EPIDEMIOLOGY

Escherichia coli, Klebsiella, Proteus, Enterobacter, Serratia, Citrobacter, Morganella, Providencia, Edwardsiella, and Acinetobacter are components of the normal animal and human colonic flora and/or of the flora of a variety of environmental habitats, including long-term-care facilities (LTCFs) and hospitals. As a result, except for certain pathotypes of intestinal pathogenic E. coli, these genera are global pathogens. In healthy humans, E. coli is the predominant species of gram-negative bacilli (GNB) in the colonic flora. GNB (primarily E. coli, Klebsiella, and Proteus) only transiently colonize the oropharynx and skin of healthy individuals. In contrast, in LTCF and hospital settings, a variety of GNB emerge as the dominant flora of both mucosal and skin surfaces, particularly in association with antimicrobial use, severe illness, and extended length of stay. This colonization may lead to subsequent infection; for example, oropharyngeal colonization may lead to pneumonia.