Mechanisms of antimicrobial action of antiseptics and disinfectants: an increasingly important area of investigation

A. D. Russell,*

Welsh School of Pharmacy, Cardiff University, Cardiff CF10 3XF, UK

The term ‘biocide’ is increasingly being used to describe compounds with antiseptic, disinfectant or, sometimes, preservative activity. A compound might be used in only one such capacity or possess two or even all of these properties.1 Until fairly recently, there were two long-held general opinions about biocides. The first was that, as long as they were effective, there was little reason (apart from academic value) to determine how they achieved their inhibitory or lethal effects. The second, widely perceived, view was that antiseptics and disinfectants acted as general protoplasmic poisons and, as such, merited little attention.

At the beginning of the twentieth century, there were few drugs available for the treatment of infections. Antiseptics and disinfectants had at that stage been employed for various purposes and in various guises, notable examples being phenol (carbolic acid), mercuric chloride, chlorine, hypochlorites and iodine. Quaternary ammonium compounds (QACs) were described in 1916 but were not used commercially for another 19 years or so.2 Early studies on the action of such compounds concentrated on the kinetics of bacterial inactivation,3 although Cooper4 notably described the relationship between phenolics (phenol and meta-cresol) and bacterial proteins as being of importance in their mechanism of disinfection. In particular, it was considered that the protein structure inside the bacterial cell was affected. Subsequently, Knaysi et al.5,6 reported on the ‘manner of death’ of bacteria, mainly Escherichia coli, exposed to mild chemical and physical agents, concluding that the order of death was determined by the distribution of resistances among the cells. Later, Jordan & Jacobs7 found the distribution of resistance of E. coli treated with phenol to be normal at all phenol concentrations used. Interestingly, specific enzymes were considered by some workers8,9 to be involved in bacterial inactivation by biocidal agents.

Penicillin (benzylpenicillin, penicillin G) began to be used as a chemotherapeutic antibiotic in the 1940s. Early studies on its mode of action were undertaken by Gardner10 and Duguid,11 and Eagle & Musselman12 demonstrated a paradoxical effect of high concentrations on staphylococci. However, it was not until the isolation of the ‘Park nucleotides’ and subsequent studies, especially those by Strominger et al.,13,14 that it was realized that not only was penicillin an important antibiotic but also that it was a valuable tool in studying bacterial peptidoglycan biosynthesis. Further discoveries, the production of other ß-lactam antibiotics and work demonstrating that tetracyclines and other antibiotics inhibited bacterial protein synthesis,15 provided a stimulus for additional and comprehensive investigations to be undertaken on bacterial inactivation mechanisms (and also, naturally, on bacterial resistance mechanisms) by such selectively toxic drugs. It was realized that these aspects, linked to structure–activity relationships, provided the key to the development of many new, improved antibiotic molecules.

Until recently, there has not been the same enthusiasm for studying the mode of action of biocidal agents. However, the literature does contain a surprising, if scattered, number of publications about the mechanisms of inhibition and inactivation of Gram-positive non-sporulating bacteria (although the information on mycobacteria is disappointing), bacterial spores (as well as germinating and outgrowing ones) and Gram-negative bacteria.16,17 Less data are available about the mechanisms of fungal and viral inactivation, with very few comprehensive studies on the mechanisms of protozoal inactivation.18–20 It is rather surprising that information on the mechanisms of viral inactivation, in particular, is sparse. Furthermore, it is not known why the MICs of biocides such as chlorhexidine and QACs are of an equivalent order for both mycobacteria and staphylococci whereas these cationic biocides possess low mycobactericidal potency but are rapidly lethal to the latter.21 Contrary to early suggestions by Rahn et al.,8,9 it is now considered to be highly unlikely that bacterial cells possess a single type of target enzyme the inhibition or inactivation of which by a biocide is responsible for a loss of cell viability.16 Thus, although triclosan has repeatedly been shown to inhibit enoyl reductase (see below),22,23 work from this laboratory has demonstrated that other events are involved in bacterial inactivation.24

It has become clear that some antiseptics and disinfectants on the one hand and antibiotics on the other have similar effects on bacteria. For example, (1) filament formation is induced in Gram-negative bacteria by both antibiotics (ß-lactams, novobiocin, fluoroquinolones) and biocides (phenoxyethanol, phenylethyl alcohol, chloroacetamide, acridines);25 (2) inhibition of enoyl reductase, involved in fatty acid synthesis, is inhibited by both isoniazid, an important antitubercular drug, and the bisphenol (phenylether) triclosan;26 and (3) autolysis brought about by low concentrations of phenolics and inorganic and organic mercury compounds has been suggested as being similar to that following bacterial exposure to penicillin.16 However, it is interesting that E. coli cells exposed to phenoxyethanol, proflavine or chloroacetamide showed the same susceptibility to ampicillin and norfloxacin as cells not pre-exposed to a biocide.25 It would be interesting to determine whether biocide-resistant E. coli cells showed differences in response to these important antibiotics. A correlation was found earlier by McKellar et al.27 as a consequence of a non-specific increase in impermeability. Additionally, it must be noted that the end result may be brought about by different inhibitory or lethal mechanisms.28

