Cosmetics: Can preservatives affect the skin microbiota?

The skin microbiota refers to the community of microorganisms living on the skin’s surface. Its composition varies according to the body site, age, sex, hormonal context, and environmental exposures. Throughout life, each body part thus represents a distinct ecosystem. This microbial community performs various roles. Indeed, it contributes to the maintenance of the skin barrier, the proper functioning of the immune system, and normal wound healing. Conversely, an unbalanced microbiota—a phenomenon known as dysbiosis—is associated with several skin disorders, such as acne, eczema or rosacea.

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

In this context, certain questions arise regarding the potential impact of preservatives in cosmetics on the balance of the skin microbiota. These molecules are essential to ensure the microbiological safety of products, in preventing the proliferation of bacteria and yeasts that could compromise the formula or pose a risk to the user. However, their antimicrobial activity raises the hypothesis of an interaction with the microorganisms naturally present on the surface of the skin.

Several studies in vitro have sought to assess the impact of preservatives on skin microorganisms. One study in particular 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 differential inhibition across species : several combinations strongly reduce the growth of S. aureus, a bacterium often involved in skin imbalances, while preserving S. epidermidis, considered beneficial for the skin’s ecosystem, and more moderately modulating C. acnes.

The study also reports, for certain preservative systems, an increase in HDAC3 expression, an enzyme involved in regulating inflammatory responses and skin homeostasis, which could reflect indirect beneficial biological effects beyond antimicrobial activity alone.

Building on these results, other studies in vitro have specifically focused on the effect of preservatives used in leave-on cosmetic products on Staphylococcus epidermidis, which plays an important role in the skin microbiota. The study conducted by CUI and colleagues analyzed 77 bacterial isolates collected from the skin of 46 healthy volunteers to evaluate the susceptibility of different strains to nine preservatives commonly used in cosmetics. The researchers measured the minimum inhibitory concentration (MIC), the mutant prevention concentration (MPC), and the bactericidal kinetics of these substances.

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 concentrations permitted in cosmetics far exceed those 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 disrupt the balance of the skin microbiota.

However, the results obtained in vitro do not always reflect the actual conditions of cosmetic product use.

Several studies in vivo indeed suggest that, despite their antimicrobial activity demonstrated in the laboratory, preservatives incorporated into complete formulations do not cause disruption of the skin microbiome under normal use. One study in vivo evaluated the impact of four cosmetic formulations containing common preservative systems (A, B, C, and D) in healthy adult women. Samples of the leg skin microbiota were collected before and after product application, then analyzed for taxonomic composition and microbial diversity. Across all studies, the flora remained dominated by the genera Staphylococcus, Cutibacterium and Corynebacterium, with other commensal bacteria present in varying proportions.

The results show that no statistically significant changes in the composition or diversity of the microbiome were observed following product use, whether rinse-off or leave-on formulations, applied over durations ranging from one day to five weeks.

In line with these findings, another randomized, double-blind clinical trial compared the effect of preservative-containing cosmetic products with that of identical preservative-free formulations in 26 volunteers over three weeks. Bacterial diversity and abundance were analyzed by 16S rRNA sequencing alongside clinical assessments using 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 induce any significant alteration of the skin microbiome or microbial imbalance.

These results confirm that, under normal usage conditions, the presence of cosmetic preservatives within a formulation does not disrupt the skin microbiota.

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

These differences betweenin vitroandin vivocan be explained in various ways. In laboratory settings, microorganisms are exposed directly to preservatives, without the skin’s natural protective mechanisms. In contrast, during 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 skin pH.

Furthermore, the skin microbiome has a high resilience capacity. Bacteria residing in hair follicles or the sebaceous glands can rapidly recolonize the skin surface following a transient disturbance. This ecological dynamic helps maintain the overall stability of the microbial community, even when exposed to low-persistence antimicrobial agents. Finally, the in vivo evaluation accounts for the complexity of the skin ecosystem — interactions among microbial species, local immune responses, and continuous epidermal renewal — which is absent from simplified experimental models.

The combination of these factors explains why the antimicrobial activity observed in vitro does not result in dysbiosis during normal use of preservative-containing cosmetic products.

The absence of preservatives in a cosmetic product is only feasible under certain precisely defined formulation conditions.

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

This need for antimicrobial protection is illustrated 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 potentially pathogenic bacteria detected, such as Staphylococcus aureus, Klebsiella or Enterobacter. These findings underscore that the absence of a preservative, when not offset by a suitable formulation or specific packaging, can expose the product—and thus the user—to a real microbiological risk.

Finally, the proper preservation of a cosmetic product does not depend solely on its formulation: the way the consumer uses it is crucial. Observance of the period after opening, storage in a cool, dry, and light-protected environment, and monitoring the product’s appearance, odor, or 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).