Escherichia coli resistant to ampicillin/sulbactam

Escherichia coli strains resistant to ampicillin/sulbactam from hospitals in 4 different geographic locations were examined with respect to type and amount of β-lactamase produced. A total of 5 strains was examined from each region. The isoelectric points of all of the involved β-lactamases were 5.4, corresponding to TEM-1. K(m) and V(max) values of the β-lactamases among the clinical isolates resembled those from the control TEM-1 strain. In an 18-hour broth culture the highly resistant isolates produced 3 times more β-lactamase as compared to the ampicillin/sulbactam-susceptible isolates. However, the highly resistant strains contained approximately the same amount of plasmid DNA (size of > 6,500 bp) as the susceptible isolates. In transformation experiments, both the resistance and the degree of resistance appeared to have been transferable by plasmids. The mechanism for resistance is likely to be a baseline overproduction of TEM-1 β-lactamase due to either an alteration in the control of gene expression or simply to an increase in the number of copies of the β-lactamase gene in the plasmids.