Determination of antioxidant and antimicrobial properties of some postbiotics

The term "postbiotic" refers to metabolites and cell wall components produced by probiotic microorganisms or released into the environment after microbial degradation, which have proven beneficial activities for the host. These metabolites include vitamins, bacteriocins, organic acids, short-chain fatty acids, enzymes, cell proteins, cofactors, peptides, teichoic acids, endopolysaccharides, and exopolysaccharides. Postbiotics exhibit potent antimicrobial, antioxidant, anti-inflammatory, immunomodulatory, and antihypertensive properties. Within the scope of this study, the effect of adding orange peel powder (a food waste rich in bioactive components) to the culture medium on the bioactive properties of postbiotics was investigated. Two probiotic strains (Lactobacillus plantarum UALp-05 and Bifidobacterium bifidum UABb-10) were inoculated into media supplemented with orange peel powder (OPP) at three different concentrations (0.5%, 1%, and 1.5%). Following incubation, ultrasound (US) treatment was applied to the culture medium to obtain postbiotics. The antimicrobial and antioxidant properties of the postbiotics were examined. For this purpose, analyses were conducted to determine total phenolic content, antioxidant activity (DPPH and ABTS), and antimicrobial activity (agar well diffusion and MIC). The flavonoid contents of postbiotics derived from media containing 1.5% OPP were identified using HPLC, and the effects of these postbiotics on Staphylococcus aureus cells were analyzed using TEM. The results showed that the addition of OPP to the medium enhanced the bioactive properties of the postbiotics, with the 1.5% OPP concentration significantly increasing both antioxidant and antimicrobial activities (p<0.05). The US treatment was found to significantly increase the total phenolic content of the postbiotics (p<0.05). Antimicrobial activity tests revealed that the highest antimicrobial activity was observed against the pathogen S. aureus. TEM images demonstrated significant cell damage in S. aureus cells treated with postbiotics derived from media containing 1.5% OPP at a concentration of 125 μg/ mL. Additionally, HPLC analyses indicated that fermentation facilitated the biotransformation of glycoside naringin in OPP into its aglycone form, naringenin, and hesperidin into its aglycone form, hesperetin.