As the cosmetic and personal care marketplaces continue to respond to the demand for more natural and ecologically friendly ingredients, including antimicrobial agents, cosmetic chemists must learn to adapt to the idiosyncrasies of the wide range of choices available today to satisfy the consumer’s wishes. In the past, cosmetic chemists typically approached the selection of the preservation agent for any new formulation as more of a reflex, rather than as an integral part of the formulation process. Because many formulators have not worked extensively with natural antimicrobial ingredients, they have not developed the same level of comfort as they have with traditional synthetic products. This difference in product familiarity makes it essential to confirm formulation compatibility early on in the product development cycle, followed by efficacy testing; a process not all that different from the past when a new synthetic preservative was introduced to the formulator. Even then, a comfort level had to be developed between the formulator and the new preservative in order to know the proper formulation conditions for maximum efficacy.

Traditional synthetic preservatives like phenoxyethanol, paraben esters, formaldehyde-releasers, methylisothiazolinone (MI) and chloromethylisothiazolinone (CMI) have been very popular with formulators for several reasons, including low-dose efficacy and compatibility in a broad spectrum of personal care formulations. These characteristics allowed formulators to reach a level of comfort where they can now select a synthetic preservative at the very end of the formulating process rather than evaluating the preservation options to ensure compatibility and efficacy. 

An increasing number of formulations are being created to meet the growing demand for natural cosmetic and personal care products. The substitution of traditional synthetic ingredients with natural ingredients of all types, regardless of their function, requires some re-thinking of the traditional formulation paradigm. It is essential that adjustments be made on the part of the formulator to successfully employ these new efficacious, natural products. This also holds true for the natural antimicrobial ingredients that are available to the formulator today.

There are a number of effective natural, alternative antimicrobial agents available to the cosmetic chemist today. One of the keys to making a successful substitution of a traditional synthetic preservative to one of the natural alternatives is to include its evaluation as an early and integral part of the overall formulating process. These antimicrobial agents should be considered early in the development of a formulation in order to better evaluate both the compatibility and efficacy in the new formulation. This may require a fresh approach, but the results can be very successful. In addition, consumers are rewarding these efforts by seeking and purchasing products which contain more natural ingredient alternatives.

Just as traditional synthetic preservatives can be grouped based on their general chemistry and function, natural antimicrobial ingredients can also be grouped into general classes based on their origin and function. Some of the natural antimicrobial ingredient options available to the cosmetic chemist today include the following general classes of compounds:

• Plant/Herbal extracts;
• Organic acids;
• Enzyme/substrate systems; and
• Antimicrobial peptides.

Plants/Herbal Extracts
A wide variety of plants contain components that have effective antimicrobial capability. Often these compounds are used as defense mechanisms by the plants against the natural activity of microorganisms, insects and herbivores. These compounds can be extracted from the leaves, flowers, fruit, seeds, stems and roots, which vary for each specific plant and the compound(s) to be isolated. Plants and herbs have been used for medicinal purposes, virtually since the beginning of time, and are still prevalent today. Some cultures embrace these medicinal treatments more than others. 

A number of plants have become the source for natural antimicrobials based on the extraction of essential oils and other antimicrobial phytochemicals. Antimicrobial phytochemicals that can be extracted from plants include compounds such as phenols, polyphenols, terpenoids and flavonoids.¹  Some of the simplest antimicrobial phytochemicals are the single substituted phenolic ring compounds, like cinnamic acid and caffeic acid. Common herbs like tarragon and thyme contain caffeic acid, which has effective antibacterial and antifungal properties. Catechol is a phenolic compound containing two hydroxyl (OH-) groups.
Pyrogallol is a phenolic compound with three hydroxyl groups. There is evidence that increased hydroxylation corresponds with increased antimicrobial efficacy.

Analysis of a number of essential oils has shown that terpenoids are some of the most abundant constituents of these oils. Many terpene compounds, which include menthol and thymol, provide broad antimicrobial properties affecting Gram positive and Gram negative bacteria, as well as fungal microorganisms.² Flavonoids are ubiquitous in photosynthesizing organisms.³ They are commonly found in fruit, seeds, stems and flowers of plants. These compounds are synthesized by plants in response to microbial infection, so it is reasonable to expect that they may have antimicrobial properties.

For centuries, crude preparations of these compounds have been used for medicinal purposes. More recently, many of these flavonoid compounds have been isolated and proven to have antimicrobial properties. 

