Antibiotic resistance profiles of representative wetland bacteria and faecal indicators following ciprofloxacin exposure in lab-scale constructed mesocosms

In this study, the effect of the antibiotic ciprofloxacin on the presentation and potential development of antibiotic resistance among selected faecal indicators (Escherichia coli and Enterococcus spp.) and the intrinsic bacterial community for a freshwater model ecosystem was investigated. Four parallel mesocosm wetlands planted with Phragmites oust rails were initially seeded with an activated sludge inoculum from a wastewater treatment plant and monitored over the course of 63 days. Two of the four mesocosms (CIP1 and CIP2) were initially exposed to a ciprofloxacin concentration of 2 mu g mL(-1) for a 5 day period, and two were considered controls (CON1 and CON2). Minimum inhibitory concentrations were determined for the culturable interstitial microbial community using ciprofloxacin concentrations ranging from 0.5 to 16 mu g mL(-1) and compared with total heterotrophic plate counts (HPCs). E. coli and Enterococcus spp. were enumerated and screened for antimicrobial resistance to twelve different antibiotics both before and after mesocosm ciprofloxacin exposure. Concentrations for total heterotrophs within the interstitial water were in the order of 10(6)-10(8) CFU 100 mL(-1), while E. coli and Enterococcus spp. were present in lower concentrations, at 10(2)-10(3) CFU 100 mL(-1). All four mesocosms had the same minimum inhibitory concentrations (MICs) of 2 mu g mL(-1) for total heterotrophic community during the first week of start-up (before ciprofloxacin exposure), followed by a sharp increase in ciprofloxacin exposed mesocosms on day 14 (after ciprofloxacin exposure), reaching a maximum MIC of 16 jig mL(-1). The resistance levels of interstitial heterotrophic counts determined by the agar dilution method peaked 7 days following the ciprofloxacin exposure, and decreased thereafter. A significant increase (p < 0.05) to the following antibiotics was observed among E. coli following ciprofloxacin exposure within mesocosm CIP2: ciprofloxacin (CIP), chloramphenicol (C), cefotaxime (CTX), ceftriaxone (CRO), and doxycycline (D). These results illustrate the potential for a single antibiotic exposure event to create a noticeable increase in the antibiotic resistance profile, not only to the imposed antibiotic, but to other classes of antibiotics. The initial background levels of resistance among Enterococcus spp. were significantly higher than those observed among E. coli, exhibiting a combined average of 96%, 96%, and 100% resistance to ciprofloxacin (CIP), linezolid (LZD), and streptomycin (S), respectively. Following the addition of ciprofloxacin, Enterococcus spp. retained high levels of resistance to ciprofloxacin, linezolid, and streptomycin, with a significant increase (p < 0.05) to vancomycin (VA), reaching 100% on Day 63. The findings of this study suggest that the transient presence of an isolated exposure event of ciprofloxacin can impact the antibiotic resistance profiles among faecal indicators in lab-scale constructed mesocosms and has the potential to impact constructed wetland treatment systems. (C) 2011 Elsevier B.V. All rights reserved.