beta-Lactam coselection of sensitive and TEM-1 beta-lactamase-producing subpopulations in heterogeneous Escherichia coli colonies.

We tested the effect of different concentrations of cefamandole, cefotaxime, cefoxitin and imipenem on homogeneous and heterogeneous bacterial stationary colonies. A reduction of not more than 100-fold in the bacterial colony population was obtained with sensitive Escherichia coli K12 strains. When an E. coli strain producing high levels of TEM-1 beta-lactamase constituted 6% or more of the population, the whole colony was resistant to cefamandole, but still sensitive to cefotaxime, cefoxitin or imipenem. Thus in the case of cefamandole the sensitive subpopulation is co-selected by the resistant one. It is suggested that the occurrence of resistant subpopulations, as a small part of an apparently sensitive E. coli population, can lead to therapeutic failures with beta-lactamase-sensitive agents.