Cosmetics: can preservatives affect the skin microbiota?

The cutaneous microbiota refers to the community of microorganisms living on the skin’s surface. Its composition varies with the body site, age, sex, hormonal context and environmental exposures. Throughout life, each area of the body thus constitutes a distinct ecosystem. This microbial community plays various roles. It contributes to the maintenance of the skin barrier, the proper functioning of the immune system and the optimal progression of wound healing. Conversely, an imbalanced microbiota – a phenomenon known as dysbiosis – is associated with several dermatoses, such as acne, eczema or rosacea.

Maintaining the balance of the skin microbiome is therefore essential for healthy skin.

In this context, certain questions arise regarding the potential impact of cosmetic preservatives on the balance of the skin microbiota. These molecules are essential for guaranteeing the microbiological safety of products, by preventing the proliferation of bacteria and yeasts liable to compromise the formulation or pose a risk to the user. However, their antimicrobial activity raises the hypothesis of an interaction with the micro-organisms naturally present on the skin’s surface.

Several studies in vitro have sought to assess the impact of preservatives on the skin’s microorganisms. In particular, one study focused on eleven preservatives commonly used in cosmetics, tested on Cutibacterium acnes, Staphylococcus epidermidis and Staphylococcus aureus using 3D skin models and microbiological cultures. The results show that certain preservative combinations exert a species-specific inhibition : several combinations markedly reduce the growth of S. aureus, a bacterium often implicated in cutaneous imbalances, while sparing S. epidermidis, considered beneficial to the skin ecosystem, and more moderately modulating C. acnes.

The study also reports, for certain preservative systems, an increase in HDAC3 expression, an enzyme involved in the regulation of inflammatory responses and skin homeostasis, which might reflect indirect beneficial biological effects beyond mere antimicrobial activity.

Building on these findings, further work in vitro has focused more specifically on the effect of preservatives used in leave-on cosmetic products on Staphylococcus epidermidis, which plays a crucial role in the skin microbiota. The study by Cui and colleagues analysed 77 bacterial isolates obtained from the skin of 46 healthy volunteers to assess the susceptibility of different strains to nine preservatives commonly utilised in cosmetics. The researchers determined the minimum inhibitory concentration (MIC), the mutant prevention concentration (MPC) and the bactericidal kinetics of these agents.

The results show that, for several preservatives, notably 2-bromo-2-nitro-1,3-propanediol, ethyl 4-hydroxybenzoate, hexadecyltrimethylammonium bromide and imidazolidinyl urea, the maximum authorised cosmetic doses far exceed the concentrations required to inhibit the growth of S. epidermidis, or even to prevent the emergence of mutants. At these levels, some preservatives have even demonstrated a rapid bactericidal activity, capable of completely eliminating a high bacterial load in less than an hour under experimental conditions.

These observations suggest that certain preservatives could, in theory, reduce the population of commensal bacteria and thus disturb the balance of the skin microbiota.

However, the results obtained in vitro do not always reflect the actual conditions under which cosmetic products are used.

Several studies in vivo have indeed suggested that, despite their antimicrobial activity demonstrated in the laboratory, preservatives incorporated in complete formulations do not cause disruption of the skin microbiome under normal use. One study in vivo thus assessed the impact of four cosmetic formulations containing common preservative systems (A, B, C and D) in healthy adult women. Samples of the skin microbiota from the lower leg were taken before and after product application, then analysed according to taxonomic criteria and microbial diversity. Across all studies, the flora remained dominated by the genera Staphylococcus, Cutibacterium and Corynebacterium, with the presence of other commensal bacteria in varying proportions.

The results show that no statistically significant change in microbiome composition or diversity was observed after using the products, whether rinse-off or leave-on formulations, applied over periods ranging from one day to five weeks.

