*2.4.3 Siderophores are important for bacterial growth in serum in the extracellular phase of salmonellosis*

Iron is an essential element for the growth of most bacteria through its involvement in a variety of metabolic and regulatory functions [119]. Studies with different iron concentrations in growth media demonstrated an effect on gene expression of the iron acquisition systems encoded both on the chromosome and plasmids at both transcriptional and translational levels [120]. Siderophores which are bacterial molecules that bind and transport iron are important for bacterial growth in serum in the extracellular stage of *Salmonella* systemic infection. They are not required after bacteria reside in SCV where siderophore-independent iron acquisition systems are sufficient for iron uptake during intracellular stage. *Salmonella* produce two major types of siderophores, high-affinity catecholate consisting of salmochelin and enterobactin the latter also known as enterochelin and a lowaffinity hydroxamate known as aerobactin which is expressed under iron-restricted conditions [121]. The synthesis, secretion, and uptake of salmochelin requires genes clustered at two genetic loci, the *fepA* gene cluster and *iroBCDEN* operon. The *fepA* gene cluster includes most *ent* genes for synthesis and export [122]. The *iroBCDEN* operon encodes gene products for enterobactin glycosylation (IroB, glycosyltransferase), export (IroC, ABC transporter protein), and utilization (IroD, esterase; IroE, hydrolase; IroN, outer membrane receptor) [122]. Mutants deficient in *iroB* or *iroC* exhibit reduced virulence during systemic infection of mice via intraperitoneal route, as indicated by lower bacterial load in liver and a delayed time of death [122]. Moreover, the enterobactin metabolite, 2, 3-dihydroxybenzoyl serine (DHBS), can also be used by *Salmonella* as sources of iron, albeit at much lower affinities, by recognizing the three catechelate receptors, FepA, IroN and Cir. The three receptors demonstrate a significant degree of functional redundancy. The Typhimurium

**181**

**4. Conclusions**

*Virulence Determinants of Non-typhoidal* Salmonellae *DOI: http://dx.doi.org/10.5772/intechopen.88904*

surable spread to the liver [123, 124].

*entB*, *fur*, *foxA*, *hemP*, and *fhuE* [126, 127].

this important foodborne and zoonotic pathogen.

**3. Future directions**

double mutant Δ*fepA iroN* were similarly virulent to the parental strain after intragastric gavage inoculation of mice, while the triple mutant *ΔfepA iroN cir* was attenuated as indicated by a significantly reduced cecal colonization and no mea-

Furthermore, *Salmonella* also utilize xenosiderophores as iron sources by utilizing the outer membrane receptors, including FhuA, FhuE, and FoxA. For example, utilization of ferrioxamines B, E, and G by Typhimurium is dependent on the FoxA receptor encoded by the Fur repressible *foxA* gene. A strain carrying the foxA mutation exhibited a significantly reduced ability to colonize rabbit ileal loops and was markedly attenuated in mice challenged by either intragastric gavage or intravenously route strain compared to the foxA+ parent [125]. The best characterized regulator for iron uptake is the iron-dependent repressor Fur that acts together with the co[-]repressor ferrous iron (Fe(II)) to regulate genes involved in the iron uptake process in response to iron restriction, including *fhuA*, *fhuB*, *fepA*, *fes*, *fepD*,

The advent of next generation sequencing (NGS) has provided an opportunity

This review provides an outline of over 200 identified virulence determinants

and details of their involvement in the four steps of *Salmonella* pathogenesis, namely: attachment, invasion, intramacrophage survival/replication and systemic dissemination. The genetic regulation of only some of the virulence determinants have been elucidated in live animal models such as mice and cattle, and this has enriched our understanding of the pathogenesis and mechanism of diarrhea and systemic disease. The majority of the current evidence on pathogenesis

to verify or improve on knowledge gained from *in vitro* and *in vivo* analyses of *Salmonella* mutants which were designed for the purpose of understanding gene function and mechanism of action. Recently, Rakov et al. [14] carried out bioinformatics analysis of 500 *Salmonella* genomes and identified 70 allelic variants virulence factors which were associated with different pathogenesis outcomes, i.e. gastrointestinal vs. invasive disease. However, the causative relationship between a putative virulence factor and disease outcome using a genomics based tool is yet to be attained. To that end, we propose the development of a comprehensive genome based tool such as a NGS AmpliSeq assay that can be used to simultaneously interrogate the presence and potential expression of over 200 virulence genes of *Salmonella* identified in this communication. The tool can be used to evaluate differences in strains and correlate the output with virulence phenotype derived from epidemiological or experimental observations which can be developed simultaneously or based on historical documentation. The tool could be used in assessing the potential risk posed by a strain of *Salmonella* given the fact that the serovars obtained from the environment are often distinct with those involved in human diseases. The technology appears suitable for dissecting the complexity associated with the redundancy and pleiotropic nature of some of the currently known virulence genes. In addition, NGS based analysis of virulence genes should provide new insights on *Salmonella* evolution and a better tool for analyzing epidemiological data that could translate to a reduction in the burden on human health posed by *Virulence Determinants of Non-typhoidal* Salmonellae *DOI: http://dx.doi.org/10.5772/intechopen.88904*

double mutant Δ*fepA iroN* were similarly virulent to the parental strain after intragastric gavage inoculation of mice, while the triple mutant *ΔfepA iroN cir* was attenuated as indicated by a significantly reduced cecal colonization and no measurable spread to the liver [123, 124].

Furthermore, *Salmonella* also utilize xenosiderophores as iron sources by utilizing the outer membrane receptors, including FhuA, FhuE, and FoxA. For example, utilization of ferrioxamines B, E, and G by Typhimurium is dependent on the FoxA receptor encoded by the Fur repressible *foxA* gene. A strain carrying the foxA mutation exhibited a significantly reduced ability to colonize rabbit ileal loops and was markedly attenuated in mice challenged by either intragastric gavage or intravenously route strain compared to the foxA+ parent [125]. The best characterized regulator for iron uptake is the iron-dependent repressor Fur that acts together with the co[-]repressor ferrous iron (Fe(II)) to regulate genes involved in the iron uptake process in response to iron restriction, including *fhuA*, *fhuB*, *fepA*, *fes*, *fepD*, *entB*, *fur*, *foxA*, *hemP*, and *fhuE* [126, 127].
