Consequences of increasing resistance to antimicrobial agents.

The correlation between in vitro bacterial susceptibility results and clinical outcome has been debated for many years. Bacterial resistance traits are more significantly correlated with failure of therapy than is an organism's susceptibility to an antimicrobial agent. We review the situations that have supported the clinical relevance of in vitro bacterial resistance. Those situations include: emergence, during therapy, of a new resistance marker not known before; selection of a resistant mutant or acquisition of a resistance gene during therapy; failure to recognize or take into account a new resistance mechanism; and superinfection with resistant bacteria. More information should be obtained in the future by performing studies oriented toward bacteriologically documented clinical failures and by better communication between microbiologists and physicians to correlate the in vitro data with host status, the pharmacokinetics of the antimicrobial agent, and the bacteriologic and clinical outcome.