Whither triclosan?

A. D. Russell*

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


    Abstract
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 Abstract
 Introduction
 Mechanism(s) of action of...
 Bacterial resistance to...
 Possible association between...
 Uses of triclosan
 References
 
Triclosan has activity against many, but not all, types of Gram-positive and Gram-negative bacteria. It is bacteriostatic at low concentrations, but higher concentrations are bactericidal. Pseudomonas aeruginosa is highly resistant, whereas methicillin-resistant Staphylococcus aureus strains are inhibited over a range of ~0.1–2 mg/L. Triclosan shows significant activity against some mycobacteria, but is not sporicidal. Its growth-inhibitory properties result from an inhibition of enoyl reductase, FabI. Membrane-destabilizing effects are likely to be responsible for bacterial inactivation by higher concentrations. Resistance can arise from mutations in, and/or overproduction of, FabI, impermeability or efflux. Whilst triclosan resistance in laboratory experiments may be associated with changes in antibiotic susceptibility, comprehensive environmental surveys have not demonstrated any association between triclosan usage and antibiotic resistance. Triclosan has several important uses, and the future aim must be to retain these applications whilst eliminating the more frivolous and unnecessary ones.

Keywords: mechanisms of action, mechanisms of resistance, antibiotic resistance


    Introduction
 Top
 Abstract
 Introduction
 Mechanism(s) of action of...
 Bacterial resistance to...
 Possible association between...
 Uses of triclosan
 References
 
The phenylether or chlorinated bisphenol, triclosan, is an antimicrobial agent that has been employed for a variety of purposes for more than 20 years. It is used clinically and in oral hygiene products, and is incorporated into many types of cosmetic formulations.1 Triclosan has a broad range of activity that encompasses many, but not all, types of Gram-positive and Gram-negative non-sporulating bacteria, some fungi,2,3 Plasmodium falciparum and Toxoplasma gondii.4 Its spectrum3,5 includes high activity (low MICs often of the order of 0.01–0.1 mg/L) against staphylococci, some streptococci, some mycobacteria, Escherichia coli and Proteus spp. Methicillin-resistant Staphylococcus aureus (MRSA) strains may or may not show an elevated triclosan MIC.6 Enterococci are much less susceptible than staphylococci,5 and Pseudomonas aeruginosa is highly resistant.3,5 Bacterial spores are unaffected. Triclosan is bacteriostatic at low concentrations but higher levels are bactericidal.7,8

In recent years, there has been renewed interest in the antibacterial properties of triclosan. This stems from the finding that triclosan had a more specific mechanism of action than hitherto realized,9 and that there might be a link between triclosan usage and antibiotic resistance.10 These aspects have since been examined extensively, and it is the purpose of this short paper to consider four aspects that pertain to this issue: (i) the mechanism(s) of action of triclosan; (ii) the mechanism(s) of bacterial resistance to triclosan; (iii) the possible association between triclosan usage in the clinical and domiciliary environments and antibiotic resistance; and (iv) its current and future usage.


    Mechanism(s) of action of triclosan
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 Abstract
 Introduction
 Mechanism(s) of action of...
 Bacterial resistance to...
 Possible association between...
 Uses of triclosan
 References
 
Earlier studies, reviewed by Russell,11 suggested that the bacterial cytoplasmic (inner) membrane was the major target for triclosan action. However, it was later demonstrated in E. coli, S. aureus and other triclosan-susceptible organisms that the growth-inhibitory activities of the phenylether resulted from blocking lipid synthesis by specifically inhibiting an NADH-dependent enoyl-acyl carrier protein (ACP) reductase, or FabI.9,12,13 The enoyl reductase, InhA, in Mycobacterium smegmatis was also found to be a target for triclosan action.14

The question then arises as to whether inhibition of a single enzyme by triclosan is responsible for its inhibitory and lethal actions.11,15,16 Triclosan is normally employed in practice at concentrations much greater than the MICs cited above against highly susceptible bacteria. At such concentrations triclosan is rapidly bactericidal,7,8 and this lethal activity extends to triclosan-resistant strains of E. coli.17 Triclosan-induced K+ leakage, indicative of membrane damage, occurs at bactericidal levels.7,8 Membrane-destabilizing effects have also been demonstrated by Villalain et al.18 Triclosan demonstrates a Z-pattern type of adsorption, which is indicative of the breakdown of a structure, presumably the membrane, and the generation of new adsorbing sites.19 As with other biocidal agents, triclosan possesses more than one type of action, and it is possible to delineate its growth-inhibitory and lethal effects.


