The family Enterobacteriaceae is a large, heterogeneous group of gram-negative bacteria. Many are normal inhabitants of the gastrointestinal tract of humans and other animals, but members also frequently cause disease in human beings. Among the Enterobacteriaceae, Salmonella, Shigella, Yersinia, and a number of specific phenotypes of Escherichia coli are important causes of gastroenteritis. In addition to diarrhea, these organisms cause a variety of extraintestinal infections. Each genera includes a heterogenous group of organisms that vary in their epidemiology and clinical characteristics. Enterobacteriaceae possess 3 major antigenic groups that react with antisera: (1) the O or somatic antigens; (2) the H or flagellar antigens; and (3) the K or capsular antigens. Serotyping has historically been an important means of subtyping these enteric pathogens; this technique is being partially superseded by our increasing ability to identify genotypic and phenotypic markers of virulence.
Salmonella are gram-negative, aerobic, non–lactose-fermenting, nonsporulating, flagellated bacilli. The genus Salmonella consists of only 2 species, S enterica and S bongori. Further, Salmonella are divided into approximately 2500 serotypes based on the somatic antigen (the major determinant) plus one or more less strongly reacting minor somatic antigens. Serotyping is performed by state health department laboratories after initial isolation of the organism. Serotyping is extraordinarily useful for epidemiologic purposes but not necessary for initial clinical management. The majority of human infections are caused by serotypes of S enterica, including Typhimurium, Enteritidis, and Typhi. The nomenclature of Salmonella has been simplified recently, with serotypes designated after species (eg, S enterica serotype Typhimurium is often simplified to S typhimurium). Because several serotypes represent the majority of isolates, additional epidemiologic subtyping can be useful. Plasmid profile analysis, bacteriophage typing, restriction endonuclease analysis, ribotyping, pulsed-field gel electrophoresis, and antimicrobial susceptibility have all been used as epidemiologic tools.
PATHOGENESIS AND EPIDEMIOLOGY
There are several distinctive steps in the development of a Salmonella infection. Upon reaching the small intestine, the bacteria must attach to the epithelium. Chromosomally encoded long, polar fimbriae; thin, aggregative fimbriae; and plasmid-encoded fimbriae are important in this process. Salmonella invade M cells (mucosal antigen-presenting immune cells) and nonphagocytic epithelial cells. Within macrophages, they may not only survive but multiply. The ability of strains to survive and reproduce in macrophages is correlated with virulence in animal models. The organism directs its own endocytosis through a complex mechanism encoded in a “pathogenicity island,” a large collection of contiguous virulence genes. Interestingly, a key component of invasion is a type 3 secretion system, which is similar to systems mediating the invasiveness of Yersinia and enteropathic and enterohemorrhagic E coli. Diarrhea is probably induced by local inflammation, by induction of inflammatory mediators, and in some strains, by one or more enterotoxins or cytotoxins. When examined histologically, the organism is prominent in Peyer’s patches. In some cases of nontyphoid salmonellosis and in all cases of typhoid, the organisms reach the regional lymphatics. Bacteremia ...