Bacterial pathogenesis

Invasive and adherent bacterial pathogens co-opt host clathrin for infection Veiga, E. et al. Cell Host & Microbe 2, 340–351 (2007)

Although many viruses enter host cells by clathrin-mediated endocytosis, it was thought that clathrin-coated vesicles were too small to internalize bacteria. So, it was a surprise when Listeria monocytogenes was shown to enter host cells by a clathrin-dependent mechanism. Building on this previously published finding, Veiga et al. investigated whether clathrin is required for the entry of other pathogens. They found that bacteria that enter cells after interactions with specific receptors, such as Staphylococcus aureus, require clathrin for entry, whereas those that inject effectors into host cells to facilitate their own entry by altering the host cytoskeleton, such as Shigella flexneri, do not. Clathrin was also required to assemble pedestals beneath adherent enteropathogenic Escherichia coli, which remain extracellular.

Molecular ecology

Mutational activation of niche-specific genes provides insight into regulatory networks and bacterial function in a complex environment Giddens, S. R. et al. Proc. Natl Acad. Sci. USA 104, 18247–18252 (2007)

The identification of environment-induced loci (EIL) is hampered by a lack of discernable phenotypes in the laboratory when EIL are mutated. Giddens et al. describe a new, broadly applicable method — SpyVet (suppressor analysis with in vivo expression technology) — to identify regulatory loci that control EIL in the rhizosphere bacterium Pseudomonas fluorescens. Promoters of EIL are fused to dapB (required for lysine biosynthesis), rendering strains prototrophic in the rhizosphere, where EIL are expressed, but auxotrophic in minimal growth media, where EIL are not expressed. Transposon mutagenesis, coupled with a simple screen for prototrophy in the laboratory, can quickly identify genes that regulate EIL–dapB fusions. A secondary screen for other members of the regulatory hierarchy was also feasible, which enabled these researchers to identify a regulatory network of seven regulators.

Bacterial secretion

A minimal Tat system from a Gram-positive organism: a bifunctional TatA subunit participates in discrete TatAC and TatA complexes Barnett, J. P. et al. J. Biol. Chem. 26 Nov 2007 (doi 10.1074/jbc.M708134200)

The Tat (twin-arginine) secretion system transports fully folded proteins across bacterial membranes. Gram-negative bacteria encode three subunits (TatABC) that are essential for secretion. TatABC form a substrate-binding complex that is thought to recruit a separate and heterogeneous TatA complex, which can form a translocation pore. Bacillus subtilis has three TatA and two TatC variants. However, it lacks tatB, as do all Gram-positive bacteria. The B. subtilis tatAd gene complemented a tatB mutant of Escherichia coli, indicating that tatAd can replace TatA and TatB. Importantly, a B. subtilis TatAdCd system secreted a Tat substrate upon expression in E. coli, even though the TatAd complex is discrete, in contrast to the heterogeneous E. coli TatA complex. Direct experiments in B. subtilis are now needed to resolve the current controversy over how Tat secretes proteins.