RESEARCH HIGHLIGHTS

BACTERIAL PHYSIOLOGY Ready, aim, fire!

The type VI secretion system (T6SS) that T6SS+ P. aeruginosa induced the same subcellular site and was not of Gram-negative bacteria powers the rounding of T6SS+ Vibrio cholerae spatially or temporally aligned with an secretion of effector proteins from cells — mainly those in direct contact attack. The pppA mutant also showed one cell into another cell, resulting with P. aeruginosa — but not of no preferential targeting of T6SS+ in antagonistic or even bacteriocidal T6SS− V. cholerae cells when these V. cholerae over T6SS− V. cholerae, activity between heterologous two V. cholerae strains were mixed suggesting that in the pppA mutant species. In addition, the system is individually with P. aeruginosa and the T6SS machinery is recycled at the associated with duelling between also, remarkably, when all three same location as it cannot be fully sister Pseudomonas aeruginosa cells, strains were mixed. Similarly, in disassembled and reassembled at a which involves directed T6SS quantitative competition assays, different site. Random firing was also firing into a target cell in the authors recovered more surviv- observed for a tagT mutant, indicating response to T6SS-mediated ing T6SS− V. cholerae cells than that the TagQRST cascade is involved attack. However, whether T6SS+ V. cholerae cells when these in sensing T6SS attack by a neighbour such targeted retaliatory strains were competed individually (possibly through TagQRST sensing firing occurs between with P. aeruginosa, indicating that of membrane perturbations) to mark heterologous species P. aerug­inosa T6SS activity targets the site of T6SS reassembly and thus and the signalling sys- cells that attack the bacterium enable a targeted response. tem behind such behav- first and does not affect ‘innocent This contact-dependent retali- iour were unknown. bystanders’. ation also occurred in response to Now, Mekalanos and The T6SS signalling cascade begins T6SS+ Acinetobacter baylyi but not colleagues show that with the cell envelope-associated in response to the T6SS− species P. aeruginosa uses the T6SS TagQRST proteins, which control Escherichia coli and is therefore likely to specifically target aggres- phosphorylation of Fha1 (a T6SS scaf- to be a general attack strategy used by sors that attack first, but leaves fold protein) to trigger T6SS assembly. P. aeruginosa. The authors propose ‘peaceful’ neighbouring cells intact. PppA-mediated dephosphorylation that such precise deployment of Duelling between sister cells of Fha1 induces disassembly. In sister T6SS effectors might have evolved is believed to be harmless because cell duelling assays, inactivation of to allow P. aeruginosa to kill harmful they encode immunity proteins pppA resulted in increased T6SS fir- aggressors while sparing harmless, to their own effectors; however, ing; however, this firing occurred at potentially helpful neighbours and Mekalanos and colleagues reasoned to reduce unnecessary energy that targeted retaliation would be expenditure on T6SS firing. biologically useful if unleashed P. aeruginosa T6SS Lucie Wootton against heterologous species, which activity targets cells that lack these immunity proteins. ORIGINAL RESEARCH PAPER Basler, M., Ho, B. T. Using target cell rounding (caused attack the bacterium first & Mekalanos, J. J. Tit-for-Tat: type VI secretion by the T6SS effector Tse1) as a system counterattack during bacterial cell-cell interactions. Cell 152, 884–894 (2013) marker for T6SS activity, they found

NATURE REVIEWS | MICROBIOLOGY VOLUME 11 | APRIL 2013

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