Characterization of alanine catabolism in Pseudomonas aeruginosa and its importance for proliferation in vivo.

The opportunistic pathogen Pseudomonas aeruginosa causes a variety of infections in immunocompromised individuals, including individuals with the heritable disease cystic fibrosis. Like the carbon sources metabolized by many disease-causing bacteria, the carbon sources metabolized by P. aeruginosa at the host infection site are unknown. We recently reported that l-alanine is a preferred carbon source for P. aeruginosa and that two genes potentially involved in alanine catabolism (dadA and dadX) are induced during in vivo growth in the rat peritoneum and during in vitro growth in sputum (mucus) collected from the lungs of individuals with cystic fibrosis. The goals of this study were to characterize factors required for alanine catabolism in P. aeruginosa and to assess the importance of these factors for in vivo growth. Our results reveal that dadA and dadX are arranged in an operon and are required for catabolism of l-alanine. The dad operon is inducible by l-alanine, d-alanine, and l-valine, and induction is dependent on the transcriptional regulator Lrp. Finally, we show that a mutant unable to catabolize dl-alanine displays decreased competitiveness in a rat lung model of infection.

More than a signal: non-signaling properties of quorum sensing molecules.

Quorum sensing in bacteria serves as an example of the adaptation of single-celled organisms to engage in cooperative group behaviors. This phenomenon is much more widespread than originally thought, with many different species 'speaking' through various secreted small molecules. Despite some variation in signaling molecules, the principles of quorum sensing are conserved across a wide range of organisms. Small molecules, secreted into the environment, are detected by neighbors who respond by altering gene expression and, as a consequence, behavior. However, it is not known whether these systems evolved specifically for this purpose, or even if their role is exclusive to information trafficking. Rather, clues exist that many quorum sensing molecules function as more than just signals. Here, we discuss non-signaling roles for quorum sensing molecules in such important processes as nutrient scavenging, ultrastructure modification and competition.

Genetics and environmental regulation of Shigella iron transport systems.

Shigella spp. have transport systems for both ferric and ferrous iron. The iron can be taken up as free iron or complexed to a variety of carriers. All Shigella species have both the Feo and Sit systems for acquisition of ferrous iron, and all have at least one siderophore-mediated system for transport of ferric iron. Several of the transport systems, including Sit, Iuc/IutA (aerobactin synthesis and transport), Fec (ferric di-citrate uptake), and Shu (heme transport) are encoded within pathogenicity islands. The presence and the genomic locations of these islands vary considerably among the Shigella species, and even between isolates of the same species. The expression of the iron transport systems is influenced by the concentration of iron and by environmental conditions including the level of oxygen. ArcA and FNR regulate iron transport gene expression as a function of oxygen tension, with the sit and iuc promoters being highly expressed in aerobic conditions, while the feo ferrous iron transporter promoter is most active under anaerobic conditions. The effects of oxygen are also seen in infection of cultured cells by Shigella flexneri; the Sit and Iuc systems support plaque formation under aerobic conditions, whereas Feo allows plaque formation anaerobically.

Anaerobic regulation of Shigella flexneri virulence: ArcA regulates Fur and iron acquisition genes.

Invasion and plaque formation in epithelial monolayers are routinely used to assess the virulence of Shigella flexneri, a causative agent of dysentery. A modified plaque assay was developed to identify factors contributing to the virulence of S. flexneri under the anaerobic conditions present in the colon. This assay demonstrated the importance of the ferrous iron transport system Feo, as well as the global transcription factors Fur, ArcA, and Fnr, for Shigella plaque formation in anoxic environments. Transcriptional analyses of S. flexneri iron transport genes indicated that anaerobic conditions activated feoABC while repressing genes encoding two other iron transport systems, the ABC transporter Sit and the Iuc/Iut aerobactin siderophore synthesis and transport system. The anaerobic transcription factors ArcA and Fnr activated expression of feoABC, while ArcA repressed iucABCD iutA. Transcription of fur, encoding the iron-responsive transcriptional repressor of bacterial iron acquisition, was also repressed anaerobically in an ArcA-dependent manner.

Iron and pathogenesis of Shigella: iron acquisition in the intracellular environment.

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.