Constructed wetland treatment increases prevalence of multidrug-resistant Escherichia coli isolates but decreases overall concentrations.

AIMS: Antibiotic resistant bacteria (ARB) are high-risk pathogens of interest in the environment. Treatment wetlands are effective in reducing microbial contamination, but little is known of their ability to remove ARB such as methicillin resistant Staphylococcus aureus (MRSA), vancomycin resistant enterococci (VRE), and low-level ceftriaxone resistant Escherichia coli (LLCR).

METHODS AND RESULTS: We measured putative MRSA, VRE, and LLCR E. coli from water column and sediments in five distinct sites within the Banklick Creek Wetland (BCW). Using a two-tier approach, we validated antibiotic resistance of putative ARB and authenticated genetic identity. We determined isolate resistance to clinically relevant antibiotics and screened for multidrug resistance (MDR). For LLCR E. coli, high degrees of antibiotic resistance (≥95%) and genetic identity verification (≥96%) were confirmed, but none of the putative VRE and MRSA isolates were confirmed genetically. LLCR E. coli from both matrices were most frequently resistant to beta-lactam antibiotics and least frequently to carbapenems. MDR E. coli isolates were significantly more prevalent at the wetland outlet (33%) compared to the inlet (19%).

CONCLUSIONS: We observed limited removal of ARB, but an increase in the proportion of MDR isolates in the later treatment stages is concerning, as it suggests that the BCW may be acting as a reservoir and a possible conduit for evolution of MDR pathogens.