Where did that bug in my water come from? How do we know?

Abstract

Biological water quality testing relies on the standard measurement of faecal contamination determined by culturing cells of thermotolerant coliform bacteria of faecal origin. Escherichia coli represents over 96.8% of thermotolerant coliforms and is the key indicator of faecal contamination. However the presence of E. coli in a water sample does not provide information about the source of contamination and is not always an effective indicator of disease risk to humans. Microbial source tracking is a developing field that involves using a specific marker such as variation in a gene, antibiotic resistance or carbon source utilization patterns to identify the host of a pathogenic or indicator organism. Genetic source tracking techniques rely on sequence differences to distinguish between strains with the assumption that the differences arise through host specificity. Therefore an E.coli from a human host will have a different genetic profile to that from other animals. The ?-glucuronidase (gusA) gene found in E. coli is one gene that has shown promise as possible genetic marker. This gene was analysed in 320 E. coli isolates that were collected from several water bodies, birds, mammals, marsupials, reptiles and humans. Phylogenetic analysis of a 500 base pair fragment of the gusA gene from these isolates revealed two distinct clades of E. coli isolates, one belonging to the humans and the other to the animals. Almost 80% of all of the known human isolates grouped in the human clade with an acceptable level of 11.4% false positives. This result suggests that the gusA gene in E. coli may be very useful in determining if faecal contamination is of human or animal origin. A description of the use of genetic markers for microbial source tracking, comparisons to other methods and the information this approach can provide from a water quality risk assessment aspect will be discussed.