, 1980). This antigenic selleck screening library variation can be observed in S. Typhimurium, but most S. Typhi strains are considered monophasic, as they lack a corresponding fljB locus (Frankel et al., 1989).
However, some S. Typhi isolates from Indonesia contain a linear plasmid encoding a novel flagellin, fljBz66, but reversion to fliC is considered irreversible due to a deletion (Baker et al., 2007a). fliB, involved in methylation of the flagellin in S. Typhimurium, is a pseudogene in S. Typhi (Parkhill et al., 2001). The Vi antigen is a polysaccharidic capsule absent in S. Typhimurium and present in S. Typhi. Vi is important for virulence and is controlled by two loci: viaA and viaB (Kolyva et al., 1992). The viaB locus located on SPI-7 is composed of two operons: tviABCDE and vexABCDE. The Vi capsule causes several differences between S. Typhimurium and S. Typhi at the level of the host’s response to infection. The Vi capsule is associated with inhibition of complement activation, resistance to serum and to phagocytosis and is involved in survival inside phagocytes (Looney & Steigbigel, 1986; Hirose et al., 1997; Miyake et al., 1998). The viaB locus lowers the invasiveness of the bacteria towards epithelial cells, as viaB mutants are superinvasive (Arricau et al., 1998; Zhao et al., 2001), and CAL-101 mouse S. Typhimurium harbouring the viaB locus is less invasive (Haneda et al., 2009). TviA
avoids interleukin-8 production in the intestinal mucosa by repressing flagellin secretion, which reduces the recognition and activation of Toll-like receptor (TLR)-5 (Raffatellu et al., 2005; Winter et al., 2008). Vi also prevents the recognition of lipopolysaccharide by TLR-4 and reduces inflammation in the intestinal
mucosa (Sharma & Qadri, 2004; Wilson et al., 2008). Salmonella enterica serovar Typhimurium sets off an immune response, which causes inflammation characterized by an important neutrophil influx that may be the result of its lack of capsule. Thus, Vi allows S. Typhi to disseminate systemically in its human host by crossing intestinal cells without activating the immune response, promotes resistance to killing by serum and contributes Glycogen branching enzyme to survival inside phagocytes (Raffatellu et al., 2006). Vi is a protective antigen and the actual constituent of the parenteral typhoid fever vaccine. Acquisition and loss of genetic material play an important role in bacterial evolution. Here, we have described the major genetic differences between S. Typhimurium and S. Typhi, two important S. enterica serovars associated with distinct diseases in humans (Fig. 1). Gene degradation in S. Typhi may be responsible for its human host restriction, but factors contributing to its systemic dispersion and survival during typhoid may be multiple and scattered, which complicates the identification of genomic regions that reflect differences in habitat and lifestyle.