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at a concentration of 4 ppm while only 2 ppm of oregano essential oil was needed (Paster et al., 1990).
II.C. Mechanism of action of antimicrobials Fundamentally, antimicrobials may act in two principal ways (Figure 24.1). They can either act as membrane perturbers or act as inhibitors of proton transfer dynamics. Examples of the latter are weak organic acids such as benzoic and sorbic acid, which have been widely used as preservatives to inhibit the growth of microorganisms in beverages (Brul and Coote, 1999; Hazan et al., 2004; Lambert and Stratford, 2004). The efficacy of these weak organic acids is a function of the environmental pH. For example, both benzoic and sorbic acid become more active with decreasing pH (Davidson et al., 2005). This is because some of the weak acid molecules dissociate in solution to form the negatively charged acid anions, while other molecules remain undissociated. Only the undissociated organic acid molecules are able to penetrate the bacterial cell envelope, whereas the dissociated anions are repelled due to electrostatic interactions. Since the concentration of the undissociated acid increases as the pH decreases, higher concentrations of active compounds can migrate into the intracellular space at lower pH to inhibit essential metabolic reactions (Brul et al., 2002). Alternatively, antimicrobials may act as membrane perturbers by inserting themselves into the
Membrane perturbation and disruption Capsule Nuclear matter
Cytoplasm Cell wall
Cell membrane
Disruption of proton transfer dynamics Diffusion/convection Addition of antimicrobial
Flagellum Pili
Extracellular space
Intracellular space DNA
Addition of antimicrobial
RNA Plasmid
Antimicrobial Intracellular space Space
COOH <->COO- H+
Figure 24.1 Illustration of the two basic mechanisms of action of antimicrobials. (Left) addition of antimicrobial leads to insertion into the membrane of bacteria, resulting in perturbation and disruption of the integrity of the membrane followed by leakage of intracellular content and permeation of extracellular content into the cell interior. (Right) Diffusion of an antimicrobial into the cell interior followed by dissociation and disruption of the proton transfer dynamics of the cell.