Antimicrobial effect of a combination of herb extract and organic acid against Bacillus subtilis spores

The sporicidal activities of the herbs were investigated to screen for novel antimicrobial substances against Bacillus subtilis spores. The bacterial inactivation effects of ethanol extracts of coriander, caraway, mace at concentrations of 0.5% (w/v) and 1.0–2.5% were about 10- and 100-fold respectively against spores. At pH 5, the antimicrobial activity was about 92%, but at pH 4 the sporicidal activity was particularly high, reducing the spore count by 99.99%. The 0.1–2.5% ethanol extract of herbs adjusted to pH 4–5 exhibited significantly marked deactivation effects, with 3–4 log CFU/mL reductions. The herb–acid combination exerted a further increase in sporicidal activity, with an additional 1–3 log CFU/mL reduction. The sporicidal mechanism was assumed to involve a two-step: (1) the hydrophobic binding of surfactants in the herbs onto the spore coat destroys its protein, and (2) the acid then penetrates into the interior, generating unstable growth conditions.