fibrinolysin and rarely produced hemolysin. The isolated organisms belonging to the coagulase-positive staphylococci corresponded by their basic properties to Staphylococcus aureus var. bovis as described in the literature. The cultures of Staphylococcus epidermidis isolated under similar conditions showed in a considerable per cent of the cases somewhat different behaviour.     

Persistence of Staphylococcus aureus strains among cystic fibrosis patients over extended periods of time

Pulsed-field gel electrophoresis (PFGE) of SmaI macrorestriction fragments of chromosomal DNA was used to confirm the persistence of methicillin-sensitive Staphylococcus aureus isolates in the sputum of 25 cystic fibrosis patients in five French hospitals. Three-to-eight consecutive isolates, with the same esterase electrophoretic type isolated from each patient over a period of 12-28 months, were analysed. Consecutive isolates with indistinguishable PFGE profiles were found in 12 patients (48%) and consecutive isolates with similar PFGE profiles showing minor differences of one-to-four fragments (similarity coefficient >/=84%) were found in 11 patients. Consecutive isolates with different PFGE profiles were obtained from only two patients, but the profiles found in each patient were more closely related to each other than to other profiles. The results were in agreement with esterase electrophoretic typing for 23 patients, and we considered that those patients were infected with a single persistent strain. For any given patient, variations in antibiotypes and phage types of consecutive isolates were not associated with major genotypic variations. PFGE is useful in confirming the persistence of S. aureus strains in cystic fibrosis patients over long periods.     

Management of chronic pain among elderly patients

OBJECTIVE: To identify the Staphylococcus aureus capsular serotypes that are not typable, using capsular serotypes 5 and 8, which are currently used to type S aureus isolated from cows with mastitis. SAMPLE POPULATION: Milk samples (n = 273) from cows with mastitis in 178 dairy herds in California, Wisconsin, Michigan, Texas, and New York that were collected by state diagnostic laboratories and S aureus-positive milk samples collected by Veterinary Health Services in the United Kingdom (15), France (22), The Netherlands (36), and Germany (21). PROCEDURE: Capsular serotyping of coded isolates was performed by use of direct cell agglutination and immunoprecipitation of cell extracts with antisera specific for capsular types 5 and 8 and a newly developed S aureus serotyping antiserum 336. RESULTS: In the United States, S aureus capsular types 5 and 8 accounted for 18 and 23% of the isolates, respectively, and type 336 accounted for 59%. Percentage of capsular serotypes in European samples were as follows: type 5 = 34%, type 8 = 34%, type 336 = 30%, and nontypable = 2%. CONCLUSIONS: Serotypes 5 and 8 accounted for only 41% of S aureus isolates from US milk samples, but accounted for 70% of isolates from European milk samples. Addition of the newly developed serotyping antiserum 336 to the typing scheme accounted for 100% of US samples and 98% of European samples and will enable development of a more comprehensive S aureus vaccine.     

Rapid identification and typing of Staphylococcus aureus by PCR-restriction fragment length polymorphism analysis of the aroA gene

The Staphylococcus aureus aroA gene, which encodes 5-enolpyruvylshikimate-3-phosphate synthase, was used as a target for the amplification of a 1,153-bp DNA fragment by PCR with a pair of primers of 24 and 19 nucleotides. The PCR products, which were detected by agarose gel electrophoresis, were amplified from all S. aureus strains so far analyzed (reference strains and isolates from cows and sheep with mastitis, as well as 59 isolates from humans involved in four confirmed outbreaks). Hybridization with an internal 536-bp DNA fragment probe was positive for all PCR-positive samples. No PCR products were amplified when other Staphylococcus spp. or genera were analyzed by using the same pair of primers. The detection limit for S. aureus cells was 20 CFU when the cells were suspended in saline; however, the sensitivity of the PCR was lower (5 x 10(2) CFU) when S. aureus cells were suspended in sterilized whole milk. TaqI digestion of the PCR-generated products rendered two different restriction fragment length polymorphism patterns with the cow and sheep strains tested, and these patterns corresponded to the two different patterns obtained by antibiotic susceptibility tests. Analysis of the 59 human isolates by our easy and rapid protocol rendered results similar to those of other assays.     

