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Department of Bacteriology, Institute of Microbiology and Immunology, School of Medicine, 11000 Belgrade, Serbia
Scottish MRSA Reference Laboratory, Microbiology Department, Stobhill Hospital, Glasgow G21 3UW, United Kingdom
School of Biochemistry and Microbiology, University of Kwa-Zulu Natal, 4000 Durban, Republic of South Africa
Department of Clinical Microbiology, Regional Hospital Píbram, CZ-26126 Píbram, Czech Republic
Department of Microbiology, Institute of Biology, University of Bialystok, 15-950 Bialystok, Poland
ABSTRACT
Staphylococcus sciuri is a principally animal-associated bacterial species, but its clinical relevance for humans is increasing. Our study aimed to provide the first insight into the prevalence of this bacterium in a hospital environment. A 3-month surveillance was conducted in a hospital located in Belgrade, Serbia, and 1,028 samples taken from hands of medical personnel, medical devices, and various hospital surfaces were screened for S. sciuri presence. In total, 108 isolates were obtained, which resulted in a relatively high rate of colonization (10.5%). These isolates, along with 7 S. sciuri strains previously isolated in the same hospital (n = 115), were phenotypically and genotypically characterized. Antimicrobial susceptibility testing revealed that 73% of the strains were resistant to one or more antibiotics, with 4.3% strains displaying multiresistance. Examination of 16S-23S ribosomal DNA intergenic spacer length polymorphism identified the strains at the subspecies level, and 74 (64.3%) strains of S. sciuri subsp. sciuri, 37 (32.2%) strains of S. sciuri subsp. rodentium, and 4 (3.5%) strains of S. sciuri subsp. carnaticus were established. Pulsed-field gel electrophoresis (PFGE) analysis showed 21 distinct pulsotypes, including 17 main types and 4 subtypes. One dominant cluster with 62 strains was found, while 19 (90.5%) of the PFGE types and subtypes identified had 5 or fewer strains. The predominance of small PFGE clusters suggests that the ubiquitous presence of S. sciuri in the outside environment presents the continuous source for colonization of the hospital environment. The presence of one dominant PFGE cluster of strains indicates that some S. sciuri strains may be capable for adaptation to hospital environment conditions and continuous existence in this environment.
INTRODUCTION
Staphylococcus sciuri is a coagulase-negative, novobiocin-resistant, oxidase-positive staphylococcal species. The organism is considered a principally animal bacterial species and is commonly present on skin and mucosal surfaces of a wide range of pets and farm and wild animals (11, 15, 16, 27) and in food of animal origin (10, 23). It is also known to occur in environmental reservoirs, such as soil, sand, water, and marsh grass (15). S. sciuri may be found as a colonizing organism in humans, with low carrier rates in the nasopharynx, skin, and urogenital tract (8, 30, 31). The clinical relevance of S. sciuri in humans appears to be increasing, since the bacterium has been associated with various infections, such as endocarditis (12), peritonitis (35), septic shock (13), urinary tract infection (30), endophthalmitis (3), pelvic inflammatory disease (31), and, most frequently, wound infections (17, 25, 28).
The capacity of this species to carry antimicrobial resistance determinants has been well documented (8, 17, 20, 25, 28). Furthermore, it was suggested that the mecA gene of methicillin-resistant strains of staphylococci originated from an evolutionary relative of the mecA homologue that has been identified in S. sciuri (7, 16).
Isolation of S. sciuri from hospitalized patients has been reported (1, 14, 18, 24), but its occurrence in hospital environment has not been investigated so far. The present study is the first evaluation of S. sciuri colonization of a hospital environment and characterization of the obtained isolates.
MATERIALS AND METHODS
Sampling. Sampling was performed over a 3-month period (October through December 2002) at the Institute for Cardiovascular Diseases—Dedinje, Belgrade, Serbia. This teaching hospital consists of 7 operating rooms, 5 intensive care units (ICUs) with 32 beds, 8 wards with 202 beds, and an outpatient department. Approximately, 20,000 patients are admitted at the outpatient department per year, and more than 8,500 operations and invasive diagnostic-therapeutic procedures are performed annually.
The sampling was performed in five operating rooms, all ICUs, six wards, and the outpatient department. Specimens were collected from the hands of nursing staff, medical devices, various hospital surfaces, and inanimate objects, such as floor areas, bed frames, over-bed tables, chairs, lockers, windowsills, door handles, light switches, nurse call buttons, telephones, urinals, bathtubs, sinks, faucet, toilet seats, stands for infusion apparatus, intravenous pump buttons, mobile instrument tables, instruments, respirators, electrocardiogram monitors, mobile monitor units, and sterilizing drums. The samples were taken with sterile cotton-tipped swabs moistened with phosphate-buffered saline (pH 7.2) and transported to the research laboratory within 2 h.
