Development and Physicochemical Characterisation of Probiotic Emulsions Containing Lactobacillus rhamnosus for Potential Dermal Applications

Background/Objectives: This study evaluated how variations in emulsion composition influence the viability of a probiotic strain Lactobacillus rhamnosus GG within biphasic systems, as well as the overall stability of the resulting formulations. Methods: Eight biphasic emulsions were prepared, each in two versions—with and without a preservative—and subsequently analysed for pH, FTIR spectroscopy, and emulsion type. The viability of L. rhamnosus GG in each formulation was determined using the plate count method. This method is regarded as the reference technique for the quantitative determination of viable bacteria, expressed as colony-forming units (CFUs). Results: The pH indicated that an emulsion with a pH of 4.65 provides the most favorable conditions for L. rhamnosus GG survival, as values below pH 6 promote its proliferation. This acidity aligns with the natural pH of human skin, although it falls slightly below the recommended 4–5 range for topical formulations. FTIR analysis confirmed the structural stability of the emulsions and revealed spectral shifts attributable to the presence of the bacteria. The spectra remained largely consistent throughout the study period, demonstrating good temporal stability. Conclusions: Microbiological evaluation showed that all produced formulations supported bacterial growth, the presence of the preservative did not inhibit viability of L. rhamnosus GG, corroborating findings from an independent assessment. All emulsions were classified as O/W systems, due to the high water content which is advantageous for microbial viability. Furthermore, O/W emulsions are user-friendly, easy to remove, limit the penetration of the active component into deeper skin layers, supporting their suitability for probiotic-based topical applications.