Comparison of different DNA fingerprinting techniques for molecular typing of Bartonella henselae isolates

Seventeen isolates of Bartonella henselae from the region of Freiburg, Germany, obtained from blood cultures of domestic cats, were examined for their genetic heterogeneity. On the basis of different DNA fingerprinting methods, including pulsed-field gel electrophoresis (PFGE), enterobacterial repetitive intergenic consensus (ERIC)-PCR, repetitive extragenic palindromic (REP) PCR, and arbitrarily primed (AP)-PCR, three different variants were identified among the isolates (variants I to III). Variant I included 6 strains, variant II included 10 strains, and variant III included only one strain. By all methods used, the isolates could be clearly distinguished from the type strain, Houston-1, which was designated variant IV. A previously published type-specific amplification of 16S rDNA differentiated two types of the B. henselae isolates (16S rRNA types 1 and 2). The majority of the isolates (16 of 17), including all variants I and II, were 16S rRNA type 2. Only one isolate (variant III) and the Houston-1 strain (variant IV) comprised the 16S rRNA type 1. Comparison of the 16S rDNA sequences from one representative strain from each of the three variants (I to III) confirmed the results obtained by 16S rRNA type-specific PCR. The sequences from variant I and variant II were identical, whereas the sequence of variant III differed in three positions. All methods applied in this study allowed subtyping of the isolates. PFGE and ERIC-PCR provided the highest discriminatory potential for subtyping B. henselae strains, whereas AP-PCR with the M13 primer showed a very clear differentiation between the four variants. Our results suggest that the genetic heterogeneity of B. henselae strains is high. The methods applied were found useful for typing B. henselae isolates, providing tools for epidemiological and clinical follow-up studies.     

Conversion of chronic staphylococcal mastitis to acute gangrenous mastitis after neutropenia in blood and bone marrow produced by an equine anti-bovine leukocyte serum

A stock strain of Staphylococcus aureus of mastitis origin, characterized by alpha-, beta-, and delta-toxins, was used to produce chronic mastitis of 20 to 300 days' duration in 6 lactating mammary quarters of 4 cows. Early acute Streptococcus agalactiae mastitis was produced in 1 additional mammary quarter of 1 cow. Equine anti-bovine leukocyte serum (EABLS) was administered to all cows by continuous intravascular drip for 12 to 32 hours. Neutropenia in blood and partial depletion of neutrophil reserve in bone marrow were produced. Chronic subclinical staphylococcal mastitis in 2 quarters of 1 cow changed to gangrenous mastitis by the 40th hour after EABLS administration and led to death of the cow. The disappearance of neutrophil leukocytes from the milk was followed by uninhibited multiplication of S aureus. Probably, staphylococcal leukocidins accelerated the destruction of neutrophils in the milk as S aureus multiplication became intensified. In another quarter of the same cow that was infected with Str agalactiae, neutrophil leukocytes were present in milk as long as 3 days after their disappearance from blood and bone marrow. This may give some indication of the extravascular life-span of the neutrophil in the udder in mastitis. The 2nd cow died at the 16th hour from the start of EABLS administration and at a time when gangrenous mastitis was in the initial stages of development. The S aureus-infected quarters of the 2 remaining cows did not become gangrenous. Administration of EABLS to these 2 cows did not significantly reduce the numbers of neutrophil leukocytes entering the milk of the 3 S aureus-infected quarters. It is concluded that continuous diapedesis of neutrophil leukocytes into the milk in chronic staphylococcal mastitis protects the gland against the development of gangrenous mastitis in the presence of a strain of S aureus capable of alpha-toxin production.     

