Evaluation of plasmid-encoded beta-lactamase resistance inEscherichia coli blood culture isolates

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Abstract
The frequency of beta-lactam resistance was determined among 313 strains ofEscherichia coli, 119 ofEnterobacter/Klebsiella/Proteus spp., and 48 ofPseudomonas spp. isolated from blood cultures (at Turku University Central Hospital and Turku City Hospital) in 1983–1987. During this period the MIC50 of ampicillin forEscherichia coli increased from 8 to 32 µg/ml, the MIC90 of piperacillin from 16 to > 32 µg/ml and the MIC90 of cefuroxime from 4–8 to 16 µg/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 inEscherichia coli occurred in a few OXA-1 producing strains as well as in strains that produced only chromosomal beta-lactamase. TwoEscherichia coli strains that overproduced chromosomal beta-lactamase had increased ceftazidime MIC values (8–16 µg/ml).