Resistance types in Escherichia coli. I. Occurrence and resistance to ampicillin, carbenicillin and cephalothin.

An investigation of the resistance types in lactose fermenting E. coli is presented. The frequency and sensitivity to beta-lactam antibiotics of different resistance types was investigated. The strains were divided into three groups according to sensitivity to penicillin derivatives. Group 1 contained the ampicillin-carbenicillin sensitive (A-s/Ca-s), group 2 the ampicillin resistant-carbenicillin sensitive (A-r/Ca-s) and group 3 the ampicillin-carbenicillin resistant (A-r/Ca-r). A-s/Ca-r were not observed. One third of the A-r strains were sensitive to carbenicillin. The distribution of resistance types was different in the three groups. Group 1 was dominated by the usual sensitive E. coli. Group 3 contained a very high proportion of multiresistant types. The IC50 against ampicillin, carbenicillin and cephalothin of 55 strains was determined. Group 3 (A-r/Ca-r, 25 strains) was much more ampicillin resistant than group 2 (A-r/Ca-s, 16 strains). Group 2 was less sensitive to carbenicillin than group 1 (A-s/Ca-s, 14 strains). Group 3 did not differ significantly from group 1 with respect to cephalothin sensitivity, while group 2 was much more resistant than the others.