An important function of the diagnostic microbiology laboratory is to help the physician select effective
antimicrobial agents for specific therapy of infectious diseases. When a clinically significant
microorganism is isolated from the patient, it is usually necessary to determine how it responds in
vitro to medically useful antimicrobial agents, so that the appropriate drug can be given to the patient.
Antimicrobial susceptibility testing of the isolated pathogen indicates which drugs are most
likely to inhibit or destroy it in vivo.
Susceptibility testing has shown that bacteria are becoming increasingly resistant to a
wide variety of antimicrobial agents. Although new antibiotics continue to be developed by pharmaceutical
manufacturers, the microbes seem to quickly find ways to avoid their effects. Two important
bacteria that have developed resistance to multiple antimicrobial agents are Staphylococcus aureus strains, especially those resistant to the drug methicillin and its relatives, and Enterococcus spp.
resistant to vancomycin. These organisms are referred to as methicillin-resistant S. aureus (MRSA)
and vancomycin-resistant enterococci (VRE), respectively. Methods for identifying staphylococci
and enterococci are described in detail in topics Staphylococci and Streptococci, Pneumococci,
and Enterococci, but antibiotic-resistant strains of both
organisms play important roles in infections acquired by hospitalized patients. The laboratory must
use methods to detect this resistance so that special precautions are quickly instituted to prevent
transfer of the resistant bacteria among patients.