Summary
Toxin-antitoxin systems are found in many bacterial chromosomes and plasmids with roles ranging from plasmid stabilization to biofilm formation and persistence. In these systems, the expression/activity of the toxin is counteracted by an antitoxin, which in type I systems is an antisense-RNA. While the regulatory mechanisms of these systems are mostly well-defined, the toxins’ biological activity and expression conditions are less understood. Here, these questions were investigated with a type I toxin-antitoxin system (AapA1-IsoA1) expressed from the chromosome of the major human pathogen Helicobacter pylori. We show that expression of the AapA1 toxin in H. pylori causes growth arrest associated with massive morphological transformation from spiral-shaped bacteria to round coccoid cells. Coccoids are observed in patients and during in vitro growth as a response to different conditions such as oxidative stress. The AapA1 toxin, first molecular effector of coccoids to be identified, targets H. pylori inner membrane without disruption, as visualized by Cryo-EM. The peptidoglycan composition of coccoids is modified as compared to spiral bacteria. No major changes in membrane potential or ATP concentration result from AapA1 expression, suggesting coccoid viability. Using single-cell live microscopy, we observed that shape conversion is associated with cell division interference. Oxidative stress represses antitoxin promoter activity and enhances processing of its transcript leading to an imbalanced ratio in favor of AapA1 toxin expression.
Our data are in favor of viable coccoids with characteristics of dormant bacteria that might be important in H. pylori infections refractory to treatment.
Significance Statement Helicobacter pylori, a gastric pathogen responsible for 800,000 deaths in the world every year, is encountered, both in vitro and in patients, as spiral-shaped bacteria and as round cells named coccoids. We discovered that the toxin from a chromosomal type I toxin-antitoxin system is targeting H. pylori membrane and acting as an effector of the morphological conversion of H. pylori to coccoids. We showed that these round cells maintain their membrane integrity and metabolism, strongly suggesting that they are viable dormant bacteria. Oxidative stress was identified as a signal inducing toxin expression. Our findings reveal new insights into a form of dormancy of this bacterium that might be associated with H. pylori infections refractory to treatment.
Footnotes
↵+ I2BC/UMR 9198 CNRS- CEA -Université Paris Sud, Bat. 430, Orsay, FRANCE.