• Enterobacter cloacae;
  • Citrobacter freundii;
  • β-lactamase production

The β-lactamase activity with ampicillin (A) and cephalothin (Ce) as substrate was examined in 28 strains of Ent. cloacae and 24 strains of C. freundii by a micro-iodometric – (MIA) and an ultraviolet assay (UVA). Carbenicillin resistance (Ca-R) was present in 32 strains (16 of each species). All Ca-resistant (Ca-r) strains had elevated amounts of β-lactamase. The conditions were very similar in the two species. The Ca-r strains could be divided into two subgroups. One group had very high ampicillinase activity and lower cephalothinase activity in MIA. In UVA the cephalothinase activity was equal to that in MIA. The strains could transfer their A- and Ca-R. The reaction velocity increased with time in MIA. The Ca-R was probably plasmid mediated. The other group had no or low ampicillinase activity in MIA. The cephalothinase activity in UVA was much higher than in MIA. Velocity decreased with time in MIA, probably due to inhibition by iodine. The strains were very Ce-r and could not transfer A-R. The Ca-and Ce-R was probably chromosomally mediated. The acquisition of Ca-R in Ent. cloacae and C. freundii was analogous to acquisition of A-R in E. coli. The fraction of strains with high ampicillinase activity (plasmid mediated among all strains with acquired β-lactam resistance was the same in Ent. cloacae and C. freundii, viz. 0.63. In E. coli the corresponding value is 0.61. This fraction thus seems rather constant between the different genera.