July 30, 2015 | BLOOMINGTON, Ind. -- Health experts have warned for years that the overuse of antibiotics is creating "superbugs" able to resist drugs treating infection.

But now scientists at Indiana University and elsewhere are finding evidence that an invisible war between microorganisms may also be catching humans in the crossfire.

This conflict is discussed in a recent article from IU biologist Farrah Bashey-Visser in the journal Philosophical Transactions of the Royal Society B.
"Bacteria aren't just evolving to resist new drugs, they are also constantly evolving due to competition with other microorganisms," said Bashey-Visser, an assistant scientist in the IU Bloomington College of Arts and Sciences' Department of Biology.

The result is that humans can be left trying to play catch-up.

The highly antibiotic-resistant bacteria MRSA, or methicillin-resistant Staphylococcus aureus, for example, has been shown to resist treatment in some cases due to competition with other microorganisms.

In the article, Bashey-Visser said a study recently conducted in Europe found a strain of MRSA became resistant to vancomycin after evolving within an infected host. A naturally occurring antibiotic reserved to fight the most serious infections, vancomycin was originally isolated by Eli Lilly and Co. in 1953 from soil collected by a missionary in Borneo.

The new mutant strain of MRSA in the overseas study overtook the original MRSA strain by producing a growth-inhibiting toxin. These toxins, called bacteriocins, are a common defense mechanism used by bacteria to compete against genetically similar microorganisms. However, in response to exposure to the bacteriocin, a third strain evolved resistance to the toxin and, coincidentally, to vancomycin.

This MRSA strain could resist the drug as a side effect of its evolutionary interactions within a host -- a process that differs from the more typical path where antibiotic resistance arises in direct opposition to treatment.

"The more scientists understand the processes that shape the evolution of potential pathogens, the more they will be able to predict the amount of time their treatments will remain effective," Bashey-Visser said.

Physicians commonly use a "reductionist approach" to fight infections, she added. They identify the pathogen, then do whatever is the most effective to stop it.

But, while effective, this approach may also have unintended consequences.

"We're realizing more and more that harmful bacteria are just one part of our body's ecosystem, or 'microbiota,'" she said. "Broad-spectrum antibiotics can wipe out numerous beneficial bacteria species too -- or worse, create an unprotected space where new species come in and wreak havoc."

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