Monthly Archives: October 2010

Spreading Salmonella—hyper-replicating bacteria act as a reservoir for dissemination

New research reveals how Salmonella enterica spread in the gut and gallbladder—a subpopulation of Salmonella primed for invasion rapidly replicate in the host cell cytosol such that bacteria-laden cells are extruded out of the epithelial-cell layer releasing invasive Salmonella into the gastrointestinal and biliary lumen. Leigh Knodler and colleagues write that other mucosal-dwelling pathogens could use this “host cell process as an exit strategy”.

Salmonella species can cause a range of infections from typhoid fever to food poisoning. Ordinarily, the intracellular bacteria Salmonella enterica resides and replicates within a membrane-bound vacuole in epithelial cells. During its life cycle, the bacteria are adapted to survive within a wide range of environmental niches within the human host (including cells such as enterocytes and macrophages and organs such as the spleen and gastrointestinal tract).

Knodler et al. observed a subpopulation of Salmonella that were ‘hyper-replicating'; these bacteria were doubling in number at almost five times the rate of the overall population of bacteria in the epithelial cell. Not only that, these bacteria were rapidly proliferating not in the Salmonella-containing vacuole, but in the host cell cytosol (which is believed to be nutrient rich) and were ready to invade other cells (they expressed type III secretion system 1 components and flagella, virulence factors that are required for invasion). Moreover, epithelial cells overloaded with these hyper-replicating cytosolic Salmonella were forced out of the apical side of the epithelial-cell layer—just as when dying cells are extruded out of the epithelium during the normal rapid turnover of epithelial cells that occurs to maintain the gut epithelium. Subsequently, invasive bacteria are released into the lumen and are primed and ready to infect new cells. The extruded host cells then die in a caspase-1-dependent manner and trigger the production of the proinflammatory cytokine interleukin 18—a process which could, in part, explain the high levels of mucosal inflammation observed in Salmonella infections of the gut and gallbladder.


ResearchBlogging.orgKnodler, L., Vallance, B., Celli, J., Winfree, S., Hansen, B., Montero, M., & Steele-Mortimer, O. (2010). Dissemination of invasive Salmonella via bacterial-induced extrusion of mucosal epithelia Proceedings of the National Academy of Sciences, 107 (41), 17733-17738 DOI: 10.1073/pnas.1006098107

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Walking with bacteria

They swim, they swarm, they twitch and glide…they even ride on comet tails, and now it seems that bacteria can ‘walk’ as Maxsim Gibiansky and colleagues demonstrate in their short but sweet research published in Science.

Gibiansky et al. studied the behaviour of Pseudomonas aeruginosa, a bacteria that is ordinarily found in soil and water, but has increasingly been associated with opportunistic infections in humans (and is a particular problem in those with cystic fibrosis). A key feature of P. aeruginosa is that these bacteria form multicellular, surface-bound communities called biofilms and are able to move within these communities by twitching motility owing to their type IV pili (hair-like structures on bacteria that can extend, tether to a surface and then retract to move bacteria along). The researchers studied microscopy movies of the P. aeruginosa biofilms and used computer software to track how the bacteria transitioned from planktonic state (that is, freely suspended in liquid) to the surface-bound biofilm.

Two different surface motility mechanisms were observed just after P. aeruginosa bacteria attached to a surface, but before a microcolony of bacteria were formed. The scientists studied mutant bacteria lacking flagella (a tail-like bacterial appendage that can also enable bacteria to move) that can only move using their type IV pili. These bacteria tended to ‘crawl’ in one direction when positioned horizontal to the surface and ‘walked’ in all directions when attached vertically to the surface by one end of the bacteria. Each movement mechanism was useful for surface exploration; crawling enabled directional movement across larger areas (6 μm distance) than walking, which enabled rapid exploration in local areas (up to 2 μm distance). Furthermore, these same movements were observed in wild-type bacteria. Moreover, the orientation of bacteria influenced biofilm morphology. Surface detachment was facilitated by type IV pili by tilting bacteria from horizontal to vertical positions and after bacterial division newborn bacteria detach and then ‘walk’ away. Finally, bacteria lacking type IV pili could neither ‘crawl’ or ‘walk’.

Scientific observations like this brevia report add to the understanding of bacterial behaviour in biofilms and could eventually lead to useful, new treatments against biofilm-forming pathogens.

 

ResearchBlogging.orgGibiansky, M., Conrad, J., Jin, F., Gordon, V., Motto, D., Mathewson, M., Stopka, W., Zelasko, D., Shrout, J., & Wong, G. (2010). Bacteria Use Type IV Pili to Walk Upright and Detach from Surfaces Science, 330 (6001), 197-197 DOI: 10.1126/science.1194238

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