Susceptibility test methods which may be able to predict the emergence of resistance to newer beta- lactam antibiotics.

Several antimicrobial susceptibility test methods in current use including the agar dilution, broth microdilution, Kirby-Bauer, and rapid automated instrument test methods have been criticized for failing to detect resistance due to chromosomally mediated beta-lactamases of certain Gram-negative bacteria. The older broth macrodilution test may be the most reliable method for recognition of low numbers of drug resistant mutants. In vitro induction of beta-lactamases by drugs such as cefoxitin may serve only to reaffirm the presence of the chromosomal gene in those species known to possess this mechanism of drug resistance.