Enhanced Carotenoid Production in Rhodococcus kroppenstedtii S12 using Photoactive Cobalt Ferrite Nanoparticles

Natural pigments, especially carotenoids, have gained significant attention due to their diverse applications in the food, pharmaceutical, and cosmetic industries. To meet the growing demand for these valuable compounds, there is a pressing need for efficient and cost-effective production strategies. Current efforts focus on the discovery of new bioactive pigment-producing strains from diverse environments and on improving their productivity. Accordingly, this study aimed to isolate and characterize a bacterium capable of producing a bioactive pigment with industrial potential and to investigate the use of nanomaterials, specifically cobalt ferrite nanoparticles (CoFe₂O₄ NPs), to enhance pigment production. Through meticulous screening and isolation techniques, Rhodococcus kroppenstedtii strain S12 was identified as a potential pigment-producing strain. Spectroscopic analyses indicated that the extracted pigment is putatively a carotenoid, specifically tentatively identified as 4-keto-γ-carotene via LC-MS. Additional mass spectral data suggested the presence of other γ-carotene derivatives, including 1'-OH-4-keto-γ-carotene and 1'-[(6-O-octadecanoyl-β-D-glucopyranosyl)oxy]-1',2'-dihydro-β,ψ-caroten-4-one. The incorporation of CoFe₂O₄ NPs at optimal concentrations significantly increased carotenoid production, achieving 37.64 ± 4.46 mg/l, a 3.23-fold increase compared to control conditions, particularly when the NPs were photoactivated. Moreover, the extracted carotenoid demonstrated moderate antibacterial activity, exhibiting a minimum inhibitory concentration (MIC) of 58 µg/ml against pathogens like Escherichia coli, Salmonella enterica serovar Typhimurium, and Staphylococcus aureus, and a MIC of 29 µg/ml against Listeria monocytogenes. The extracted carotenoid also showed notable antioxidant activity, with an IC50 value of 66.97 ± 1.34 µg/ml.