Influence of ciprofloxacin on microbial community structure and function in soils.

Microcosm experiments were designed to investigate the effects of the widely used antibiotic ciprofloxacin (CIP) on soil microbial communities by using four different concentrations (0, 1, 5, and 50 mg kg-1 of soil) and five sampling times (1, 3, 9, 22, and 40 days). Untreated controls only received water. The addition of CIP significantly decreased microbial biomass (p < 0.05) but did not affect soil respiration at high doses. Potential nitrification rates were stimulated at low CIP concentrations (1 mg kg-1) and inhibited at high CIP concentrations (50 mg kg-1) after 9 days of incubation. The nitrate and ammonium contents of soil were not altered after CIP addition at any time. The structure of soil microbial communities was assessed by phospholipid fatty acid (PLFA) analysis. The addition of CIP decreased the ratio of bacteria to fungi and increased the ratio of Gram-positive to Gram-negative bacteria. Principal component analysis of the PLFA data clearly distinguished among the different CIP concentrations. Redundancy analysis indicated that the CIP concentration and incubation time explained 33.5 % of the total variance in the PLFA data. These results confirmed that a single addition of CIP can influence structure and function of microbial communities in soil