Antibiotic selection is simplified when bacteria are accurately identified, as many organisms have predictable susceptibility patterns. Certain antibiotics will yield predictable results with most staphylococcus (usually S pseudintermedius) and anaerobes. Pseudomonas aeruginosa, Enterobacter, Klebsiella, Escherichia coli, and Proteus, however, can show resistance to many common antibiotics and susceptibility testing is recommended. It is now more common for laboratories to directly measure the minimum inhibitory concentration (MIC) of organisms using an antimicrobial dilution test, rather than the agar disk diffusion (Kirby-Bauer) test. Once the MIC is determined, the laboratory may use the "SIR" classification for breakpoints: S, susceptible; I, intermediate; R, resistant. The intermediate category reflects a possibility of error when the MIC borders between susceptible and resistant. It does not mean "moderately susceptible" and therapy using a drug with a MIC value in this category is discouraged as it is likely the drug concentration will be inadequate for a cure. To achieve cure, drug concentrations should be maintained above MIC for at least part of the dose interval. Rather than bactericidal or bacterostatic, drugs are now grouped more as either concentration-dependent or time-dependent in action. There is really no such thing as "good penetration" and "poor penetration" when referring to most drugs in most tissues; diffusion to the site of infection can be achieved as long as adequate drug concentration is achieved in the plasma and the tissue has adequate blood supply. In some tissues (central nervous system, eye, prostate) a lipid membrane presents a barrier to drug diffusion and drugs must be sufficiently lipid soluble to reach effective concentrations in tissues.

In recent years there have been important changes in antimicrobial therapy. There are new antimicrobials available and there is a greater database of pharmacokinetic and pharmacodynamic information available for antimicrobials used in veterinary medicine. Concerns over the development of bacterial resistance to antimicrobials have heightened the awareness of rational use of antimicrobials. This article will be a good synopsis to get veterinarians "up to speed" on the pharmacokinetic/pharmacodynamic approach to optimizing antimicrobial use in practice.

Optimizing antimicrobial therapy strategies for improving patient care and minimizing resistance. Papich MG. NAVC PROC 2009, pp 1187-1189.