Evaluation of plasmid-encoded beta-lactamase resistance in Escherichia coli blood culture isolates.

The frequency of beta-lactam resistance was determined among 313 strains of Escherichia coli, 119 of Enterobacter/Klebsiella/Proteus spp., and 48 of Pseudomonas spp. isolated from blood cultures (at Turku University Central Hospital and Turku City Hospital) in 1983-1987. During this period the MIC50 of ampicillin for Escherichia coli increased from 8 to 32 micrograms/ml, the MIC90 of piperacillin from 16 to greater than 32 micrograms/ml and the MIC90 of cefuroxime from 4-8 to 16 micrograms/ml. Among 172 ampicillin-resistant isolates beta-lactamase-mediated resistance was characterized by DNA hybridization with TEM-1, SHV-1, OXA-1, OXA-2, PSE-1, PSE-2 and PSE-4 beta-lactamase probes and by isoelectric focusing. Beta-lactamase types found were TEM-1, TEM-2, SHV-1 and OXA-1. Isoelectric focusing did not show any other plasmid-mediated beta-lactamase varieties. Piperacillin-resistant strains showed mostly TEM-1 activity, but also produced OXA-1 and chromosomal beta-lactamase. Interestingly, a decrease in cefuroxime susceptibility in Escherichia coli occurred in a few OXA-1 producing strains as well as in strains that produced only chromosomal beta-lactamase. Two Escherichia coli strains that overproduced chromosomal beta-lactamase had increased ceftazidime MIC values (8-16 micrograms/ml).