Conditional filamentation as an adaptive trait of bacteria and its ecological significance in soils.

Bacteria can regulate cell morphology in response to environmental conditions, altering their physiological and metabolic characteristics to improve survival. Conditional filamentation, in which cells suspend division while continuing lateral growth, is a strategy with a range of adaptive benefits. Here, we review the causes and consequences of conditional filamentation with respect to bacterial physiology, ecology and evolution. We describe four major benefits from conditional filamentation: stress tolerance, surface colonization, gradient spanning and the facilitation of biotic interactions. Adopting a filamentous growth habit involves fitness trade-offs which are also examined. We focus on the role of conditional filamentation in soil habitats, where filamentous morphotypes are highly prevalent and where environmental heterogeneity can benefit a conditional response. To illustrate the use of information presented in our review, we tested the conditions regulating filamentation by the forest soil isolate Paraburkholderia elongata 5NT . Filamentation by P. elongata was induced at elevated phosphate concentrations, and was associated with the accumulation of intracellular polyphosphate, highlighting the role of filamentation in a phosphate-solubilizing bacterium. Conditional filamentation enables bacteria to optimize their growth and metabolism in environments that are highly variable, a trait that can impact succession, symbioses, and biogeochemistry in soil environments.

Lytic transglycosylases RlpA and MltC assist in Vibrio cholerae daughter cell separation.

The cell wall is a crucial structural feature in the vast majority of bacteria and comprises a covalently closed network of peptidoglycan (PG) strands. While PG synthesis is important for survival under many conditions, the cell wall is also a dynamic structure, undergoing degradation and remodeling by 'autolysins', enzymes that break down PG. Cell division, for example, requires extensive PG remodeling, especially during separation of daughter cells, which depends heavily upon the activity of amidases. However, in Vibrio cholerae, we demonstrate that amidase activity alone is insufficient for daughter cell separation and that lytic transglycosylases RlpA and MltC both contribute to this process. MltC and RlpA both localize to the septum and are functionally redundant under normal laboratory conditions; however, only RlpA can support normal cell separation in low-salt media. The division-specific activity of lytic transglycosylases has implications for the local structure of septal PG, suggesting that there may be glycan bridges between daughter cells that cannot be resolved by amidases. We propose that lytic transglycosylases at the septum cleave PG strands that are crosslinked beyond the reach of the highly regulated activity of the amidase and clear PG debris that may block the completion of outer membrane invagination.

Genetic Determinants of Penicillin Tolerance in Vibrio cholerae.

Many bacteria are resistant to killing (tolerant) by typically bactericidal antibiotics due to their ability to counteract drug-induced cell damage. Vibrio cholerae, the cholera agent, displays an unusually high tolerance to diverse inhibitors of cell wall synthesis. Exposure to these agents, which in other bacteria leads to lysis and death, results in a breakdown of the cell wall and subsequent sphere formation in V. cholerae Spheres readily recover to rod-shaped cells upon antibiotic removal, but the mechanisms mediating the recovery process are not well characterized. Here, we found that the mechanisms of recovery are dependent on environmental conditions. Interestingly, on agarose pads, spheres undergo characteristic stages during the restoration of rod shape. Drug inhibition and microscopy experiments suggest that class A penicillin binding proteins (aPBPs) play a more active role than the Rod system, especially early in sphere recovery. Transposon insertion sequencing (TnSeq) analyses revealed that lipopolysaccharide (LPS) and cell wall biogenesis genes, as well as the sigma E cell envelope stress response, were particularly critical for recovery. LPS core and O-antigen appear to be more critical for sphere formation/integrity and viability than lipid A modifications. Overall, our findings demonstrate that the outer membrane is a key contributor to beta lactam tolerance and suggest a role for aPBPs in cell wall biogenesis in the absence of rod-shape cues. Factors required for postantibiotic recovery could serve as targets for antibiotic adjuvants that enhance the efficacy of antibiotics that inhibit cell wall biogenesis.