Objective: Contamination of the hospital environment with MRSA has been implicated in patient acquisition and infection. With limited isolation facilities in Irish hospitals, continued pressure on bed availability and resulting increased use of cohort nursing practices, it is important that the hospital environment remains MRSA free. Improved environmental testing regimens are required so that decontamination of surfaces can occur in real time. The aim of this study was to evaluate a new rapid screening tool, Baclite™ MRSA assay, for the detection of MRSA from hospital environmental swabs.

Methods: MRSA was cultured on chromogenic agar plates, and confirmed using slide/tube coagulase tests and methicillin sensitivity tests. Baclite™ detection of bacterial strains in pure culture was determined by seeding Baclite™ broth with 10⁵cfu/ml of the test organism and processing in the Baclite™ instrument. Environmental swabs were processed according to assay instructions initially, followed by optimization of broth incubation times in the assay, for recovery of distressed environmental microrganisms.

Results: The Baclite™ assay could detect current prevalent Irish MRSA strains (n=64) incorporating all known SSCmec types, including 21 PVL positive strains. The limit of detection (LoD) of the assay, based on 4 MRSA isolates exhibiting a range of oxacillin MIC's, was equivalent to culture techniques, at 10²cfu/ml. The LoD in the presence of 5 other test microorganisms was not affected by a high background (10⁵cfu/ml) of these organisms. An extended incubation period, (16h) compared with clinical specimens (2h) was required to achieve optimum sensitivity with environmental specimens. 137 environmental swabs were processed using 16h incubation, 42 were wrongly identified as MRSA (30% false positive rate). Twenty-two of these contaminating organisms were identified, eighteen were methicillin-resistant coagulase negative staphylococci (MRCNS) and the remaining four were Enterococcus faecalis. The predominant MRCNS included S. epidermidis and S. haemolyticus. The addition of desferoxamine, an inhibitor of some MRCNS, improved assay specificity by 36%. Assay specificity and sensitivity with environmental specimens, (using extended incubation and desferoxamine supplement) was calculated as 100% and 72.3%.
Conclusion: The Baclite™ assay detects MRSA in environmental specimens but further work is required to optimise the assay for this application.