Streptomyces Pigmented Extract-Loaded Nanoemulsions: Stability, Antibacterial Activity, and Cytotoxicity Assessment...

Streptomyces-derived pigments are promising bioactive metabolites for cosmeceutical applications due to their antimicrobial potential; however, their stability may be affected by environmental conditions. This study evaluated the antibacterial activity, physicochemical characterization, pH stability, cytotoxicity, and nanoemulsion-based protection of a pigmented extract from Streptomyces. Pigment production was maintained during culture scale-up, with the highest antibacterial response observed between the third and fourth weeks, mainly in the supernatant fraction. The extracted pigment showed concentration-dependent activity against methicillin-resistant Staphylococcus aureus (MRSA), with a minimum inhibitory concentration (MIC) of 0.05% and an IC₅₀ of 0.293%. FTIR analysis suggested the presence of phenolic hydroxyl groups, aromatic structures, and conjugated systems, while pH analysis indicated greater stability between pH 5 and 8. A Box-Behnken response surface design was used to develop macroemulsions and nanoemulsions by microfluidization, evaluating microfluidization pressure, oil proportion, and pigmented extract concentration. Nanoemulsions showed droplet sizes from 243.4 to 902.4 nm, polydispersity indices from 0.326 to 0.471, and zeta potential values below -30 mV. After two months of refrigerated storage, average droplet size, polydispersity, and zeta potential remained stable, although antibacterial activity decreased slightly over time. The most stable pigment-loaded nanoemulsions were non-cytotoxic to HaCaT and HDFa skin cells under in vitro conditions, maintaining cell viability above 78%. These findings support microfluidized nanoemulsions as protective delivery systems for Streptomyces-derived pigments with potential cosmeceutical applications.