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Escherichia coli, Klebsiella, Proteus, Enterobacter, Serratia, Citrobacter, Morganella, Providencia, Cronobacter, and Edwardsiella are gram-negative enteric bacilli that are members of the family Enterobacteriaceae. Salmonella, Shigella, and Yersinia, also in the family Enterobacteriaceae, are discussed in Chaps. 62, 63, and 68, respectively. These pathogens cause a wide variety of infections involving diverse anatomic sites in both healthy and compromised hosts. Increasing antimicrobial resistance in this group has put them at the forefront of an evolving public health crisis. In addition, new infectious syndromes have emerged. Therefore, a thorough knowledge of clinical presentations and appropriate therapeutic choices is necessary for optimal outcomes.


image E. coli, Klebsiella, Proteus, Enterobacter, Serratia, Citrobacter, Morganella, Providencia, Cronobacter, and Edwardsiella are components of the normal animal and human colonic microbiota and/or the microbiota 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. The incidence of infection due to these agents is increasing because of the combination of an aging population and increasing antimicrobial resistance. In healthy humans, E. coli is the predominant species of gram-negative bacilli (GNB) in the colonic flora; Klebsiella and Proteus are less prevalent. GNB (primarily E. coli, Klebsiella, and Proteus) only transiently colonize the oropharynx and skin of healthy individuals. In contrast, in LTCFs and hospital settings, a variety of GNB emerge as the dominant microbiota of both mucosal and skin surfaces, particularly in association with antimicrobial use, severe illness, and extended length of stay. LTCFs are emerging as an important reservoir for resistant GNB. This colonization may lead to subsequent infection; for example, oropharyngeal colonization may lead to pneumonia. Interestingly, the use of ampicillin or amoxicillin was associated with an increased risk of subsequent infection due to the hypervirulent variant of Klebsiella pneumoniae in Taiwan; this association suggests that changes in the quantity or prevalence of ­colonizing bacteria may be important. Serratia and Enterobacter infection may be acquired through a variety of infusates (e.g., medications, blood products). Edwardsiella infections are acquired through freshwater and marine environment exposures and are most common in Southeast Asia.


Enteric GNB possess an extracytoplasmic outer membrane, which consists of a lipid bilayer with associated proteins, lipoproteins, and polysaccharides (capsule, lipopolysaccharide). The outer membrane interfaces with the external environment, including the human host. A variety of components of the outer membrane are critical determinants in pathogenesis (e.g., capsule) and antimicrobial resistance (e.g., permeability barrier, efflux pumps).


Multiple bacterial virulence factors are required for the pathogenesis of infections caused by GNB. Possession of specialized virulence genes defines pathogens and enables them to ...

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