Effect Of Ciprofloxacin On Diversity Of Resistance Genes And Bacterial Communities In Soil

Antibiotics and heavy metals are often used as feed additives in large-scale to livestock and poultry farming processes,resulting in residues of antibiotics and heavy metals in livestock and poultry manure.Antibiotics and heavy metals can exert selective pressure on microorganisms to induce the production of new pollutant antibiotic resistance genes（ARGs）.However,the incorporation of contaminated organic manures in agricultural farmland becomes an important repository for ARGs.ARGs may enter the human body through the food chain,threatening human health and have certain ecological risks.Therefore,it is great significance to study the distribution of ARGs in soil system and plant endophytes.In this present study,in-situ pot experiment was performed to investigate the effects of ciprofloxacin（CIP）at different concentrations viz.(0,20 and 80 mg?kg^-1)on ryegrass endophytic bacterial,soil bacterial community and physico-chemical properties of contaminated soil.The obtained main results and conclusions are as follows:（1）The effect of CIP on the ryegrass was high concentration(80 mg?kg^-1),which inhibited the growth and reduced the plant height（21.8%）,the number of tillers（29.4%）and the biomass（24.6%）of ryegrass respectively compared to the control,indicating that high concentrations of CIP showed some biological toxic effects on the growth of ryegrass.（2）During the mature period（D50）,the abundances of ARGs in endophytes of ryegrass decreased with the increase of CIP concentration,and there was no difference between treatments in the harvest（D75）.The total relative abundance of copA and pcoA in endophytes of ryegrass in the harvest period was reduced by 28.2%–54.5%.The abundance of ARGs was lowest in the treatment with low concentration of ciprofloxacin,but the abundance of CRGs was the highest,indicating that low concentrations of CIP produced selective pressure on copper-tolerant microorganisms,leading to an increase in the abundance of CRGs.（3）The effect of CIP on endophytic bacteria in ryegrass was small,and the endophytic bacteria in the treated ryegrass had similar changes.Cyanobacteria,Proteobacteria and Firmicutes are the dominant microorganisms of ryegrass endophytes.The high concentration of CIP(80 mg?kg^-1)had changed the soil microbial community structure,which significantly differentiated the soil microorganisms treated with high concentrations of CIP followed by other treatments,in which increased the abundances of Acidobacteria and Thaumarchaeota,but decreased the Firmicutes abundance.（4）The differences in resistance genes among different soil samples were mainly caused by differences in microbial community,pH,and TN.Potential host strains of resistance genes in soil and ryegrass endophytes belong mainly to Firmicutes and Actinobacteria.IntI2 was positively correlated with four genes（copA,pcoA,qnrA,and qnrS）and three MGEs（intI1,ISCR1,and Tn916/1545）.Therefore,intI2 plays an important role in the spread of resistance genes.