Resistance types in Escherichia coli. III. beta-lactamase production

The beta-lactamase activity with ampicillin (A) and cephalothin (Ce) as substrate was examined in 22 E. coli strains by a micro-iodometric - (MIA) and an ultraviolet assay (UVA). The strains were divided into three groups according to their sensitivity to A and carbenicillin (Ca). Group 1 contained 6 A-Ca sensitive (A-s/Ca-s) strains, group 2 contained 8 A-resistant (A-r)/Ca-s strains and group 3 eight A-r/Ca-r ones. MIA with A as substrate showed high activity in group 3. The activities in groups 1 and 2 did not differ and were very low. Group 1 had the lowest and group 3 the highest IC50 with A. UVA with Ce as substrate showed high activity in group 2, lower in group 3 and lowest in group 1. IC50 corresponded to this order. MIA with Ce as substrate showed high activity (measured 25 min after enzyme was added) in group 3, lower activity in group 2 and lowest in group 1. The initial velocity was highest in group 2, but decreased with time. In group 3 the velocity increased with time. Group 2 (A-r/Ca-s) strains contained cephalothinases that apparently were inhibited by iodine. The beta-lactam resistance was probably chromosomally mediated. Group 3 (A-r/Ca-r) strains contained beta-lactamases with predominantly ampicillinase activity and were not inhibited by iodine. The A-resistance was probably plasmid mediated.