Cross-colonisation with Pseudomonas aeruginosa of patients in an intensivecare unit

Background-Ventilator-associated pneumonia (VAP) caused by Pseudomonas aeru ginosa is usually preceded by colonisation of the respiratory tract. During outbreaks, colonisation with P aeruginosa is mainly derived from exogenous sources. The relative importance of different pathways of colonisation of P aeruginosa has rarely been determined in non-epidemic settings. Methods-In order to determine the importance of exogenous colonisation, all isolates of P aeruginosa obtained by surveillance and clinical cultures fr om two identical intensive care units (ICUs) were genotyped with pulsed fie ld gel electrophoresis. Results-A total of 100 patients were studied, 44 in ICU 1 and 56 in ICU 2. Twenty three patients were colonised with P aeruginosa, seven at the start of the study or on admission and 16 of the remaining 93 patients became col onised during the study. Eight patients developed VAP due to P aeruginosa. The incidence of respiratory tract colonisation and VAP with P aeruginosa i n our ICU was similar to that before and after the study period, and theref ore represents an endemic situation. Genotyping of 118 isolates yielded II strain types: eight in one patient each, two in three patients each, and on e type in eight patients. Based on chronological evaluation and genotypical identity of isolates, eight cases of cross-colonisation were identified. E ight (50%) of 16 episodes of acquired colonisation and two (25%) of eight c ases of VAP due to P aeruginosa seemed to be the result of cross-colonisati on. Conclusions-Even in non-epidemic settings cross-colonisation seems to play an important part in the epidemiology of colonisation and infection with P aeruginosa.