Biocides: Definition, Mode of action

Biocides are inorganic or synthetic organic molecules used to disinfect, sanitize, or sterilize objects and surfaces, and to preserve materials or processes from microbiological degradation(Chapman, 2003). Because biocides range in antimicrobial activity, other terms maybe more specific, including “-static,” referring to agents which inhibit growth (e.g., bacteriostatic, fungistatic, and sporistatic) and “-cidal,” referring to agents which kill the target organism (e.g., sporicidal, virucidal, and bactericidal) (McDonnell and Russell, 1999).
Biocides are used extensively in hospitals for a variety of topical and hard-surface applications. In particular, they are an essential part of infection control practices and aid in the prevention of nosocomial infections (McDonnell and Russell, 1999).
Table (1) shows the chemical structure and examples of the common groups of disinfectants and antiseptics:

The mechanisms of the antibacterial action of biocides are still not perfectly understood. Biocides are likely to have multiple target sites within a bacterial cell. Biocides are known to interact with bacterial cell walls or envelopes (e.g. glutaraldehyde), produce changes in cytoplasmic membrane integrity (cationic agents), dissipate the proton-motive force
(organic acids and esters), inhibit membrane enzymes (thiol interactors), act as alkylating agents (ethylene oxide), cross-linking agents (aldehydes) and intercalating agents (acridines), or otherwise interact with identifiable chemical groups in the cell (Russell, 2002b).
A summary of the mechanisms of antibacterial action of antiseptics and disinfectants is shown in table (2).

Because the mechanisms of action of biocides are often poorly understood, detailed evaluation of bacterial resistance mechanisms remains disappointing. Nevertheless, it is known that at least some (efflux, impermeability, modification of target sites) of the general mechanisms responsible for antibiotic resistance are also applicable to biocides. The possibility exists of ‘cross-resistance’ arising between antibiotics and biocides(Russell, 2002b).
Research on antibiotics and biocides has traditionally proceeded along separate lines. The reasons for this is probably because antibiotics rely on selective toxicity for their activity, although this does not imply that they are without side effects on human and animal cells. Selective toxicity, on the other hand, is not a prerequisite for the use of biocides, although their actual and potential toxicity should never be ignored (Russell, 2002a).
By the very nature of the usage of the two types of antimicrobial agents, tests for evaluating their antibacterial potency differ considerably. With antibiotics, bacterial susceptibility is determined mainly by disc sensitivity and minimum inhibitory concentration (MIC) procedures. MICs can be linked to blood or serum levels and peak drug concentration and mutant prevention concentration in vivo. By contrast, such methodology has limited applications for most types of biocide evaluations. Many biocides diffuse poorly in agar, some biocides interact with agar constituents, and MICs often provide little more than a starting point for the information needed about the lethal effects of ‘in-use’ concentrations. Therefore, Standard European tests are increasingly being made available to measure such lethal effects for a variety of purposes(Russell, 2002a).