Dynamics of Bacterial Assisted Phytoremediation of Soil Contaminated by Quinolone Antibiotics

Recently there has been growing concern about public health and ecosystems due to the presence of trace quantities of quinolone antibiotics in the environment. These antibiotics enter the environment through irrigation by contaminated waste water, particularly in the areas that are arid or semi-arid. Another route of these antibiotics is the application of sludge and manure containing antibiotics as soil amendments on agricultural land. Once they enter the environment they can cause ecotoxic effect. In the current study, the concentration of ciprofloxacin, enrofloxacin, and levofloxacin in waste water and sludge were relatively higher than reported in many countries. To degrade the aforementioned antibiotics in agricultural soil, eight bacterial strains were isolated and characterized. The isolated bacterial strains possess multiple antibiotic resistance and degraded ciprofloxacin by whole cell activity. No DNA transformation were observed when naked DNA of the isolated bacterial strain were exposed to indigenous soil microbes. The isolated bacterial strains survived well under aerobic conditions in soil treated with ciprofloxacin at the concentration of 5 mg kg-1, 50 mg kg-1 and 100 mg kg-1. The quinolone antibiotics did not affect germination of wheat even at 400 mg L-1 concentration but the young wheat seedlings were sensitive to the antibiotics at relatively low concentrations (5 mg L-1). The toxicity of the quinolone antibiotics ciprofloxacin (CIP), enrofloxacin (ENR), levofloxacin (LEV), and a mixture (MIX) of the same were tested in a greenhouse study with wheat for 20 days. Antioxidants, which play a vital role in detoxifying reactive oxygen species (ROS) during stress, were xxii also studied. The highest antibiotic concentrations, 100 and 300 mg L-1, significantly decreased plant growth and caused oxidative stress. The quinolone antibiotics ciprofloxacin (CIP), enrofloxacin (ENR), levofloxacin (LEV), and a mixture (MIX) in agricultural soil were readily sorbed when tested in agricultural soil. The antibiotics showed high sorption coefficient (Kd) values at low concentration and low Kd values at high concentrations, which means sorption was concentration dependent and time for sorption decreased with increasing concentrations. In both the individual and MIX treatment the order of sorption was CIP > ENR > LEV and for desorption it was LEV > ENR > CIP. In a bacterial assisted phytoremediation study, wheat was grown in greenhouse conditions in soil supplemented with CIP, ENR, and LEV at 5, 50 and 100 mg kg-1 in individual and MIX treatments. CIP, ENR and LEV accumulated in plant roots and shoots from soil and more accumulation was observed in roots. The order of accumulation in plant parts was LEV > ENR > CIP both in individual and MIX treatments. The pots having only plants enhanced the recovery of antibiotics from soil compared to those where there were only bacteria but greater degradation was observed where plants were grown in the presence of bacteria. CIP, ENR, and LEV were completely degraded in individual treatments at 5 and 50 mg kg-1 level in the presence of plants and bacteria after 30 days compared to the control. In the MIX treatment all three antibiotics were less degraded compared to individual treatments in the presence of plants and bacteria after 30 days because of the higher concentration of each antibiotic in the pots.