The presence of a variant of type-IIIa beta-lactamase in a series of strains isolated in a burns unit.

An outbreak of carbenicillin-resistant Pseudomonas aeruginosa infection in a Burns Unit in 1968-69 occurred in two phases separated by a period when no resistant pseudomonas were detected. Nevertheless, strains of Klebsiella aerogenes carrying R factors similar to those in the pseudomonas strains were encountered in the intervening period. The similarity of the plasmids occurring in all stages of this outbreak has been confirmed by the fact that each specifies a novel variant of type-IIIa beta-lactamase.