## 4 Discussion
Addition of any of these sugars to BPW or other complex pre-enrichment media might aid in improving the growth of Enteritidis and Typhimurium strains when competing with serovar Kentucky and/or Give strains in enrichments; this will be addressed in future studies. The number of different compounds that may be used as phosphorus sources and the better growth with the individual phosphorus sources by \(S\). Kentucky strains is curious. Phosphorus is usually assimilated by bacteria in the form of inorganic phosphates (Pi) [60]. Phosphorus is not predominant in nature, which is why excess phosphates (as well as nitrates and sulfates) in agricultural and sewage runoff lead to eutrophication in surface waters and grasslands [61]. The concentration of phosphates used in laboratory culture media is artificial, and many bacteria are stressed or inhibited by high phosphate concentrations [62; 63; 64]. \(S\). Kentucky might be better at scavenging for phosphorus in various conditions. Therefore, we thought a closer look at phosphate metabolism in these strains might be useful. Of the nine genes assessed, the gene product that showed differences based on serovar was _phoR_. The PhoB/PhoR two-component regulatory signal transduction system regulates the expression of genes involved in the transport of Pi and other P-containing compounds [65]. These are not the only genes comprising the Pho regulon, which is estimated to contain at least 137 gene products in _Escherichia coli_[65; 66]. Proteomic analysis of the \(S\). Typhimurium strain 14028s revealed over 300 proteins that are regulated by Pi-limiting conditions, and at least 27 of these are regulated by PhoB [26]. PhoR autophosphorplates upon reduced Pi levels and transfers that phosphoryl group to PhoB, thereby activating PhoB. When Pi levels are sufficient, PhoR removes the phosphoryl group from PhoB [29]. It was reported recently that _Salmonella_ limits Pi uptake to maintain cellular processes involving Mg\({}^{2+}\) homeostasis. Excess Pi uptake leads to increased ATP, which in turn chelates intracellular Mg\({}^{2+}\)[28]. Therefore, regulation of aspects of cell metabolism that control nutrient assimilation, cellular processes, and survival of stresses and transition into the stationary phase are complex. Sequence variability in a central two-component regulatory system might result in slight differences in functionality of the proteins. The increased variability in _phoR_ compared to the other genes led to examination of more _phoR_ gene sequences from additional _Salmonella_ strains. The highest _phoR_ sequence variability was among the \(S\). Kentucky strains. \(S\). Kentucky is a polyphyletic serovar with at least two different subgroups [67], and our comparison of 15 \(S\). Kentucky strains found three different predicted protein sequences for PhoR. Interestingly, \(S\). Kentucky ST152 and ST198 each had unique PhoR substitutions. In a study of another polyphyletic serovar (_S_. Mississippi), [PERSON] et al. concluded that the O and H antigenic elements that determine serovar may be inherited from different ancestors, leading to different sequence types with the same serovar [68]. More study of the phylogenetically distinct \(S\). Kentucky strains is needed to determine if similar independent acquisitions of antigenic elements occurred. Nearly all of the variability in PhoR sequences was in the CA domain, i.e., the part of the protein involved in ATP binding and catalysis. This region is located between amino acid residues 267 and 431. Immediately upstream of the CA domain is the DHp domain (located between amino acid residues 193 and 267), which is responsible for dimerization and histidine phosphorylation. The autophosphorylation of PhoR involves the transfer of a phosphoryl group from the CA domain to an essential His-213 residue in the DHp domain [47].