Escherichia coli has a highly clonal population structure, which allows them to be delineated into phylogroups (A, B1, B2, C, D, E, F). Strains of different phylogroups vary in their ecological niche, life history characteristics, and propensity to cause disease. Of all the phylogroups, B2 is commonly isolated from human faeces, and is also the most abundant phylogroup in developed countries like Australia. It is clinically significant, as strains belonging to this phylogroup are most often the cause of extra-intestinal infections. Although strains belonging to phylogroup B2 are highly diverse, relatively few B2 lineages represent the great majority of B2 strains isolated from humans. Although phylogroup B2 strains are usually a small fraction of the E. coli recovered from water samples, their presence may be indicative of human faecal contamination and they may represent human health risk.
The aim of this study was to determine what fraction of phylogroup B2 strains isolated from water samples belonged to human associated B2 lineages. To this end, over 10,000 isolates of E. coli from more than 900 water samples collected in the Sydney and Gold Coast regions were characterised for their phylogroup membership. A total of 825 strains belonging to phylogroup B2 were assigned to one of the human associated B2 lineages using a recently developed allele-specific PCR method for lineage assignment.
Overall, 40% of the isolates were found to potentially belong to one of the human associated lineages. However, only 12% represent three of the human associated B2 lineages (ST 73, ST 95 and ST131) that are grossly over represented among human B2 isolates.
The results of this study demonstrate that human associated phylogroup B2 strains represent a small fraction of the E. coli strains present in Australian waters.