Beta-lactamase types and beta-lactam resistance ofEscherichia coli strains with chromosomally mediated ampicillin resistance

On the basis of isoelectric focusing six -lactamase types could be distinguished in ampicillin-resistant and ampicillin-sensitive strains ofEscherichia coli. More than 90% of the ampicillin-resistant strains produced the same -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 -lactam antibiotics of the cephalosporin group.