Previous studies have shown thatSalmonellamutants lackingspaOfailed to assemble the needle complex, leading to its inability to secrete proteins and invade cells [41,42]. This suggests that the SpaO protein is essential for needle complex assembly and protein secretion critical for bacterial entry. However, its expressionin vivohas not been reported. Our findings of the differential expression of SpaO preferentially bySalmonellacolonizing the cecum but not spleen

raises the possibility that efficient expression of this protein may not be needed bySalmonellain the spleen, find more possibly because bacteria entry can be accomplished with phagocytosis by splenic macrophages. Furthermore, tissue-specific differential regulation of the expression of SpaO, a protein essential for the secretion machinery [41,42], JAK inhibitor should provide another mechanismin vivofor the regulation of the amounts of effector proteins to be secreted into the host cells. Recent studies have revealed hierarchical transport of different effectors duringSalmonellaentry and extensive ordered synergistic and antagonist relationships between these effectors following

their delivery into the host cells [5,39,40]. Thus, it is conceivable that differential expression of SpaO may dictate the amounts of needle complexes available during bacterial entry. This may result in hierarchical transport of specific effectors

and specific functional interplay (synergistic or antagonist relationships) among these proteins in the host cells, leading to specific pathological consequences in different tissues. We note that some of the protein expression results in our study may not be consistent with those from the expression of the transcripts of the SPI-1 genes that have been recently published [19,20]. The expression of SPI-1 genes is tightly controlled transcriptionally and post-transcriptionally [13]. Thus, we believe that our results of the SPI-1 protein expressionin vivomay not necessarily correlate with the previous observationsin Methane monooxygenase vitro. Furthermore, the amounts of proteins expressed from the SPI-1 genesin vivoare in a delicate balance as there are hierarchical transports of different effectors duringSalmonellaentry and extensive ordered synergistic and antagonist relationships between these effectors following their delivery into the host cells. An SCH727965 in vitro imbalance of the amounts of these factors available during infection would seriously compromise the ability of the bacteria to establish successful infection. Our results complement and further extend previous findings of the expression of these SPI-1 factors, and demonstrate the importance of examining protein expressionin vivoin the context of infection.