Antibiotic pharmacodynamics for clinicians: A summary of the basics

For an antimicrobial agent to be effective, it must reach the site of infection in sufficient concentration and duration to inhibit pathogen growth. While the minimum inhibitory concentration (MIC) reflects an organism's in vitro susceptibility to an antibiotic, it does not predict drug concentration at the site of infection. Certain classes of antibiotics are more effective in killing pathogens as their peak serum concentration above the MIC is increased (concentration-dependent killing), while others are more effective the longer the antibiotic concentration remains above the pathogen's MIC (time-dependent killing). The fluoroquinolones and aminoglycosides exhibit concentration-dependent killing. Penicillins, carbapenems, cephalosporins, and macrolides - with the exception of azithromycin - display time-dependent killing. In the treatment of pulmonary infections, an antibiotic must cross the blood-bronchoalveolar barrier and enter the epithelial lining fluid. Antibiotics that accomplish this include azithromycin, clarithromycin, and the fluoroquinolones.