    Bacterial resistance to triclosan
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 Mechanism(s) of action of...
 Bacterial resistance to...
 Possible association between...
 Uses of triclosan
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In the laboratory, triclosan-resistant bacteria can be produced fairly readily by serial passage in increasing triclosan concentrations17 or by isolation of resistant colonies within growth inhibition zones around a triclosan disc.7,8 Resistance may be due to overproduction of enoyl reductase, an insensitive form of enzyme or to changes in cellular impermeability in E. coli.17

In P. aeruginosa, which is intrinsically resistant to triclosan, resistance could be due to a non-susceptible enoyl reductase (both triclosan-susceptible and -non-susceptible enzymes have been found20), an outer membrane permeability barrier or efflux. The permeabilizer, ethylenediamine tetraacetic acid (EDTA), does not increase susceptibility to triclosan,21 and efflux has been stated to be the major reason for triclosan non-susceptibility.2224

MRSA strains may or may not show elevated triclosan MICs.68,25 Fan et al.26 found that all S. aureus strains with elevated triclosan MICs overexpressed FabI production by three- to five-fold, and that strains with the highest MICs (1–2 mg/L) also had mutations in FabI.


    Possible association between triclosan and antibiotic resistance
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 Introduction
 Mechanism(s) of action of...
 Bacterial resistance to...
 Possible association between...
 Uses of triclosan
 References
 
The overexpression of marA, soxS or acrAB in laboratory or clinical strains of E. coli reduces their susceptibility to triclosan, and also to ampicillin, tetracyclines and fluoroquinolones.10 The increases in resistance in terms of MIC are, however, low. Exposure of a triclosan-susceptible mutant of P. aeruginosa to triclosan switches on an efflux pump that renders the cells highly resistant to ciprofloxacin.22 However, there is no evidence that triclosan usage is responsible for clinical ciprofloxacin resistance in either human or veterinary medicine.23

Susceptibility of MRSA strains to triclosan has changed little over a 10 year period,6 and there does not seem to be any association between triclosan response in MRSA and other strains of S. aureus and antibiotic susceptibility or resistance.7,8

Both triclosan and the important antitubercular drug, isoniazid, inhibit enoyl reductase in M. smegmatis and Mycobacterium tuberculosis.14,27,28 In the former, mutations in the inhA gene produce increases in resistance to both triclosan and isoniazid. Triclosan binds in a similar manner to the enoyl reductase (InhA) of mycobacteria and that of E. coli. The InhA from M. tuberculosis is 36% identical and 65% similar to EnvM, and 87% identical and 97% similar to the InhA from M. smegmatis.14 Nevertheless, there are differences in the effects of triclosan and isoniazid27,28 such that inhibitors targeted at the enoyl substrate binding site could produce effective drugs against isoniazid-resistant strains of M. tuberculosis.27

It has been emphasized that laboratory studies have a useful role to play in evaluating mechanisms of action of and resistance to biocides, including triclosan, but that these should, wherever possible, be related to the clinical and other uses of these agents.15,16 Do biocides therefore select for antibiotic resistance?29 Certainly, there are some similarities in the manner in which bacteria resist the action of both types of antibacterial agents.30 Several authors have purported to show a relationship between the use of triclosan (or other biocides) and antibiotic resistance.3133 Others have cast doubt on this proposal,16,29,34,35 whilst emphasizing that, like all biocides, triclosan should be used only when appropriate.

Some recent surveys on the use of triclosan and other biocides add weight to these doubts. In the first,36 conducted over a 10 year period, it was found that there was no relationship between triclosan usage and antibiotic resistance in MRSA and P. aeruginosa. In the second,37 it was shown that there were no significant differences in overall titres of bacteria, potential pathogens or frequencies of antibiotic resistance in a single-time analysis of homes that did or did not use surface antibacterial agents. A comprehensive survey by Cole et al.38 could find no relationship between the use of triclosan and other biocides and antibiotic resistance in homes where biocidal products were or were not being used.

