Objectives:

To develop methods to measure the initial response of S. aureus after exposure to antimicrobial agents. Such an approach has the potential to allow both the sensitivity and mechanism of resistance to be rapidly determined from isolated bacterial strains.

Methods:

mRNA was extracted from a selection of S. aureus isolates either with or without 30 min exposure to antimicrobial agents (including oxacillin and mupirocin). The mRNA extracted was then used to produce labelled nucleic acid suitable for hybridisation to a low-density flow through oligonucleotide array targeting specific genes. These arrays are suitable for high throughput screening and provide very rapid hybridisation kinetics.

Results:

Distinctive changes in mRNA levels were detected for each agent tested and for isolates with different phenotypic susceptibilities. Oxacillin resulted in a significant increase in the levels of penicillin binding protein 2 (PBP2) mRNA in both sensitive and resistant isolates and an increase in the levels of PBP2prime mRNA in resistant isolates only. In contrast mupirocin resulted in very high levels of ile-tRNA synthetase in both strains with high- or low-level mupirocin resistance but not in sensitive strains.

Conclusion:

Future developments in RNA extraction and labelling as well as the increased availability of DNA array technology will allow this approach to be more widely used. This and similar methods have the potential to provide information on both the resistance phenotype of the isolate and the mechanism of resistance, in contrast to ‘classical’ molecular tests for drug resistance which generally target known genotypes.