Antibacterial activity of ethanol plant extract combined with organic acid against pathogenic and spoilage bacteria at different pH conditions

The major foodborne pathogens have caused significant foodborne illness and even death, while spoilage microorganisms lead to remarkable public health issues worldwide. To address these concerns, combination of the natural preservative with potential organic acid is the superior solution because the use of synthetic preservatives alone are aware of health impacts to consumers. Therefore, this research aimed to identify the combined effect of these two substances against pathogenic and spoilage bacterial strains in different pH conditions (non-adjusted, mildly acidic, and neutral pH conditions). Eighteen ethanol plant extracts with five organic acids were used in this study to test for antibacterial activities through disk diffusion method to determine the zone of inhibition, and plate dilution with broth dilution method to determine the MIC and MBC respectively. The potential plant extract and organic acid that demonstrated stronger activity were selected for the combinatorial study. In all tested pH conditions, clove extract demonstrated high potency when lactic acid or acetic acid was incorporated. At mildly acidic conditions, combinations were more effective in reducing the MICs of clove extract combined with acetic acid against all tested strains. The combination resulted as a synergistic effect with the FIC index of 0.5 for the test on methicillin-resistant Staphylococcus aureus 11D1677, Listeria monocytogenes 4b, and Salmonella Typhimurium 12529. Therefore, the amplified activity was established when clove extract combined with acetic acid. The obtained data was evaluated to have a combinatorial effect, which could be a potential object for controlling pathogenic and spoilage bacteria.