Medium shows advantages in MRSA detection

Rapid isolation of Methicillin-resistant Staphylococcus aureus (MRSA) is essential for the timely implementation of appropriate treatment and infection control measures.

Comparative studies undertaken in Canada and the UK show that Oxoid's Oxacillin resistance screening agar base (Orsab) has advantages over mannitol salt agar (MSA) for the isolation of MRSA from clinical specimens.

Orsab is a novel medium from Oxoid which uses aniline blue to detect mannitol fermentation in staphylococci.

Its formulation includes a dual antibiotic supplement (polymixin B and oxacillin) and 5.5% concentration of sodium chloride which combine to reduce the growth of non-staphylococcal organisms and select for MRSA.

A study conducted by a team from Sunnybrook and Women's College Health Sciences Centre, at the University of Toronto, found that Orsab more easily differentiated presumptive MRSA from other organisms and gave lower false presumptive-positive colonies.

While the yield of MRSA on Orsab was comparable to that obtained on MSA, Orsab was also found to be less labour-intensive, as MSA plated specimens required more subcultures and investigations for identification of MRSA.

In the UK, a clinical evaluation conducted jointly by the Public Health Laboratory at the University Hospital of Wales and Oxoid showed that Orsab recovered a greater number of MRSA isolates from clinical samples after 48 hours incubation in a direct comparison with two MSA media.

Orsab also isolated more strains after 24 and 48 hours incubation from an MRSA panel, compared with MSA.

Orsab is regularly used at Great Ormond Street Hospital for Children, London, for MRSA screening.

Oxoid's publication 'Setting Standards' (folio 842), contains the Canadian evaluation and a case study from Gt Ormond Street, the UK evaluation is also available as a poster, both can be obtained from Val Kane at Oxoid.