Very low concentrations of antibiotics and heavy metals in the environment contribute to antibiotic resistance, a new Swedish study has demonstrated.

The researchers, led by Professor Dan I. Andersson of Uppsala University, showed that plasmids containing genes that confer resistance to antibiotics can be enriched by very low concentrations of antibiotics and heavy metals such as arsenic, copper, silver, lead and mercury.

As much as half of the antibiotics used in treating humans and animals are excreted in the urine, in unchanged and active form.

"These antibiotics then disperse, usually in very low concentrations, through sewerage systems into water and soil, where they can remain active in the environment for a long period and so contribute to the enrichment of resistant bacteria," Professor Andersson pointed out.

Additionally, huge quantities of biocides and heavy metals are present in the environment. These come from natural sources as well as from contamination caused by human activities.

Plasmids — small fragments of DNA that can be transferred between bacteria — can contain not only antibiotic resistant genes but also genes conferring resistance to biocides and heavy metals. Results of the study revealed that very low concentrations of both heavy metals and antibiotics, separately or in combination, were able to enrich resistant plasmid-bearing bacteria.

"When these chemicals spread in the environment, bacteria with resistant plasmids will be selected. This indirectly results in antibiotic resistance increasing as well. What's more, in most environments there are complex mixtures of antibiotics, biocides and heavy metals that, together, have intensified combination effects," Andersson explained.

These findings suggest that substances other than antibiotics that are present in very small quantities in the environment can also drive development of resistance.

"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," the professor concluded.

The findings of the study have been published in the journal mBio.