The swabs were inoculated into STS broth (29) and agar (27), the selective media specifically designed for the isolation of S. sciuri. The swabs were first inoculated onto STS agar and then into STS broth enriched with 5 g/liter of yeast extract. The inoculated STS agar plates were incubated at 35°C for 3 days and subsequently at room temperature for 2 days. Samples in STS broth were incubated at 35°C for 3 days and then streaked onto STS agar plates and further incubated at 35°C for 3 days and at room temperature for 2 days. STS agar plates were examined daily for the presence of colonies resembling S. sciuri. Both direct inoculation onto STS agar and the enrichment method were conducted with the first set of 450 samples, but all S. sciuri isolates obtained were recovered only by the growth enrichment method. Hence, direct inoculation onto STS agar was excluded for all the following samples.
Preliminary identification of S. sciuri was based on colony morphology and positive esculin hydrolysis on STS agar. Esculin-positive colonies were checked for oxidase activity by oxidase diagnostic tablets (Rosco, Taastrup, Denmark). Oxidase-positive colonies were finally identified as S. sciuri in accordance with the previously described recommendations (16, 26).
Bacterial isolates. Isolates obtained during this study, as well as a set of seven isolates previously isolated in the same hospital (26, 28), were further analyzed.
Antimicrobial susceptibility testing. Susceptibility testing was performed by the disk diffusion method on Mueller-Hinton agar (Oxoid Limited, Basingstoke, Hampshire, United Kingdom). The antibiotics included penicillin, 10 U; oxacillin, 1 μg; vancomycin, 30 μg; gentamicin, 10 μg; erythromycin,15 μg; clindamycin, 2 μg; tetracycline, 30 μg; chloramphenicol, 30 μg; ciprofloxacin, 5 μg; and trimethoprim-sulfamethoxazole, 1.25/23.75 μg (Bioanalyse Co., Ltd., Ankara, Turkey). Performance and evaluation of the susceptibility testing followed the recommendations given by the National Committee for Clinical Laboratory Standards (21).
PFGE. Molecular typing of the isolates was performed by pulsed-field gel electrophoresis (PFGE) as described previously (2, 33).
16S-23S ribosomal DNA intergenic spacer length polymorphism. Molecular identification of the isolates to the species and subspecies levels was based on PCR amplification of the 16S-23S rRNA intergenic spacer region according to the previously described protocol (6, 25). The patterns of the PCR products were visually compared with the patterns of reference strains obtained from the Czech Collection of Microorganisms (CCM, Brno, Czech Republic): Staphylococcus vitulinus CCM 4511, Staphylococcus pulvereri CCM 4481, Staphylococcus lentus CCM 3472, S. sciuri subsp. sciuri CCM 3473, S. sciuri subsp. rodentium CCM 4657, S. sciuri subsp. carnaticus CCM 4835, and Staphylococcus fleurettii CCM 4922.
PCR-based detection of mecA. The presence of the mecA gene was determined by a previously described PCR procedure (4, 19).
RESULTS
Isolation of S. sciuri in the hospital environment. A total of 50 samples from the hands of nursing staff and 978 samples from environmental sources and medical devices were screened for S. sciuri. A total of 108 isolates were obtained from various environmental samples and medical devices. The hand specimens and samples from the operation rooms yielded no isolate. One strain was recovered from the floor in one of the ICUs. The overall isolation rate of S. sciuri in the hospital environment was 10.5%. The distribution of the isolates from various samples is presented in Table 1.
All strains isolated during this investigation, as well as one strain isolated during 1998 and six strains isolated during 2001, making a total of 115 isolates, were further studied.
Antimicrobial susceptibility. Out of 115 isolates tested, 84 (73%) were resistant to one or more antibiotics, while 31 (27%) were fully susceptible. All strains were susceptible to vancomycin, ciprofloxacin, and chloramphenicol. Seventy-five isolates (65.2%) were resistant to penicillin, 74 (64.3%) to oxacillin, 11 (9.6%) to gentamicin, 4 (3.5%) to tetracycline, and 2 (1.7%) to trimethoprim-sulfamethoxazole, 9 were resistant and 1 intermediately resistant to clindamycin (in total, 8.7%), and 1 was resistant and 7 intermediately resistant to erythromycin (in total, 6.9%). Only 5 (4.3%) were multiresistant, expressing resistance to more than three different classes of antibiotics.
PFGE. PFGE patterns of 115 strains revealed 21 different pulsotypes (Table 2), including 17 main types and 4 subtypes within the major pattern A (A2, A3, A4, and A5). The dominant cluster A included 62 strains, while the majority of remaining types and subtypes identified (n = 19; 90.5%) had 5 or fewer strains. There was no complete correlation between resistotyping and PFGE.
Genotypic characterization. 16S-23S ribosomal DNA intergenic spacer length polymorphism and PCR-based detection of mecA were performed for one representative of each PFGE pulsotype. PCR analysis of 16S-23S ribosomal DNA intergenic spacer length polymorphism enabled identification of the isolates to the subspecies level, and the overall numbers of isolates per subspecies were as follows: 74 (64.3%) S. sciuri subsp. sciuri isolates, 37 (32.2%) S. sciuri subsp. rodentium isolates, and 4 (3.5%) S. sciuri subsp. carnaticus isolates (Table 2). Phenotypic resistance to oxacillin was observed in 9 (42.8%) representatives, but the mecA gene was detected in 8 (38.1%).
