In some pathogenic bacteria, antibiotic resistance is caused by genes carried on plasmids – small, transferable fragments of DNA. The plasmids typically contain multiple genes that confer resistance not only to antibiotics but also to the effects of biocides and heavy metals.
In a series of in vitro experiments, researchers at Uppsala University in Sweden have shown that very low, sublethal levels of antibiotics and heavy metals (such as arsenic) in the environment can select for bacteria carrying multidrug-resistant plasmids.
“These results are worrying,” says Dan Andersson, who led the research, published in mBio
(online, 7 October 2014). “The results underline the importance of reducing the use of antibiotics, but also suggest that our high use of heavy metals and biocides in various contexts should decrease too.”
 Gullberg E, Albrecht LM, Karlsson C et al. Selection of a Multidrug Resistance Plasmid by Sublethal Levels of Antibiotics and Heavy Metals. mBio 2014;5 doi:10.1128/mBio.01918-14 (accessed 7 October 2014).