Describing resistance

Minimum inhibitory concentration (MIC) values describe the susceptibility of a specific bacterium to a specific antibiotic.

If the concentration of antibiotic required to inhibit growth exceeds a certain level, the breakpoint, the antibiotic in question becomes useless as a therapeutic agent because the breakpoint signifies the maximum dose of that antibiotic which can be given safely to patients.

Higher doses, even if they might be effective on the agar plate in the laboratory, would be hazardous to the patient. The breakpoint is the dividing line between the bacterium being sensitive or resistant to the antibiotic. Medical microbiology laboratories will usually report bacteria as either sensitive or resistant to an antibiotic, even if a more precise measurement has been made such as zone size around an antibiotic disc or MIC.

A set of terms has been developed to describe bacteria resistant to a number of unrelated antibiotic classes. These progress through resistance to increasing numbers of antibiotic classes from multi-drug resistant (MDR), extensively drug resistant (XDR) to pan drug resistant (PDR). The relationship between these classes is illustrated in the Venn diagram below with the definitions for each.

Diagram adapted from article “Multidrug-resistant, extensively drug-resistant and pan drug-resistant bacteria: an international expert proposal for interim standard definitions for acquired resistance” by Magiorakos et al. This article is available as a pdf to download below.