5.1 Use of indicator antimicrobials

Indicator antimicrobials can be used alongside normal AST. The principle of this test is to use the minimum inhibitory concentration (MIC) to a specific indicator antimicrobial to which resistance implies the mechanism you are looking for. Organisms with an MIC above the ECOFF or clinical breakpoint (measured by inhibition zone size) may have the mechanism of concern.

In these instances, once you know that the organism is resistant to the indicator, you can then extrapolate that it will be resistant to every antimicrobial in that class. However, this only works in a few specific cases. For example, cefoxitin resistance in S. aureus implies mecA is present, so the Staphylococcus must be an MRSA (see Section 3.2.1).

Certain indicator antimicrobials can also be used to detect non-wild type strains, without knowing the precise resistance mechanism. Resistance to these antimicrobials means a resistance mechanism of some sort is present. For example, a nalidixic acid, norfloxacin or pefloxacin MIC higher than the ECOFF or clinical breakpoint implies a fluoroquinolone resistance mechanism in organisms including Salmonella, Streptococcus and H. influenzae – the specific antibiotic used will depend on the species being tested. Similarly, oxacillin is used to look for penicillin resistance mechanisms in S. pneumoniae.