This study compares the performance of three chromogenic culture agar plates, chromID MRSA, MRSA-Screen and MRSA-Select, by challenging with a collection of Staphylococcus aureus strains and screening samples obtained from hospitalised patients. All chromogenic media showed excellent sensitivity (>95%) and specificity after 18 h on the methicillin-resistant Staphylococcus aureus (MRSA) collection strains, but the specificity of MRSA-Screen decreased markedly after 42 h. Sixty-eight of 1,002 screening specimens yielded MRSA on at least one medium. The sensitivity of all media to detecting MRSA after 18 h was <50%, but this increased to 75% (chromID MRSA), 81% (MRSA-Screen) and 72% (MRSA-Select) after 42 h and 85% after enrichment and plating on the same media. The specificity at 18 h was excellent, but was significantly lower for MRSA-Screen after 42 h and enrichment. In conclusion, all media showed equivalent sensitivities after 18 h of incubation and performed better when enriched before inoculation. MRSA-Screen was more sensitive but less specific than the two other media after 42 h of incubation.

MDRAB strains were negative for qnrA, qnrB and qnrS and other studies have shown that qnr in A. baumannii is not widespread. 4 Four of the eight qnr genes were found in ciprofloxacin-resistant isolates and three in isolates with intermediate susceptibility. C. freundii harboured qnrB8 and was ciprofloxacin-susceptible. Six of the eight (75%) qnr-positive isolates were ESBL-producing strains representing 11% of all the ESBL-positive isolates (n¼53).x 2 analysis showed that qnr was associated with ESBL-producing strains (P,0.01). Quinolone conjugation studies have revealed that the presence of qnr does not confer high-level resistance, although it has been shown that the qnr gene is able to co-localize with other antibiotic resistance determinants on a plasmid. 5 This implies that although they may not be the cause of resistance, qnr genes are associated with isolates that have reduced susceptibility.Each ESBL-positive isolate harboured only one type of qnr gene. The sequences of qnrA and qnrS genes detected in this study were 100% identical to the published sequences. Within qnrB, however, there were mutations noted in all three of the sequences resulting in two amino acid changes. The change in the amino acids was at the same position irrespective of the qnrB subtype or bacterial species harbouring the gene. Six variants of qnrA have been identified worldwide compared with 19 variants of the qnrB gene and 3 variants of qnrS. 6 This study describes for the first time the presence of qnrB8 in the UK. Although the full clinical significance of qnr-mediated quinolone resistance remains unclear, this relatively new resistance genotype is clearly expanding in prevalence and repertoire.

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