12 June 2023

Heavy metals in soil promote antibiotic resistance in bacteria


Bacteria isolated in contaminated soils appear to be more antibiotic resistant, says a study from New Zealand. A phenomenon that is still unclear and worrisome because of the risks to human health

by Matteo Cavallito


The growth of antibiotic resistance by bacteria could be a consequence of the elevated presence of heavy metals in soil. This is the hypothesis that emerges in light of recent research by scientists at Massey University in Palmerston North, New Zealand.

Mechanism behind this relationship is still unknown. The link between the two phenomena, however, appears to be evident. The study focused on pastoral soils in the Waikato region in the north of the country. Where a significant presence of cadmium and zinc was found.

The investigation

Soil samples with a different history of usage, including pastoral and arable with high levels of HMs (e.g. cadmium and zinc) were collected from the Waikato region (WR), as well as soil from an area of native bush (background) as control,” the study says.

Analyses conducted in the laboratory revealed different levels of resistance by the bacteria isolated in the different soils. The data, in particular, “showed there were significantly greater number of bacterial isolates resistant to antibiotics in soils with the higher initial levels of heavy metals.”

A mechanism still unknown

According to Barry Palmer, lead author of the research, quoted in Farmers Weekly magazine, the origin of this correlation is unclear. However, it is possible to speculate that the bacteria use similar mechanisms to process both heavy metals and antibiotics in the soil.

And more importantly, that they are able to transmit this ability “through a process known as horizontal gene transfer.”

“Cadmium resistance genes located on mobile genetic elements were able to be transferred horizontally form donor bacterial strains to recipients and the transconjugants showed resistance not only to Cadmium, Zinc and/or Mercury, but also to a range of antibiotics,” the study points out.

Antibiotic and fertilizer, a dangerous combination

Antibiotic resistance remains a major issue. As researchers from the School of Geographic Sciences at East China Normal University in Shanghai recently observed, the spread of resistant genes in pathogenic bacteria would generate problems in drug treatment. This endangers the health of humans and animals. Under scrutiny for some time has been the massive use of antibiotics themselves on farms and their transfer into the soil through animal manure.

The hypothesis of a link to the presence of heavy metals represents a new development that call into question another problematic factor: the use of phosphate-based mineral fertilizers.

“Soils in Waikato have been found to have cadmium concentrations five times higher than background levels after 70 years of superphosphate application,” Farmers Weekly further remarks. “Zinc levels have doubled over the past 30 years.” In addition, “A third of soils in New Zealand have too-high levels of phosphorous due to an oversupply of fertilisers, causing issues around water quality and toxic algae growth,” the magazine concludes.