Copper from industrial pollution increases antibiotic resistance in forest floor soil bacteria

Emerging antibiotic resistance is a central threat to global food security and health. It has been well established that metal pollution can cause antibiotic resistance in agricultural settings, but data is lacking on most other habitats. This study investigates emerging antibiotic resistance in a European forest, providing new data towards the development of accurate environmental risk assessments.

Focusing on the longer-term impact of heavy metal pollution on a forest-floor microbial community, the researchers examined a forest in Sweden, in which a smelter had polluted the soils over an extended period. They chose to look at metal concentrations, microbial function and community resistance to copper and antibiotics in soil samples from this site. They also conducted a lab-based experiment to see whether further metal and antibiotic resistance could be experimentally induced in soil samples with differing levels of background metal pollution.

The European Commission is committed to reducing the emergence and spread of antimicrobial resistance, providing a framework to that end within the 2017 One Health Action Plan for Member States. The researchers highlight monitoring and research of the environment as a priority to inform risk assessments regarding human and animal health.

The researchers took soil samples along a transect running north and south from the Rönnskär smelter in Skellefteå, Sweden. The soil samples were taken from forest areas containing stands of Norway spruce, European birch and Scots pine.

Soil samples were analysed for biological, physical and chemical characteristics, including organic matter, water content, bulk density, fungal/bacterial growth rates, soil respiration rate and pH. Soil carbon and nitrogen were measured using an elemental analyser, whilst arsenic, cadmium, lead, copper, zinc, mercury and silver concentrations were analysed by inductively coupled plasma optical emission spectrometry – a technique used to detect chemical elements.

The most toxic form of copper ion found in the soil (Cu²⁺) was chosen to act as a proxy for copper toxicity. The researchers used chemical markers of microbial taxonomic groups (phospholipid fatty acids - PLFAs), to assess the diversity of the microbial community in the soil samples. These samples were exposed to different concentrations of copper, tetracycline and vancomycin, and following this exposure, the researchers noted the effect on soil bacterial growth rates, to highlight the resistance levels of the microbiome to these chemicals.

The study found that copper pollution has affected the microbial community structures of the Swedish forest soil samples, lowering their ability to decompose organic matter. Additionally, as the concentration of copper increased in the forest-floor soil samples, however (i.e. nearer to the smelter), so too did the microbial community resistance to this metal. Crucially, this pattern was mirrored by an induced resistance to tetracycline antibiotics.

However, adding copper to samples from the low and high end of the pollution gradient induced greater bacterial community resistance to copper, but did not enhance antibiotic resistance. The researchers note that the difference between the impact of metal levels on microbial antibiotic resistance, as compared with the lab-based results, suggests a difference in chronic versus short-term exposure within the soil’s bacterial community. The researchers highlight that the ‘small multi-drug resistance family (SMR)’ mechanism in bacteria – which are bacterial proteins that provide protection against metal ions and tetracycline antibiotics – could take time to initiate, conferring the different microbial patterns of short-term to long-term exposure to copper.

The study shows that metal pollution in temperate forests promotes soil bacterial antibiotic resistance, illustrating a previously unknown reservoir of microbial antibiotic resistance. The researchers recommend that environmental risk assessments for any activity giving rise to increased soil metal concentrations should also consider the induction of microbial antibiotic resistance.

Further information:

The study’s findings highlight that short-term, lab-based experiments are not the most suitable model for investigating soil bacterial community resistance to antibiotics – as a result of exposure to industrial metal pollutants. Instead, longer-term exposure models of bacterial communities – as found in soil communities polluted by nearby industrial sites – are required. This research would then allow researchers to observe the development of antibiotic resistance over a longer time frame, and ascertain how long before the suggested small multi-drug resistance family mechanism for antibiotic resistance in bacteria occurs.

Source:

Rabow, S., Soares, M., and Rousk, J. (2022) Can heavy metal pollution induce soil bacterial community resistance to antibiotics in boreal forests? Journal of Applied Ecology 60(2): 237–250.

To cite this article/service:

“Science for Environment Policy”: European Commission DG Environment News Alert Service, edited by the Science Communication Unit, The University of the West of England, Bristol.

Notes on content:

The contents and views included in Science for Environment Policy are based on independent, peer reviewed research and do not necessarily reflect the position of the European Commission. Please note that this article is a summary of only one study. Other studies may come to other conclusions.