Poster Presentation Australian Microbial Ecology 2017

Genotypic and phenotypic characteristics of Escherichia coli strains responsible for bloom events in Australia (#119)

Buddhie S. Nanayakkara 1 , David M. Gordon 1
  1. The Australian National University, Canberra, ACT, Australia

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, be unable to multiply outside a host, and all cells of the species should be identical in their ability to survive in the external environment. However, some E. coli strains have acquired the ability to multiply outside a host, and as a consequence may temporarily achieve counts from 10,000 – 100,000 CFU/100 ml, well above typical E. coli counts of around 200 CFU/100 ml.   

The strains responsible for these elevated counts (‘bloom’ strains) belong to E. coli phylogenetic groups A and B1.  Using comparative genomics and phylogenetic analyses, the genome sequence of the B1 bloom strain E267 was compared to strains representing major E. coli phylogroups and S. boydii, S. sonnei and S. flexneri.  The growth rates of bloom strains were studied in microcosm experiments, and stationary phase cell densities and fitness of the 3 types of strains were investigated.

E267 has a reduced genome size and is phylogenetically closely related to Shigella.  However, it does not carry the Shigella specific virulence factors and its closest relatives are typical lactose positive E. coli. E267 has undergone a considerable gene loss and from core genes of the strains studied, the operons cai, csg, efe, fli and gat are absent or impaired in E267.  B1 bloom strains have a significantly lower growth rate compared to A0 and A1 blooms. Stationary phase cell densities of B1 bloom strains were an order of magnitude lower than those of A bloom strains. In fitness experiment, A strains outcompeted B1.

The gain and loss of genes suggest E267 has evolved to become ‘planktonic’ and not form aggregates and biofilms. This would explain their occurrence being restricted to reservoirs. The competitive advantage of A strains over B1 strains explains why the B1 bloom strain is seldom detected when using the Colilert system.