DISCUSSION
Hospitals provide a reservoir of microorganisms, many of which are multiply resistant to antimicrobials (9). Although hospital staff and patients are considered the most important source of nosocomial microorganisms, there is growing evidence that the colonized hospital environment is also of substantial importance (9, 32). This short-term study aimed to provide the first insight into the prevalence of S. sciuri in a hospital environment. The results presented showed a relatively high rate of colonization by S. sciuri of the hospital environment tested. The floor areas accounted for 20% of the total number of isolates recovered, while there was no yield from hand specimens of medical personnel.
Microbial density is important in establishing infection, and likewise, colonization density is presumably important in the assessment of environmental colonization by microorganisms (22). Although the number of samples positive for S. sciuri was large, we assume that the density of colonization is low, since all the isolates were recovered after the enrichment step only. The low population density of S. sciuri may partly explain the absence of this organism on hands of the nursing staff, as well as the rare occurrence of intrahospital infections caused by this bacterium. Only one case of surgical wound infection has been associated with S. sciuri in the hospital tested in the present study since 1998 (28). The strict infection control measures in the operating rooms and ICUs most probably explain the absence of the bacterium in these sections of the hospital.
PFGE analysis revealed one dominant clone A (with 4 subtypes) and 16 pulsotypes within the population of S. sciuri strains. No correlation between the PFGE types and various sampling sites was observed. The dominant clone A accounted for more than 50% of all isolates. Such clear predominance of one clone indicates that S. sciuri may be capable of permanent existence within a hospital environment. It is interesting that four strains isolated in 2002 displayed PFGE profiles, namely, the pattern E, identical to that of the one isolated in 1998 in the same hospital. The identical PFGE profiles of the strains isolated over a 4-year period could be explained either by recolonization of the hospital environment with bacteria from the outside environment or by continued existence of the clone in the hospital. However, we consider the latter a less-likely alternative, since a permanently present and well-adapted cluster of bacteria would have been isolated more frequently. It seems more reasonable to assume that the four strains displaying the same PFGE pattern as the previously isolated strain were most probably reintroduced into the hospital from the community. This finding, and in particular, the predominance of small PFGE clusters in our sample, suggests that the ubiquitous presence of S. sciuri in the outside environment presents the continuous source for colonization/recolonization of the hospital environment.
As far as routes of nosocomial transmission of S. sciuri are concerned, transmission of this animal bacterial species to humans may occur via close contact with animals, but this is not likely to be of great importance in hospital settings. The role of food products of animal origin, which is also considered to be an important route for S. sciuri colonization and infection, could not have been confirmed. Our study did not include analysis of food and fecal specimens, and moreover, the occurrence of this bacterium in human fecal samples has not been investigated so far. Since S. sciuri was not isolated from samples taken from hands of medical personnel, we presume that dust containing S. sciuri could be the vehicle for dispersal of this bacterium. It is well known that staphylococci can withstand desiccation and thus are a frequent component of hospital dust (9, 34). Moreover, it has been reported that S. sciuri does not require an organic source of nitrogen and is capable of a free-living existence (15). Shared medical devices may also be of certain importance for S. sciuri spread within a hospital environment, since we recovered a number of the isolates from such samples.
Most clinical isolates of S. sciuri reported previously were resistant (8, 20, 28, 30, 35) or even multiresistant (17, 25) to antibiotics. Since more than 70% of the S. sciuri strains we isolated were resistant to antimicrobials, it is clear that the isolates originating with the hospital environment displayed a high level of resistance similar to that of clinical isolates of this bacterium. More than 60% of tested strains were resistant to oxacillin, as determined by the disk diffusion method. Detection of the mecA gene was carried out in randomly selected strains from each PFGE pattern, and phenotypic and genotypic testing for methicillin resistance displayed identical results. The only exception was the single strain exhibiting PFGE profile S, which was resistant to oxacillin as determined by the disk diffusion method, but the presence of the mecA gene was not demonstrated by PCR. Since detection of the mecA gene is considered to be the "gold standard" for examination of methicillin resistance in staphylococci (5), this strain was classified as methicillin susceptible.
The results of PCR analysis of 16S-23S intergenic spacer polymorphism showed that S. sciuri subspecies sciuri was most frequently isolated from the hospital environment (64.3%), followed by subspecies rodentium and subspecies carnaticus. Although the same distribution pattern of S. sciuri subspecies was reported for clinical isolates of this bacterium (20), the current number of studies is insufficient for any strong conclusions about variation in clinical and/or epidemiological significance among the three subspecies.
The present study suggests that S. sciuri is a relatively frequent colonizing organism in a hospital environment, although the presumed density of the colonizing population is low. The PFGE typing results presented indicate that the hospital environment is most probably repeatedly colonized by S. sciuri strains present in the community. In addition, according to the presence of one dominant PFGE cluster of strains, some S. sciuri strains introduced from the outside environment may be capable of adaptation to hospital environment conditions and continuous existence in this environment. A long-term follow-up study, however, is needed to fully elucidate the origin of S. sciuri strains found in the hospital environment.
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