Organic Acids
A number of organic acids and their associated salts can be useful in the preservation of cosmetic formulations. Because the free-acid forms of a number of these compounds typically have very limited water solubility, it is most common to incorporate these antimicrobial ingredients into the formulation while in their water-soluble salt form. The formulation pH can then be lowered to convert the salt into the acid form. The degree of dissociation from the salt form into the active acid form is a function of each acid’s dissociation constant (pKa), combined with the final pH of the formulation. In general, the lower the pKa value for a given acid, the lower the formulation pH needs to be in order to have a sufficient concentration of the acid form to provide antimicrobial efficacy.
This can be a limiting factor for organic acids like salicylic, benzoic, dehydroacetic and sorbic, which typically require the formulation pH to be 5 or less to provide sufficient efficacy. In contrast, even at a pH of 7, caprylhydroxamic acid and undecylenic acid are essentially 100% dissociated, and therefore in their acid form. Most of these organic acids are especially effective against fungal microorganisms.

It is important to note that several of these organic acids can be obtained from natural sources. Natural salicylate salts and esters can be extracted from willow bark (Salix alba and Salix nigra), aspen bark (Populus tremuloides), and wintergreen leaves (Gaultheria procumbens). These naturally occurring compounds impart effective antimicrobial properties and due to their greater water solubility compared to their synthetic counterparts, are more effective for aqueous-based and emulsion-type cosmetic formulations. Sorbic acid can be sourced from the fruit of rowan trees (Sorbus aucuparia), also known as mountain ash. The natural sorbic acid from the rowan berries of these trees can provide efficacious antifungal properties to cosmetic formulations.
Undecylenic acid can be derived from castor oil, obtained from castor beans (Ricinus communis). Like sorbic acid, undecylenic acid is an organic acid that has excellent antifungal properties. All three of these natural organic acids (salicylic, sorbic and undecylenic) are multifunctional ingredients. For example, in addition to their antimicrobial properties, these organic acids can also provide exfoliating, emollient, moisturizing and conditioning properties.

Coconut oil and palm kernel oil are sources of two very effective organic fatty acids that also can be useful as antimicrobial ingredients. Both coconut and palm kernel oils contain approximately 6 to 8% caprylic acid. Caprylic acid is an effective antifungal agent. The second and more abundant organic fatty acid found in coconut and palm kernel oil is lauric acid. Coconut and palm kernel oils contain approximately 50% lauric acid, which has broad antibacterial properties. It has also been tested for efficacy specifically against Propionibacterium acnes, with good results.^4,5

Enzyme/Substrate Systems
Another natural antimicrobial option is an enzyme/substrate system.^6,7  This multi-component system involves first adding the substrate solution containing glucose, iodide ions and thiocyanate to the cosmetic formulation.  At the end of the manufacturing process, the enzyme solution containing glucose oxidase and lactoperoxidase is added. The glucose oxidase enzyme interacts with the glucose and any free oxygen in the formulation, generating hydrogen peroxide. This effectively results in scavenging or removal of the usable oxygen needed for microbial activity. The lactoperoxidase reacts with the hydrogen peroxide generated to catalyze the oxidation of the iodide and thiocyanate anions to create hypoiodite and hypothiocyanite, which both have antimicrobial activity.

This is a relatively complex, multi-component system, comprised of five ingredients that individually do not have antimicrobial efficacy. The system relies on all five ingredients working in tandem to provide microbial control. The system has broad antibacterial properties and also contributes to antifungal efficacy. Because the system is based on enzyme activity, once the enzyme solution is added to the formulation, the temperature should not be allowed to exceed 40°C. It is also very important to add the substrate component solution and the enzyme component solution at a ratio of 20 parts substrate to 1 part enzyme, by weight.

Antimicrobial Peptides
Antimicrobial peptides are relatively short, protein-like compounds that are typically 30 to 60 amino acids in length.⁸ In plants and animals, they are often produced to defend themselves against microbial activity. In the case of antimicrobial peptides produced by bacteria, they are typically produced as defense mechanisms to gain a competitive advantage against other microorganisms within their environment.  

The Lactic Acid Bacteria (LAB) group, which includes microorganisms such as Lactobacillus, Enterococcus, Pediococcus and Leuconostoc, produces a variety of antimicrobial peptides. As a class of compounds, they are commonly referred to as bacteriocins. Numerous antimicrobial peptides have been identified since the initial discovery 1925. Antimicrobial peptides are “generally recognized as safe” (GRAS) by the FDA. Today, antimicrobial peptides have become commonly used ingredients in the preservation of certain food products.

Using controlled fermentation processes with microorganisms such as Lactobacillus and Leuconostoc, sufficient concentrations of antimicrobial peptides can be generated to provide effective broad-spectrum antimicrobial ingredients for use in cosmetic formulations. Antimicrobial peptides are also multifunctional compounds. Peptides are known for their moisturizing benefits and the antimicrobial peptides produced by these microorganisms are no exception. An example of this type of fermentation process is Leuconostoc, grown in the presence of macerated radish root. The resulting fermentation filtrate contains antimicrobial peptides that also provide the additional benefit of moisturization. A similar end product can be achieved through fermentation of Lactobacillus in a traditional microbial growth medium. Although a unique antimicrobial peptide is generated by Lactobacillus as compared with that of Leuconostoc, the Lactobacillus fermentation filtrate also provides both moisturizing properties and antimicrobial efficacy.

