An evaluation of membrane faecal coliform agar and Colilert-18^® for the enumeration of E. coli bacteria in surface water samples

Aims: To evaluate and review two methods for enumeration of E. coli bacteria in surface water samples.

Methods and results: Filtration using membrane faecal coliform (mFC) agar and a defined substrate technology^™ method (Colilert-18^®) was evaluated. E. coli BioBall^™ bacteria were seeded into autoclaved surface water samples. In addition, 266 surface water samples from South East Queensland were analysed in parallel using both mFC and Colilert-18^®.

Conclusions: E. coli is the bacterium of choice when analysing water samples for faecal contamination. An overall lower mean recovery was demonstrated using mFC agar than Colilert for both seeded surface water and in parallel testing of surface water samples. There was a statistically significant difference between BioBall bacteria on mFC agar and values on the QA certificate (p≤0.0001) but there was no significance difference between Colilert for E. coli using the BioBall (p=0.8488). There was a significant difference between mFC agar and Colilert in parallel testing of surface water samples (p≤0.0001) with greater sensitivity demonstrated by the Colilert procedure.

This data confirms the conclusions of other researchers that Colilert is a suitable substitute for membrane filtration for surface water analysis for the detection of E. coli as faecal indicator bacteria.