Differentiation of Australian isolates of Mycobacterium paratuberculosis using pulsed-field gel electrophoresis

OBJECTIVES: To examine strain variation amongst Australian isolates of Mycobacterium paratuberculosis. DESIGN: Pulsed field gel electrophoresis was optimised for differentiation of M paratuberculosis strains, and this typing technique was then applied to a collection of Australian isolates. PROCEDURE: DNAs from 35 Australian isolates of M paratuberculosis and a UK reference strain were digested with one or other of three restriction endonucleases. The banding patterns obtained after pulsed field gel electrophoresis of the DNA fragments were compared. RESULTS: The Australian isolates were divided into two groups on the basis of their DNA banding pattern. Both were different from the UK reference strain. Seven isolates from cattle in Victoria and the Northern Territory had the same pattern as five isolates from alpacas in Victoria and Western Australia. Another 20 isolates from cattle in Victoria, Western Australia and the Northern Territory had the same pattern as isolates from two sheep and a goat in New South Wales. CONCLUSION: Pulsed field gel electrophoresis was a useful tool for strain typing of M paratuberculosis, and could be used to study the transmission of strains in Australia.     

Molecular epidemiology of Vibrio cholerae O1 isolated from sporadic cholera cases in Okinawa, Japan

In July 1994, 6 cholera cases due to Vibrio cholerae O1 El Tor Ogawa sporadically appeared in Okinawa. All 6 patients had no history of traveling abroad. In the period of this cholera outbreak, a strain of V. cholerae O1 El Tor Ogawa was detected from an imported fish at the Naha port quarantine station. The isolates were characterized to clarify whether or not, they belonged to a common clone. Phenotypes were identical except that one strain revealed cured Celebes and the others were original Celebes in kappa phage typing. The restriction fragment patterns of DNA of the isolates hybridized with an enzyme-labeled oligonucleotide probe for cholera toxin gene (ctx) were identical. Randomly amplified polymorphic DNA of the isolates were identical when a primer was used, but 2 patterns were seen when another primer was used. Pulsed-field gel electrophoresis of the chromosomal DNA digested with NotI restriction enzyme showed 3 patterns. The DNA fragment pattern of the strain isolated from the imported fish was different from the clinical isolates. These results suggested that there was no epidemiological relation among the strains of V. cholerae O1 isolated during this period.     

Random amplified polymorphic DNA typing versus pulsed-field gel electrophoresis for epidemiological typing of vancomycin-resistant enterococci

Sixty vancomycin-resistant vanA mutant Enterococcus faecium (VRE) isolates, collected during a 40-month period from 48 patients hospitalized in a French Cancer Referral Center, were typed by using random amplified polymorphic DNA (RAPD), and the results were compared with those previously obtained by typing with SmaI pulsed-field gel electrophoresis (PFGE), which is currently recognized as the "gold standard." The discriminating power of RAPD typing, with seven primers and 11 combinations of primers, was tested on 18 strains, and only the most discriminating combination was further tested on the whole collection. We compared the epidemiological usefulness of RAPD typing of 60 clinical VRE isolates with that of SmaI PFGE typing. With primers AP4 and ERIC1R, RAPD generated 30 patterns versus the 36 patterns generated by SmaI PFGE. However, this did not hamper the epidemiologically correct clustering of 15 related strains and the detection of multiple colonization in nine patients. We conclude that this simple RAPD technique is well suited to the epidemiological typing of VRE and the monitoring of its nosocomial spread.     

Molecular typing of environmental and patient isolates of Aspergillus fumigatus from various hospital settings

Fingerprinting of more than 700 clinical and environmental isolates of Aspergillus fumigatus from four differential hospital settings was undertaken with a dispersed repeated DNA sequence. The analysis of the environmental isolates showed that the airborne A. fumigatus population is extremely diverse, with 85% of the strains being represented as a single genotype isolated once. The remaining 15% of the strains were isolated several times and were able to persist for several months in the same hospital environment. No strains were found to be associated with a specific location inside the hospital, and identical strains were isolated from different buildings of the hospital and outdoors. Isolation of the same strain both from patients and from the environment of the same hospital is highly suggestive of a nosocomial infection. The characteristics of the environmental fungal population explains the two main results obtained from the typing of the clinical isolates: (i) the absence of a common strain responsible for an invasive aspergillosis outbreak results from the extreme diversity of the environmental population of A. fumigatus in contact with the patients, and (ii) patients hospitalized in different wards of the same hospital can be infected with the same strain since every patient might inhale the same spore population.     

