| In recent years, the application amounts of antibiotics have greatly increased and lots of antibiotics were discharged into environment. Besides the chemical pollution, the ecological effects caused by antibiotics have arisen more concern. The increasing levels of antibiotics in environment may cause strong antibiotic selective pressure and accelerate the development and the dissemination of ARGs and antibiotic resistant bacteria, increasing the chances of human pathogens for acquiring resistance, which shade health risks to humans.Resistance determinants present in gene-transfer units are auto-replicate elements that can be maintained and dissemination in different microbial populations. ARGs can be excreted along the fecal waste of animals from farms and the sanitary wastewater from the sewage treatment plants, eventually they can get into the natural water system. Once those genes are in the environment, they can migrate between connected aquatic systems. The natural water body has become the reservoir for various ARGs. Therefore, it is absolutely necessary to investigate the environmental behaviors of ARGs in aquatic environment.Pearl River Delta region is seriously polluted by antibiotics according to our previous studies. In order to access the ARGs pollution in natural water system in Pearl River Delta region, 20 surface samples were collected along the Beijiang River during December 2009. Some important results are obtained as follows:1) The existences of antibiotic resistant bacterial were tested on culture-based approach. Strains were classified as"resistant"if they form colonies on agar with antibiotics. It was found that 17 of the 20 samples had bacterial colonies grown on LB agar with sulfadimidine (280 mg/L) and 14 samples had bacterial resistant to tetracycline (20 mg/L), while all the control experiments were negative.2) The efficiencies of two DNA extraction methods were evaluated. The yield and purity of the DNA extracted by water DNA kit was better than that extracted by SDS-CTAB method presenting that water DNA kit was a reliable method to extract DNA from various micro organisms in water.3)The occurrence of two sul-ARGs (sulI , sulII) and nine tet-ARGs, including four genes encoding efflux pumps (tetA, tetC, tetE and tetG), four genes encoding target modification (tetM, tetO, tetS and tetQ) and one gene encoding inactivating enzymes (tetX), were investigated using Polymerase Chain Reaction (PCR) technique. Both sulI and sulII were found in more than 85% of the samples. Most of tet-ARGs could be found in the water samples excluding tetE and tetS. The detection frequencies for tet A, tet C, tetG and tetO were over 70%. The PCR results have proved that the sul-ARGs and tet-ARGs studied in this work were commonly existed in the surface water of the Beijiang River.4) Two sul-ARGs and seven tet-ARGs in water samples according to the PCR presence/absence assays were further quantified by Q-PCR using a SYBR Green I approach. To compare the ARG levels among different sampling sites, the ARGs copies detected were normalized to the copies of 16S rDNA. The results showed that tetC and tetA were dominant pollutants and their abundances ranged from 0 to 1.32 for tetC and 0 to1.37×10-1 for tetA, respectively; the abundances of sulI (0-4.16×10-2) were higher than those of sulII (0-7.09×10-3) in the water of the Beijing River. Higher levels of sul-ARGs found in the Beijiang than those obtained from Northern Colorado River showed ARG pollution in PRD region should not overlooked.5) The sulI level at each site was correlated to the concentration of sulfa drug in river water, indicating that the existence of the antibiotics was one of the factors affecting the contribution of ARGs. Although no tetracyclines were detected, various tet-ARGs could be found in the water samples presenting that ARGs may be"persistent"and can be much hazardous compared to the antibiotics pollutants in the environments.
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