Antimicrobial therapy: interpreting susceptibility results (Proceedings)
The design of antimicrobial regimens is addressed in the next section in these proceedings ("Antimicrobial Therapy: Regimen Design"), but the concepts within regimen design related to determining the concentration of drug required to inhibit growth of bacterial pathogens deserve a more thorough discussion. Antimicrobial susceptibility testing must not be viewed as a black box into which a veterinarian places a clinical sample of an infected site and receives a "yes" or "no" from the diagnostic laboratory.
Susceptibility and resistance
These are related terms to describe the same concept: the concentration of antimicrobial required to inhibit growth of an isolate of bacteria as it relates to the likelihood of clinical success. "Susceptible" and "resistant" are qualitative terms used to simplify the results of tests of growth inhibition. "Susceptible" suggests that the pathogen's growth is likely to be inhibited and clinical success is likely with labeled doses of the antimicrobial drug, and "resistant" suggest that the concentration of antimicrobial drug required to inhibit the pathogen is not achievable in the patient. Problems with interpretation of laboratory or published results can occur when the definitions underlying the qualitative terms are not universally agreed upon or when different thresholds are used by the individual that performs the test and the individual that uses the results of the test.Differentiating inherent and acquired resistance is important from a clinical and epidemiological standpoint, since the concern for resistance associated with treatment failure is generally due to bacteria acquiring resistance genes from other bacteria, rather than a gene that has always been present in a particular bacterial species. Acquired resistance is most often generated by the acquisition of new bacterial DNA by various mechanisms, including transfer of plasmids (extra-chromosomal DNA) between a resistant and a susceptible organism, or transfer of an integron or other type of moveable gene between a resistant and a susceptible organism. Transfer of resistance can occur between bacteria of the same species, but also between bacteria of different species and even of different genera. The significance of this transfer is that the bacteria containing these resistance genes may be selectively targeted for survival by the presence of an antimicrobial, and the resistance DNA is then carried on the next generation or is transferred to other bacteria.
Determining susceptibility or resistance
Since we have described "resistance" as requiring more antimicrobial than can be delivered to the infection site, we need a way to determine how much of a particular antimicrobial is required to inhibit the growth of a particular organism. We are all familiar with susceptibility testing as performed by most clinical microbiology labs, but it is useful to briefly review these tests.
The two major types of susceptibility testing performed by veterinary diagnostic laboratories are disk diffusion and broth microdilution. Disk diffusion testing uses paper disks containing known quantities of antimicrobials, and the zone around which no growth of bacteria occurs correlates with a particular range of antimicrobial concentrations. The correlation between zone and MIC is a qualitative rather than a quantitative one: zones of inhibition do not linearly correspond to minimum inhibitory concentrations of antimicrobial.
Broth microdilution testing is used to characterize the quantity of antimicrobial required to inhibit bacterial growth. Varying concentrations of antimicrobial are mixed with the broth used to grow bacterial isolates, and the lowest concentration which demonstrates no growth is the MIC. This type of testing is usual performed with 96-well plates so multiple drugs can be tested or even more than one isolate can be tested on a plate. Concentrations of antimicrobial which are clustered around the breakpoint are generally selected for testing, with the idea that these concentrations are also actually clinical achievable in the animal.