Oxytetracycline is a tetracycline that is widely used to treat animals during poultry farming in China.Approximately 25% to 90% of antibiotics administered to animals are subsequently excreted,after which they enter the soil as manure.Due to the persistence and accumulation of such antibiotics in soil,oxytetracycline pollution may lead to a series of changes in soil microbial concentrations,microbial species,microbial antibiotic resistance,microbial community structure,and soil enzyme activities.In this study,relationships between antibiotic exposure dosages and the concentrations,species and distribution characteristics of antibiotic resistant bacteria and the activities of soil enzymes in wheat rhizosphere soil were investigated.To accomplish this,isolation and liquid cultivation methods were employed in the presence of different exposure dosages of oxytetracycline.Exposure to various concentrations of oxytetracycline resulted in a significant increase in the concentrations of oxytetracycline resistant bacteria to one to three times that of control samples that were not exposed to antibiotics.Exposure also led to an increase in the concentration of oxytertracycline-streptomycine double resistant bacteria to one to two times that of control samples that were not exposed to antibiotics.Additionally,the majority of double resistant bacteria(60%—80%) were colored bacteria.As the microbial community structures changed in response to oxytetracycline exposure,the activities of some soil enzymes such as alkaline phosphatase,acidic phosphatase and dehydrogenase in the wheat rhizosphere decreased;however,there was no clear relationship between the change in enzymes and oxytetracycline dose.A total of 50 strains of dominant antibiotic resistant bacteria were isolated from wheat rhizosphere soil samples treated with different doses of oxytetracycline.These bacteria were assigned to five groups,Actinobacteria,Bacilli,Alphaproteobacteria,Gammaproteobacteria and Sphingobacteria,based on morphological observations,RFLP grouping and their full length 16S rRNA gene sequences.Among them,Actinobacteria(15 strains) was the most commonly observed genus,accounting for 30% of the total antibiotic resistant isolates.This was followed by Bacillus sp.(9 strains) and Pseudomonas sp.(8 strains),which accounted for 18% and 16% of the total antibiotic resistant isolates,respectively.Additionally,several important human opportunistic pathogenic bacteria with antibiotic resistance were frequently isolated from antibiotic treated soil,including Pseudomonas sp.,Sphingomonas sp.and Stenotrophomonas sp.,which accounted for 16%,8% and 4% of the total resistant isolates,respectively.Moreover,the dominant bacteria with antibiotic resistance in the wheat rhizosphere were Bacillus sp..These organisms were generally isolated from samples of soil that had been treated with low doses of oxytetracycline(0,100 μg/L),while very few were isolated from samples of soil that had been treated with 2000 μg/L.Taken together,these findings suggest that exposure to high concentrations of antibiotics would inhibit the wheat growth-promoting bacteria in the rhizosphere,thereby indirectly inhibiting the growth of wheat.Conversely,human opportunistic pathogenic bacteria with antibiotic resistance(Pseudomonas sp.,Sphingomonas sp.and Stenotrophomonas sp.) were generally isolated from soil samples treated with high doses of oxytetracycline exposure(2000 μg/L).These antibiotic resistant opportunistic pathogenic bacteria and their antibiotic resistant genes pose a risk because they have the potential to spread within plants and humans.