Anti-quorum-sensing activity of Plectranthus aliciae in multispecies Cutibacterium acnes-Staphylococcus epidermidis...

Quorum sensing (QS) is a bacterial communication process mediated by the production, release, and detection of autoinducers, regulating gene expression linked to virulence and biofilm formation. Cutibacterium acnes and Staphylococcus epidermidis exhibit opportunistic pathogenic behavior on the skin, complicating treatment strategies for inflammatory skin conditions such as acne vulgaris and wound-associated infections. The interspecies relationship between C. acnes and S. epidermidis, particularly within multispecies biofilms and under different oxygen conditions, remains insufficiently explored. This study aimed to investigate QS-related interactions between these species and to evaluate the anti-virulence potential of the South African medicinal plant Plectranthus aliciae.

Interactions between C. acnes and S. epidermidis were investigated in single- and multispecies systems under aerobic and anaerobic conditions. Biofilm formation was quantified using crystal violet staining, while extracellular DNA (eDNA) release, lipase activity, and autoinducer-2 (AI-2) production were measured as QS-associated phenotypes. The antibacterial, anti-biofilm, and anti-QS activities of the ethanolic extract of P. aliciae and its hexane (Hex) and ethyl acetate (EA) partitions were assessed. Wound-healing activity was evaluated using a HaCaT keratinocyte scratch assay, and major constituents were identified using UPLC-QTOF analysis.

Multispecies cultures demonstrated significantly higher biofilm biomass, eDNA release, and lipase activity than single-species systems, especially under aerobic conditions. The ethanolic extract of P. aliciae (PA_EtOH)) and its hexane (Hex) and ethyl acetate (EA) portions exhibited significant antibacterial efficacy against C. acnes (MIC 7.8-15.6 µg/ml) and preferentially suppressed biofilm formation in multispecies environments, with biofilm reduction exceeding 80 % in some models. The inhibition of biofilm was associated with reduced extracellular DNA release, consistent with interference in biofilm structural components. At 0.5×MIC, PA_EtOH, Hex, and EA reduced AI-2 production by approximately 50-89 %, accompanied by decreased extracellular DNA release and lipase activity, particularly in multispecies systems. These observations are consistent with modulation of quorum-sensing-associated virulence phenotypes; however, direct molecular targets within QS pathways were not evaluated. Lipase inhibition showed variability across systems, with more pronounced effects observed in multispecies cultures. Furthermore, PA_EtOH, Hex, and EA statistically significantly improved wound closure in HaCaT keratinocytes after 18 h (p< 0.01), with closure increases of approximately 14-19% compared to the control. UPLC-QTOF analysis identified luteolin and rosmarinic acid as major constituents, both exhibiting selective anti-virulence activity in association with reduced QS-linked phenotypes CONCLUSION: Multispecies interactions were associated with enhanced virulence-related phenotypes, consistent with previous reports suggesting that S. epidermidis supports C. acnes persistence. P. aliciae extracts modulated quorum-sensing-associated behaviors and biofilm formation in vitro, supporting further investigation of plant-derived anti-virulence strategies in multispecies skin models. However, in vivo validation and molecular mechanistic studies are required to determine clinical relevance.