Tetracycline Resistant Bacteria (TRB) And Tetracycline Resistance Genes (TRGs) In Cucumber Planting Soils Under Different Fertilizer Treatments

The increasing use of pharmaceutical antibiotics all over the world has been triggering the problem of antibiotic resistance, which is becoming a major threat to animal health. In this study, tetracycline resistant bacteria (TRB) and tetracycline resistance genes (TRGs) in cucumber rhizosphere and bulk soils under different fertilizer treatments were determined. The results are showed as below.(i) The population of TRBs in the manure from pig farm is much higher than that from free-range pig manure. Among 15 TRGs, seven of which (tetM, tetO, tetS, tetT, tetQ, tetB/P, and tetW) were positively detected in both pig manures using polymerase chain reaction (PCR) method; tetA gene was not detected in both samples; tetD and tetK genes were not found in the free-range pig manure and pig farms’s manure respectively.(ⅱ) The application of commercial organic fertilizer (FC and FB) increased the population of TRBs in cucumber bulk soil. FB treatment significantly stimulated the population of TRBs at low concentrations of tetracycline at 35 d and the TRBs at high concentrations of tetracycline at 65 d. FC treatment significantly increased the population of TRBs, which was declined as cucumber growth, and gradually reached to the control levels. The application of commercial organic fertilizer (FC and FB) also increased the population of TRBs in cucumber bulk soils at 35 d and 65 d. There was no significant differences between treatments and the control. The application of fresh manure fertilizer increased the population of TRBs, which was positively correlated with the levels of fresh manure fertilizer. The population of TRBs in soil under the treatment with different levels of manure fertilizer were higher than the control during the cucumber growth period. The normal fertilizer levels (PG8) had great impact on the population of TRBs at 65 d. The number of tetracycline resistant E. coli in soil increased with the increasing application of fresh pig manure. The number of tetracycline resistant E. coli in cucumber rhizosphere soil was fewer than that in bulk soil. The use of fresh manure fertilizer increased the population of tetracycline resistant Pseudomonas in both cucumber rhizosphere and bulk soil. The population of tetracycline resistant Pseudomonas in cucumber rhizosphere soil was higher than that in bulk soil,(ⅲ) Using RT-PCR method, the copies of tetZ and tetC genes in cucumber rhizosphere and bulk soils were quantitatively determined. The results showed that fertilizers, especially the fresh manure fertilizer, increased the copies of the two tetracycline resistance genes. The copies of the two tested genes in cucumber rhizosphere soil was significantly higher than that in bulk soil; With the passing of time, tetZ gene copies decreased in all treatments. The copies of tetC gene were less than the tetZ gene. Thirty-five days after fertilization, the tetC genes was undetectable in the bulk soil of the control and FT treatments;(ⅳ) Compared to different treatments of fertlizers, the use of fresh manure fertilizer significantly increased the diversity of TRBs in cucumber rhizosphere and bulk soils. The impact of fresh manure fertilizer on the diversity of TRBs was more obvious than other fertilizer treatments.