Purpose: This study evaluated the comparative fitness of outbreak-related clinical and environmental strains to resist protozoan predation and survive in soil from a spinach field in the general vicinity of isolation of strains genetically indistinguishable from the 2006 outbreak strains.
Methods: The decreases in the populations of EcO157 strains during predation by Vorticella microstoma and Colpoda aspera, isolated previously from dairy wastewater, in decreased strength Sonneborn medium were measured. Strains with different proportions of curli-positive (C+) and curli-negative (C-) phenotypes were evaluated to determine whether protozoa are selective in grazing on them. We compared if specific phenotype subpopulations that evade predation are increased in fitness for survival in soil.
Results: Environmental strains from soil and feral pig feces survived longer (11 to 35 days for 90% decreases, D-value) with both protozoa; these D-values correlated (P < 0.05) negatively with protozoan growth. Similarly, strains from cow and pig feces survived significantly longer in soil compared to genetically indistinguishable clinical isolates. The C+ phenotype, a fitness trait linked with attachment in ruminant and human gut, decreased after exposure to protozoa and in soils only C- cells remained after 7 days. The C+ phenotype correlated negatively with D-values of EcO157 exposed to soil (rs = -0.683; P = 0.036), Vorticella (rs = -0.465; P = 0.05) or Colpoda (rs = -0.750; P = 0.0001). In contrast, protozoan growth correlated positively with C+ phenotype (Vorticella, rs=0.730, P = 0.0004; Colpoda, rs = 0.625, P = 0.006) suggesting a preference for C+ cells.
Significance: We speculate that the C- phenotype is a selective trait for survival and possibly transport of the pathogen in soil and water environments.