Bovine clinical mastitis due to coagulase-negative staphylococci and their susceptibility to antimicrobials

A total of 168 coagulase-negative staphylococci (CNS) strains were isolated from milk samples taken from cows with clinical mastitis. The samples were collected between January 1990 and August 1992 from cows in the veterinary surveillance area of the Ambulatory Clinic, College of Veterinary Medicine, Hautj?rvi, Finland. In 100 cases the effect of antibiotic treatment was evaluated 3-4 weeks after initial sampling. Clinical symptoms of the animals were recorded, and the inflammatory status of their udders was evaluated using the CMT test and assessing milk NAGase activity. CNS mastitis was most common in young cows during early lactation. Staphylococcus hyicus, S. simulans and S. epidermidis were the most frequently isolated CNS. Clinical symptoms were most severe with S. hyicus. Cure rates for CNS induced mastitis were high.     

Sensitivity of Streptococcus agalactiae and Staphylococcus aureus to antibiotics in the Slovak Socialist Republic in the year 1974

In 1974, the State veterinary institutes in the Slovak Socialist Republic studied the sensitivity of 4420 strains of Streptococcus agalactiae and 1056 strains of Staphylococcus aureus to eight antibiotics. The strains were isolated from milk samples obtained from dairy cows suffering from mastitis. 100 per cent of the strains of Streptococcus agalactiae was sensitive to ampicillin, 99.23% to erythromycin, 98.70% to oxytetracycline, 93.02% to bacitracin, 90.77% to chloramphenicol, 41.97% to penicillin, 12.39% to neomycin, 9.73% to streptomycin. As to the strains of Staphylococcus aureus, 98.68 were sensitive to chloramphenicol, 98.50% to ampicillin, 97.92% to erythromycin, 94.98% to oxytetracycline, 93.85% to neomycin, 92.67% to bacitracin 87.50% to streptomycin, and 46.24% to penicillin. The results are discussed in relation to the use of antibiotics in the treatment of mastitis in dairy cows.     

Accuracy of methods using somatic cell count and N-acetyl-beta-D-glucosaminidase activity in milk to assess the bacteriological cure of bovine clinical mastitis

This study examined the capability of milk somatic cell count (SCC) and NAGase activity to discriminate between quarters that had been cured versus those that had not been cured at 4 wk after antimicrobial therapy for clinical mastitis. The distribution of microorganisms that were isolated before therapy from 630 quarters with mastitis was as follows: 225 strains of Staphylococcus aureus, 96 strains of coagulase-negative staphylococci, 152 strains of streptococci (Streptococcus dysgalactiae and Streptococcus uberis), and 157 strains of coliform bacteria. Bacteriological cure rates were 35% for mastitis caused by Staph. aureus, 75% for mastitis caused by coagulase-negative staphylococci, 66% for mastitis caused by streptococci, and 72% for mastitis caused by coliforms. Diagnostic accuracy of milk SCC and NAGase and their interquarter ratios for predicting bacteriological status of the control samples was assessed by calculating sensitivity, specificity, and accuracy and by means of receiver operating characteristic analysis. The efficiency of milk SCC and NAGase for predicting bacteriological cure was greatest for cows that had been infected with Staph. aureus. The main problem in detecting coagulase-negative staphylococci was low sensitivity, and the main problem in detecting streptococci and coliforms was low specificity. Receiver operating characteristic analysis is not completely suitable for the detection of mastitis because reference method bacteriology and indirect tests can never fully agree. To assess the recovery of cows from mastitis caused by Staph. aureus, bacteriology should be supplemented with an examination of milk SCC or NAGase activity at threshold values such as those presented here.     

