Salmonella Virulence Mechanisms and their Genetic Basis

The genus Salmonella is named after the American veterinarian Daniel E. Salmon, who fi rst isolated Bacterium cholerae-suis (Salmonella enterica serotype Choleraesuis abbreviated to S. enterica serotype Choleraesuis or S. Choleraesuis) from a pig (Salmon and Smith, 1885). The members of this genus are gramnegative facultative-anaerobic and peritrichously fl agellated rods. Salmonella serotypes are distinguished from members of other genera of the Enterobacteriaceae by a combination of biochemical reactions, including the production of hydrogen sulfi de, the use of tetrathionate as a terminal electron acceptor and others. The genus Salmonella contains 2579 serotypes, which can be distinguished serologically based on antigenic properties of their lipopolysaccharide (LPS) sugar repeat units (O-antigens) and their structural protein subunits of fl agella (H-antigens) (Grimont and Weill, 2007). Members of the genus Salmonella can be divided into two species, termed Salmonella enterica (Le Minor and Popoff, 1987), which contains 2557 serotypes, and Salmonella bongori (Reeves et al., 1989), which comprises the remaining 22 serotypes. All Salmonella serotypes are considered potential pathogens of humans and/or animals (Kelterborn, 1967). In a fi rst approximation, diseases caused by Salmonella serotypes can be divided into two broad categories, those that remain localized to the intestinal tract (self-limiting gastroenteritis) and those that are associated with systemic dissemination of the bacteria. Localized and systemic infections are associated with different Salmonella serotypes. One goal of this overview is to illustrate that in immunocompetent hosts, these two different syndromes are a result of distinct virulence strategies being employed by the respective pathogens. Research on Salmonella pathogenesis suggests that these two distinct virulence strategies refl ect differences in how the respective pathogens ensure their transmission to a new susceptible host. After a brief overview of the two major disease manifestations associated with Salmonella serotypes, this chapter will discuss the virulence factors responsible for host–pathogen interactions observed in both settings.