Antibiotics Pharmacodynamics
The term antibiotic pharmacodynamics refers to the manner in which antibiotics interact with their target organisms to exert their effects: Does the antibiotic kill the organism or just slow its growth? Is it better to give a high dose of antibiotics all at once or to achieve low concentrations for a long time? Clinicians increasingly recognize such considera-tions as important in maximizing the success of therapy, especially for difficult-to-treat infections or in immunocompromised patients.
Susceptibility Testing
Typically, one judges the susceptibility of a particular organism to an antibiotic based on the minimum inhibitory concentration (MIC) for the organism-antibiotic combination. The clinician determines the MIC by mixing a standard concentration of the organism that the patient has grown with increasing concentrations of the antibiotic in a broth solution. Classically this was done in test tubes (see Figure 3-1), but today it is done more commonly on microdilution plates. The mixture is incubated for about a day, and the clinician examines the tubes or plates (with the naked eye or with a computer) for signs of cloudiness, indicating growth of the organism. The mixture with the lowest concentration of antibiotic where there is no visible growth is deemed to be the MIC. For each organism-antibiotic pair there is a particular cutoff
MIC that is considered susceptible. This particular MIC is called the breakpoint. Table 3-1 provides examples of breakpoints for different organism/ pathogen combinations. Note that just because an antibiotic has the lowest MIC for a pathogen, that does not mean it is the best choice—different antibiotics achieve different concentrations in the body. Thus, antibiotic MICs for a single organism generally should not be compared across different drugs in selecting therapy. Finally, be aware that other methods of susceptibility testing exist, including disk diffusion and E-tests, but that broth dilution methods are generally considered the gold standard.