Electrophoretic karyotype and in vitro antifungal susceptibility of Cryptococcus neoformans isolates from AIDS patients

Electrophoretic karyotype (EK) was used to type 13 clinical isolates of Cryptococcus neoformans from eight AIDS patients. All of the isolates were also tested for their in vitro susceptibilities to fluconazole, itraconazole, D0870, flucytosine, and amphotericin B by a broth macrodilution technique performed according to the National Committee for Clinical Laboratory Standards recommendations. Although all strains were isolated from a limited geographic area, DNA typing showed a wide genetic variation in this group of patients, yielding seven different patterns. Two patients in whom C. neoformans was isolated in the same time period shared similar EK profiles, suggesting the possibility of cross-infection. In three patients, sequential isolates were evaluated: in two of them, EK analysis showed the persistence of the same genotype throughout the infection, whereas from the third, two isolates of C. neoformans with two different DNA profiles were obtained. Despite the small number of strains considered in this study, our susceptibility data indicate that C. neoformans isolates are very susceptible to the new triazoles.     

A contingency-based approach for assessing policies on aging

The analysis of genomic DNA fragment patterns has revealed as a powerful tool for strain discrimination in Staphylococcus aureus; for use as an epidemiological marker, stability during the course of an outbreak is an essential prerequisite. Genomic DNA fragment patterns (SmaI restriction, pulsed-field electrophoresis) of four different epidemic MRSA strains were compared along with intra- and interhospital and country-wide spread over more than 12 months in Germany. Strain I was isolated from infections in 8 hospitals. In one hospital a subclone arised which differed from the original strain by 4 fragments. Strain II was spread among 4 hospitals, isolates from three of these hospitals exhibited a variability of one to three fragments in the 150-200 kb range. Two hospitals in the Hannover-area were affected by strain III; in 17 isolates of this strain a variability up to three fragments was found in the 170-200 kb range. Strain IV was isolated from 19 cases of infections in 3 hospitals in Berlin. The fragment patterns were completely stable. When S. aureus strains are typed by genomic DNA fragment patterns, a variability in a definite range of molecular masses during the course of an epidemic should be taken into consideration.     

Antigenic crossreactivity and lipopolysaccharide neutralization properties of bovine immunoglobulin G

We investigated a possible mechanism by which immunization against core and lipid A determinants of lipopolysaccharide reduced clinical cases of mastitis and symptoms commonly associated with heterologous Gram-negative IMI. The IgG fraction of sera from cows immunized with either Escherichia coli J5 bacterin, E. coli J5 lipopolysaccharide conjugate vaccine, or unimmunized controls was purified by precipitation with caprylic acid and ammonium sulfate. The degree of IgG crossreactivity with Gram-negative bacteria that were isolated from clinical quarters was greater than that with Gram-positive isolates of Staphylococcus aureus. The highest magnitude of crossreactivity was against smooth strain E. coli isolates, followed by heterologous species of Enterobacter, Serratia, and Klebsiella isolates. Serum IgG from cows immunized with conjugate was highly crossreactive to E. coli J5, E. coli O111:B4, Serratia marcescens, Klebsiella pneumoniae, and Salmonella typhimurium lipopolysaccharides. The magnitude of antibody crossreactivity with lipopolysaccharides coincided with the ability of IgG to suppress the mitogenic effect of lipopolysaccharides on bovine lymphocytes.     

