Molecular characterization of the R factors implicated in the carbenicillin resistance of a sequence of Pseudomonas aeruginosa strains isolated from burns.

An outbreak of R-factor-mediated carbenicillin resistance in Pseudomonas aeruginosa in burned patients in March 1969 was followed by a second outbreak 6 months later. No R-factor-carrying P. aeruginosa strains were detected in the intervening period but R-factor-determined lactamase was commonly encountered, particularly in Klebsiella aerogenes strains. A comparison of the molecular properties of the R factors in pseudomonads from the first and second phases with those in the Klebsiella strains from the intervening period showed them to be very closely related. A single R-factor type therefore may have been maintained in the Burns Unit between the two Pseudomonas outbreaks as a plasmid conferring resistance to ampicillin in K. aerogenes.