## 4 Discussion
The PhoR protein has opposing kinase and phosphatase functions in the CA and DHp domains, and it was predicted that these two activities are controlled by physical constraint and a swing movement in the protein to prevent access of the CA domain to the DHp domain [65]. Such constraint between the domains has been described in studies of the crystal structure of similar sensor histidine kinases [69; 70]. The mechanism by which PhoR receives the environmental phosphate signal is unknown [46], but in _Salmonella_, the _phoBR_ operon is partly responsible for maintaining intracellular phosphate levels between 1 and 10 mM [65; 71; 72]. Phosphate homeostasis, in turn, involves multiple functions in the cell [26; 28; 65]. While the different PhoR proteins in these _Salmonella_ are obviously functional (as the operon is essential), it might be possible that sequence differences in this CA domain have variable kinetics. We could not find any studies in the literature that involved direct testing of amino acid variations in the CA domain of PhoR, but it is tempting to speculate that changes in this region might affect the efficiency of function. Because of the complexity of the interactions of the proteins involved in phosphate metabolism and transport as well as the central role phosphate homeostasis plays in cellular metabolism, small differences in efficiency might affect competition between strains when in complex culture backgrounds. In addition to the nutritional and genomic differences reported here, there are other factors beyond culture media that could contribute to unequal enrichment of _Salmonella_ serovars. _Salmonella_ strains were shown to enter into a viable but non-culturable (VBNC) state when incubated on dried fruit [73]. While a cocktail of _Salmonella_ strains was not detectable via enrichment culture from dried apple slices after 46 days of incubation, live/dead staining showed 25% and 32% of the total population were viable after storage for 50 days at 25 \({}^{\circ}\)C and 110 days at 4 \({}^{\circ}\)C, respectively [73]. Bacteria can enter the VBNC state after exposure to stresses such as temperature, sanitizers, osmotic pressure, pH changes, and nutrient starvation [74; 75; 76]. Similar conditions may simply lead to stressed cells that are less likely to survive enrichment [77]. Several of these conditions may be encountered when _Salmonella_ is present as a contaminant in and on foods. Additionally, _Salmonella_ can survive inside of protozoa, which might mask its presence and render it more resistant to disinfectants [74; 78]. In enrichment culture, the target organism must compete with other microbes present on or in the foods. This natural microbiota can vary depending on several factors, including the type of food, location, time of year, or natural variation [79]. Similarly, other microorganisms present in food samples may produce compounds such as bacteriocins that kill _Salmonella_[80]. Concentration effects of different _Salmonella_ serovars that may contaminate the same food product may also affect the serovars that result after the enrichment process. _S._ Kentucky and _S._ Give may dominate enrichment cultures simply because there are many more of those types in the samples being tested. Surveillance for pathogens in foods is a complex process with many variables that affect the outcome. The comparison of different _Salmonella_ serovars in growth characteristics is the beginning of a process to determine if the methods might be altered to achieve a fuller picture of the subtypes that might be present in contaminated foods.