The Effect of Gastric Acidity on Escherichia coli Isolates Recovered from Poultry Litter-amended Soils

Introduction: Pathogenic E. coli can contaminate fruit and vegetable commodities, and may be introduced to soils through biological soil amendments.  It is unknown how survival in manure-amended soils affects the tolerance of E. coli to low pH environments.  Gastric acidity in humans is one of the barriers pathogenic E. coli need to overcome to cause infection.

Purpose: To determine if E. coli recovered from poultry litter (PL)-amended soils have greater tolerance to gastric acidity than laboratory strains. 

Methods: Several field studies conducted between 2011 and 2014 examined the survival of a three-strain inoculum (TVS353, TVS354, and TVS355) of rifampicin-resistant E. coli (rEc) laboratory isolates (LI) in PL-amended soils in Maryland and Pennsylvania. Samples of PL-amended soils were taken for up to 341 days post inoculation (dpi) and analyzed for rEc populations. Colonies of field isolates (FI) were identified by BOX-PCR-typing. FI of TVS353 and TVS354 were recovered at 120 dpi and 150 dpi, respectively, while two FIs of TVS355 were recovered at 150 dpi and another recovered at 341 dpi. FIs and LIs of each strain were exposed to synthetic gastric fluid (SGF, pH 2.3) for 0, 4 and 6h and changes in populations (log CFU/ml) were statistically compared by Student’s- t-Test.

Results: All LI and FI E. coli populations declined by 0.8 – 2.6 log CFU/ml and by 2.0 – 3.6 log CFU/ml after 4 and 6 h, respectively, of exposure to SGF. There were no significant (P > 0.05) differences between LI and FI isolates of either TVS354 or TVS355. However, at 4 h, for TVS353, FI population declines (2.6 log CFU/ml) were significantly greater than those of LI (1.5 log CFU/ml).

Significance:  Extended survival of E. coli in PL-amended soils did not affect the acid tolerance of E. coli strains compared to laboratory strains; however, extended survival in manure-amended soils may induce other phenotypic changes in E. coli.