Most E. coli bacteria are harmless and an important part of a healthy intestinal tract, said co-author Edward Dudley, associate professor of food science at Penn State’s College of Agricultural Sciences.
However, some are pathogenic and can cause illness by producing toxins that result in bloody diarrhea, severe abdominal cramps or impaired kidney function.
In some cases, the disease progresses to hemorrhagic colitis (HC) and hemolytic uremic syndrome (HUS), due to the expression of one or more Shiga toxins (Stx).
Researchers said the study may help doctors to predict how an E. coli-infected patient will fare by evaluating a stool sample and analyzing the presence or absence of various strains of non-pathogenic E. coli.
However, this would require follow-on studies which are being done in Dudley's laboratory, determining which non-pathogenic strains amplify production of E.coli O157:H7 "Shiga" toxin.
Pathogenic and non-pathogenic interaction
Researchers co-cultured the pathogenic E. coli O157:H7 serotype with a non-pathogenic strain of the bacteria and inoculated mice over a four year period in work supported by the US Department of Agriculture (USDA).
These mice got much sicker than those infected with just the pathogenic strain.
An estimated 265,000 STEC infections occur each year in the US. STEC O157 causes about 36% and non-O157 STEC cause the rest, according to the Centers for Disease Control and Prevention.
Dudley said within the intestine, each person carries several hundred different types of bacteria, including E. coli, which comes in a large number of varieties.
“These range from organisms that just naturally colonize our intestines and provide us with benefits to organisms like the ones I focus on that have evolved to be very virulent,” he said.
"This research suggests that some strains of harmless E. coli in our intestines can interact with pathogenic E. coli in ways that will either increase or decrease how much toxin the pathogen produces. And that may dictate how sick one gets with an E.coli infection, or even if an infection proves to be fatal."
How findings were discovered
The study characterized an E. coli O157:H7 strain, designated as PA2, which belongs to the hypervirulent clade 8 cluster.
However, during co-culture with the non-pathogenic strain E. coli C600, PA2 produced Stx2a levels that were two to 12-fold higher than those observed during co-culture with Sakai and EDL933.
Germ-free mice co-colonized by PA2 and C600 showed greater kidney damage, increased Stx2a accumulation in feces, and more visible signs of disease than mice given PA2 or C600 alone, found the study.
Researchers examined the animals' kidneys, intestines and livers after death, using molecular biology and DNA-sequencing techniques and biochemistry procedures.
Dudley said one of the issues with the pathogen is that by the time people are infected, not much can be done for them.
"We can't use antibiotics because antibiotics make E.coli O157:H7 more virulent - the only treatment is just to monitor the individual and make sure he or she doesn't become dehydrated and be sure the kidneys stay functional,” he said.
"What our findings suggest is that by looking carefully at the gut flora of someone who is sick - while we can't necessarily treat them right away - we soon may be able to make a prediction about what the outcome of the disease is going to be.
“We can see if the patient is going to clear the organisms and have mild symptoms, or if they are likely to have something that is more serious."
“Co-culturing Escherichia coli O157:H7 with a non-pathogenic E. coli strain increases toxin production and virulence in a germ-free mouse model”
Authors: Kakolie Goswami, Chun Chen, Lingzi Xiaoli, Kathryn A. Eaton and Edward G. Dudley