To survive in hostile environments, bacteria attach to one another, forming a supportive framework known as a biofilm. In biofilms of Salmonella  – a major cause of food-borne diarrheal illness – a key component of this framework is curli amyloid protein.

Now, in new research, scientists at the Lewis Katz School of Medicine at Temple University show that the repression of curli by an environmental factor in the intestine plays a critical role in freeing Salmonella bacteria of strain S. Typhimurium from their biofilms, enabling them to cause active infection. The environmental cue is nitrate, which both represses curli and modulates levels of an intracellular molecule known as cyclic-di-GMP. These events ultimately lead to the activation of S. Typhimurium flagella, which in humans is a critical step in allowing individual S. Typhimurium bacteria to swim toward and infect intestinal cells.

“It had been unclear what factors trigger S. Typhimurium to switch between a sessile, biofilm lifestyle to a motile, free-swimming lifestyle in the intestine,” explained Çagla Tükel, PhD, Director of the Center for Microbiology and Immunology at the Katz School of Medicine and senior investigator on the new study. “Our study shows for the first time that nitrate produced in the intestinal lumen of the host serves as an environmental cue driving this switch.”

The new findings were described online February 8 in the journal mBio.

Building on evidence that bacteria grown in biofilm conditions are able to sense nitrate levels in the surrounding environment, Dr. Tükel and colleagues began by investigating the effect of nitrate on S. Typhimurium biofilms and curli production in vitro. They found that when the biofilms were exposed to nitrate, not only did the average size of biofilm biomass decrease, but curli expression also dropped. Moreover, nitrate exposure led to a decrease in intracellular levels of c-di-GMP – a signaling molecule known to regulate genes involved in biofilm formation.

In experiments in mice, Dr. Tükel’s team observed that blocking the production of nitrate resulted in increased curli expression, confirming that S. Typhimurium biofilm integrity is regulated in response to nitrate produced by the host during intestinal inflammation. “Simply treating the animals with a nitrate inhibitor resulted in significant biofilm growth,” Dr. Tükel said.

The new research suggests that while biofilms help S. Typhimurium survive in the hostile environment of the intestinal tract, switching to a motile lifestyle could be advantageous, allowing establishment of infection and ensuring eventual transmission to a new host.

“Our identification of nitrate as an environmental cue gives us new insight into how bacterial pathogens establish infection and how they move through the gastrointestinal tract,” Dr. Tükel said.

Dr. Tükel and her team plan next to search for other inflammatory molecules produced in the host intestinal tract that might also play a role in helping S. Typhimurium establish infection. “We suspect that in addition to nitrate, other molecules in the metabolic landscape of the gut can also trigger the bacteria’s switch from biofilm to motile phase or vice versa,” she said.

Other researchers who contributed to the new study include Amanda L. Miller, Lauren K. Nicastro, Shingo Bessho, and Kaitlyn Grando, Center for Microbiology and Immunology, Katz School of Medicine; Aaron P. White, Vaccine and Infectious Disease Organization-International Vaccine Centre, Saskatoon, Saskatchewan, Canada; Yi Zhang, Fels Institute for Cancer Research and Molecular Biology, Katz School of Medicine; Gillian Queisser, Department of Mathematics, College of Science and Technology, Temple University; and Bettina A. Buttaro, Thrombosis Research Center, Katz School of Medicine.

The research was supported in part by funding from the National Institutes of Health grants AI153325, AI151893, and AI148770.