Inhibition Effects Of Typical Antibiotics On Denitrification Process In Groundwater

The nitrate co-contamination with antibiotics has been given rise to significant concerns,especially for the influences on nitrogen conversion in the water and soil environment.But whether the low-level antibiotics will affect denitrification and how to affect are still poorly understood in groundwater.To effectively solve this problem,both direct antimicrobial susceptibility testing and microbial batch-culture experiments were conducted to assess the difference in the sensitivity of denitrifying bacteria against9 typical antibiotics.Then selected Lomefloxacin?LOM?,which is more sensitive at low concentration,was to carry out simulation experiments of nitrate degradation in groundwater.By monitoring the nitrogen content,microbial population and activity,community structure,denitrification enzyme activity,functional genes and resistance genes in employed bioreactors,the inhibition of LOM on each denitrification sub-processes and the microbial response mechanism were characterized and explored.The main results of the paper are as follows:?1?The antimicrobial susceptibility testing indicated that the sensitivity of denitrifying bacteria,mainly Pseudomonas?46.17%?,to antibiotics has a significant relationship with the type and concentration of antibiotics.For five different types of antibiotic,the sensitivity ranking,from high to low,was Enrofloxacin?ENR?>Tetracycline>Sulfamethoxazole>Erythromycin>Clindamycin.And Fluoroquinolones represented by ENR revealed high and low concentration effects on denitrifying bacteria sensitivity.The critical value is about 100?g/L.Exposure to high concentrations had a significant inhibitory effect on bacterial growth and exhibited a dose dependency.Oppositely,the effect of low concentration may be stimulation or inhibition,which was not significant and independent of concentration.But relatively speaking,the inhibition of LOM was more effective than other antibiotics.?2?The results of nitrate degradation simulation experiment in groundwater showed that the inhibition rates on the entire denitrification process,exposed to 100ng/L-100?g/L LOM,were positively correlated with LOM initial concentration?R²=0.99,p<0.005?,which be from 30.5%to 96.2%.This trend was dominated by the inhibitory effects on the nitrate reduction and nitrite reduction.The inhibitory effects on the NO and N₂O reduction were minimal.For the nitrate reduction,in addition to 10?g/L,the inhibition rate of LOM also increased with the concentration.While the nitrite inhibition was mainly observed in 10?g/L system,accounting for 47.1%.?3?Microbial analysis revealed that LOM inhibited microbial population and activity and nitrite reductase activity,and induced the production of some resistance genes and transposase genes,which exerted a certain concentration effect.However,in the system of 10?g/L LOM,the inhibition rate of microbial population and activity was only 5.8%.The microbial abundance and diversity in the 10?g/L system were relatively higher than others,where containing nearly 10-10⁴ times the resistance gene,and lacking the critical nitrite reductase-encoding gene nirS.Therefore,these microbial differences were the main reasons that the NO₂-N reduction was more easily inhibited in the 10?g/L system.It should be noted that whether the specificity of the 10?g/L system is accidental or inevitable needs to be determined by further research.