Phenotypic characteristics contributing to the enhanced growth of Escherichia coli bloom strains

During bloom events, Escherichia coli cell counts increase to between 10,000 and 100,000 cfu/100 ml of water. The strains responsible for bloom events belong to E. coli phylogenetic groups A and B1, and all have acquired a capsule from Klebsiella. A pan‐genome comparison of phylogroup A E. coli revealed that the ferric citrate uptake system (fecIRABCDE) was overrepresented in phylogroup A bloom strains compared with non‐bloom E. coli. A series of experiments were carried out to investigate if the capsule together with ferric citrate uptake system could confer a growth rate advantage on E. coli. Capsulated strains had a growth rate advantage regardless of the media composition and the presence/absence of the fec operon, and they had a shorter lag phase compared with capsule‐negative strains. The results suggest that the Klebsiella capsule may facilitate nutrient uptake or utilization by a strain. This, together with the protective roles played by the capsule and the shorter lag phase of capsule‐positive strains, may explain why it is only capsule‐positive strains that produce elevated counts in response to nutrient influx

Phenotypic and Genotypic Characteristics for Escherichia coli strains responsible for bacterial bloom events in Australia

Escherichia coli is widely used as an indicator of recent faecal contamination of drinking and recreational waters. However, evidence suggests that E. coli can proliferate in the environment outside a host, confounding its use as a faecal indicator. E. coli strains that produce significantly elevated counts of 10,000 - 100,000 cells/100 ml of water (bloom events) are reported from freshwater reservoirs and recreational lakes in Australia. Bloom strains are not faecal associated and may represent free-living E. coli. A limited number of strains are responsible for bloom events and all belong to E. coli phylogroups A and B1. Bloom strains have acquired a capsule originating from Klebsiella. Diversity and distribution of Klebsiella capsules in E. coli overall and in bloom strains were investigated. A PCR-based protocol was developed to detect capsule-positive E. coli and discriminate strains that harbour bloom strain-associated capsule types. Bloom strain attributes that could lead to the elevated cell densities observed in bloom events were experimented. The B1 bloom strain from the east coast (termed B1-001) which is numerically dominant and always present in bloom events was further characterised, and why B1-001 strain has not been detected in recent bloom events assessed using the Colilert-18 system was investigated. Frequency of Klebsiella capsules in E. coli was only 7% and 23 different Klebsiella capsule types were detected. All bloom strains were encapsulated and seven Klebsiella capsule types were detected among the eight bloom strains isolated to date. Capsules were observed only in strains from E. coli phylogroups A, B1, and C, and all encapsulated strains were of O-serogroups O8, O9, and O89. Capsule gene region and the adjacent O-antigen gene region in encapsulated E. coli are a result of a horizontal gene transfer event that occurred between E. coli and Klebsiella. The PCR accurately detected known bloom, non-bloom encapsulated, and capsule-negative strains. A pan genome comparison of phylogroup A E. coli revealed that the iron uptake system encoded by fecIRABCDE operon was over-represented among bloom strains (100%) compared to non-bloom E. coli (<39%). Growth assays however showed that the fec operon is unlikely to contribute to the elevated cell densities. In contrast, strains that were encapsulated had a growth rate advantage compared to capsule-negative strains. B1-001 bloom strain was closely related to Shigella but its closest relatives were lactose-positive E. coli. Several features that are beneficial for a free-living lifestyle such as flagella and curli were disrupted in B1-001. In Colilert-18, B1-001 was heavily outcompeted by the two phylogroup A bloom strains from the east coast, explaining why B1-001 was not detected in recent bloom events assessed using Colilert-18. The current study suggests that any E. coli strain that harbours a Klebsiella capsule may be able to produce elevated counts under conducive environmental conditions. The recurrence of bloom events across Australia confounds the use of E. coli as a water quality indicator and urges a shift to alternative indicators

Phenotypic characteristics contributing to the enhanced growth of Escherichia coli bloom strains

During bloom events,Escherichia colicell countsincrease to between 10,000 and 100,000 cfu/100 mlof water. The strains responsible for bloom eventsbelong toE. coliphylogenetic groups A and B1,and all have acquired a capsule fromKlebsiella.Apan-genome comparison of phylogroup AE. colirevealed that the ferric citrate uptake system(fecIRABCDE) was overrepresented in phylogroupA bloom strains compared with non-bloomE. coli.A series of experiments were carried out to investi-gate if the capsule together with ferric citrateuptake system could confer a growth rate advan-tage onE. coli. Capsulated strains had a growthrate advantage regardless of the media composi-tion and the presence/absence of thefecoperon,and they had a shorter lag phase compared withcapsule-negative strains. The results suggest thattheKlebsiellacapsule may facilitate nutrientuptake or utilization by a strain. This, together withthe protective roles played by the capsule and theshorter lag phase of capsule-positive strains, mayexplain why it is only capsule-positive strains thatproduce elevated counts in response to nutrientinflux.This study was funded in part by an Australian Research Council Linkage Grant (Grant No. LP120100327)