Unlike more complex proteins and enzymes, these relatively small antimicrobial peptides are much less susceptible to temperature and pH extremes. As a result, temperatures well above 40°C are typically tolerated, as are the range of pH values commonly found in cosmetic products. The small, relatively simple structure of antimicrobial peptides also helps to maintain the mode of efficacy over the broad range of temperature and pH often encountered during cosmetic manufacturing processes, as well as in the final cosmetic formulation. Antimicrobial peptides are efficacious at pH values between 3 and 8. Another distinct difference with the antimicrobial peptides is that while many of the plant extracts exhibit color and odors that must often be addressed in order to be useful in a formulation, the antimicrobial peptides produced by fermentation typically impart neither significant color nor odor to the final formulation. Because of their stability, the antimicrobial peptide ingredients are also not likely to cause changes in color of the formulation during the final product’s shelf life. These characteristics of antimicrobial peptides provide the flexibility needed to be effective in a wide variety of cosmetic and personal care formulations.

Modes of Antimicrobial Action
A significant amount of research has been done to identify the specific mode of antimicrobial activity for many of the natural ingredients described above. In general, the majority of natural antimicrobial compounds initially interact with the microbial cell walls and membranes, ultimately interfering with membrane permeability so that transport across the membrane is negatively affected. This interference with membrane permeability results in the loss of cellular osmotic control, which is an essential protective mechanism for the microbial cell. If a microbial cell loses its osmotic control, it ultimately results in an inability to protect itself from its environment. The loss of this critical cellular function also impairs the cell’s ability to transport essential nutrients in and waste products out. Under these circumstances the cell cannot survive. Some natural antimicrobial agents further interfere with various internal cellular functions, ultimately blocking energy generation and/or cellular replication, resulting in cell death.

In summary, there are a variety of natural antimicrobial ingredients available to the cosmetic formulator who is seeking to respond to today’s market demand for more natural and environmentally friendly cosmetics and personal care products. The key to making substitutions of traditional synthetic ingredients with today’s natural alternatives is to evaluate their potential impact on, and their interaction with, other components in the formulation as an early step in the formulating process.  Some traditional ways of doing things may need to be adjusted or modified slightly to accommodate these natural ingredients, including natural antimicrobials. This early integration into the formulating process will ensure product stability from both a traditional chemical perspective, as well as a microbiological perspective. The cosmetic formulator is no longer limited to traditional synthetic preservatives like parabens, isothiazolinones and formaldehyde donors. They now have a number of effective natural antimicrobial tools in their formulating toolbox from which to choose. 

References:

1. Cowan, M.M. 1999. Plant products as antimicrobial agents. Clinical Microbiology Reviews 10: 564-582.
2. Trombetta, D., F. Castelli, M.G. Sarpietro, V. Venuti, M. Cristani, C. Daniele, A. Saija, G. Mazzanti and G. Bisignano. 2005. Mechanisms of antibacterial action of three monoterpenes. Antimicrobial Agents and Chemotherapy 49(6):2474-2478.
3. Cushnie,T.P.T. and A.J. Lamb. 2005. Antimicrobial activity of flavonoids. International Journal of Antimicrobial Agents 26:343-356.
4. Huang, W.C.,T.H. Tsai, L.T. Chuang, Y.Y. Li, C.C. Zouboulis and P.J. Tsai. 2014. Anti-bacterial and anti-inflammatory properties of capric acid against Propionibacterium acnes: A comparative study with lauric acid. Journal of Dermatological Science. 73:232-240.
5. Nakatsuji, T., M.C. Kao, J.Y. Fang, C.C. Zouboulis, L. Zhang, R.L. Gallo and C.M. Huang. 2009. Antimicrobial property of lauric acid against Propionibacterium acnes: Its therapeutic potential for inflammatory Acne vulgaris. Journal of Investigative Dermatology 129(10):2480-2488.
6. Thomas, E.L. and T.M. Aune. 1978. Lactoperoxidase, peroxide, thiocyanate antimicrobial system: Correlation of sulfhydryl oxidation with antimicrobial action. Infection and Immunity 5:456-463.
7. Bosch, E.H>, H. Van Doorne, and S. De Vries. 2000. The lactoperoxidase system: The influence of iodide and the chemical and antimicrobial stability over the period of about 18 months. Journal of Applied Microbiology 89:215-224.
8. Settanni, L. and A. Corsetti. 2008. Application of bacteriocins in vegetable food biopreservation. International Journal of Food Microbiology 121:123-138.

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