6. Conclusions of the SCCS
- 6.1 Does triclosan pose a risk, and what are the current gaps in knowledge?
- 6.2 Is it still safe to use triclosan as a preservative in cosmetic products?
6.1 Does triclosan pose a risk, and what are the current gaps in knowledge?
The SCCS opinion states:
9. DATA GAPS ON SCIENTIFIC KNOWLEDGE
In the course of this work, several important gaps were noted. These can be divided into scientific and technical gaps:
- Environmental studies focussing on the identification and characterisation of resistance and cross-resistance to antibiotics following use of triclosan.
- In vitro studies to demonstrate whether triclosan, used at sub-lethal concentrations, triggers the emergence of antibiotic resistance and/or select bacteria resistant to antibiotics. This has only been demonstrated in a limited number of bacterial genera. Further information for other genera should be obtained.
- Despite in vitro evidence of the effect of triclosan on the emergence of antibiotic resistance and on the selection of bacteria resistant to antibiotics, epidemiological data indicating public health relevance are lacking.
- There is no information available on the maintenance and transferability of resistance and virulence markers in the presence of triclosan.
- Standardisation of methodologies to measure resistance and cross-resistance is needed.
- Information on production, use volumes is required to assess the exposure of bacteria to triclosan in various matrices.
- Data on the fate and bioavailability of triclosan in the environment are sparse. Information on environmental concentrations, contact time, microbial population present in the field and bacterial exposure, is insufficient to determine whether expression of resistance actually occurs in situ.
- No validated methodologies are available for the determination of the dose-response relationships and of the threshold triggering the emergence of antibiotic/biocide resistance and/or the selection of resistant bacteria.
- The role of bacterial biofilm in resistance to triclosan has been shown. Furthermore, bacterial biofilms are very common in the environment. Yet, most laboratories are not using biofilm tests to assess the efficacy of biocides (Cookson 2005). There are, currently, no European standards for the testing of disinfectants against biofilms for health care applications.
10. RISK ASSESSMENT
Triclosan is the most studied biocide with respect to antimicrobial resistance. Such a level of information, notably on its activity on bacteria, the identification of mechanisms of microbial resistance, including genomic and proteomic aspects, is commendable.
However, in spite of this level of information on mechanisms, information on the interaction between triclosan and microbial cells/communities including data on exposure and bioavailability in situ is lacking. Thus, a full risk assessment of triclosan cannot be performed. However, a number of points can be made:
- a hazard has been identified concerning the effect of triclosan on the regulation of resistance genes in bacteria
- mechanisms which can promote resistance and cross-resistance to biocides and antibiotics in bacteria have been identified
- high concentrations of triclosan (compared to concentrations known to select for resistance in in vitro experiments) have been measured in certain environmental compartments, however a link with cosmetic or other specific product uses could not be made.
- bacterial biofilms are widespread in the environment and are able to survive exposure to adverse environmental factors.
10.1. Limitation in activity
Bacteria can be classified according to their intrinsic resistance to biocides. Bacterial endospores are considered to be most resistant, followed by mycobacteria, Gram-negative bacteria and Gram-positive bacteria (Maillard 2005a). Triclosan is not sporicidal. It is not bactericidal against certain bacteria such as P. aeruginosa and Burkholderia sp. (Rose et al. 2009).It might also have limited activity against certain mycobacteria as these micro- organisms are considered to be less susceptible to biocides than Gram-negative bacteria.
Recent laboratory studies indicate that, during short exposures of mid-logarithmic growth phase to MIC concentrations (30 min at 0.12 mg/L), triclosan can trigger a genetic response in Gram-negative bacteria (e.g. E. coli, S. enterica) inducing expression of genes involved in biocide and antibiotic resistance. In addition, in Listeria monocytogenes triclosan concentrations of 19 mg/L to 150 mg/L activate the expression of virulence factors (Kastbjerg et al. 2010).
Concerning the genetic aspects; genetic mobile elements play an important role in bacterial resistance response since they contain resistance genes (coding for pump, enzyme, qnr factors, etc) which can confer resistance to different drug families. The gene pool encoding for various mechanisms that confer resistance to antimicrobials has been shown to be present in soil bacteria (Dantas et al. 2008). Although exposure to some biocides (such as quaternary ammonium compounds) favours the dissemination and maintenance of such genetic mobile elements in bacteria and subsequently may facilitate the exchange of key genes between bacterial species (Paulsen et al. 1998, Pearce et al. 1999, Sidhu et al. 2001 2002, Bjorland et al. 2001, Noguchi et al. 2002), this has not been reported for triclosan.
10.3. Environment point of view
Several recent studies have clearly demonstrated the widespread presence of triclosan in the environment, especially in wastewater, in wastewater treatment plant effluents, in rivers and in sediments. However, there is limited information from the EU. The reported concentrations range from less than 0.001 ng/L (seawater) to 133 mg/kg (biosolids from WWTP) (see Table 4). The following information is also necessary for the risk assessment:
- The bioavailability of triclosan in these environments,
- The microflora in contact with triclosan in these environmental compartments,
- Whether this microflora contains bacterial species in which triclosan is able to trigger a genetic response. If not, could the environmental bacteria in contact with triclosan transfer genetic elements (containing resistance genes) to "target" bacteria?
Regulation cascades and corresponding resistance genes are present in the soil bacteria. These bacteria may both serve as original source/reservoir of genetic mobile elements (horizontal transfer) and as genetic manipulator (exchange between chromosomal and mobile genes) of resistance genes in the presence of a selective pressure.
10.4. Biofilm formation in specific environmental conditions
Bacterial biofilms are widespread in the environment including waters, plants, etc. They deserve a special attention because of three main characteristics: the decrease in bioavailability of antibacterial agents within the biofilm, the presence of dormant/persister bacteria, and in complex biofilms the presence of various bacterial species in close contact that facilitate exchange of genetic material.
