Antibiotic Resistome In Aquaculture Sediment And Agricultural Soil

Over the last decades, there has been growing concerns about antibiotic resistance. Thelong-term abuse of antibiotics has induced antibiotic resistant strains in humans and animals,which could be discharged into the environments, and result in the increase of antibioticresistance genes. On the other hand, high concentration of antibiotics used in aquaculture andmedical treatments can also induce antibiotic resistant genes when released to the environments.Antibiotic resistance genes are new environmental pollutants and can disseminate world widelyby Horizontal Gene Transfer (HGT).To explore antibiotic resistance genes in nonclinical environment, we isolated DNA directlyfrom sediment and soil samples, which were sampled from shrimp pond, paddy fields irrigatedwith fresh and waste water, following by construction of genomic DNA library and screening ofantibiotic resistance genes.From these three libraries, we identified28clones expressing resistance to antibiotics usedin this study.11clones expressed resistance to minocycline,8to tetracycline,4to kanamycin,and2to rifampicin. As for the oxycycline, streptomycin and chloramphenicol, each of them had1resistant clone respectively.Resistance to antibiotics can occur through a number of mechanisms, and the results are asfollows:(1) Bacterial resistance to the aminoglycside antibiotics was most frequently associatedwith the expression of modifying enzymes. Based on the predicted amino acid sequences of theresistance genes,4genes encoded proteins that were most similar to aminoglycosideacetyltransferases, aminoglycoside phosphotransferase and aminoglycoside adenylyltransferase,and1encoded protein that was most similar to efflux pump.(2) Tetracycline resistance genes weidentified had no homology to previously known genes at the amino acid level.5genes encodedproteins that were most similar to efflux pump,4encoded proteins that were most similar tomodifying enzymes,5genes’ products were most similar to proteins related to DNA replication,transcription or translation, and6clones contained ORFs most similar to hypothetical proteins ofunverified function in GeneBank. There were few reports about tetracycline resistance genesencoding modifying enzymes, and our results perhaps indicated the underestimation of thesegenes.(3) We identified1gene conferring resistance to chloramphenicol, whose protein wasmost similar to active efflux system TonB. The predominant mechanism of resistance tochloramphenicol is degration of antibiotics by acetylase, however, with the limitation of heterology expression, no genes encoding acetylase were found.(4)2genes, encoding proteinsmost similar to ADP-ribosyltransferase and ABC transporter related protein, were found conferresistace to rifampicin.Most of the antibiotic resistance genes we identified had low homology to proteins inGeneBank. This may imply that environment was a rich reservoir of anticiotic resistance genes,and we should expand the resistome and explore environmental diversity to prompt discover ofnew drugs. On the other hand, this indicated that antibiotic resistome can be used to survey theantibiotic resistance genes from uncultured microorganisms in the environment rapidly.