Distribution of tetracycline resistance genes and AmpC β-lactamase genes in representative non-urban sewage plants and correlations with treatment processes and heavy metals

The mixed development of livestock breeding and industry in non-urban zones is a very general phenomenon in China. Distribution of antibiotic resistance genes (ARGs) in non-urban sewage treatment systems has not been paid enough attentions. In this study, eleven tetracycline resistance genes (tetA, tetB, tetC, tetE, tetG, tetL, tetM, tetO, tetQ, tetS and tetX), four AmpC β-lactamase genes (EBC, MOX, FOX and CIT) and four heavy metals (Cu, Zn, Cd and Pb) were detected and analyzed in four non-urban sewage plants with different sewage sources and different treatment processes in Guangzhou. The results showed that tetA and tetC were the most prevalent tetracycline resistance genes with the same detection frequency of 85% and EBC was the most prevalent AmpC β-lactamase gene with a detection frequency of 75%. The relative abundance of tetracycline resistance genes was approximately 1.6 orders of magnitudes higher than that of AmpC β-lactamase genes in all samples. A/O was the most effective process for the non-urban sewage plant receiving industrial or agricultural wastewater. Sedimentation was the most key process to eliminate ARGs from liquid phase. Most ARGs were carried in excess sludge rather than effluent. Significant correlation was found between the tet gene and Zn (r = 0.881, p < 0.01), followed by the AmpC gene and Cu (r = 0.847, p < 0.01), the tet gene and Cu (r = 0.714, p < 0.05). Therefore, the pollution of ARGs in the sewage treatment systems of non-urban zones co-polluted by heavy metals should be paid more attentions.

Antibiotic resistance genes (ARGs) in duck and fish production ponds with integrated or non-integrated mode

Antibiotic resistance genes (ARGs) are emerging micropollutants with environmental persistence. Aquaculture environments are considered as potential reservoirs for ARGs pollution and horizontal gene transfer (HGT). This study analyzed water and sediment from eight culture ponds (integrated culture: duck-fish pond; monoculture: duck pond and fish pond) and a control pond (without any aquaculture activity) in Zhongshan, South China. Seventeen types of ARGs were detected in all ponds, which conferring resistance to four classes of antibiotics including tetracycline (tetA, tetB, tetC, tetE, tetG, tetL, tetA-P, tetM, tetO, tetS, tetW and tetX), AmpC beta-lactamase products (EBC and FOX), sulfonamide (sul1 and sul2) and erythromycin (ermA), with class 1 integron (intI1) as motility gene. The total concentrations of detected ARGs in culture pond water were much higher than control (about 1.6-4.0 times). Integrated culture showed lowest absolute abundance of ∑ARGs in water (3.686 × 10⁷ copies mL^-1) and the highest in sediment (4.574 × 10⁸ copies g^-1). Monoculture ponds showed higher relative abundance of ∑ARGs both in water (fish pond: 0.5149) and sediment (duck pond: 0.4919). As the main contributor to the ARGs abundance and significant correlations with ∑tet, ∑ARGs and intI1 (P < 0.01), tetA was suggested to be a potential indicator for the abundance of tetracycline resistance genes in these classes of aquaculture modes in the Pearl River Delta. This study provides a case for the ARGs abundance in aquaculture and as a reference for the upcoming health risk assessment in aquatic environment.

Occurrence of antibiotic resistance genes in landfill leachate treatment plant and its effluent-receiving soil and surface water

The antibiotic resistance genes (ARGs) from urban waste may spread to the environment with the discharge of leachate. Fifteen types of ARGs, including tetracycline, sulfonamides, AmpC β-lactamase and the class 1 integron gene were detected in the samples from the largest leachate treatment plant (LTP) in Guangzhou and its effluent receiving bodies (soil and surface water). The results showed that ARGs in leachates were in high levels and varied with seasons. The abundance of ARGs in the influent from high to low was in the turn of summer, winter, spring. About 2 to 4 orders of magnitude of ARGs were eliminated by the whole leachate treatment process. The predominant ARGs in the receiving soil were intI1, tetB, sul2, tetA and tetX, while those in the receiving surface water were sul2, intI1 and sul1, and the concentrations of ARGs in the receiving bodies were higher than those in the other natural bodies by 1 to 2 orders of magnitude. In addition, the results of bivariate correlation analysis showed that the abundances of ARGs (tetC, tetW, sul1, sul2, intI1 and FOX) were in significant correlation with the concentrations of heavy metals (Cu, Zn, Ni and Cr) (p < 0.05). LTPs are more likely to be sources of ARGs than wastewater treatment plant (WWTP) and need to be focused on.

Combined toxic effects of heavy metals and antibiotics on a Pseudomonas fluorescens strain ZY2 isolated from swine wastewater

A Pseudomonas fluorescens strain ZY2, isolated from swine wastewater, was used to investigate the synergistic effects of five heavy metals (Pb, Cu, Zn, Cr(VI) and Hg) on bacterial resistance to antibiotics. Results indicate that the combined effects of antibiotic type, heavy metal type and concentration were significant (p < 0.01). Cross-resistance to Hg and antibiotics was the most noticeable. Moreover, the resistance to Hg and cefradine or amoxicillin, and Cr and amoxicillin were synergistic for low heavy metal concentrations, and turned antagonistic with increasing concentrations, while the resistances to Cr or Cu and cefradine, Pb or Cu and amoxicillin, Cu and norfloxacin showed reverse effects. In addition, resistance to Zn and amoxicillin were always synergetic, while resistance to Pb and cefradine or norfloxacin, Cr or Hg and norfloxacin as well as all the heavy metals and tetracycline were antagonistic. These results indicate that bacterial resistance to antibiotics can be affected by the type and concentration of co-exposed heavy metals and may further threaten people’s health and ecological security severely via horizontal gene transfer.