Touch-Tone Tran­scrip­tion: A Pathogen's Sense of Touch

by Janie

In a world buzzing with sen­sory in­put of all stripes and col­ors, a bac­terium isn't one to miss out on any of the lat­est sur­round­ing scoop. The sense of touch is not ig­nored. Me­chan­i­cal feed­back in­flu­ences how a bac­terium might move or stay in place, whether the bug in ques­tion is a piezo­phile at the bot­tom of the ocean or a pathogen on the sur­face of a tar­get cell.

A re­cent pa­per by Sirisaeng­taksin et al. delved into the path­way from me­chan­i­cal stim­u­lus to vir­u­lence in en­tero­he­m­or­rhagic E. coli O157:H7 (EHEC). EHEC causes di­ar­rhea by form­ing at­tach­ing and ef­fac­ing (A/E) le­sions on gut ep­ithe­lia, ad­her­ing to the cells atop an el­e­vated pedestal of poly­mer­ized actin. This sticky trou­ble is due in large part to EHEC's "lo­cus of entero­cyte ef­face­ment" (LEE). LEE is a path­o­genic­ity is­land en­cod­ing a type III se­cre­tion sys­tem, which in­jects ef­fec­tor pro­teins into the host cell to fa­cil­i­tate at­tach­ment and cy­to­skele­ton-hi­jack­ing. Me­chan­i­cally stim­u­lated ac­ti­va­tion of LEE re­quires the mas­ter reg­u­la­tor Ler and its own global reg­u­la­tor, GrlA.

Fig­ure 1. Elec­tron mi­cro­graph of at­tach­ing and ef­fac­ing (A/E) le­sions on ep­ithe­lial cells. Scale bar: 0.5 µm. Source. Frontis­piece.

When EHEC is atop a hard sur­face, GrlA is me­chan­i­cally stim­u­lated and goes off to ac­ti­vate ler tran­scrip­tion. By gen­er­at­ing mu­tant forms of GrlA, the in­ves­ti­ga­tors dis­covered how this me­chan­i­cal sens­ing works. Prior to ac­ti­va­tion, GrlA is as­so­ci­ated, through its ba­sic C‑term­inal residues, to acidic lipids in the mem­brane, for ex­am­ple car­di­olipin. Docked at the mem­brane, GrlA is poised to sense and re­spond to me­chan­i­cal stim­uli.

Upon me­chan­i­cal stim­u­la­tion, GrlA re­lo­cates to the cy­to­plasm to ac­ti­vate ler. The au­thors showed that this hap­pens by ob­serv­ing GFP-GrlA lo­cal­iza­tion in three con­di­tions rang­ing from low to high me­chan­i­cal stress − plank­tonic EHEC, sur­face-at­tached EHEC, and sur­face-at­tached EHEC ex­posed to fluid flow. The mem­brane-se­questered GrlA in plank­tonic EHEC re­dis­trib­uted to the cy­to­plasm when the cells were sur­face-at­tached, and to an even higher de­gree when the cells were ad­di­tion­ally ex­posed to fluid shear force (mim­ic­k­ing con­di­tions in the GI tract). Thus, GrlA's ler-ac­ti­va­tion hinges upon a force-trig­gered mi­gra­tion be­tween mem­brane and cy­to­plasm. This re­lo­cal­iza­tion hap­pens rapidly, ten min­utes af­ter me­chan­i­cal stim­u­la­tion, and in­duc­tion of ler fol­lows hot on its heels.

The au­thors also parsed apart the two in­hibitory path­ways of GrlA − mem­brane se­ques­tra­tion and the bind­ing of re­pres­sor GrlR − and demon­strated that the two are in­de­pen­dent. Elim­i­nat­ing ei­ther mode of in­hi­bi­tion caused sig­nif­i­cantly in­creased ler tran­scrip­tion, and the EHEC cells stuck to HeLa cells like glue. Wild-type EHEC could col­o­nize ze­brafish lar­vae like nobody's busi­ness, but EHEC with­out GrlA or GrlR or with mu­tated GrlA were less able. Al­to­gether, this GrlA story sug­gests that for EHEC to col­o­nize and per­sist within the tu­mult of a GI tract, it must finely tune into me­chan­i­cal cues from its en­vi­ron­ment.