Influence of r-Lactam Antibiotics on Serum Resistance of Ki-Positive Blood Culture Isolates of Escherichia coli

The Kl-positive strains of Escherichia coli are a group with considerable clinical importance, serum resistance being a common virulence factor of these strains. In the present paper, the influences of cephaloridine, iminpenem, and ceftazidime on the serum resistance of eight serum-resistant Kl-positive E. coli blood culture isolates with smooth-type lipopolysaccharide were studied. Al strains were rendered more serum sensitive by treatment with subinhibitory concentrations of antibiotics. The amount of the reduction of serum resistance was dependent on the concentration of the antibiotic. Amounts of Kl produced under the influence of the antibiotics were measured and were found to be reduced for almost aDl strains tested. To further test the hypothesis that antibiotic-induced reduction of serum resistance is mediated by inhibition of Kl expression, isogenic mutants of one strain were produced by selection for resistance against infection with Kl-speciflc bacteriophages. These mutants were found to be highly serum sensitive. We conclude from this study that ,-lactam antibiotics can render Kl-positive serum-resistant strains of E. coli highly serum sensitive and that this effect is mediated by inhibition of Kl expression.