Triclosan is the most studied biocide with respect to bacterial resistance. Such a level of information, notably on its activity against bacteria, the identification of mechanisms of microbial resistance including genomic and proteomic aspects, is commendable and should be extended to other biocides. This information allows better understanding of triclosan interactions with bacterial cells and should be applied to ensure that its use is sustainable for human health. Based on the available scientific information, it is not possible to quantify the risk of development of antimicrobial resistance induced by triclosan applications, including its use in cosmetics. However, there are environmental concentrations in a number of geographically distinct areas high enough to suggest that triggering of bacterial resistance could also occur in the environment. The applications of triclosan which contribute to those high environmental concentrations cannot be properly identified nor quantified at present. This should be taken into account when considering the current and future uses of triclosan in all applications so as to ensure that the demonstrable benefits for human health in certain applications are not compromised.
Low concentrations of triclosan can trigger the expression of resistance and cross-resistance mechanisms in bacteria in vitro. Some environmental concentrations reported in a number of geographically distinct areas are high enough to give plausibility to this scenario occurring outside of the laboratory and warrant further investigation. The presence of other chemicals (e.g. antibiotics, surfactants, other biocides, etc.) in the environment, which may also affect microbial populations, would preclude assessing the effects of triclosan alone.
The emergence of resistance induced/selected by triclosan is related to the genetic control on the resistance gene(s) present on chromosomal and genetic mobile elements in vitro. This represents the origin for a hazard about selection and dissemination of cross-resistance with other anti-bacterial molecules including biocides and antibiotics.
Bacterial biofilms are widespread in the environment including waters, plants, etc. They deserve special attention because of three main characteristics: the decrease in bioavailability of antibacterial agents within the biofilm, the presence of dormant/persister bacteria, and in complex biofilms the presence of various bacterial species in close contact that facilitates some genetic exchange.
Triclosan, like any other biocide, contributes to the selection of less susceptible bacteria in a complex microcosm in vitro. The impact of such a selection is unclear, as is the fitness of the “selected” bacterial species following triclosan exposure. The few in situ studies investigating long-term triclosan exposure (i.e. at least 6 months) did not indicate changes in resistance susceptibility in the predominant bacteria selected for monitoring but the changes in the entire flora were not evaluated.
When used appropriately, biocides, including triclosan, have an important role to play in disinfection, antisepsis and preservation. Information on the expression/triggering of bacterial resistance mechanisms should be considered to (re-)assess the uses of triclosan in order to preserve its efficacy.
6.2 Is it still safe to use triclosan as a preservative in cosmetic products?
The SCCS opinion states:
Does the SCCS consider a continued use of triclosan as a preservative in cosmetic products as safe taking into account the new provided documentation of resistance development by certain micro-organisms and cross-resistance?
At present, several distinct hazards have been identified: (i) the effect of triclosan on the triggering/regulation of resistance genes in bacteria (ii) the existence of mechanisms which can promote resistance and cross-resistance to biocides and antibiotics in bacteria, (iii) high concentrations of triclosan (compared to concentrations known to select for resistance in in vitro experiments) have been measured in certain environmental compartments and (iv) bacterial biofilms are widespread in the environment and are able to survive exposure to adverse environmental factors. The first two of these hazards have been identified in vitro. The presence of resistance genes in soil bacteria should be investigated further.
The six in situ studies and the one meta-analysis quoted in this document have failed to demonstrate an increase in antibiotic resistance following triclosan use. While these results are at first sight reassuring, the differences of methodologies used to measure “resistance” and to analyse the data make it premature at this stage to conclude that triclosan exposure never leads to developing microbial resistance. These studies were state-of-the art at the time they were performed but they did not have the modern tools (e.g. proteomic or genomic analysis) available today to investigate the complete bacterial population and the bacterial response to biocides. These useful in situ studies do not provide information on expression of genes involved in resistance, maintenance of resistance and virulence genes and transfer of resistance determinants. Thus the SCCS strongly recommends performing additional in situ studies looking at these aspects and bacterial phenotypes where known concentrations of triclosan have been found in the environment.
This opinion concerns the safety of triclosan in terms of microbiology, i.e. generation of bacterial resistance harmful for human health. Based on the available scientific information including recent data from in vitro investigations (proteomic and genomic analyses), it is not possible to quantify the risk associated with triclosan (including its use in cosmetics) in terms of development of antimicrobial resistance (i.e. selection for less susceptible population), genetic basis for resistance and dessemination of resistance. In view of the concentrations of triclosan reported to trigger resistance in vitro, some of the environmental concentrations found in a number of geographical distinct areas are high enough to suggest that bacterial resistance could be triggered. However, no studies have been conducted on this aspect. The applications of triclosan which contribute to those high environmental concentrations cannot be properly identified nor quantified at present and the presence of other chemicals (e.g. antibiotics, surfactants, other biocides, etc.) in the environment, which may also affect microbial populations, would preclude assessing the effects of triclosan independently.
Due to the limited number of in situ studies of resistance induced by triclosan to date, SCCS can only recommend the prudent use of triclosan, for example in applications where a health benefit can be demonstrated. However, conclusions from in vitro studies cannot be ignored, notably the role of triclosan (and other biocides) in triggering resistance and in the dissemination (or lack of) resistance determinants. Hence, the SCCS appreciates that research investment from industry will be maintained to contribute to a better understanding of the potential risks associated with triclosan applications. Research in triggering mechanisms of resistance, maintenance of the gene pool and the transfer of resistance and virulence determinants, and improving the translational application of laboratory results to situations in situ are needed.