Third-generation cephalosporins resistance in clinical isolates of Enterobacteriaceae.

Seven clinical isolates of Enterobacteriaceae resistant to third-generation cephalosporins were investigated in order to assess the role played in resistance by permeability barrier and by beta-lactamase production. The addition of subinhibitory concentrations of EDTA increased susceptibility to ceftriaxone in five strains showing that the permeability barrier is involved. All strains produced different amounts of beta-lactamases that were always increased by cefoxitin induction. Hydrolytic activity of investigated enzymes varied in different cephalosporins; ceftriaxone and ceftazidime are the most stable compounds.