The Francisella pathogenicity island (FPI) is a cluster of 16-19 genes, which is found duplicated in most of the Francisella genomes that have been sequenced. Although 16 FPI genes are highly conserved there are 2-3 putative genes that are absent or interrupted by stop codons in some strains. Francisella strains with experimentally induced mutations in FPI genes are highly attenuated in virulence and show defects in intramacrophage growth. There is experimental evidence indicating that the regulation of most FPI genes is affected by the presence of the virulence regulator MglA and by the concentration of iron in the growth medium. Although studies of mRNA expression show that essentially all FPI genes are transcribed, only a handful of FPI-encoded proteins have been detected by biochemical methods. The cumulative biochemical and genetic data to date have not yet been able to ascribe a biochemical function to any of the FPI-encoded proteins. However, bioinformatics analysis suggests that some of the FPI-encoded proteins are part of a type VI secretion system.

The Francisella pathogenicity island (FPI) encodes proteins thought to compose a type VI secretion system (T6SS) that is required for the intracellular growth of Francisella novicida. In this work we used deletion mutagenesis and genetic complementation to determine that the intracellular growth of F. novicida was dependent on 14 of the 18 genes in the FPI. The products of the iglABCD operon were localized by the biochemical fractionation of F. novicida, and Francisella tularensis LVS. Sucrose gradient separation of water-insoluble material showed that the FPI-encoded proteins IglA, IglB and IglC were found in multiple fractions, especially in a fraction that did not correspond to a known membrane fraction. We interpreted these data to suggest that IglA, IglB and IglC are part of a macromolecular structure. Analysis of published structural data suggested that IglC is an analogue of Hcp, which is thought to form long nanotubes. Thus the fractionation properties of IglA, IglB and IglC are consistent with the current model of the T6SS apparatus, which supposes that IglA and IglB homologues form an outer tube structure that surrounds an inner tube composed of Hcp (IglC) subunits. Fractionation of F. novicida expressing FLAG-tagged DotU (IcmH homologue) and PdpB (IcmF homologue) showed that these proteins localize to the inner membrane. Deletion of dotU led to the cleavage of PdpB, suggesting an interaction of these two proteins that is consistent with results obtained with other T6SSs. Our results may provide a mechanistic basis for many of the studies that have examined the virulence properties of Francisella mutants in FPI genes, namely that the observed phenotypes of the mutants are the result of the disruption of the FPI-encoded T6SS structure.

Burkholderia cenocepacia is a Gram-negative aerobic bacterium that belongs to a group of opportunistic pathogens displaying diverse environmental and pathogenic lifestyles. B. cenocepacia is known for its ability to cause lung infections in people with cystic fibrosis and it possesses a large 8 Mb multireplicon genome encoding a wide array of pathogenicity and fitness genes. Transcriptomic profiling across nine growth conditions was performed to identify the global gene expression changes made when B. cenocepacia changes niches from an environmental lifestyle to infection. In comparison to exponential growth, the results demonstrated that B. cenocepacia changes expression of over one-quarter of its genome during conditions of growth arrest, stationary phase and surprisingly, under reduced oxygen concentrations (6% instead of 20.9% normal atmospheric conditions). Multiple virulence factors are upregulated during these growth arrest conditions. A unique discovery from the comparative expression analysis was the identification of a distinct, co-regulated 50-gene cluster that was significantly upregulated during growth under low oxygen conditions. This gene cluster was designated the low-oxygen-activated (lxa) locus and encodes six universal stress proteins and proteins predicted to be involved in metabolism, transport, electron transfer and regulation. Deletion of the lxa locus resulted in B. cenocepacia mutants with aerobic growth deficiencies in minimal medium and compromised viability after prolonged incubation in the absence of oxygen. In summary, transcriptomic profiling of B. cenocepacia revealed an unexpected ability of aerobic Burkholderia to persist in the absence of oxygen and identified the novel lxa locus as key determinant of this important ecophysiological trait.

Francisella tularensis is a highly infectious, facultative intracellular bacterial pathogen that is the causative agent of tularemia. Nearly a century ago, researchers observed that tularemia was often fatal in North America but almost never fatal in Europe and Asia. The chromosomes of F. tularensis strains carry two identical copies of the Francisella pathogenicity island (FPI), and the FPIs of North America-specific biotypes contain two genes, anmK and pdpD, that are not found in biotypes that are distributed over the entire Northern Hemisphere. In this work, we studied the contribution of anmK and pdpD to virulence by using F. novicida, which is very closely related to F. tularensis but which carries only one copy of the FPI. We showed that anmK and pdpD are necessary for full virulence but not for intracellular growth. This is in sharp contrast to most other FPI genes that have been studied to date, which are required for intracellular growth. We also showed that PdpD is localized to the outer membrane. Further, overexpression of PdpD affects the cellular distribution of FPI-encoded proteins IglA, IglB, and IglC. Finally, deletions of FPI genes encoding proteins that are homologues of known components of type VI secretion systems abolished the altered distribution of IglC and the outer membrane localization of PdpD.

Hopanoids are pentacyclic triterpenoids that are thought to be bacterial surrogates for eukaryotic sterols, such as cholesterol, acting to stabilize membranes and to regulate their fluidity and permeability. To date, very few studies have evaluated the role of hopanoids in bacterial physiology. The synthesis of hopanoids depends on the enzyme squalene-hopene cyclase (Shc), which converts the linear squalene into the basic hopene structure. Deletion of the 2 genes encoding Shc enzymes in Burkholderia cenocepacia K56-2, BCAM2831 and BCAS0167, resulted in a strain that was unable to produce hopanoids, as demonstrated by gas chromatography and mass spectrometry. Complementation of the ⌬shc mutant with only BCAM2831 was sufficient to restore hopanoid production to wild-type levels, while introducing a copy of BCAS0167 alone into the ⌬shc mutant produced only very small amounts of the hopanoid peak. The ⌬shc mutant grew as well as the wild type in medium buffered to pH 7 and demonstrated no defect in its ability to survive and replicate within macrophages, despite transmission electron microscopy (TEM) revealing defects in the organization of the cell envelope. The ⌬shc mutant displayed increased sensitivity to low pH, detergent, and various antibiotics, including polymyxin B and erythromycin. Loss of hopanoid production also resulted in severe defects in both swimming and swarming motility. This suggests that hopanoid production plays an important role in the physiology of B. cenocepacia.