A gene Salmonella picked up from a virus apparently enables it to tweak the human gut to get what it needs to outcompete its neighbors, according to the latest study published in mBio this week.
A multi-drug resistant strain of Salmonella that caused an outbreak among both humans and cattle in the 1980's carried a gene called sopE. sopE is not your average Salmonella gene: it's a virulence factor the bacterium acquired from a phage. Overall, sopE is present in only a small fraction of Salmonella isolates, but it was all over the human-and-cattle-epidemic strain like white on rice. It's an effector protein of the invasion-associated type III secretion system and it's a potent inducer of inflammation in the intestine, but what sort of advantage does it offer to Salmonella that made it so successful in this outbreak? What does sopE do, exactly? Figuring this out could help control outbreaks in the future (and rest assured there are plenty of Salmonella outbreaks in our future).
Researchers at UC Davis (Go Aggies!) studied the action of sopE in mice. They found that by inducing the host to make inducible nitric oxide synthase, sopE enhances the production of host-derived nitrate, enabling the bacterium to live off nitrate, a valuable electron acceptor in this dog-eat-dog anaerobic habitat. This provides a growth advantage for Salmonella, boosting its numbers in the lumen.
In a tidy little turn of events, when Salmnonella has access to nitrate in the gut it turns down the respiration of another, less valuable electron acceptor: tetrathionate. Without sopE, Salmonella uses tetrathionate for its "breathing" needs, expressing genes for tetrathionate respiration so that it can survive in the anaerobic gut. But when sopE is on hand to help Salmonella extract nitrate from the host, the bacterium reduces the expression of tetrathionate respiration genes. This ensures the bacterium uses the optimal electron acceptor, nitrate, and maximizes its growth rate, allowing it to outcompete other bacteria in the gut.
Salmonella gooses the human host to boost its own fitness
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