Even negligible levels of antibiotics in chicken blood can cause bacterial resistance and sicken people with hard-to-treat infections, suggests new research based on a study of antibiotic resistance in leech's gut.

Microbiologists have long known that the overuse of antibiotics in people and animals leads to antibiotic resistance or the proliferation of germs that do not respond to usual treatments. Antibiotic resistance can develop in the environment, too, as hospitals and pharmaceutical companies create favorable conditions for resistance by discharging large quantities of medications. But what concentration of antibiotic exposures boost the growth of resistant microbes in the wild? The new study, published in the journal mBio, suggests the threshold is low.

The researchers found resistant bacteria thriving in leeches exposed to less than four-hundredths of a milligram, per millilitre, of ciprofloxacin, an important antibiotic, in the environment. That level represents less than one per cent of the "clinical resistance breakpoint," or concentration in the gut that selects for resistance. For the study, the international team of researchers took a deep dive into the microbiome of blood-sucking medicinal leeches.

They found that low levels of antibiotics in the animal's environment improved the survival of antibiotic-resistant bacteria in its gut. Those resistant bacteria, in turn, displaced healthy bacteria. The findings could help explain why antibiotic resistant infections have been found in patients who undergo medicinal leech therapy. In addition, "it suggests that contamination with very low levels of antibiotics in other environments can lead to the increase in resistant bacteria," said microbiologist Joerg Graf at the University of Connecticut in the US who led the study.