Evaluation of media available for testing the susceptibility of Pseudomonas aeruginosa by BSAC methodology

BSAC methodology was used to study media available from four manufacturers for susceptibility testing of Pseudomonas aeruginosa in the UK and Ireland. Fifty isolates of P. aeruginosa, including ATCC and NCTC control strains, were studied by disc testing methods by 20 centres around the UK and Ireland and by MIC determination at the BSAC Standardized Method Development Centre (SMDC) in Birmingham, UK. Although many of the antibiotics tested gave similar zone diameters for a particular strain-antibiotic combination irrespective of the medium used for testing, significant differences in media performance were noted for the aminoglycosides, imipenem and colistin. The data generated showed that for testing aminoglycosides, zone diameters on two of the agars were significantly larger than those on Oxoid IsoSensitest agar-the recommended medium for the BSAC method. For imipenem, zones were smaller on one of the agars and for colistin, zones were larger on one of the agars in comparison with those on Oxoid agar. Scattergram analysis of zone diameters and MICs of colistin indicated that the zone diameter breakpoints should be adjusted. Media purported to be equivalent to Oxoid IsoSensitest agar do not give the same zone sizes with some agents and should not be used until standards for media performance have been established and the media have been shown to perform adequately.