It has also become increasingly obvious that insusceptibility mechanisms to biocides and antibiotics may be similar although not necessarily identical.15 A natural (intrinsic) insusceptibility to both groups may be shown by Gram-negative bacteria and mycobacteria, with outer membrane or cell wall impermeability being responsible. Additionally, efflux systems may remove toxic drug and biocide molecules from the cell, although a key issue here relates to the concentrations at which the compounds are used. Thus, with drugs it is usually necessary to equate MICs or minimum bactericidal concentrations (MBCs) with blood serum levels. By contrast, biocides are essentially used for external purposes at concentrations likely to be considerably higher than MICs or MBCs; such concentrations are unlikely to be effluxed from bacterial cells. Nevertheless, the possibility remains that low, ‘residual’ concentrations could act as a focus for the survival of organisms containing efflux genes or for the gradual or rapid development of biocide-resistant bacteria.29 This reinforces the argument that the effects of biocides on bacterial (and, indeed, other types of microbial) cells should be examined over a wide range of concentrations.30

There are other reasons for studying the mechanisms of biocides. At present, comparatively little is known about the uptake of biocides into bacterial (and other microbial) cells. The probability exists that targeted drug delivery, whereby a biocide can readily reach its target site(s) within a cell, could lead to greater efficacy. An aspect that needs to be considered is the possible design of new biocidal molecules based on known effects of current molecules; there is little evidence that this is happening. The only significant new biocides to be introduced in the past few years have been ortho-phthalaldehyde (OPA)31 and those based on peracetic acid.32 However, both OPA and peracetic acid are themselves ‘old’ molecules that have been examined in a new, antimicrobial context.

In recent years, rotation of disinfectants in hospitals and elsewhere, e.g. in the pharmaceutical and food industries, has been advocated to prevent the development of bacterial resistance. It has been claimed that, ideally, one disinfectant should be replaced by another having a dissimilar mechanism of action.33 Clearly, a knowledge of the ways in which such agents act is an essential component of such a policy.

In conclusion, there is an urgent need to investigate more fully the nature of the inhibitory and lethal effects of antiseptics and disinfectants on a range of microorganisms and microbial entities. Possible multiple target sites and concentration-dependent effects would form an important aspect of such studies, which would also provide a better understanding of intrinsic and acquired bacterial resistance mechanisms and of the possible linkage between biocide usage and antibiotic resistance.

Notes

* Tel: +44-2920-875812; Fax: +44-2920-874149; E-mail: russellD2{at}cardiff.ac.uk Back

REFERENCES

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2 . Russell, A. D. (2002). Introduction of biocides into clinical practice and the impact on antibiotic resistant bacteria. Journal of Applied Microbiology, Symp. Suppl. (in press).

3 . Kronig, B. & Paul, T. L. (1897). The chemical foundations of the study of disinfection and of the action of poisons. Zeitschrift für Hygiene 25, 1–112.

4 . Cooper, E. A. (1912). On the relationship of phenol and m-cresol to proteins: a contribution to our knowledge of the mechanism of disinfection. Biochemical Journal 6, 362–87.

5 . Knaysi, G. (1930). Disinfection. I. The development of our knowledge of disinfection. Journal of Infectious Diseases 47, 293–302.

6 . Knaysi, G. & Morris, G. (1930). The manner of death of certain bacteria and yeast when subjected to mild chemical and physical agents. Journal of Infectious Diseases 47, 303–17.

7 . Jordan, R. C. & Jacobs, S. E. (1944). Studies on the dynamics of disinfection. I. New data on the reaction between phenol and Bact. coli using an improved technique, together with an analysis of the distribution of resistance amongst the cells of the bacterial population studied. Journal of Hygiene (Cambridge) 43, 275–89.

8 . Rahn, O. & Schroder, W. R. (1941). Inactivation of enzymes as the cause of death in bacteria. Biodynamica 3, 199–208.

9 . Roberts, M. H. & Rahn, O. (1946). The amount of enzyme inactivation at bacteriostatic and bactericidal concentrations of disinfectants. Journal of Bacteriology 42, 639–44.

10 . Gardner, A. D. (1940). Morphological effects of penicillin on bacteria. Nature 146, 837–8.

11 . Duguid, J. P. (1946). Sensitivity of bacteria to the action of penicillin. Edinburgh Medical Journal 53, 401–12.

12 . Eagle, H. & Musselman, A. D. (1948). Rate of bactericidal action of penicillin in vitro as a function of its concentration and its paradoxically reduced activity at high concentrations against certain organisms. Journal of Experimental Medicine 88, 99–131.

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32 . Block, S. S. (2001). Peroxygen compounds. In Disinfection, Sterilization and Preservation, 5th edn, (Block, S. S., Ed.), pp. 185–204. Lippincott, Williams & Williams, Philadelphia, PA.

33 . Murtough, S. M., Hiom, S. J., Palmer, M. & Russell, A. D. (2001). Biocide rotation in the healthcare setting: is there a case for policy implementation? Journal of Hospital Infection 48, 1–6.[ISI][Medline]