Significance and context
Successful infection by pathogenic bacteria often depends on their ability to survive and multiply within host cells. To do so, they alter or adapt to the host-cell environment. To these ends, pathogenic bacteria contain a variety of secretion systems, including type I, II and III secretion systems, which can export virulence factors to the environment or into the infected host cell. A type IV secretion system has also recently been identified, which is, for example, involved in the transfer of T-DNA from Agrobacterium tumefaciens into a host plant cell.
The VirB system of B. suis is an example of a type IV secretion system that is important in the virulence of medically important pathogenic bacteria. B. suis is a member of a genus of Gram-negative pathogenic bacteria that cause brucellosis in pigs, dogs and rodents, and Malta fever in humans. Relatively little is known about the genetics of Brucella virulence or about factors that facilitate bacterial survival and multiplication within host cells. The virB operon of B. suis, which encodes the VirB system, is essential for bacterial survival and multiplication in macrophages and epithelial cells. Boschiroli et al. studied the regulation of this operon and found that phagosome acidification is a key signal required for induction of virB-operon expression. This contributes to our understanding of how B. suis has adapted to and exploits an intracellular environment intended to destroy it.
The previously isolated virB region of B. suis strain 1330 contains 12 genes - virB1 - virB12. Boschiroli et al. showed that transcription of all 12 genes is controlled by the same promoter, demonstrating that these genes are indeed part of the virB operon. No vir genes were found either immediately upstream of virB1 or downstream of virB12. A mutation in virB1 had an effect on the expression of the downstream virB genes, suggesting that no internal promoters were present. In contrast to mutations upstream of virB1 and downstream of virB12, a mutation in virB12 had a severe effect on survival and multiplication in macrophages and HeLa epithelial cells. To study virB promoter activity, a plasmid carrying a transcriptional fusion between the putative virB promoter and a gene encoding green fluorescent protein (GFP) was introduced into B. suis. Using fluorescence-activated cell sorting, Boschiroli et al. showed that the virB promoter is not activated in free-living bacteria, but is specifically activated soon after infection of macrophage cell lines. One of the major induction stimuli was acidification for 3 hours to give a pH of 4. When phagosomal acidification was blocked, no activation of the virB promoter after infection could be observed. It has also recently been shown by other workers that phagosome acidification is required for intracellular multiplication of B. suis.
Determination of the entire genome sequence of B. suis is in progress and can be followed at the NCBI Microbial Genomes page
Boschiroli et al. showed that the virB operon of B. suis is specifically activated shortly after infection in macrophages or epithelial cell lines and that this activation is dependent on the phagosomal acidification that occurs rapidly after infection. This is a convincing example of how B. suis adapts to the new environmental conditions within the host cell. As Boschiroli et al. indicate, the next step will be to identify the virulence factors secreted by the virB system. If the virB system of B. suis is involved in the secretion of particular proteins, comparative proteomics might be helpful to determine these virulence factors, now that the induction conditions for the virB operon are well described. Once the genome sequence of B. suis is available, microarray-based analysis can be used to discover other genes whose expression is induced by low pH. Characterization of virulence factors secreted by the virB system and proteins produced upon acidification of the environment will be a significant step towards unraveling how B. suis influences processes in the host cell and, consequently, to finding ways to block the early stages of infection.