Salmonella enterica serovar Typhimurium bacteria commonly cause human gastroenteritis. The CDC estimates Salmonella bacteria cause about 1.35 million infections, 26,500 hospitalizations, and 420 deaths in the United States every year. Food is the source for most of these illnesses.

There are many studies that have shown that Salmonella use a “run-and-tumble” method of short swimming periods, but how they move within the gut is not well understood. Now, scientists at the National Institutes of Health (NIH) and their colleagues believe they have identified a Salmonella Typhimurium protein, methyl-accepting chemotaxis protein C (McpC), that allows the bacteria to swim straight when they are ready to infect cells.

Their findings were recently published in the journal Nature Communications, in a paper titled, “Regulatory protein HilD stimulates Salmonella Typhimurium invasiveness by promoting smooth swimming via the methyl-accepting chemotaxis protein McpC.”

“Chemotaxis allows motile bacterial cells to navigate through complex environments, such as the mammalian gastrointestinal tract. In the model organisms Escherichia coli and Salmonella, bacteria swim in a random pattern produced by alternating counterclockwise (CCW) and clockwise (CW) flagellar rotation,” the researchers wrote. Chemoreceptors detect attractants or repellents and stimulate responses through a signaling cascade that controls the direction of the flagellar motor. Attractant gradients extend the length of time flagellar motors rotate CCW, resulting in more smooth swimming in a favorable direction, while repellents cause an increase of CW rotations, resulting in more tumbling and changes in direction. Chemotaxis is required for Salmonella enterica serovar Typhimurium (STm) growth in the lumen of the inflamed gut, however, non-chemotactic smooth swimming mutants are more invasive suggesting that repression of chemotaxis could be advantageous under certain conditions.”

The researchers hypothesize that controlled smooth swimming could be a widespread bacterial infection strategy. Similar smooth swimming behavior can be seen in unrelated enteric bacteria, such as Vibrio, which can cause infection when undercooked seafood is eaten. These findings may inform the development of novel antibiotics.

“Our results highlight the importance of smooth swimming for Salmonella Typhimurium invasiveness and indicate that McpC can act via a ligand-independent mechanism when incorporated into the chemotactic receptor array,” concluded the researchers.

The study authors suggest that McpC is a potential target for developing new antibacterial treatments to hinder the ability of Salmonella Typhimurium to infect intestinal epithelial cells and colonize the gut.

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