¹Department of Land, Air and Water Resources, University of California, Davis, CA, USA

Correspondence: AL Córdova-Kreylos, Department of LAWR, University of California, 1110 PES Building, One Shields Avenue, Davis, CA 95616, USA. E-mail: anacordova@ucdavis.edu

Received 11 May 2007; Revised 23 July 2007; Accepted 23 July 2007; Published online 6 September 2007.

Abstract

Fluoroquinolones, a widely used class of antibiotics, are frequently detected in sediments and surface waters. Given their antimicrobial properties, the presence of these compounds may alter the composition of microbial communities and promote antibiotic resistance in the environment. The purpose of this study was to measure sorption, and effects of ciprofloxacin on microbial community composition, in sediment samples from three California salt marshes. Sediments were exposed to a ciprofloxacin concentration gradient (0–200 g ml^-1 ciprofloxacin) and microbial community composition characterized using phospholipid fatty acid (PLFA) analysis. Sorption coefficients, expressed as log K_d, were calculated from fits using the Freundlich isotherm model. Ciprofloxacin strongly sorbed to all sediments and had log K_d values, ranging from 2.9 to 4.3. Clay content was positively (r²=0.98) and pH negatively (r²=0.99) correlated to K_d values. Biomass, PLFA richness, sulfate reducer and Gram-negative bacteria markers increased with ciprofloxacin concentrations, while the 17 cy/precursor and saturated/unsaturated biomarker ratios, indicators of starvation stress, decreased. The magnitude of the effect of ciprofloxacin on microbial communities was inversely correlated to the degree of sorption to the sediments. Despite the fact that ciprofloxacin is a wide-spectrum antibiotic, its impact on sediment microbial communities was selective and appeared to favor sulfate-reducing bacteria and Gram-negative bacteria.