Evaluation of single vs pooled swab cultures for detecting MRSA colonization.

BACKGROUND: The costs and laboratory workload arising from meticillin-resistant Staphylococcus aureus (MRSA) screening could be reduced markedly by processing nose, throat and skin swabs from one person in a single culture broth (specimen pooling). The purpose of this study was to evaluate the sensitivities and times for MRSA detection using a variety of approaches to processing of individual and pooled swabs. METHODS: Four hundred and seventeen swabs from 139 subjects with a history of MRSA colonization (three swabs per subject - nose, throat and skin) were submitted. Swabs were suspended in 200-µL sterile saline, and these suspensions were used individually and as pooled samples to inoculate two different chromogenic media [MRSA SMART (bioMerieux, Marcy-l'Étoile, Paris, France) and CHROMagar MRSA (CHROMagar, Paris, France)] and Todd-Hewitt Broth; the latter cultures were then subcultured on to the same chromogenic media. RESULTS: MRSA was detected from at least one specimen in 75 subjects (50.4%). The diagnostic sensitivities of pooled surveillance cultures compared with single cultures were 97% and 93% for direct and enrichment cultures, respectively. Enrichment culture of either individual or pooled samples had no benefit compared with direct culture (P>0.05). CONCLUSIONS: Pooling of MRSA screening swabs for either direct culture on chromogenic agar or enrichment culture is suitable for routine use.

Survey of community-associated-methicillin-Resistant Staphylococcus aureus in Slovenia: identification of community-associated and livestock-associated clones.

The epidemiology of methicillin-resistant Staphylococcus aureus (MRSA) in Slovenia is poorly documented. The aim of this study was to investigate susceptibility patterns, virulence gene profile and clonality among MRSA isolates with positive screened resistance phenotype for CA-MRSA collected from patients in Slovenia, from January 2010 to December 2010. We included only MRSA isolates that were resistant to cefoxitin and oxacillin, and susceptible to at least two of the following four antibiotics: ciprofloxacin, erythromycin, clindamycin or gentamicin (presumptive CA-MRSA). Altogether 151 isolates fulfilled our screening phenotypic definition, 126 MRSA isolates were classified as CA-MRSA and 25 as HA-MRSA. Thirty-six per cent of them were resistant to ciprofloxacin, 24% to clindamycin, 33% to erythromycin and 13% to gentamicin. The mecA gene was detected in 150 isolates, while the mecC gene only in 1 isolate. The MRSA isolates were classified to 19 different clones. The most prevalent sequence types were ST5 (26.4%), ST45 (25.2%), ST22 (10.6%), ST398 (9.9%), ST8 (5.9%), ST7 (4.6%), ST1 (3.9%), ST152/377 (3.3%), ST228 (2.6%) and ST2883 (1.3%). The ST6, ST9, ST30, ST72, ST88, ST111, ST130, ST225 and ST772 were identified sporadically. The Panton-Valentine leukocidin (PVL) gene was detected in 13 (8.6%) isolates that belonged to ST5, ST7, ST8, ST22, ST72, ST88, ST 152/377 and ST772. Our results show high variability of CA-MRSA circulating in Slovenia and also the presence of LA-MRSA clones.

Detection of methicillin-resistant Staphylococcus aureus carrying the mecC gene in human samples in Slovenia.

SUMMARY Following the recognition of a mecC MRSA isolate from a patient hospitalized in the northeastern region of Slovenia, a national collection of 395 community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) isolates from 2006 to 2013 was screened. An additional six mecC MRSA strains were found and characterized as spa types t843, t9397 and t10009, and multilocus sequence type ST130. The low oxacillin minimum inhibitory concentrations and absence of the mecA gene make recognition of these MRSA strains problematical for diagnostic laboratories. In such strains the presence of mecC should be determined.

Evaluation of separate vs pooled swab cultures, different media, broth enrichment and anatomical sites of screening for the detection of methicillin-resistant Staphylococcus aureus from clinical specimens.

Early identification of methicillin-resistant Staphylococcus aureus (MRSA) carriers is a major component of an MRSA control programme. The cost and laboratory workload could be markedly reduced by processing multiple swabs from one person in one culture broth (specimen pooling). We evaluated the sensitivity for MRSA detection and the growth rate of pooled swabs compared with individual processing. In total, 1254 swabs from 423 subjects (two to five swabs per subject) were submitted for detection of MRSA. Swabs were suspended in 2-mL volumes of sterile Todd-Hewitt Broth and divided into two 1-mL aliquots. One aliquot of the suspension was processed as a single specimen, and the other aliquot was mixed (pooled) with other suspensions in which swabs from the same patient were suspended. Forty-four (10%) pooled samples were positive for MRSA. Specimens from seven additional patients that were negative when pooled were positive when processed separately. There was no case where the pooled specimen was positive but the separate specimens were negative. The diagnostic sensitivity of pooled surveillance cultures compared with single cultures, when only subjects colonized by MRSA were considered, was 86% and the false-negative rate was 14%. Eighty percent of the pooled positive cultures were detected by the third day and all were detected by the fourth day. Fifty-four percent of the specimens processed separately were detected by the second day and all were detected by the fourth day. Pooling of specimens decreases the sensitivity of MRSA detection compared with processing each swab separately, particularly in swabs with a low number of colony-forming units. In all subjects whose pooled samples were negative but whose swabs examined separately were positive, the swabs examined separately were negative on primary plates and positive only after culturing in enrichment broth.