Evidence for growth of enterococci in municipal oxidation ponds, obtained using antibiotic resistance analysis

The Christchurch wastewater treatment plant uses a series of six oxidation ponds to reduce the bacterial load of treated effluent before it is discharged into the local estuary. To ensure that this discharge does not adversely affect water quality in the receiving environment, local regulations specify maximum levels in the discharge for a number of parameters, including enterococci. Between 2001 and 2006, regulations required fewer than 300 enterococci per 100 ml in summer. During this period, the discharge intermittently exceeded this limit, with unexplained levels of enterococci of up to 180,000/100 ml. Characterization of these enterococci by antibiotic resistance analysis showed that enterococci sampled over 4 months had almost identical resistance profiles. In contrast, enterococci from raw sewage and wildfowl from around the oxidation ponds had a diverse range of antibiotic resistance profiles that could be distinguished from each other and also from those of enterococci from the discharge. The hypothesis of a clonal nature of the enterococci in the discharge was supported by molecular genotype analysis, suggesting that these bacteria may have replicated in the pond environment rather than being reflective of breakthrough in the sewage treatment process or the result of recent wildfowl inputs to the ponds. This study highlights the usefulness of antibiotic resistance analysis in identifying this phenomenon and is the first report of apparent replication of a specific type of enterococci in an oxidation pond environment.