Acid Adaptation Enhances Tolerance of Escherichia coli O157:H7 to High Voltage Atmospheric Cold Plasma in Raw Pineapple Juice

Abstract

Pathogens that adapt to environmental stress can develop an increased tolerance to some physical or chemical antimicrobial treatments. The main objective of this study was to determine if acid adaptation increased the tolerance of Escherichia coli O157:H7 to high voltage atmospheric cold plasma (HVACP) in raw pineapple juice. Samples (10 mL) of juice were inoculated with non-acid-adapted (NAA) or acid-adapted (AA) E. coli to obtain a viable count of ~7.00 log₁₀ CFU/mL. The samples were exposed to HVACP (70 kV) for 1-7 min, with inoculated non-HVACP-treated juice serving as a control. Juice samples were analyzed for survivors at 0.1 h and after 24 h of refrigeration (4 °C). Samples analyzed after 24 h exhibited significant decreases in viable NAA cells with sub-lethal injury detected in both NAA and AA survivors (p < 0.05). No NAA survivor in juice exposed to HVACP for 5 or 7 min was detected after 24 h. However, the number of AA survivors was 3.33 and 3.09 log₁₀ CFU/mL in juice treated for 5 and 7 min, respectively (p < 0.05). These results indicate that acid adaptation increases the tolerance of E. coli to HVACP in pineapple juice. The potentially higher tolerance of AA E. coli O157:H7 to HVACP should be considered in developing safe juice processing parameters for this novel non-thermal technology.

Keywords: Escherichia coli O157:H7; acid adaptation; cold plasma; pineapple juice; sub-lethal injury.

Figure 1

Schematic of the components of the dielectric barrier, high voltage atmospheric cold plasma (HVACP) system used to treat pineapple juice.

Figure 2

Effect of holding time (0.1 h and 24 h) following HVACP (70 kV) treatment on the viability of non-acid-adapted (A) and acid-adapted (B) Escherichia coli O157:H7 survivors in pineapple juice. The survivors were recovered on sorbitol MacConkey (SMAC) agar. For each treatment time, different letters (A or B) above the bars indicate a significant difference in the number of survivors at 0.1 h and 24 h post-treatment (p < 0.05). Asterisk (*) indicates that the pathogen was not detected.

Figure 3

Effect of holding time (0.1 h and 24 h) following HVACP (70 kV) treatment on the viability of non-acid-adapted (A) and acid-adapted (B) Escherichia coli O157:H7 survivors in pineapple juice. Survivors were recovered on a thin agar layer (TAL) medium. For each treatment time, different letters (A or B) above the bars indicate a significant difference in the number of survivors at 0.1 h and 24 h post-treatment (p < 0.05). Asterisk (*) indicates that the pathogen was not detected.

Figure 4

Effect of physiological state (NAA and AA) on the survival of Escherichia coli O157:H7 in raw pineapple juice held at 4 °C for 24 h after HVACP (70 kV) treatment. Survivors were recovered on sorbitol MacConkey (SMAC) agar (A) and TAL medium (B). For each treatment time, different first letters (A or B) above the bars indicate a significant difference in the viable counts of NAA and AA cells (p < 0.05). Asterisk (*) indicates that the pathogen was not detected.