What, then, of the incorporation of triclosan into oral products? Will the use of triclosan in dental hygiene products result in the development of triclosan-resistant bacteria with reduced susceptibility to important antibiotics? An Expert Panel review concluded in 2000 that there was no evidence of resistant, opportunistic or pathogenic microorganisms developing.39 The short-term use of triclosan had no major impact on normal oral microflora or on the streptococcal susceptibility to antibiotics.40 Chronic exposure to triclosan did not demonstrate significant decreases in antibiotic susceptibility in dental bacteria.41 In general terms, the use of antimicrobial agents in dental care products in order to reduce plaque is considered to be justified.42

Overall, there is no convincing evidence to support the contention that triclosan usage has resulted in the clinical development of antibiotic-resistant Gram-negative bacteria, antibiotic-resistant cocci or isoniazid-resistant M. tuberculosis.16 Nevertheless, it would be wise to restrict the use of triclosan to areas where it has been shown to be effective.43


    Uses of triclosan
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 Mechanism(s) of action of...
 Bacterial resistance to...
 Possible association between...
 Uses of triclosan
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There remain concerns about the unnecessary use of triclosan and other biocides in the home and in clinical settings.44,45 Furthermore, triclosan has been found in human milk and in the aquatic environment in Sweden.46 Triclosan has been used in skin-care products for some 30 years. It has also been employed as surgical scrubs, handwashes, body washes to control MRSA and in dental hygiene products. More recent uses include the suggestion that it could be employed to control encrustation and blockage of Foley catheters caused by Proteus mirabilis, an organism that is highly susceptible to triclosan.47 In addition, novel inhibitors of FabI are being developed.48 Clearly, in view of the concerns expressed about the phenylether, the future aim should be to retain its important and valuable applications whilst eliminating the more frivolous and unnecessary ones.

Finally, although it is sometimes stated that resistance to triclosan and other biocides is increasing,44 this conclusion is generally based upon MICs rather than bactericidal estimations. Bacterial resistance to disinfectants in general is most certainly not a new phenomenon, and there are known examples of reduced susceptibility being described over a century ago.49 Triclosan, of course, is of more recent vintage. Consequently, it is necessary to continue to monitor whether reduced susceptibility to it and to antibiotics occurs.


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


    References
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 Abstract
 Introduction
 Mechanism(s) of action of...
 Bacterial resistance to...
 Possible association between...
 Uses of triclosan
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1 . Parfitt, K. (1999). In Martindale, The Complete Drug Reference, p. 1127. Pharmaceutical Press, London, UK.

2 . Jones, R. D., Jampani, H. B., Neman, J. L. et al. (2000). Triclosan: a review of effectiveness and safety in health care settings. American Journal of Infection Control 28, 184–96.[CrossRef][ISI][Medline]

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4 . McLeod, R., Muench, S. P., Rafferty, J. B. et al. (2001). Triclosan inhibits the growth of Plasmodium falciparum and Toxoplasma gondii by inhibition of apicompexan FabI. International Journal of Parasitology 31, 109–13.[CrossRef][ISI][Medline]

5 . Ciba-Geigy. (1988). Document: Irgasan DP 300. Ciba, Wyhlen, Germany.

6 . Al-Doori, Z., Morrison, D., Edwards, G. et al. (2003). Susceptibility of MRSA to triclosan. Journal of Antimicrobial Chemotherapy 51, 185–6.[Free Full Text]

7 . Suller, M. T. E. & Russell, A. D. (1999). Antibiotic and biocide resistance in methicillin-resistant Staphylococcus aureus and vancomycin-resistant enterococcus. Journal of Hospital Infection 43, 281–91.[CrossRef][ISI][Medline]

8 . Suller, M. T. E. & Russell, A. D. (2000). Triclosan and antibiotic resistance in Staphylococcus aureus. Journal of Antimicrobial Chemotherapy 46, 11–8.[Abstract/Free Full Text]

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10 . McMurry, L. M., Oethimger, M. & Levy, S. B. (1998). Overexpression of marA, soxS or acrAB produces resistance to triclosan in Escherichia coli. FEMS Microbiology Letters 186, 305–9.

