Vibrio cholerae has multiple iron acquisition systems, including TonB-dependent transport of heme and of the catechol siderophore vibriobactin. Strains defective in both of these systems grow well in laboratory media and in the infant mouse intestine, indicating the presence of additional iron acquisition systems. Previously uncharacterized potential iron transport systems, including a homologue of the ferrous transporter Feo and a periplasmic binding protein-dependent ATP binding cassette (ABC) transport system, termed Fbp, were identified in the V. cholerae genome sequence. Clones encoding either the Feo or the Fbp system exhibited characteristics of iron transporters: both repressed the expression of lacZ cloned under the control of a Fur-regulated promoter in Escherichia coli and also conferred growth on a Shigella flexneri mutant that has a severe defect in iron transport. Two other ABC transporters were also evaluated but were negative by these assays. Transport of radioactive iron by the Feo system into the S. flexneri iron transport mutant was stimulated by the reducing agent ascorbate, consistent with Feo functioning as a ferrous transporter. Conversely, ascorbate inhibited transport by the Fbp system, suggesting that it transports ferric iron. The growth of V. cholerae strains carrying mutations in one or more of the potential iron transport genes indicated that both Feo and Fbp contribute to iron acquisition. However, a mutant defective in the vibriobactin, Fbp, and Feo systems was not attenuated in a suckling mouse model, suggesting that at least one other iron transport system can be used in vivo.
Hfq plays an important role in cellular physiology by regulating the expression of several genes. Hfq synthesis in Escherichia coli is subject to auto-repression at translational level. Studies with Shigella flexneri show that hfq transcription is regulated by a pleiotropic regulator, DksA. Comparison of gene expression profiles of wild type and dksA mutant S. flexneri determined that hfq expression was reduced in the dksA mutant. As DksA is required for stress resistance and plaque formation in cultured cell monolayers, a measure of virulence, we assessed the role of Hfq in the dksA virulence phenotype. Expression of hfq in the dksA mutant restored plaque formation, and an hfq mutant failed to form plaques. Thus, DksA plays a role in regulating hfq gene expression and this regulation is important for S. flexneri virulence. In an in vitro transcription assay, addition of DksA increased transcription of hfq and this effect was greatest with one of the known hfq promoters. Addition of ppGpp, a stringent response molecule, along with DksA in the in vitro transcription assay resulted in a further increase in transcription of hfq, indicating that DksA is required for maximal transcription of hfq during both exponential and stringent response growth conditions.
Shigella species are able to grow in a variety of environments, including intracellularly in host epithelial cells. Shigella have a number of different iron transport systems that contribute to their ability to grow in these diverse environments. Siderophore iron uptake systems, heme transporters, and ferric and ferrous iron transport systems are present in these bacteria, and the genes encoding some of these systems appear to have spread among the Shigella species by horizontal transmission. Iron is not only essential for growth of Shigella but also plays an important role in regulation of metabolic processes and virulence determinants in Shigella. This regulation is mediated by the repressor protein Fur and the small RNA RyhB.
Most inbred mice carry germline proviruses of the retrovirus, mouse mammary tumor virus (MMTV) (called Mtvs), which have multiple replication defects. A BALB/c congenic mouse strain lacking all endogenous Mtvs (Mtv-null) was resistant to MMTV oral and intraperitoneal infection and tumorigenesis compared to wild-type BALB/c mice. Infection of Mtv-null mice with an MMTV-related retrovirus, type B leukemogenic virus, also resulted in severely reduced viral loads and failure to induce T-cell lymphomas, indicating that resistance is not dependent on expression of a superantigen (Sag) encoded by exogenous MMTV. Resistance to MMTV in Mtv-null animals was not due to neutralizing antibodies. Further, Mtv-null mice were resistant to rapid mortality induced by intragastric inoculation of the Gram-negative bacterium, Vibrio cholerae, but susceptibility to Salmonella typhimurium was not significantly different from BALB/c mice. Susceptibility to both MMTV and V. cholerae was reconstituted by the presence of any one of three endogenous Mtvs located on different chromosomes and was associated with increased pathogen load. One of these endogenous proviruses is known to encode only Sag. Therefore, Mtv-encoded Sag appears to provide a unique genetic susceptibility to specific viruses and bacteria. Since human endogenous retroviruses also encode Sags, these studies have broad implications for pathogen-induced responses in mice and humans.