The process was discovered following an infection of one patient, who had to undergo four amputations during a life-saving procedure after the disease turned deadly.
“Now that we have the capability of identifying strains and substrains of a bacterial species, it is highly likely we will find that polymicrobial infections are very common,” co-author Rita Colwell, a Distinguished University Professor at the University of Maryland Institute for Advanced Computer Studies, told Newsweek.
The patient described in the paper was diagnosed with necrotizing fasciitis, a rare but potentially fatal infection that requires fast antibiotic treatment and surgery. According to the Centers for Disease Control and Prevention (CDC), one in three people who receive treatment for the disease will die. Others may be left with life-long disabilities.
In this instance, medics used traditional diagnostics to identify the bacteria responsible for the infection—Aeromonas hydrophila—and were surprised when the patient’s condition took a turn that required life-saving amputations.
It was only when the study’s authors completed a genetic analysis of the patient’s bacterial culture that they identified differences strains within the culture.
“This research provides clear evidence that a very severe infection considered to be caused by a single species of a naturally occurring bacterium actually had two strains,” Colwell said in a statement.
“One of the strains produces a toxin that breaks down muscle tissue and allows the other strain to migrate into the blood system and infect the organs.”
Previous studies have shown that there is not one but two strains of Aeromonas hydrophila that can cause necrotizing fasciitis—necrotizing fasciitis 1 (NF1) and necrotizing fasciitis 2 (NF2). Both interact with the body in different ways but neither appears to cause a deadly infection on their own. It is only when both strains are present and cooperating that the most lethal infection occurs.
Now, researchers have discovered how.
Colwell and colleagues used mouse models to determine how genetic tweaks to NF1 and NF2 can affect each strain’s ability to cause infection and interact with the opposite strain.
The results show that NF1 on its own will remain localized until it is handled by the body’s immune system. When NF2 is on its own, it releases a toxin that weakens muscle tissue, which means it can spread to other parts of the body. But when the two are combined, NF2 will break down muscle tissue and NF1 will spread. NF1 kills NF2 so the latter remains localized. NF1, in contrast, becomes more dangerous and can cause life-threatening conditions in human hosts.
This may explain why antibiotics in these instances would prove ineffective.
“When we treat with a given antibiotic, we’re clearing an organism out of the body,” said Colwell. “But if there’s another organism that’s participating in the infection and that’s also pathogenic, then any antibiotic treatment that doesn’t also target that organism may just be clearing ground for it to grow like crazy.”
Instead, Colwell says a combination of antibiotics and other drugs may be needed.
The results of the study challenge traditional assumptions that infections are monomicrobial—i.e. caused by a single strain of microbe—and adds to the body of studies that show that often infections that are hard to treat and progress quickly are caused by strains of bacteria interacting with one another as opposed to a single strain.
Another example, reported by is Campylobacter jejuni—a bacteria that can cause food poisoning—and Pseudomonas putida—a bacteria that manages oxygen levels—combining together to cause an unpleasant stomach bug.
“We’re excited by this very elegant detective work,” said Colwell. “We now have the ability through metagenomics to determine the individual infectious agents involved in polymicrobial infections. With these powerful new methods we can determine how microbes work together, whether they’re bacteria, viruses or parasites.”
While this study shows how two bacteria might cooperate, it is “very likely” that more three or more strains could work together in a similar manner, Colwell told Newsweek.
