Some researchers in the field of green and sustainable chemistry have proposed replacing volatile organic solvents in industrial reactions with a class of solvents called ionic liquids. Most of these organic salts are not volatile, so they have been touted as a safe, environmentally friendly alternative to conventional solvents. But a new study shows that they could create problems of their own when released into the environment. Chinese researchers report that one ionic liquid helps bacteria share an antibiotic resistance gene, causing the gene to proliferate among the microbes (Environ. Sci. Tech. Lett., DOI: 10.1021/ez500103v).
One proposed environmental advantage for ionic liquids is that most don’t emit harmful volatile organic compounds (VOCs). However, environmental scientists have only recently begun to study how these solvents might affect ecosystems after they’re disposed of. Daqing Mao, an environmental engineer at Tianjin University in China, and his colleagues recently investigated the toxicity of the solvents in bacteria. When they added the solvent to the microbes, they noticed that the levels of antibiotic resistance genes spiked unexpectedly.
Normally, scientists point to widespread usage of antibiotics as the main pathway for the increase in antibiotic resistance genes among bacteria, which can lead to dangerous, untreatable infections. But some chemicals, including detergents and pesticides, can enhance this proliferation. So Mao’s team wanted to see if the ionic liquids had the same effect.
To do this, the team filled flasks with water collected from an urban park in Tianjin, and added a commonly used ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate, to some of the flasks at levels up to 2.5 g/L. Then they sampled the flasks over a period two weeks, extracting the DNA from the samples and analyzing it using quantitative polymerase chain reaction.
A resistance gene to the antibiotic sulfonamide was up to 500 times more abundant in flasks treated with the ionic liquid than in flasks with just the park water. The treated flasks also had much higher concentrations of a gene encoding an integron, a mobile genetic unit that facilitates horizontal gene transfer among bacteria. When the scientists sequenced the DNA in bacterial strains isolated from the treated samples, they found that the integron carried the resistance gene, helping it hop between bacteria.
To determine just how strong an effect the ionic liquid had on integron-mediated gene transfer, the team exposed bacteria collected from the park that did not carry the integron gene to bacteria that did. Cultures that included the ionic liquid showed 88-fold higher antibiotic resistance gene transfer than cultures without the solvent.
Mao’s team thought the ionic liquid might enhance gene transfer by making the bacteria’s cell membranes more permeable. To test their hypothesis, they added a dye to the bacteria that indicates the permeability of cell membranes and examined them using flow cytometry. The membranes of ionic-liquid-treated bacteria were 230% more permeable than untreated ones.
“I would never have guessed that ionic liquids stimulate antibiotic resistance,” says Amy Pruden, an environmental engineer at Virginia Tech. Calling chemicals environmentally friendly can imply that it is fine to dump them down the drain, she says. But the study suggests that ionic liquids could create problems in wastewater treatment plants, which are critical points for controlling the spread of antibiotic resistance genes. “It’s ideal to look at these issues before we start using ionic liquids widely,” she says.