Purpose: In this study E. coli isolates that previously exhibited environmental fitness were subjected to gastric juice and compared to laboratory-control strains.
Methods: E. coli isolates (2) were recovered from manure and leafy greens at 10 days post inoculation from previously conducted field trials or from survival in wildlife feces over 12 months (O104:H4). Five E. coli isolates, including 2 laboratory-control strains, were individually compared for survival on spinach leaves (0 or 1 h) first and then in synthetic gastric juice pH 2.0 (0.5 or 2 h) or in 0.1% peptone water. Spinach plants (n = 6 of 3 pooled plants) were inoculated with 7 log CFU per strain and recovered bacteria enumerated on TBX agar.
Results: Bacteria were recovered from spinach plants and the field isolates survived exposure to gastric juice significantly better than control strains despite presence of rpoS in the controls (P < 0.003). All isolates decreased from initial concentrations by an average of 1.7 - 2.6 ± 0.3 log CFU after plant recovery. Laboratory-control strains of rpoS positive and deficient E. coli, decreased by an average of 3.91 and 3.99 log CFU after gastric juice exposure (P = 0.008); while field isolates and O104:H4 decreased by an average of 0.79,1.20, and 0.82 log CFU after 2 h of gastric acid exposure. No differences were observed between gastric juice exposure times. No significant decrease was observed in any strain following leaf recovery and 2 h in peptone water. Research extending time points will be conducted.
Significance: Field isolates survived on leaves and in synthetic gastric juice better than laboratory-strains, suggesting these isolates have enhanced persistence which merits further attention.