“Many strains of E. coli, including EPEC which is a major cause of diarrheal disease, are becoming increasingly resistant to the antibiotics commonly used to treat these infections,” Professor Begoña Heras at La Trobe University of Australia said in a study published in the journal Gut Microbes. Her remark reflects a growing alarm among microbiologists and other health experts: The emergence of multidrug-resistant pathogens that not only resist treatment but also wreak havoc on human health.
Enteropathogenic Escherichia coli (EPEC), responsible for a quarter of cases of severe diarrhea in infants and young children, has long perplexed scientists with its ability to destroy the lining of the gut. New research by a multidisciplinary team at La Trobe University sheds light on one piece of that puzzle the EspC toxin, a serine protease enzyme that acts somewhat like a pair of molecular scissors. Yet another E. coli enzyme, EspC, goes after intestinal epithelial cells, disassembling their protein scaffolding and putting into action a molecular subroutine of death. This damage results in the hallmark features of EPEC infections: dehydration, electrolyte abnormalities and death in severe cases.
The most critical aspect is the structural biology of EspC that provides insight into its unique architecture and potential mechanisms of action. It consists of a serine protease domain followed by a β-helix scaffold and other subdomains that assist entry into host cells. EspC is the only autotransporter described with dual Type V and Type III secretion systems. Our Type III system promotes EspC uptake into epithelial cells and consequently enhances EspC cytotoxic activity. In this regard, research comparing EspC to its homolog Pet cleaves cytoskeletal proteins (ex. superstructure and also associated with numerous sites of disruption of cellular integrity and basal detachment from adjacent cells (imaging B) (Kimbrough et al., 2017).
More broadly, and in addition to elucidating EPEC pathogenesis, this finding has considerable implications. Therapy against E. coli infections, based on antibiotic susceptibility, is now restricted to such broad spectrum classes that additional therapy is often not helpful in AMR infections. “We’re running out of options to treat bacterial diseases, with some bacterial pathogens now resistant to all antibiotics,” said Dr. Jason Paxman, who was a co-lead on the study. E. coli already has a short generation time and a high capacity of adaptation, and can also access resistance genes from the environment on plasmid vectors. Infections caused by resistant strains often rely on last-resort antibiotics, which are toxic and unsustainable options.
The various structures of EspC elucidate a blueprint for the development of niched therapies that neutralise this toxin while preserving the commensal gut microbiota. In this regard, distinctive characteristics of this protein such as β-helix domain and serine protease activity are the targets of the researchers and attention is directed towards the development of pharmaceutical agents, which needs to prioritize inhibition of the function of the protein and spare epithelia damage and consequent EPEC pathogenesis severity [56]. All of this is in line with this big vision for precision medicines, where you are taking aim at target pathogens, and avoiding the collateral damage, so to speak, to the microbiome.
The La Trobe crew paper additionally illustrates a pattern in fashionable science: the collaborative multidisciplinary method. The new work highlighted the critical need to connect the fields of structural biology, molecular microbiology and biochemistry to shine light on the mechanism of EspC, said Dr. Akila Pilapitiya, first author of the study. “By using a multidisciplinary approach, I was able to determine the 3D structure of EspC toxin, which shows how it’s built and the role each of its parts play to make it work,” she said. This study not only enhances our understanding of EPEC but establishes a framework for addressing the challenge posed by additional bacterial pathogens possessing complex virulence strategies.
If these are deep findings, the social implications are staggering. An estimated mortality of diarrheal disease, including that caused by EPEC, is 1.3 million children under the age of five annually, mainly in developing countries. These infections are also associated with extreme dehydration and malnourishment, contributing to health inequalities and placing additional burden on health systems. Wholesome “broad-spectrum” intervention could potentially save far more lives and affect the global burden of diarrheal disease if it could target the causes of EPEC virulence at its source.
Molecular studies of EspC and other related bacterial virulence factors will continue and we can only hope that these data will ultimately transcend to new ways to fight MDR micro-organisms that are currently evolving faster than we are. A big step forward in this work has been the characterization of EspC’s “molecular scissors” that enhances our knowledge of pathogenic E. coli and may offer some hope of how to tame this nasty bug.
