Analysis of beta-lactamase production in ampicillin-resistant Escherichia coli isolated from blood cultures 1983-1989.

From 1983 to 1989, 520 Escherichia coli blood culture pathogens were isolated from two hospitals in Turku, Finland. Ampicillin resistance (MIC greater than or equal to 16 micrograms/ml) of these isolates increased from 33% in 1983 to 66% in 1987, but decreased to 38-49% in 1988-1989. Occurrence of TEM-1 beta-lactamase producing isolates increased only slightly from 14% in 1983 to 25% in 1989 among all Escherichia coli strains studied. Strains with ampicillin MIC values of 16 micrograms/ml and 32 micrograms/ml were mostly responsible for the increase in resistance. Among these isolates, TEM-1 or any other of the well known plasmid-mediated beta-lactamases were not found by hybridization or isoelectric focusing.