Escherichia coli is widely used as an indicator of faecal contamination of drinking and recreational waters.  There is a growing body of evidence demonstrating that some E. coli strains become naturalized to their secondary habitat: soil, water and sediments.  Consequently the presence of E. coli in water may not always indicate recent faecal contamination.  E. coli strains responsible for significantly elevated counts (10,000 – 100,000 cells/100 ml of water) have been reported from fresh water reservoirs across Australia.  Multiple strains belonging to phylogenetic groups A and B1 are responsible for these elevated counts, and all possess a Group I capsule originating from Klebsiella.

Phylogroup A strains represent the majority of bloom strains.  Pan genome analysis based on whole genome sequence data for 330 strains revealed that the ferric citrate uptake system (fecIRABCDE) was over-represented among phylogroup A encapsulated strains.  The growth rate of strains of four capsule and fec operon genotype combinations; [cap+fec+], [cap+fec-], [cap-fec+] and [cap-fec-] in media containing iron and/or citrate was determined.  Growth rates were also estimated using carbon sources that differed in their uptake mechanism, and at low (0.01 mM) and high (10 mM) glucose concentrations.  The duration of the lag phase was also measured.

Encapsulated strains had a statistically significant growth rate advantage over non-encapsulated strains at low glucose concentrations, irrespective of media composition.  The effect of fecIRABCDE operon was variable.  The strains grew better in trehalose and maltose due possibly to enhanced outer membrane uptake.  Lag time of encapsulated strains was significantly shorter than non-encapsulated strains.

The capsule is known to provide protection against adverse environmental conditions and predation, thus enhancing cell persistence in the environment.  The experimental results show that encapsulated strains have a shorter lag phase and higher growth rates than the non-encapsulated strains.  Thus the enhanced persistence of encapsulated strains means that they are more likely to be present during nutrient influx events, and are also able to outcompete the co-occurring non-encapsulated strains.