Objectives:

MRSA ID is a new chromogenic medium, which relies upon alpha-glucosidase detection for the visualisation of methicillin-resistant Staphylococcus aureus (MRSA) as green colonies. The medium is selective against Gram-negative bacteria, enterococci and fungi and also inhibits methicillin-sensitive staphylococci due to the inclusion of a beta-lactam antibiotic. The aim of this project was to evaluate the effectiveness of MRSA ID for the isolation of MRSA from nasal swabs. Its performance was compared with that of two commercially available agars.

Method:

A total of 192 nasal swabs from distinct hospital patients were each emulsified in 750 mL of saline (0.85%). A 50 ml sample of the resulting suspension was inoculated onto MRSA ID, CHROMagar MRSA and ORSAB medium. The inoculum was spread for single colonies and all media were incubated at 37^[compfn]C. The culture plates were examined for the presence of MRSA after 20–22 h incubation and were examined again after a total of 48 h incubation. All suspect colonies of MRSA on all media were confirmed using standard methods including the tube coagulase test and detection of the mecA resistance determinant.

Results:

A total of 28 strains (14.6%) were isolated on at least one of the three media. After 20–22 h incubation 25 of these strains (89%) were isolated as coloured colonies on MRSA ID compared with 20 (71%) on CHROMagar MRSA and 21 (75%) on ORSAB medium. After 48 h incubation, 26 strains (93%) were isolated as coloured colonies on MRSA ID and ORSAB medium compared with 25 (89%) on CHROMagar MRSA. The chromogenic reactions of all three media were highly specific for MRSA after 20–22 h incubation. The specificities of the media were 100, 100 and 95% after 20–22 h for MRSA ID, CHROMagar MRSA and ORSAB, respectively. After 48 h, some strains of coagulase-negative staphylococci generated pale green colonies on MRSA ID, they were clearly distinguishable from S. aureus colonies.

Conclusions:

MRSA ID is a highly effective medium for the detection of MRSA from nasal swabs and has an excellent sensitivity and specificity after 20–24 h incubation.