Antimicrobial Effect of Acetic Acid, Sodium Hypochlorite, and Thermal Treatments against Psychrotolerant Bacillus cereus Group Isolated from Lettuce (Lactuca sativa L.)

Abstract

Various food products distributed throughout the cold chain can present a health risk for consumers due to the presence of psychrotolerant B. cereus group species that possess enterotoxin genes and antibiotic resistance. As these bacteria can grow at the low temperatures used in the food industry, this study evaluated the antimicrobial efficacy of acetic acid, sodium hypochlorite, and thermal treatments for inhibition of psychrotolerant strains and the effect that differences in activation temperature (30 °C and 10 °C) have on their efficacy. The minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), and bacterial growth assay of acetic acid and thermal treatment showed an equal or higher antimicrobial efficacy in isolates activated at 10 °C than in those activated at 30 °C. In particular, psychrotolerant strains from the B. cereus group were completely eliminated with 0.25% acetic acid, regardless of the activation temperature. The possibility of tolerance was determined by observing responses in cells activated at 10 and 30 °C when exposed to different concentrations of sodium hypochlorite. Five isolates activated at 10 °C exhibited enhanced survivability in sodium hypochlorite compared to isolates activated at 30 °C, and these isolates were able to grow in sodium hypochlorite at concentrations of 250 ppm or higher. Although a significant difference in antimicrobial efficacy was observed for psychrotolerant B. cereus group strains depending on the activation temperature, acetic acid may be the most effective antimicrobial agent against psychrotolerant B. cereus species isolated from food products distributed in a cold chain.

Keywords: Bacillus cereus group; acetic acid; antimicrobial activity; antimicrobials; mild heat treatment; psychrotolerant; sodium hypochlorite.

Figure 1

Growth curve experiments of seven psychrotolerant B. cereus group strains activated at 30 °C (a-1, BCG34; b-1, BCG68; c-1, BCG76; d-1, BCG86; e-1, BCG88; f-1, BCG89; g-1, BCG102) and 10 °C (a-2, BCG34; b-2, BCG68; c-2, BCG76; d-2, BCG86; e-2, BCG88; f-2, BCG89; g-2, BCG102) with different concentrations (0.0625, 0.125, 0.25, 0.5, and 1%) of acetic acid during 24 h. The error bars indicate standard deviation from triplicate determinations.

Figure 2

Growth curve experiments of seven psychrotolerant B. cereus group strains activated at 30 °C (a-1, BCG34; b-1, BCG68; c-1, BCG76; d-1, BCG86; e-1, BCG88; f-1, BCG89; g-1, BCG102) and 10 °C (a-2, BCG34; b-2, BCG68; c-2, BCG76; d-2, BCG86; e-2, BCG88; f-2, BCG89; g-2, BCG102) with different concentration (50, 100, 200, 250, and 300 ppm) of sodium hypochlorite during 24 hr. The error bars indicate standard deviation from triplicate determinations.