The type II protein secretion system of Legionella pneumophila promotes growth at low temperatures

lbtA and lbtB are required for production of the Legionella pneumophila siderophore legiobactin

Under iron stress, Legionella pneumophila secretes legiobactin, a nonclassical siderophore that is reactive in the chrome azurol S (CAS) assay. Here, we have optimized conditions for legiobactin expression, shown its biological activity, and identified two genes, lbtA and lbtB, which are involved in legiobactin production. lbtA appears to be iron repressed and encodes a protein that has significant homology with siderophore synthetases, and FrgA, a previously described iron-regulated protein of L. pneumophila. lbtB encodes a protein homologous with members of the major facilitator superfamily of multidrug efflux pumps. Mutants lacking lbtA or lbtB were defective for legiobactin, producing 40 to 70% less CAS reactivity in deferrated chemically defined medium (CDM). In bioassays, mutant CDM culture supernatants, unlike those of the wild type, did not support growth of iron-limited wild-type bacteria in 2',2'-dipyridyl-containing buffered charcoal yeast extract (BCYE) agar and a ferrous iron transport mutant on BCYE agar without added iron. The lbtA mutant was modestly defective for growth in deferrated CDM containing the iron chelator citrate, indicating that legiobactin is required in conditions of severe iron limitation. Complementation of the lbt mutants restored both siderophore expression, as measured by the CAS assay and bioassays, and bacterial growth in deferrated, citrate-containing media. The lbtA mutant replicated as the wild type did in macrophages, amoebae, and the lungs of mice. However, L. pneumophila expresses lbtA in the macrophage, suggesting that legiobactin, though not required, may play a dispensable role in intracellular growth. The discovery of lbtAB represents the first identification of genes required for L. pneumophila siderophore expression.

Adaptation of Legionella pneumophila to the host environment: role of protein secretion, effectors and eukaryotic-like proteins

The intracellular pathogen Legionella pneumophila has evolved sophisticated mechanisms that enable it to subvert host functions, enter, survive and replicate in amoebae or alveolar macrophages, and to finally evade these hosts. Protozoa are essential for the growth of Legionella and the interaction with amoeba seems to be the driving force in the evolution of its pathogenicity. This is reflected in the genome of this pathogen, which encodes a high number and variety of eukaryotic-like proteins that are able to interfere in the various steps of the infectious cycle by mimicking functions of eukaryotic proteins. Central to the pathogenicity of L. pneumophila are the many secretion systems delivering these and other effectors to the host cell. Recent studies have highlighted the multi-functional role of these factors secreted by L. pneumophila, in host-pathogen interactions.

Type II secretion: a protein secretion system for all seasons

In Gram-negative bacteria, type II secretion (T2S) is one of five protein secretion systems that permit the export of proteins from within the bacterial cell to the extracellular milieu and/or into target host cells. An analysis of numerous sequenced genomes now reveals that T2S genes are common, but by no means universal, in Gram-negative bacteria. Recent functional studies indicate that T2S can promote the virulence of human, animal and plant pathogens, as well as the physiology of various environmental bacteria. Thus, it is an opportune time to highlight the new and different ways in which T2S serves bacterial function.

The Legionella pneumophila tatB gene facilitates secretion of phospholipase C, growth under iron-limiting conditions, and intracellular infection

Our previous mutational analysis of Legionella pneumophila demonstrated a role for type II protein (Lsp) secretion and iron acquisition in intracellular infection and virulence. In gram-negative bacteria, the twin-arginine translocation (Tat) pathway is involved in secretion of proteins, including components of respiratory complexes, across the inner membrane to the periplasm. To assess the significance of Tat for L. pneumophila, tatB mutants were characterized. The mutants exhibited normal growth in standard media but grew slowly under low-iron conditions. They were also impaired in the Nadi assay, indicating that the function of cytochrome c oxidase is influenced by tatB. Consistent with this observation, a subunit of the cytochrome c reductase was shown to be a Tat substrate. Supernatants of the tatB mutants showed a 30% reduction in phospholipase C activity while maintaining normal levels of other Lsp secreted activities. When tested for infection of U937 macrophages, the tatB mutants showed a 10-fold reduction in growth. Double mutants lacking tatB and Lsp secretion were even more defective, suggesting tatB has an intracellular role that is independent of Lsp. tatB mutants were also impaired 20-fold in Hartmannella vermiformis amoebae cultured in the presence of an iron chelator. All mutant phenotypes were complemented by reintroduction of an intact tatB. Thus, L. pneumophila tatB plays a role in the formation of a respiratory complex, growth under low-iron conditions, the secretion of a phospholipase C activity, and intracellular infection.