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- Portuguese Natural Resources as Modulators of Acne Vulgaris HallmarksPublication . Oliveira, Ana Sofia Domingues ; Oliveira, José António Martinez Souto de; Oliveira, Ana Cristina Palmeira de; Teixeira, João Paulo FernandesAcne vulgaris, commonly known as acne, is a chronic skin disease of the pilosebaceous unit, that affects millions worldwide, particularly adolescents and young adults. Clinically, acne is characterized by non-inflammatory and inflammatory lesions, predominantly on the face, but also affecting the neck, chest, and upper back. Beyond its signs and symptoms, acne is also associated with a substantial psychological and emotional burden. The pathogenesis of the disease is multifactorial and involves a complex interplay of several biological events or hallmarks. Traditionally, four mechanisms have been associated with its development, namely: increased sebum production, follicular hyperproliferation/hyperkeratinization, colonization by Cutibacterium acnes bacterium (formerly Propionibacterium acnes), and the release of inflammatory mediators. However, recent research has further complicated the disease’s pathophysiology, with new factors such as changes in sebum composition, differential involvement of innate and adaptive immunity, and dysbiosis of the follicular microbiota, being increasingly described by the scientific community. Given its multifactorial nature, different therapeutic strategies are adopted depending on the severity of the disease. Topical treatments, when used alone, are typically reserved for comedonal or non-inflammatory acne, while in combination topical therapy is considered the first-line approach for mild to moderate acne. In more severe cases, systemic treatments are used as first-line options, with isotretinoin being the standard choice despite its severe risks and side effects. These adverse effects associated with classical acne therapies, together with a behavioral shift among consumers who increasingly prefer more “natural” and/or “green” solutions, have driven both the cosmetic and pharmaceutical industries to invest in the research and development of alternatives or adjuvants to conventional treatments that meet these expectations. Plant-derived ingredients have gained particular attention and, among them, essential oils (EOs), which are secondary metabolites from aromatic plants, stand out for their spectrum of bioactive properties and are extensively used across various health fields, including dermatology. Based on this premise, the aim of this doctoral project was to investigate the bioactive potential of preparations from aromatic and medicinal plants produced in Portugal, with a particular focus on their ability to modulate the main hallmarks involved in acne pathophysiology. Two autochthonous Portuguese species, namely Thymus mastichina (L.) L. (Thymus mastichina; TM) and Cistus ladanifer L. (Cistus ladanifer; CL), and the allochthonous hybrid Thymus × citriodorus (Pers.) Schreb. (Thymus × citriodorus; TC) were investigated for their anti-acne potential, through the study of their EOs in addition to their hydrolates, by-products of EO production, thus aligning this work with the principles of circular economy. Initial screenings of the autochthonous taxa revealed that CL EO, rich in α-pinene, exhibited the highest anti-inflammatory potential (EC₅₀ = 0.002% v/v), although with associated cytotoxicity in murine macrophage cell model (RAW 264.7; IC₅₀ = 0.012% v/v). TM preparations, mainly composed by 1,8-cineole, showed balanced anti-inflammatory effects with superior biocompatibility in both murine macrophages and fibroblasts (L929). CL EO showed the highest antimicrobial activity, with minimum inhibitory concentrations (MICs) ranging from 0.06% (v/v) to 2% (v/v) against the tested microorganisms, which included those used in the Challenge test for cosmetics and representatives of skin microbiota. Regarding the preparations from the allochthonous hybrid, TC EO, mainly composed of geraniol, stood out for its antimicrobial activity against acne-associated bacteria (C. acnes, Staphylococcus epidermidis and S. aureus) and for its ability to prevent and disrupt C. acnes biofilms, even at sub-inhibitory concentrations. TC hydrolate retained some bioactive potential, particularly in modulating C. acnes biofilms, still being deprived of relevant antimicrobial activity. Considering the higher anti-acne potential of TC EO, and to address variability concerns relevant to large-scale application of EOs, four TC EOs from different Portuguese regions were compared. Although all shared geraniol as a major compound, significant differences in yield, efficacy, and cytotoxicity were observed, underscoring the need for standardization protocols. Still, all EOs exhibited antimicrobial activity, with particular effectiveness against C. acnes, as evidenced by MICs ranging from 0.016% (v/v) to 0.031% (v/v), corroborating the anti-acne potential of this hybrid. Building upon previous results and considering the anti-acne potential of the studied preparations, their ability to modulate key acne-related events was further explored using more complex in vitro models that better mimic in vivo conditions. These closer-to-disease models further explored the preparations’ ability to modulate sebum production in human sebocytes (SZ95 cell line), C. acnes-induced inflammation and oxidative stress, keratinocyte proliferation, and bacterial adhesion. Additionally, potential antimicrobial interactions between the different preparations along with their cytotoxic and mutagenic potential were also investigated. Regarding anti-acne efficacy, comprising the above-described models, EOs exhibited overall greater bioactivity compared to their corresponding hydrolates. The EOs of TC and CL stood out for their ability to modulate all the studied hallmarks, demonstrating a multitarget potential. Although less potent, the hydrolates still showed relevant activity, particularly the one from CL, displaying multi-target potential, failing only in inhibiting lipase activity. Regarding the combined antimicrobial activity of the preparations against C. acnes, EO–EO combinations showed consistent synergistic effects (FICIs between 0.250 and 0.375), while the presence of additive interactions depended on the specific combinations. As for safety, TC and CL EOs were the least biocompatible upon human keratinocytes and sebocytes, and all preparations were considered non-mutagenic, except for CL EO, which showed a “likely-positive” result in one of the Salmonella typhimurium strains included in the Ames test. Based on the higher overall efficacy of the tested EOs, the final experimental work focused on potential phylotype-selective efficacy of EOs against clinical C. acnes strains, with different virulence patterns. Phylotype IA1 strains (typically associated with acne lesions), exhibited higher lipase activity and biofilm formation than phylotype II strains (more associated with healthy skin). However, not all virulence factors were phylotype-dependent, which supports the multifactorial nature of this disease. TC EO showed the lowest MICs across all strains (MICs between 0.03% v/v and 0.06% v/v) when compared to the other EOs, demonstrating less variation in MIC values between phylotypes. It was also able to reduce both biofilm biomass and metabolic activity from IA1 phylotype strains, being effective at lower concentrations. These results further support the relevance of TC EO for this application, despite its lack of phylotype-dependent selectivity. In conclusion, this work provides evidence for the anti-acne potential of Portuguese plant preparations, particularly TC and CL EOs, supported by their capacity to modulate different pathophysiological mechanisms associated with disease development. This research also contributes to the scientific valorization of Portuguese natural resources and supports their integration into sustainable dermocosmetic formulations aligned with the principles of circular economy.