Ge et al. BMC Genomics 2010, 11:456 http://www.biomedcentral.com/1471-2164/11/456 synthesis; it introduces negative supercoils into DNA during replication [51]. The induction of gyrA and gyrB perhaps indicates that more DNA was synthesized by Salmonella resulting in increased bacterial proliferation in HLA-B27-transfected cells. Salmonella substrate and energy metabolism Growth in a nutrient-limited environment may be echoed in changes in gene expression. Expression of genes involved in substrate and energy metabolism, including nuoJKMN encoding NADH dehydrogenase I chains (J, K, M and N components), cyoCD encoding cytochrome O ubiquinol oxidase subunits III and IV, and aceF encoding a putative dehydrogenase was up-regulated in HLA-B27 cells compared to HLA-A2 cells. This suggests that more energy was produced in HLA-B27 cells under aerobic conditions, which could be connected to the enhanced bacterial replication observed in HLA-B27 cells [52,53]. Up-regulation of amino acid metabolism by Salmo- nella growth in HLA-B27 cells compared to HLA-A2 cells was evident from the induction of gltB, gltD and glnA encoding glutamate synthase/glutamine synthetase, and glnGL encoding a two-component regulator of glutamine synthetase [54]. Glutamate is an important precursor of other amino acids and of pyrimidine and purine synthesis in bacteria [55]. Moreover, sugar catabolism genes such as aceF, fruA, fruF and pykF were elevated in HLA-B27 cells. Collectively, these data suggest that HLA-B27 affects intracellular substrate metabolism and energy production by Salmonella during infection of U937 cells, probably due to the increased bacterial proliferation in these cells. HLA-B27 modified ionic transport systems Magnesium is involved in the stabilisation of the cellu- lar membrane and functions as a coenzyme. MgtA and MgtB are two inducible transporters, and high up-regu- lation of mgtB and mgtA in both HLA-B27 and HLA- A2 macrophages might suggest that Mg2+ is extremely limited within the SCV of infected U937 cells [56,57]. From 1 to 8 h after infection, the degree to which these two genes were up-regulated was increased in both cell lines, indicating that the amounts of magnesium decreased during the course of infection. MgtC is also involved in Mg2+ transport in the intracellular survival of Salmonella and other bacteria in macrophages [58,59]. The two genes mgtC and mgtB are located in one operon, mgtCB, which belongs to SPI-3 on the Salmonella chromosome [58]. Our data showed that both mgtC and mgtB were less induced in HLA-B27- positive cells than in HLA-B27-negative cells, suggesting that HLA-B27 modulates the acquisition of Mg2+ in macrophages. Page 7 of 13 Phosphate availability is limited in macrophage vacuoles [60]. These observations are supported by our microarray data showing that the phoBR regulon, the pstC and pstS genes responsible for phosphate uptake and/or transport, were up-regulated during intracellular growth. The genes were induced to similar levels in both cell lines at 2 h after infection but the expression of pstC was down-regulated in HLA-B27 cells compared to HLA-A2 cells at 8 h postinfection. Taken together, these results suggest that HLA-B27 affects the intracel- lular ionic status of the SCV. Conclusions We have elucidated the global gene expression profile of S. Enteritidis PT4 KS8822/88 during intracellular growth in human monocyte/macrophages for the first time. The gene expression profile shown by S. Enteritidis during intracellular growth is very different from that exhibited in LB broth. However, it is similar to the patterns of gene expression reported for S. Typhimurium bacterial intra- cellular growth in macrophages [52]. Approximately one quarter of S. Enteritidis genes showed up- or down-regu- lation in host cells compared to LB broth. Among the genes up-regulated intracellularly were SPI-2 virulence genes, nutrient acquisition system genes, and ionic uptake and/or synthesis genes. Similar differences in intracellular S. Typhimurium gene expression were also seen between macrophages and medium in vitro [52]. SPI-2 genes are broadly induced to evade immune responses by host cells. Acquisition and transport system genes were elevated to take up the necessary elements. Among the down-regulated genes, some were related to central metabolism, such as the tricarboxylic acid cycle and oxidative phosphorylation, indicating that the level of oxygen is extremely low in the SCV intracellular envir- onment, which seems contrary to the observation of intracellular S. Typhimurium growth [52]. Also, Salmo- nella metabolism and energy production were slower intracellularly than during growth in LB medium. These changes reflect the intracellular environment that the bacteria encounter during infection of macrophages [61]. The survival and proliferation of pathogens within macrophages is critical for establishing systemic infec- tion, and this process is related to bacterial gene expres- sion [62,63]. The expression of HLA-B27 in the U937 cells appeared to generate a favourable environment for intracellular Salmonella growth compared with HLA- A2-transfected cells, resulting in more bacteria in HLA- B27-transfected cells. The increased ability of Salmo- nella to survive is probably due to the altered gene expression. The expression of 118 genes at 8 h after infection differed significantly between HLA-B27 cells and HLA-A2 cells, suggesting that HLA-B27 modified Salmonella gene expression and/or affected the