Selection of enhanced cefotaxime resistance in Enterobacter spp

Four strains of Enterobacter spp. with different chromosomal beta-lactamase expression (inducible, constitutive or negligible) were grown in broth containing either cefoxitin or cefotaxime, then plated on to agar containing 20 mg cefotaxime per litre to quantitate the cefotaxime-resistant mutants present in the population. Spontaneous resistant mutants were initially isolated from each strain at frequencies of 10(-4) to 10(-5). These high frequencies of spontaneous mutation suggested that more than one type of mutational event could yield cefotaxime resistance. Induction of a high level of beta-lactamase in broth cultures was not in itself sufficient to confer a high level of cefotaxime resistance on the population, and increased resistance following selection of resistant mutants did not necessarily correlate with any significant increase in beta-lactamase activity.