Research by scientists in Australia could help open up new possibilities to treat enterohemorrhagic E. coli (EHEC) infections.

University of New South Wales (UNSW) microbiologists discovered a molecular pathway that controls Shiga toxin production. The findings were published in the journal Proceedings of the National Academy of Sciences (PNAS).

EHEC is a foodborne pathogen that releases Shiga toxins during infection and can result in a type of kidney failure called hemolytic uremic syndrome. It is also known as Shiga toxin producing E. coli (STEC). Children younger than five years of age and older people are at highest risk of developing infections.

Antibiotics not recommended
Jai Tree, senior author of the study, said findings were important because there is no commercially available treatment for EHEC infections.

“Antibiotic treatment of these infections is generally not recommended because antibiotics stimulate production of the Shiga toxin, leading to an increased risk of kidney failure, neurological damage, and death,” he said.

“The new pathway that we have found reduces toxin production and is not expected to be stimulated by antibiotic treatment. So, our results identify a potential new target for the development of drugs that can suppress Shiga toxin production during EHEC infection.

“It’s still early days, however, and we need to conduct a lot more research to understand if our findings apply to a broad range of clinical EHEC isolates and to both types of Shiga toxins produced by human EHEC isolates.”

EHEC outbreaks occur sporadically in Australia and worldwide, according to Tree.

“The most significant outbreak occurred in South Australia in 1995 and was caused by contaminated mettwurst, a semi-dry fermented sausage made from raw minced pork preserved by curing and smoking,” he said.

“In that outbreak, 143 people were infected – 23 of them suffered kidney and neurological damage. Many of these severe cases were in infants who suffered permanent kidney damage and later required kidney transplants. A four-year-old girl suffered multiple strokes and died three days after admission to hospital. This episode triggered a major food safety investigation and outbreaks since 1995 have been smaller.”

Tree also referenced a large STEC O104:H4 outbreak in Europe in 2011 linked to raw sprouts produced from fenugreek seeds.

“The strain in Germany was spread mostly via consumption of contaminated sprouts and in several cases, from close contact with an infected person. During this outbreak more than 4,000 people were infected and 50 people died.”

Well studied pathogen
Tree said it was the first discovery in almost 20 years of a new pathway that controls the Shiga toxins.

“In 2001, researchers at Tufts and Harvard universities first showed how production of the Shiga toxin was controlled by a bacterial virus, known as a bacteriophage, within the genome,” he said.

“We have extended that work to show a new mechanism of toxin control that is, surprisingly, buried within the start of the DNA sequence that encodes the Shiga-toxin messenger RNA – a working copy of the gene. We discovered a very short piece of the toxin messenger RNA is made into a regulatory non-coding RNA that silences the toxin and promotes growth of the pathogen.”

The findings were a surprise because Shiga toxin genes have been well studied, with almost 7,000 studies in the past 40 years.

“only recently have we been able use advances in RNA sequencing technology to detect the presence of the new regulatory non-coding RNA embedded within the Shiga toxin messenger RNA. This new regulatory non-coding RNA had been hiding in plain sight for almost 20 years,” said Tree.

He said the research is moving to the next stage of testing interventions.

“Our work shows a new mechanism for controlling toxin production that may be amenable to new RNA-based therapeutics to inhibit toxin production during an infection. We anticipate this would expand intervention options and potentially allow use of antibiotics that are currently not recommended because they stimulate Shiga toxin production.”

link between subtype and symptoms
Another piece of research looked at the association between Shiga toxin gene (stx) subtype and disease severity for 3,000 patients with E. coli O157:H7 infections in England from 2009 to 2019.

The STEC pathotype is defined by the presence of the genes encoding Shiga toxin type 1, type 2, or both. Stx1 and Stx2 can be further divided into subtypes Stx1a-1d and Stx2a-2g.

The study in Emerging Infectious Diseases found that STEC O157:H7 with stx profiles that included stx2a only or with other stx subtypes were more likely to be isolated from patients reporting bloody diarrhea, HUS, or both.

However, researchers also observed that strains of O157:H7 that had stx1a and stx2a only, or in combination with other stx subtypes, were significantly more associated with severe disease than those strains of STEC O157:H7 that had stx2c only.

This is significant as clinical and public health risk assessment algorithms in many counties, including the United Kingdom, are based on using detection of stx2 as a predictor of severe disease.