Building on these data, another double-blind randomised clinical trial compared the effects of cosmetic products containing preservatives with those of identical preservative-free formulations in 26 volunteers over three weeks. Bacterial diversity and abundance were analysed by 16S rRNA sequencing, alongside clinical assessments via 3D imaging. As expected, the microbiome remained dominated by major taxa such as Cutibacterium acnes, Corynebacterium and the family Neisseriaceae. More importantly, the use of preservative-containing formulations did not result in any significant alteration of the skin microbiome nor in microbial imbalance.

These results confirm that, under normal conditions of use, the presence of cosmetic preservatives in a formulation does not disturb the skin microbiota.

How can the differences observed between tests in vitro and tests in vivo be explained?

These discrepancies between findings in vitro and in vivo can be explained in various ways. In the laboratory, micro-organisms are exposed directly to preservatives, without the skin’s natural protective mechanisms. In contrast, during a topical application, preservatives are often more diluted, their contact time with the microbiota is limited, and their activity can be modulated by the overall formulation, sebum or even the skin pH.

Moreover, the skin microbiome has a high capacity for resilience. Bacteria residing in hair follicles or sebaceous glands can rapidly recolonise the skin’s surface following a transient disturbance. This ecological dynamic helps to maintain the overall stability of the microbial community, even in the presence of low-persistence antimicrobial agents. Finally, the in vivo evaluation takes into account the complexity of the cutaneous ecosystem — microbial species interactions, local immune responses and the continuous renewal of the epidermis — absent from simplified experimental models.

All of these factors explain why the antimicrobial activity observed in vitro does not result in dysbiosis during the normal use of cosmetic products containing preservatives.

The absence of preservatives in a cosmetic product can only be envisaged under very specific formulation conditions.

Micro-organisms responsible for contamination require water to grow. Consequently, the anhydrous products or those composed solely of an oil phase can be formulated without preservatives while maintaining a natural microbiological stability. This is the case for many preparations based on vegetable oils, butters or waxes, where the low-water environment limits microbial proliferation. Conversely, as soon as an aqueous phase is present, as with emulsions, gels and lotions, the addition of at least one preservative system becomes essential to ensure safe use.

This need for antimicrobial protection is further evidenced in the scientific literature. A study on preservative-free eye drops showed that microbial contamination can occur after just a few days of use, with the detection of potentially pathogenic bacteria such as Staphylococcus aureus, Klebsiella or Enterobacter. These findings highlight that the absence of a preservative, unless offset by an appropriate formulation or specific packaging, can expose the product, and therefore the user, to a real microbiological risk.

Finally, the proper preservation of a cosmetic product does not rely solely on its formula: the way it is used by the consumer is crucial. Observing the period after opening, storing it in a cool, dry environment away from light, and monitoring the product’s appearance, odour and texture all contribute to preventing contamination.

• RAMAESH K. & al. Microbial contamination of preservative free eye drops in multiple application containers. British Journal of Ophtalmology (2006).

• HE C. & al. Effect of cosmetic chemical preservatives on resident flora isolated from healthy facial skin. Journal of Cosmetic Dermatology (2018).

• RINALDI F. & al. Effect of commonly used cosmetic preservatives on skin resident microflora dynamics. Scientific Reports (2021).

• CAMPBELL-LEE S. & al. In-vivo impact of common cosmetic preservative systems in full formulation on the skin microbiome. PLOS One (2021).

• GILLBRO J. M. & al. Randomized comparative double-blind study assessing the difference between topically applied microbiome supporting skincare versus conventional skincare on the facial microbiome in correlation to biophysical skin parameters. International Journal of Cosmetic Science (2022).

• CUI S. & al. Effect of leave-on cosmetic antimicrobial preservatives on healthy skin resident Staphylococcus epidermidis. Journal of Cosmetic Dermatology (2023).

• GILLBRO J. M. & al. Microbial dynamics: Assessing skincare regimens’ impact on the facial skin microbiome and skin health parameters. Microorganisms (2024).