Escherichia coli is almost universally used as an indicator of faecal contamination of water. An ideal indicator should be absent in the absence of faecal contamination and be unable to multiply outside a host. There have been reports of E. coli strains which have acquired the ability to multiply outside a host, viz. E. coli bloom strains. The typical E. coli counts in a water body are around 200 cells/100 ml of water, whereas in bloom events can rise up to 10,000 – 100,000 cells/100 ml of water. Elevated counts have been reported regularly from water bodies in East Coast of Australia and recently for the first time in Western Australia (WA). Bloom strains belong to the E. coli phylogroups A and B1.
A characteristic of all bloom strains is the production of a group 1 capsule. It aids cells to withstand desiccation, UV radiation and other stress conditions in the environment outside a host. The bloom strain capsule region represents an approximately 25 kbp insertion in between the his operon and galF that originates from Klebsiella. This is the region that determines the O antigen characteristics and colonic acid production in E. coli K12.
Given our understanding that it is the capsule region that enables a strain to produce blooms, the capsule gene region of the phylogroup A1, A0, B1 and WA bloom strains, along with several human and mammal strains was characterised. The results indicate that a Klebsiella capsule has been acquired, with one exception, as a result of independent horizontal gene transfer events. One bloom strain appears to have arisen as a consequence of a within-species transfer of the capsule region found in the B1 bloom strains to a phylogroup A bloom strain. The phylogenetic distribution and diversity of Klebsiella capsule types among E. coli lineages suggests a specific genetic background is required for the capsule transfer event to produce a strain capable of causing bloom events.