Effect of pH on the Fate of Novel Bacteriophages Targeting Non-O157 Shiga-toxigenic Escherichia coli

Introduction:  Shiga-toxin-producing Escherichia coli(STEC) are responsible for multiple foodborne illness outbreaks. It is therefore crucial to devise effective control strategies. Bacteriophages are advantageous over traditional antimicrobials by virtue of their high-specificity against target bacteria. The FDA-approval of bacteriophages, in ready-to-eat meat and poultry products, has paved way for development of phage-based antimicrobials. However, designing their application method as bio-preservatives in the food systems requires knowledge of critical points such as pH-inactivation parameters. Modeling pH-inactivation of STEC-specific bacteriophages would enable their predicted survival in acidic and alkaline environments.

Purpose:  Evaluation of inactivation models for selected bacteriophages and analysis of optimum pH treatment parameters.

Methods:  Bacteriophages isolated from cattle farms in Oklahoma, exhibiting inhibition towards non-O157 STEC (O26, O45, O103, O111, O121, and O145) were used. The pH-inactivation of phages was tested over a pH range: 1-11. Surviving population was enumerated at 0, 1, 2, 4, 6, 12 and 24 hours, using double-agar-layer technique. Applicability of first order and Weibull distribution models was evaluated to understand degradation kinetics through regression module, using SigmaPlot13^TM (Systat Software, US). Models were evaluated and compared on the basis of regression coefficient (r²), mean square error (MSE), and standard error of means (SEM).

Results:  The results confirmed that Weibull model provided a perfect description of the data. The r²values ranged from 0.82-0.99 for all phages. Predicted inactivation reliable time (Tr), analogous to D-value, of tested phages ranged from 3.17 h at pH 1 to 1,214,889 h (138 years) at pH 5. Optimum activity of all phages ranged from pH 5.07-5.93 except J-4 (O121) showing optimum activity only at pH 7.13. Positive skewness indicated that bacteriophages showed more stability at slightly acidic pH (5.07-5.90) than near neutral and alkaline pH.

Significance:  pH-inactivation model parameters reveal that isolated bacteriophages could be successfully used as biopreservatives in acidic and low alkaline foods.