The epidemiology of antibiotic resistance in hospitals.

Changes in antibiotic susceptibility among different species are a function of the interaction of bacterial genomes (chromosomes, plasmids, transposons) with specific host and environmental factors. Computer-assisted surveillance of resistance patterns and plasmid fingerprinting with restriction endonucleases have facilitated resistance gene tracking. Certain nosocomial plasmids have persisted for many years within specific centres, others are widely distributed by species and location, and others may have a narrow host range or undergo rapid molecular evolution. Klebsiella pneumoniae is most often the index species for the dissemination of nosocomial plasmids, which are frequently multiresistant. Acute increases in antibiotic resistance may be due to antibiotic selection pressures, introduction of new, well-adapted species, over-crowding and other environmental changes. The most resistant organisms are found in units where patients are immunocompromised, are subject to invasive procedures or remain for long periods. Although large-scale studies have not shown uniform increases in the prevalence of resistant bacteria, acute outbreaks in specific niches have resulted in increased patient morbidity, necessitating rapid changes in antibiotic usage. Further molecular epidemiological studies are required in order to understand better how to anticipate the spread of antibiotic resistance.