Oxacillin- and quinolone-resistant Staphylococcus aureus in Sao Paulo, Brazil: a multicenter molecular epidemiology study

OBJECTIVE: To investigate the possibility of interhospital spread of multiresistant Staphylococcus aureus in Sao Paulo, Brazil. DESIGN: We evaluated 13 nosocomial S aureus strains selected because of resistance to oxacillin and ciprofloxacin. SETTING: The strains were collected between March 1991 and September 1991 from four different hospitals in Sao Paulo. Two were teaching hospitals, and two were private hospitals. PATIENTS: Each strain was isolated from a different patient. All patients were hospitalized when the strains were isolated. INTERVENTIONS: The strains were typed by restriction endonuclease analyses of plasmid DNA (REAP) using EcoRI, HindIII, RsaI, and AluI and by extended antibiogram profile (34 drugs). RESULTS: All strains had identical plasmid and antibiogram profile. They demonstrated the same plasmid pattern as previously described in one of the hospitals studied. CONCLUSIONS: Our results suggest the dissemination of a unique oxacillin- and quinolone-resistant strain of S aureus in several hospitals of Sao Paulo, Brazil.     

Recovery of Staphylococcus aureus from centrifuged quarter milk samples

The identification of cows that are positive for mastitis caused by Staphylococcus aureus is difficult under field conditions. The frequency of isolation of S. aureus from quarter milk samples was compared with the frequency of recovery of S. aureus from sediment after centrifugation of those same samples. Overall, 776 quarter milk samples from 194 cows were studied. Cultures that were positive for S. aureus were obtained from 82 samples; 153 sediments from quarter milk samples were also positive for S. aureus. The results of this investigation showed that cultures of the sediment of quarter milk samples increased the number of positive outcomes up to 145.5%, depending on the herd. Using a different group of samples, including samples taken 1 to 5 d or 7 to 10 d after calving and samples taken after intramammary therapy, a 94% increase in cultures that were positive for S. aureus after centrifugation was found compared with cultures of the same quarter milk samples that were not centrifuged. Sedimented cultures may be useful in S. aureus control programs that require the segregation, selective treatment, or culling of cows that are positive for S. aureus.     

Nasal carriage of staphylococcus aureus and epidemiology of surgical-site infections in a Sudanese university hospital

Surgical site infections (SSI) due to Staphylococcus aureus among 256 male and 158 female patients (mean age, 28 years) undergoing elective surgery at the Soba University Hospital (Khartoum, Sudan) were studied. During an 11-month study period all patients were analyzed for nasal carriage of S. aureus at the time of admission. Follow-up of the development of SSI proceeded until 4 weeks after the operations. In addition, nasal swabs were obtained periodically during the same period from 82 members of the staff. In order to discriminate autoinfection from cross infection, bacterial isolates were typed by random amplification of polymorphic DNA (RAPD), pulsed-field gel electrophoresis (PFGE) of DNA macrorestriction fragments, and restriction fragment length polymorphism analysis of the protein A and coagulase genes. Preoperative cultures revealed the presence of S. aureus in the noses of 98 patients (24%). The overall number of postsurgical wound infections in the entire group was 57 (14%), 24 of which were due to S. aureus. Only 6 of the 98 nasal S. aureus carriers suffered from wound infections by the same species. In these six cases the infecting strain could not be genetically discriminated from the nasal inhabitant, substantiating autoinfection. However, nasal carriage of S. aureus is not a significant risk factor for the development of SSI in this setting (6 of 98 patients with autoinfection versus 18 of 316 patients [414 - 98 patients] with cross infection; P = 0.81), most probably due to the fact that noncarriers are at a significant and relatively large risk for acquiring an independent S. aureus SSI. The other S. aureus strains causing SSI showed a high degree of genetic heterogeneity, demonstrating that it is not an epidemic strain that is causing the SSI. Among the staff personnel screened, 47.4% did not carry S. aureus in the nose at any time during the study period, whereas 13. 2% persistently carried a single strain in the nose. Another 39.5% could be classified as intermittent carriers. When strains derived from staff personnel were genetically typed, it was demonstrated that most of the strains represented genetic variants clearly differing from the isolates causing SSI. On the other hand, possible cross colonization among staff personnel and even cross infection from staff personnel to patients or from patient to patient were demonstrated in some cases, but epidemic spread of a single strain or a few clonally related strains of S. aureus could be excluded.