Killing of Klebsiella pneumoniae mediated by conjugation with bacteria carrying antibiotic-resistance plasmids of the group N

Escherichia coli K-12 strains carrying any one of a number of different plasmids of the incompatibility group N have been found to kill Klebsiella pneumoniae, M5a1. A simple spot test is described that could be of value in diagnosing the presence of these plasmids. Killing is closely associated with the ability of the N⁺ strains to mate with M5a1 and all conjugation-deficient (Tra^?) mutants were also unable to kill M5a1 (Kil^?1). At a low frequency, some Kil^? mutants could be isolated that showed little or no transfer deficiency. Plasmids of the groups P and W which have been suspected (on other grounds) to specify conjugative systems interrelated in some manner to that specified by N plasmids, also kill M5a1 but to a lesser degree.