Exposure to quaternary ammonium disinfectant promotes bacterial antibiotic resistance in soil

As antibiotics are restricted or even banned in animal production, more disinfectants are used to maintain proper hygiene and prevent bacterial diseases. The use of disinfectants is more than that of antibiotics, especially during the flu pandemic. Widespread quaternary ammonium disinfectants have exacerbated the problem of antimicrobial resistance due to co- and cross-resistance, but changes in disinfectant-induced antibiotic resistance in the soil environment are unclear.

In a study published in Agriculture, Ecosystems & EnvironmentA research group led by Prof. Yao Huaiying and Prof. Su Jianqiang from the Institute of Urban Environment of the Chinese Academy of Sciences has made progress in understanding the concentration gradients of quaternary ammonium disinfectants caused by the co-selection of soil antibiotic resistomes be induced.

“The appropriate concentration of disinfectant is one of the key factors in selecting resistant bacteria. In particular, concentrations at the environmental level require a special focus,” said Prof. Yao.

A short-term soil microcosm was established to study the profile dynamics of the antibiotic resistance gene (ARG) in agricultural soils treated with sulfamethazine (SMZ, 10 mg kg^-1) and gradient concentrations of benzalkonium chloride (0, 1, 10, 100 mg kg^-1BC(C12), a widely used class of quaternary ammonium disinfectants).

A unique concentration effect on changes in ARG profiles in agricultural soils exposed to different BC concentrations was found. With increasing BC concentration, the number of ARGs detected in soil increased while the overall abundance of ARGs decreased.

The highest number of ARGs was observed when exposed to high BC concentrations. Won’t a high concentration of disinfectant directly damage bacteria? The researchers speculated that this phenomenon is related to the adsorption and storage of BC in the soil.

Quaternary ammonium disinfectants are cationic surfactants, and their high adsorption affinity for soil particles leads to a decrease in their bioavailability, reducing mineralization and their acute toxicity. However, soil adsorption and isolation increases the persistence of BC, and the gradual release of BC induces the emergence of more antibiotic resistance.

“Co-selection for antimicrobial resistance occurs only when concentrations of bioavailable disinfectants are below the critical biocide concentration,” said Prof Yao. “The complexity of the soil environment and the concentration effect of the disinfectant have led to the development of soil ARG profiles.”

SMZ was added by the researchers to investigate whether SMZ and BC would synergistically promote co-selection of soil ARG against the background of combined pollution. Unexpectedly, SMZ and BC had no apparent interaction in shaping the ARG profile in soils. Compared to the broad spectrum fungicide BC, the specificity of SMZ significantly affected the microbial community. SMZ mainly affected vertical gene transfer from ARGs by altering bacterial host abundance.

This work contributes to future assessments of public health risks associated with quaternary ammonium disinfectant-induced antibiotic resistance. Disinfectants enter surface water or soil via sewage pipes and fertilizers. Dilution and degradation lead to environmental concentrations much lower than those used at the point of application. “Future research still needs to focus on the lower BC concentration range,” said Prof. Su.