*2.2.4 SPI-4 is involved in colonization*

*Microorganisms*

*ssaB* (*spiC*), *ssaC* (*spiA*), *ssaD* (*spiB*), *ssaR* (*yscR*).

**Table 3.**

*macrophage survival and replication.*

*SsaABCDEFGHIJKLMNOPQRSTUV*

inside host cells—both epithelia cells and macrophages—within the SCV [75]. Non-functional SPI-2 mutants are unable to colonize internal target organs such as spleen and liver of mice, although they penetrate the intestinal barrier as efficiently as the wild type strain [76]. These mutants were attenuated by at least five orders of magnitude compared with the wild type strain after either oral or intraperitoneal inoculation of mice [75]. The SPI-2 related events are triggered by the action of effector proteins with its own T3SS known as T3SS-2, which also encodes its proper translocon machinery named SseBCD [77]. Gene sequence similarity to the known components of other T3SS has been used to propose functions for *SsaN*, *SsaR*, *SsaS*, *SsaT*, *SsaU* and *SsaV* as coding for putative proto-channel components, SsaD/SpiB, SsaJ, SsaK and SsaQ appear to code for basal components, whereas SsaC/SpiA may code for an outer ring protein [78]. Generally, SPI-2 contains four types of virulence genes: *ssa* encodes T3SS-2 apparatus; *ssr* encodes regulators; *ssc* encodes the chaper-

*Location and function of the major proteins and virulence determinants contributing to Salmonella* 

**Virulence genes Location Functions** *CsrA* RNA chaperones

*Hfq* SPI-2 SPI-2 regulator (transcriptional

*mgtABCD* SPI-3 A hydrophobic membrane

*sscAB* Chromosome Putative type III secretion

*sseABCDFGIJL* SPI-2 Translocation; sif formation in

*ssrAB* (*ssrA/SpiR*) SPI-2 Regulates SPI-2 gene expression

and post-transcriptional), RNA

protein; Mg2+ transporter (Mg2+ transporting P-type ATPase)

translocon proteins under conditions that simulate the vacuolar environment; interferes with vesicular trafficking; intracellular bacterial proliferation; secretion

system chaperone protein or pathogenicity island effector

epithelial cells; SCV maturation and positioning; SCV membrane dynamics; nuclear response-gene

chaperones

SPI-2 Regulate the secretion of

protein

expression;

Unlike SPI-1 and SPI-2, only four ORFs within SPI-3 have been shown to contribute to replication in macrophages via a high-affinity Mg2+ uptake system [81]. The *mgtC* gene encoding a 22.5-kDa hydrophobic membrane protein, is the major virulence gene factor found within this locus, and is responsible for growth in Mg2+ limiting environment, intramacrophage survival, and systematic virulence in mice [82]. The transcription of *mgtC* is followed by activation of PhoP-PhoQ in response

ones and *sse* encodes the effectors (**Table 2**) [79, 80].

*2.2.3 SPI-3 contributes to intramacrophage proliferation*

**176**

to low Mg2+ levels [81].

The fourth SPI contributes to *Salmonella* colonization in the intestine of cattle, but not of chicks [83]. Loss of SPI-4 attenuates the oral but not intraperitoneal virulence of serovars Typhimurium and Enteritidis in mice [84]. Three genes namely *SiiC*, *SiiD*, and *SiiF* produce proteins that form the type 1 secretion system (T1SS); the fourth gene, *siiE* codes for a giant non-fimbrial adhesion exported by the T1SS and mediates contact-dependent adhesion to polarized epithelial cells rather than to non-polarized cells. In contrast, SiiA and SiiB are not secreted but represent inner membrane proteins whose function is unknown [55, 85]. Recently, transmembrane mucin MUC1 was shown to be required for *Salmonella* siiE-mediated entry of enterocytes via the apical route [86].
