Antibiotics are primarily secondary metabolites or artificially synthesised analogues produced by microorganisms (including bacteria, fungi, and Actinomycetes) with anti-pathogen or other activities. Recently, the extensive abuse of veterinary antibiotics in the fields of excessive medical treatment and animal husbandry has contributed to the surge of antibiotic residues in the environment and the emergence of many drug-resistant bacteria. Antibiotic resistance genes (ARGs) in the soil primarily come from the "internal resistance" of bacteria in the environment, soil microorganisms, human or animal manure, etc., and the main way to enter the soil is animal manure fertilisation. Most antibiotics used to prevent or treat infections in humans or animals and to promote fast growth in livestock are only partially metabolised and subsequently excreted in the excrement (Martinez 2009). Human activities have a serious impact on the microbial community and ARGs (Yan, et al. 2020). Antibiotics have become the new cause for concern in environmental pollution. Increase in the number of antibiotics produced, the emergence of ARGs in human body, the effect of antibiotic treatment, and the stability of the ecological environment has caused serious damage. ARGs as a new type of environmental pollutant was formally defined in 2006 (Pruden, et al. 2006), and has attracted increasing attention from the public.
Manure from the aquaculture industry contains large amounts of antibiotic residues, and fertilising the soil with this manure induces soil microorganisms to produce a large number of resistance genes. ARGs migrate through horizontal gene transfer of mobile genetic elements (MGEs) such as plasmids, transposons, and integrons, leading to the spread of antibiotic-resistant bacteria. Horizontal transfer of ARGs is the transfer of mobile genetic factors through conjugation, transformation, transduction, etc. The mobile genetic factors are primarily integrons, plasmids, transposons, etc. Among them, the integron, as a gene-assisted capture system, is located on plasmids and chromosomes, and is also an important molecular element for the horizontal transfer of resistance genes (Yang, et al. 2013). Owing to the unique structure that can capture and integrate resistant genes, bacteria develop resistance or even multi-resistance, such that integrons are considered to play an important role in the propagation of ARGs (Ruiz, et al. 2003). (Zhu, et al. 2019)analysed residual levels and spatial distribution characteristics of four types of quinolone antibiotics. The results showed that the detection rates of fluxacin, ciprofloxacin, and enfloxacin were 100%, and that of ofloxacin was 98.6%. Quinolone antibiotics were widely distributed, and the concentration of antibiotics was the highest in farmland soil along the middle reaches of the Fenhe River. Four types of quinolone antibiotics and one type of tetracycline antibiotic were detected in eel culture wastes, and the correlation between residual antibiotics and ARGs was not significant (P ≥ 0.05); however, the change in microbial community had a significant impact on the occurrence of environmental ARGs (P < 0.05) (Huang, et al. 2020). The results showed that the pollution of ARGs in eel culture waste is extremely serious, and the change of microbial community has a significant effect on environmental antibiotic resistance. Currently, there are numerous reports on the distribution characteristics of ARGs in soil. (Ji, et al. 2012) detected eight types of resistance genes in soils around breeding farms in Shanghai, and the ARGs with high abundance were sulfonamide ARGs SulA and SulⅡ. (Wu, et al. 2010) analysed soil around farms in Beijing, Tianjin, Jiaxing, and Zhejiang and found 15 tetracycline resistance genes, among which the most abundant ARGs were tetM, tetO, tetQ, tetW, and tetT. Quantitative polymerase chain reaction (PCR) was used to detect tetracycline and macrolide ARGs in manure application soil in Harbin, and it was found that tetM and tetW tetracycline ARGs and emrB and emrF genes were in high abundance (Peng, et al. 2019). (Liu, et al. 2021) showed that 17 strains of Escherichia coli showed varying resistance to β-lactams, aminoglycosides, tetracyclines, quinolones, and chloramphenicol; however, they were primarily resistant to β-lactams and tetracyclines. The detection rate of tetA resistance genes was relatively high, indicating overuse of cephalosporins and tetracycline. According to the results, the abundance of ARGs in both the farm and fertilised soil increased.
To study the distribution characteristics and correlation of ARGs and MGEs in facility soils in Liaoning Province. In this study, the distribution characteristics of ARGs and MGEs in agricultural soils were analysed in Panjin, Jinzhou, Chaoyang, Tieling, Xinmin, Shenyang, Fushun, Liaozhong, and Zhuanghe counties of Liaoning Province.