Sensitivity of Planktonic Cells of Staphylococcus aureus to Elevated Hydrostatic Pressure as Affected by Mild Heat, Carvacrol, Nisin, and Caprylic Acid

Efficacy and mechanism of carvacrol with octanoic acid against mastitis causing multi-drug-resistant pathogens

In the present investigation, we determined the in vitro antimicrobial activity of eight essential oils (EOs) and three medium-chain fatty acids (MCFAs) alone and in combination against Staphylococcus aureus ATCC 700698, Klebsiella pneumoniae ATCC 700603, and E. coli FcW5. The interactions between EOs and MCFAs were determined by fractional inhibitory concentration indices. Moreover, mode of action of selected bioactive components was studied by changes in bacterial surface charge, morphology, and membrane integrity assays. Among EOs, carvacrol (CAR), trans-cinnamaldehyde (TC), and thymol (TM) showed strong antimicrobial activity. In combination study, CAR+OA (octanoic acid), CAR+DA (decanoic acid), and TM+OA were observed as the most significant (P≤0.05) which were also confirmed through time-kill plots. Based on these results, CAR+OA were found to be most efficacious in terms of killing time (P≤0.05). Changes in the surface charge, morphology, and membrane integrity upon the combined treatment of CAR+OA were also observed, which ultimately leads to cell death. Results suggest that CAR+OA when used in combination offer a significant (P≤0.05) additive antimicrobial activity against the selected pathogenic bacteria. Therefore, these natural bioactive molecules could be interesting alternatives to conventional therapy for the control of mastitis caused by multi-drug-resistant pathogens in bovine animals to ensure the milk safety.

Synergistic Effects of Nisin, Lysozyme, Lactic Acid, and CitricidalTM for Enhancing Pressure-Based Inactivation of Bacillus amyloliquefaciens, Geobacillus stearothermophilus, and Bacillus atrophaeus Endospores

The inactivation of bacterial endospores continues to be the main curtailment for further adoption of high-pressure processing in intrastate, interstate, and global food commerce. The current study investigated the effects of elevated hydrostatic pressure for the inactivation of endospore suspension of three indicator spore-forming bacteria of concern to the food industry. Additionally, the effects of four bacteriocin/bactericidal compounds were studied for augmenting the decontamination efficacy of the treatment. Elevated hydrostatic pressure at 650 MPa and at 50 °C was applied for 0 min (untreated control) and for 3, 7, and 11 min with and without 50K IU of nisin, 224 mg/L lysozyme, 1% lactic acid, and 1% Citricidal^TM. The results were statistically analyzed using Tukey- and Dunnett’s-adjusted ANOVA. Under the condition of our experiments, we observed that a well-designed pressure treatment synergized with mild heat and bacteriocin/bactericidal compounds could reduce up to >4 logs CFU/mL (i.e., >99.99%) of bacterial endospores. Additions of nisin and lysozyme were able, to a great extent, to augment (p < 0.05) the decontamination efficacy of pressure-based treatments against Bacillus amyloliquefaciens and Bacillus atrophaeus, while exhibiting no added benefit (p ≥ 0.05) for reducing endospores of Geobacillus stearothermophilus. The addition of lactic acid, however, was efficacious for augmenting the pressure-based reduction of bacterial endospores of the three microorganisms.

Inactivation of Shiga toxin-producing Escherichia coli O157: H7 and mesophilic background microbiota of meat homogenate using elevated hydrostatic pressure, mild heat, and thymol

A six-strain mixture of E. coli O157:H7 was exposed to 0 to 9 min of six treatments: (i) hydrostatic pressure (400 MPa) at 4 °C; (ii) hydrostatic pressure and thymol at 4 °C; (iii) thymol at 4 °C; (iv) heat at 40 °C; (v) hydrostatic pressure at 40 °C; and (vi) hydrostatic pressure and thymol at 40 °C. Pressure intensity level of 400 MPa and thymol concentration of 0.15% (w/v) were used for the experiments of inoculated pathogen (4.0 to 5.0 log CFU/mL) in a homogenate (10% nonsterilized beef in 90% sterilized distilled water). Temperature was precisely monitored by stainless steel water jacket surrounding pressure chamber (16 mL volume), mechanically linked to a refrigerated circulating water bath. Analyses of variance were conducted followed by Tukey- and Dunnett's-adjusted mean separations. Pathogen counts before treatment were 4.08 ± 0.7 log CFU/mL and were reduced (P < 0.05) to 0.67 ± 0.2 log CFU/mL after 6 min of pressure treatment. Thymol and mild heat (40 °C) further augmented decontamination efficacy of pressure treatments where in their presence, the mesophilic background microbiota counts of pressure-treated samples after 3, 6, and 9 min were reduced (P < 0.05) by 2.1, 2.5, and 3.1 log CFU/mL, respectively. Results of the current study indicate that thymol and mild heat could enhance decontamination efficacy of elevated hydrostatic pressure for pasteurization of food commodities. This could be of great significance for industry practitioners to assure microbiological safety of a product and cost optimization by benefiting from synergism of antimicrobials, mild heat, and elevated hydrostatic pressure. PRACTICAL APPLICATION: Thymol and mild heat could enhance decontamination efficacy of pressure-based pasteurizer that could be of great significance for practitioners. Application of pressure coupled with antimicrobial and mild heat could assure microbiological safety of a product, lead to cost optimization, and assist in meeting regulatory requirements of food commerce such as Hazard Analysis and Critical Control Point and Preventive Control for Human Food rule of Food Safety Modernization Act. Addition of an antimicrobial could have further co-benefits for the product due to residual protective effects during shelf-life and minimizing potential undesirable organoleptic changes associated with pressure treatments of >400 MPa.