The COVID-19 pandemic made wearing facemasks mandatory, leading to various adverse effects, including a skin disorder known as ‘maskne’ (mask + acne)1. While the alteration of the skin environment, such as increased moisture and temperature caused by facemask wearing, could be a main factor for maskne, microbial contamination of facemasks could also be a potential risk factor for this symptom. Bacterial contamination, including Staphylococcus aureus, in facemasks has been reported. However, these previous studies only covered bacterial contamination and did not investigate the possibility of this pathogenesis. Therefore, we aimed to reveal the causative relationship between mask bacterial contamination and “maskne” pathogenesis. Notably, we focused on contamination by bacteria capable of growing under anaerobic conditions, which were excluded in previous studies, because skin disorders like acne can be caused by anaerobic bacteria that survive in the epidermis and skin hair follicles2
Initially, we inferred that the oral microbiome contaminated in facemasks could be the main potential skin-virulent bacteria, as most oral bacteria are anaerobes. However, contrary to our expectation, the facemask microbiome was more similar to that of the skin than the oral microbiome (Fig. 1), with more than 70% of the bacteria found in facemasks originating from the skin, and only 3% were exclusively from the oral microbiome. Additionally, in a mice skin virulence test aimed at evaluating the anaerobically isolated facemask bacteria, skin microbiota belonging to Staphylococcus aureus, S. capitis, S. epidermidis, and Cutibacterium acnes were found to be the most virulent. Among these, S. aureus and C. acnes are representative causative bacteria inducing skin disorders, including acne, acne-like diseases, and atopic dermatitis. Notably, we could first determine that the obligate anaerobic C. acnes isolated from facemasks induced skin inflammation, which had been excluded in previous studies because isolation was conducted under only aerobic conditions. Persistent exposure to these skin pathogens contaminated in the environment, including beds or floors, has been demonstrated as a key factor determining the severity of skin disorders3. Therefore, our results suggest that facemasks can act as reservoirs for skin pathogenic bacteria.
Secondly, we tried to understand the unique microbial ecology established in facemasks, which are composed of skin and oral microbiomes. We hypothesized that this unique composition could create novel interactions between skin pathogens and other microbiota isolated from facemasks. Therefore, we investigated the growth pattern of the pathogen when co-cultured with the cell-free supernatant from anaerobically isolated facemask bacteria. Firstly, “pathogen helpers” or “pathogen inhibitors,” which directly promoted or inhibited the growth of the pathogen, were identified (Fig. 2). These effects were validated by an in vivo intradermal infection model, showing aggravation or attenuation of skin lesions when co-infected with “pathogen helpers” or “pathogen inhibitors.” Notably, most pathogen helpers promoting pathogen growth were identified as C. acnes, and these strains had no direct skin virulence. More than 80% of the pathogen helper bacteria C. acnes isolated from facemasks belonged to the IB type, which has been previously reported as the predominant commensal phylogroup. This result suggests that commensal C. acnes, not directly inducing skin disorders, can contribute to the aggravation of skin lesions through interaction with the counterpart pathogen. Moreover, the unique microbiological ecology in facemasks contributed to finding the oral anaerobic bacterium Streptococcus parasanguinis, which inhibited the skin pathogen IFM12. This inhibitor strain could inhibit all tested skin-virulent S. aureus strains, including reference strains like USA300 and ATCC25923.
Collectively, the presence of “pathogen helpers” or “pathogen inhibitors” led us to explore a novel therapeutic strategy against skin disorders via modulation of these bacteria. Upto date, most studies have focused on speculating the direct interaction between pathogens and their counterparts and identifying individual microbes and microbial byproducts that inhibit target pathogens. We attempted one step further. We re-screened helpers and inhibitors of “pathogen helpers” and “pathogen inhibitors”.
Finally, we isolated a facemask strain promoting the pathogen inhibitor S. parasanguinis. Unfortunately, we failed to isolate such a strain. Instead, we could identify the M6 strain as an inhibitor of the pathogen helper (IPH), which inhibited the “pathogen helper” C. acnes without inducing any alteration in pathogen growth (Fig. 3). This IPH strain could successfully attenuate the lesion exacerbated by co-infection with the pathogen and pathogen helper. To the best of our knowledge, this indirect therapeutic approach to overcome skin bacterial infection was reported for the first time. Only one recent study demonstrated that disease development induced by the plant pathogenic Ralstonia solanacearum could be indirectly attenuated through the inhibition of the “pathogen helper,” not direct inhibition of the pathogen4. Moreover, we identified that the IPH strain produced high concentrations of phenyllactic acid, which is made by fermentation with the shikimate or phenylalanine metabolic pathway. Notably, phenyllactic acid had an inhibitory effect specifically against the pathogen helper strain C. acnes, not the pathogenic strain IFM12 (Fig. 3).
Collectively, we consider that therapeutic strategy for inhibiting pathogen helper bacteria provides many advantages in clinical trials. Firstly, such IPH strains may reduce the side effects associated with the direct induction of pathogen defense mechanisms, including the development of antimicrobial resistance. Secondly, screening for further pathogen helper strains may facilitate the identification of a single IPH strain capable of suppressing multiple pathogens. Lastly, the specific determinant molecule, phenyllactic acid, produced by IPH, holds potential as a postbiotic candidate, which could offer a safer and more accessible alternative to probiotics.
Further work
We discovered that phenyllactic acid can inhibit pathogen helper bacteria. If this molecule demonstrates broad antimicrobial activity against these strains, it could be developed as the first-in-class microbiome-based drug candidate for treating S. aureus-associated skin diseases, such as atopic dermatitis and acne.
References
Malczynska, I.U., Krych, G., Baran, A., Kaminski, T.W. & Flisiak, I. Maskne—Dermatosis of a Pandemic. A Survey on the Impact of PPE on Facial Skin Among HCW and N-HCW in Poland. Dermatology and Therapy 12, 2297-2308 (2022).
Han-Hee Na, S.K., Jun‐Seob Kim, Soohyun Lee, Yeseul Kim, Su-Hyun Kim, Choong-Hwan Lee, Dohyeon Kim, Sung Ho Yoon, Haeyoung Jeong, Daehyuk Kweon, Hwi Won Seo, Choong-Min Ryu Facemask acne attenuation through modulation of indirect microbiome interactions. npj Biofilms and Microbiomes in press (2024).
Leung, A.D., Schiltz, A.M., Hall, C.F. & Liu, A.H. Severe atopic dermatitis is associated with a high burden of environmental Staphylococcus aureus. Clin Exp Allergy 38, 789-793 (2008).
Li, M. et al. Indirect reduction of Ralstonia solanacearum via pathogen helper inhibition. The ISME Journal 16, 868-875 (2022).