Beta-lactamase types and beta-lactam resistance of Escherichia coli strains with chromosomally mediated ampicillin resistance.

On the basis of isoelectric focusing six beta-lactamase types could be distinguished in ampicillin-resistant and ampicillin-sensitive strains of Escherichia coli. More than 90% of the ampicillin-resistant strains produced the same beta-lactamase type. The serotypes found in a group of ampicillin-resistant urinary tract infection strains did not represent the distribution usually found in urinary tract isolates. Chromosomal ampicillin resistance was always associated with high cephalothin MIC values and increased resistance to other beta-lactam antibiotics of the cephalosporin group.