Enzyme Vulnerability Opens Door to a Combined Diarrheal Vaccine

Enterotoxigenic E. coli and Shigella together cause hundreds of millions of infections annually and rank among the leading causes of diarrheal death, particularly in children. Decades of vaccine development efforts have stalled, in part because the usual targets vary too much between strains. New research from Washington University School of Medicine in St. Louis, published in PNAS, identifies a shared biological vulnerability in both pathogens that could make a single combination vaccine possible. 

To infect the gut, both bacteria must first penetrate a thick mucus layer that lines the intestine and keeps even healthy resident bacteria at bay. They accomplish this using closely related enzymes that cut through the main structural protein in gut mucus. Once past that barrier, the bacteria deliver the toxins that cause diarrhea.

The WashU Medicine team, working with collaborators at the University of Missouri and the International Centre for Diarrhoeal Disease Research in Bangladesh, found that antibodies targeting one shared region of these enzymes can neutralize all three biomolecules and block the bacteria from breaching the mucus barrier.

"For something so common and so deadly to young children, it's striking that we still don't have a vaccine for either of these pathogens," said co-senior author James M. Fleckenstein. "What's exciting here is that we've found a kind of Achilles' heel or weak point they share that we might be able to target to protect against both." 

Fleckenstein's lab had previously identified one such enzyme in ETEC, called EatA, which degrades the primary structural component of mucus. The team has now shown that two related enzymes—SepA and Pic, produced by Shigella and some other diarrhea-causing bacteria perform the same function. Working with co-author Ali Ellebedy, the team isolated antibodies from patients in Bangladesh naturally infected with ETEC and from volunteers intentionally infected in controlled studies. Antibodies blocking EatA also neutralized SepA and Pic.

Structural biologists at the University of Missouri then used cryo-electron microscopy to pinpoint exactly where the most effective antibodies attached to the enzymes, identifying a shared region across all three. That region now gives vaccine designers a precise target.

"This study establishes EatA as a viable vaccine candidate capable of providing protection across multiple pathogens," said co-senior author Zachary Berndsen. "By identifying the key regions of EatA that are targeted by neutralizing antibodies capable of inhibiting its enzymatic function, we've established a foundation for rational vaccine design."

The need extends beyond the developing world. Enterotoxigenic E. coli has caused large foodborne outbreaks in the United States, and because it is difficult to distinguish from harmless E. coli in most clinical labs, cases frequently go unrecognized. Antibiotic reliance to treat these infections also contributes to resistance. The team is now working toward vaccine development.

"These bacteria have evolved right alongside us, and they've gotten very good at breaching our defenses," Fleckenstein said. "If we can block that first step, we have a chance to stop these infections before they ever take hold."