Hospital outbreaks caused by Pseudomonas aeruginosa: importance of serogroup O11.

Suspected outbreaks caused by pseudomonas aeruginosa in 19 hospitals and two motels were studied. On the basis of epidemiological analysis, serological typing, and antibiotic resistance patterns, 17 were classified as single-strain outbreaks. Six were classified as common-source outbreaks: of these, three were caused by contaminated urological instruments or solutions, two involved bathing in contaminated whirlpools, and one was caused by contaminated lens prostheses implanted during eye surgery. The ability of P. aeruginosa to survive or grow in wet environments was important in each of these six outbreaks. Eight outbreaks were classified as cross-infection. Two involved the urinary tract and were caused by antibiotic-resistant strains. Six involved the respiratory tract, but only one was caused by an antibiotic-resistant strain. In 2 of the 17 single-strain outbreaks, the exact mode of transmission could not be determined. One was an outbreak of pseudobacteremia in which patient blood cultures were contaminated with a single strain, presumably during collection of specimens or culture processing, P. aeruginosa serogroup O11 caused 9 of 17 (53%) single-strain outbreaks, a surprising finding since this serogroup represents only about 8% of endemic hospital isolates of this species. Serotyping was very useful in epidemiological analysis, but antibiotic susceptibility patterns were less useful.