According to University of Connecticut scientists, tiny levels of antibiotics found in the environment can encourage bacterial resistance. This finding, published in mBio today, is based on research with leeches.
UConn microbiologist Joerg Graf is intimately familiar with leech guts. He studies how bacteria can live inside animals' digestive tracts without making the animals sick, and leeches are a perfect study case; with only two major types of bacteria inside them.
The genesis of this study was driven by two events. First, one of his graduate students had difficulty growing a strain of Aeromonas that usually does really well in leeches. Second, plastic surgeons began to report problems with patients getting infected with Aeromonas bacteria resistant to ciprofloxacin. The patients had all been treated with leeches to improve blood flow at surgical sites.
The leeches used in medical settings are raised on specialty farms and fed a controlled diet, typically poultry blood. Graf and his team thought the chicken blood could be contaminated with antibiotics and analyzed the gut contents of leeches from the farm that fed poultry blood, and found traces of both ciprofloxacin and enrofloxacin, Cipro's veterinary analog. But the amount of antibiotic present was low. Just around 0.01 micrograms per milliliter, four hundred times less than the concentration a bacteria must survive in order to be considered "resistant". Still, leeches from other farm—in whose guts the test strain of Aeromonas still grew fine—didn't have any detectable antibiotics in their intestines.
The team then isolated strains of Aeromonas from leeches contaminated with antibiotics, and sequenced their genomes. They found that these genomes contained the three bits of DNA, two genes with mutations and a plasmid, necessary for resistance to Cipro.
When the Cipro-resistant Aeromonas were grown alongside the test strain of Aeromonas in a clean lab medium or inside a leech, the test strain grew all over them. But if there was even a tiny bit of antibiotic added into the mix, as low as 0.01 micrograms/mL, the Cipro-resistant variety dominated.
"This was the first time such low levels of antibiotics were observed doing this in the natural environment. Levels of antibiotic 100x below the clinical breakpoint allowed the resistance bacteria to outgrow the sensitive bacteria a million-fold!" Graf says. This is worrisome, because ciprofloxacin and related drugs don't breakdown very well in the environment. They persist. They're found in hospital wastewater, in effluent from pharmaceutical manufacturers and farms, sewage, as well as poultry blood.