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.”