Non-Antibiotic Antimicrobial Agents: Mode of Action and Resistance

NON-ANTIBIOTIC ANTIMICROBIAL AGENTS: MODE OF ACTION AND RESISTANCE

INTRODUCTION

The group of agents which comprises antiseptics, disinfectants, chemical sterilants and preservatives (often collectively called biocides) have frequently been classified as non-specific protoplasmic poisons. Such a broad generalization is, however, far from the true position.

It is often convenient to consider the modes of action of biocides in terms of their targets within the bacterial cell, in particular the region of the cell in which their activity is deemed to predominate. Thus agents have been described variously as cell wall, membrane and cytoplasm-active. This characterization, whilst having the benefits of simplicity, does not necessarily describe their mechanism of action; this is best classified by effects on functional structures and cellular processes. The range and complexity of the reactions involved will become apparent from this account and Table 20.1, and it is worth emphasizing here that many of these substances exhibit concentration-dependant dual or even multiple effects.

Mechanisms Of Interaction

For a chemical to exhibit antimicrobial activity it usually has to undergo a sequence of events that begins with adsorption on to the microbial cell surface. This initial uptake is a physicochemical phenomenon which can be generally characterized into one of several uptake isotherms (Figure 20.1); it bears a relationship to the concentration exponent  which describes the influence of concentration on activity (Table 20.2). In the many cases where the chemical has an intracellular site of action, adsorption must be followed by passage through porin channels in Gram-negative cells, diffusion across, or into, the lipid-rich cytoplasmic membrane, and finally, interaction with proteins, enzymes, nucleic acids or other targets within the cytoplasm. These processes are markedly influenced by the physicochemical characteristics of the biocide, e.g. ionization constant and lipid solubility, so the wide diversity of structures exhibited by biocide molecules complicates the prediction of antimicrobial potency and explanation of their mechanisms of action. Despite this, it is important to recognize that there is a basis upon which the mode of action might be deduced, because there are certain molecular features of biocides that are associated with activity against particular cellular targets.

Antimicrobial Effects

Antimicrobial activity is often strongly influenced by the affinity of the biocide for structural or molecular components of the cell, and this, in turn, may depend upon the attraction of dissimilar charges or on hydrophobic interactions. Antimicrobial drugs whose active species is positively charged, e.g. quaternary ammonium compounds (QACs) and chlorohexidine, display an affinity for the negative charges of sugar residues on the microbial cell surface or phosphate groups on the membrane(s); adsorption of these biocides, and thus their antimicrobial activity, is increased as the pH rises and the cell surface becomes more electronegative. Antimicrobial chemicals possessing a long alkyl chain, on the other hand, may integrate into the hydrophobic region of phospholipid molecules within the membrane and so cause membrane disruption and fatal permeability changes. Further examples of structure-activity relationships are afforded by aldehydes, particularly glutaraldehyde, which is an electrophile that is able to react with molecules possessing thiol (SH) or amino groups, e.g. proteins. This reaction, too, increases with pH, so aldehydes are more active in alkaline conditions. Biocides containing heavy metal ions, e.g. silver or mercury, also damage or inactivate enzymes and structural proteins by virtue of interactions with thiol groups. A number of phenols and bisphenols incorporate a hydroxyl group that is capable of generating a labile proton, i.e. they are weak acids. A weakly acidic nature combined with significant lipid solubility are properties associated with uncoupling agents, i.e. those molecules that can disrupt the proton-motive force that is responsible for oxidative phosphorylation in the cell. It is thought that these molecules dissolve in the lipid bilayer of the membrane and act as proton conductors by virtue of their ionizability. This property, possessed by biocides such as phenoxythanol and fentichlor, results in the failure of many important energy-requiring processes in the cell, including the concentration and retention of sugars and amino acids.