Correlation between the genetic diversity of nosocomial pathogens and their survival time in intensive care units.

Bacteria differ in their ability to survive in the hospital environment outside the human host. Species remaining viable and infectious have a higher chance of being transmitted, giving them a fitness advantage in hospitals. This differential fitness could be expected to alter the genetic population structure of bacterial populations in hospitals, and should be reflected by the relative abundance of several successful clones. The objective of this study was to test for a potential correlation between tenacity, i.e. environmental survival, and clonal abundance determined by the genetic diversity in different bacterial species from prospectively collected isolates of intensive care patients. A literature review was performed to identify mean environmental survival times for the most important pathogens in intensive care units (ICUs): Staphylococcus aureus, enterococci, Acetinobacter baumannii, Pseudomonas aeruginosa, Enterobacter spp., Escherichia coli, Klebsiella pneumoniae and Stenotrophomonas maltophilia. To determine the genetic diversity of the natural population of these species in ICUs, a prospective 18-month study was conducted in five units with median nosocomial infection rates. All clinical isolates were collected, and highly discriminatory DNA fingerprinting techniques were used to identify specific clones. A diversity index for each species was calculated as the number of distinguishable genotypes in the population divided by size. The correlation between survival times and the diversity indices for the individual pathogens was investigated using non-parametric methods. Although 21 studies were identified in the literature, only two were relevant. They showed median survival times between 1.5 days (P. aeruginosa) and 60.0 days (Enterococcus faecium). During the prospective ICU study, 1264 pathogens were investigated and simple diversity indices between 49.1 (Enterococcus faecalis) and 89.8 (E. coli) were found. A correlation between survival times and the diversity indices for the individual pathogens was found (correlation coefficient 0.821, P=0.024). Environmental survival may be an important factor contributing to the ecological fitness of some nosocomial pathogens in ICUs. Infection control measures should consider this finding.