11 . Russell, A. D. (2003). Similarities and differences in the responses of microorganisms to biocides. Journal of Antimicrobial Chemotherapy 52, 750–63.[Abstract/Free Full Text]

12 . Levy, C. W., Roujeinikova, A., Sedelnikova, S. et al. (1999). Molecular basis of triclosan activity. Nature 398, 383–4.[CrossRef][ISI][Medline]

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14 . McMurry, L. M., McDermott, P. F. & Levy, S. B. (1999). Genetic evidence that InhA of Mycobacterium smegmatis is a target for triclosan. Antimicrobial Agents and Chemotherapy 43, 711–3.[Abstract/Free Full Text]

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21 . Lear, J. C., Maillard, J.-Y., Dettmar, P. W. et al. (2002). Chloroxylenol- and triclosan-tolerant bacteria from industrial sources. Journal of Industrial Microbiology and Biotechnology 29, 238–42.[CrossRef][Medline]

22 . Chuanchen, R., Beinlich, K., Hoang, T. T. et al. (2001). Cross-resistance between triclosan and antibiotics is mediated by multidrug efflux pumps: exposure of a susceptible mutant strain to triclosan selects nfxb mutants overexpressing MexCD-OprJ. Antimicrobial Agents and Chemotherapy 45, 428–32.[Abstract/Free Full Text]

23 . Beinlich, K. I., Chuanchen, R. & Schweizer, H. P. (2001). Contribution of multidrug efflux pumps to multiple antibiotic resistance in veterinary isolates of Pseudomonas aeruginosa. FEMS Microbiology Letters 198, 129–34.[CrossRef][ISI][Medline]

24 . Chuanchen, R., Karkhoff-Schweizer, R. R. & Schweizer, H. P. (2003). High-level triclosan resistance in Pseudomonas aeruginosa is solely a result of efflux. American Journal of Infection Control 31, 124–7.[CrossRef][ISI][Medline]

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27 . Parikh, S. I., Xiao, G. & Tonge, P. J. (2000). Inhibition of InhA, the enoyl reductase from Mycobacterium tuberculosis, by triclosan and isoniazid. Biochemistry 39, 7645–50.[CrossRef][ISI][Medline]

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29 . Russell, A. D. (2000). Do biocides select for antibiotic resistance? Journal of Pharmacy and Pharmacology 52, 227–33.[CrossRef][ISI][Medline]

30 . Poole, K. (2002). Mechanisms of bacterial biocide and antibiotic resistance. Journal of Applied Microbiology 92, 55S–64S.[CrossRef][ISI][Medline]

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36 . Lambert, R. J. W., Graf, J. F. & Sedlak, R. I. (2002). Antimicrobial resistance and cross-resistance in several bacterial species between 1989 and 2000. In Program and Abstracts of the Forty-second Interscience Conference on Antimicrobial Agents and Chemotherapy, San Diego, CA, USA, 2002. Abstract C2-1127, p. 103. American Society for Microbiology, Washington, DC, USA.

37 . Marshall, B. M., Robleto, E., Dumont, T. et al. (2003). The frequency of bacteria and antibiotic resistance in homes that use and do not use surface antibacterial agents. In 103rd General Meeting of the American Society for Microbiology, Washington, DC, USA, 2003. Abstract A-147. American Society for Microbiology, Washington, DC, USA.

38 . Cole, E. C., Addison, R. M., Rubino, J. R. et al. (2003). Investigation of antibiotic and antibacterial agent cross-resistance in target bacteria from homes of antibacterial product users and nonusers. Journal of Applied Microbiology 95, 664–76.[CrossRef][ISI][Medline]

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47 . Stickler, D. J., Jones, G. L. & Russell, A. D. (2003). Control of encrustation and blockage of Foley catheters. Lancet 361, 1435–7.[CrossRef][ISI][Medline]

48 . McDevitt, D., Payne, D. J., Holmes, D. J. et al. (2002). Novel targets for the future development of antibacterial agents. Journal of Applied Microbiology 92, 28S–34S.[CrossRef][ISI][Medline]

49 . Russell, A. D. (2004). Bacterial adaptation and resistance to antiseptics, disinfectants and preservatives is not a new phenomenon. Journal of Hospital Infection, in press.