Question

Use the figure below to describe Salmonella enterica respiration on tetrathionate. What is the advantage of using this anaerobic respiratory mechanism?

Transmission vi Gut bacteria release H2S. Hydrogen + sulfide--+ S203 (thiosulfate) (tetrathionate) ROS IV Mononuclear cell Neutrophil

Answer 1

Salmonella enterica is a Gram-negative bacterial pathogen that is a common cause of food-borne illnesses. It causes acute inflammation in the gut, but manages to survive among the normal gut flora.

The selective advantage of these microbes lies in the ability to use sulfur-rich compound called tetrathionate. They use tetrathionate for their energy metabolism under anaerobic conditions.

Intestinal tissue, normal bacterial flora and S. enterica help in the production of tetrathionate. Normal bacterial flora present in the intestine make large amount of hydrogen sulfide (H₂S). But this H₂S is quite toxic to the body tissues. As a result, the intestinal epithelial cells detoxify H₂S to thiosulfate (S₂O₃^2-). When S. enterica induce inflammation, neutrophils migrate into the intestinal lumen where they generate reactive oxygen species (ROS) that further oxidize thiosulfate to tetrathionate (S₄O₆^2-).

These progressions have been described in the figure.

(i) S. enterica uses its virulence factors (like flagella) to invade the epithelium and survive in mononuclear cells.

(ii) The ensuing inflammatory response causes the cells to release antimicrobial lipocalin-2 that isolates enterobactin (produced by microbiota). But it does not sequester salmochelin (produced by S. enterica). When inflammation is induced, neutrophils migrate into the intestinal lumen to generate ROS.

(iii) Abundant H₂S generated by gut bacteria is detoxified by the intestinal epithelial cells to thiosulfate.

(iv) ROS (chemically reactive chemical species containing oxygen) oxidizes the endogenous sulfur compound (thiosulfate) to generate respiratory electron acceptor (tetrathionate).

(v) This enables S. enterica to edge out of fermenting biota.

(vi) In this way, transmission of the pathogen occurs.

Advantages of using anaerobic respiratory mechanism

(1) The ability of S. enterica to use tetrathionate as an electron acceptor during anaerobic respiration helps them to compete with the biotic environment of intestine. Hence, anaerobic respiratory mechanism is useful for S. enterica to outgrow its competitors.

(2) The gut is abundant in ethanolamine, a derivative of a membrane lipid called phosphatidylethanolamine. Ethanolamine cannot be fermented and can only be utilized by respiration. Fermentative organisms present in gut flora are not able to oxidize ethanolamine. But S. enterica is able to oxidize it under anaerobic conditions. Hence, this ability of S. enterica again becomes a selective advantage for them which helps it to outgrow its competitors.