Antibiotic resistance in triclosan tolerant fecal coliforms isolated from surface waters near wastewater treatment plant outflows (Morris County, NJ, USA)

Triclosan (TCS) is a common antimicrobial agent that has been detected in wastewater treatment plant (WWTP) effluent outflows. A link between TCS exposure and increased antibiotic resistance in microbes has been postulated. The purpose of this study was to evaluate whether fecal coliforms (FC) isolated from surface waters located near (WWTP) outflows display TCS resistance and, if so, whether such organisms exhibit increased resistance to antibiotics. Water samples were collected at two streams in Morris County, NJ that receive WWTP effluent: Loantaka Brook and the Whippany River. Water samples were collected at three sites within each location near the WWTP effluent outflow. Abiotic river parameters were measured and FCs were enumerated for each sample. River parameters were analyzed to determine if TCS or antibiotic resistance was correlated to water quality. Triclosan resistance levels were determined for individual isolates, and isolates were screened against seven classes of antibiotics at clinically relevant levels to assess cross-resistance. At Loantaka Brook, 78.8% of FC isolates were resistant to TCS with an average minimum inhibitory concentration (MIC) of 43.2 μg ml−1. In addition, 89.6% of isolates were resistant to four classes of antibiotics and all were identified as Citrobacter freundii. There was a significant effect of stream location on mean TCS MIC values in the Loantaka Brook, with effluent isolates maintaining significantly higher MIC values compared to upstream isolates. At Whippany River sites, TCS resistant isolates were detected on 94% of sampling dates with a significant relationship between TCS resistance and multiple antibiotic resistances (≥three antibiotic classes, p<0.001). TCS resistant isolates were significantly more resistant to chloramphenicol (p=0.007) and to nitrofurantoin (p=0.037) when compared to TCS sensitive isolates. Environmental FC isolates resistant to high level TCS included species of Escherichia, Enterobacter, Serratia and Citrobacter. There was no correlation between river water quality and resistance of isolates to TCS. Presence of isolates not resistant to TCS, but resistant to other antibiotics, were significantly correlated to increased river flow, precipitation, and decreased nutrient levels, suggesting that observed resistance is due to run-off events. This study demonstrates that TCS resistant FC are common in river systems receiving WWTP effluent and display multiple drug resistance.

► TCS resistant FCs were isolated near WWTP outflows.
► High level (≥256 μg ml−1) TCS resistant E. coli were isolated.
► TCS resistant FC exhibited significantly higher multiple antibiotic resistance.
► A significant increase in resistance to chloramphenicol in TCS resistant FC isolates.
► A significant increase in resistance to nitrofurantoin in TCS resistant FC isolates.