Candidate-Probiotic Lactobacilli and Their Postbiotics as Health-Benefit Promoters

<i>Lactobacillus</i> species are widely recognized for their probiotic potential, focusing on their mechanisms of health benefits and protection. Here we conducted an in vitro investigation of the probiotic potential with a role in microbiome homeostasis of four strains: <i>Lactiplantibacillus plantarum</i> L6 and F53, <i>Ligilactobacillus salivarius</i> 1, and <i>Lactobacillus helveticus</i> 611. A broad spectrum of antibacterial and antifungal activity was determined. The strain-specific inhibition of <i>Staphylococcus aureus</i>, <i>Streptococcus mutans</i>, <i>Escherichia coli</i>, <i>Pseudomonas aeruginosa</i>, and saprophytic/toxigenic fungi makes them promising as protective cultures. DPPH (2,2-diphenyl-1-picrylhydrazyl) and ABTS (2,2′-azino-bis-(3-ethylbenzothiazoline-6-sulfonic) acid) measurements showed that tested samples had strain-specific capacity for scavenging of radicals. The molecular base for the antioxidant potential of two lyophilized forms of active strains was investigated by electron paramagnetic resonance spectroscopy. The MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay, with fractions of the most active postbiotics obtained by SEC-FPLC (fast protein liquid chromatography) analysis, showed a wide variety of effects on the growth of a K562 myeloid leukemia cell line. The IC₅₀ (half-maximal inhibitory concentration) of <i>L. salivarius</i> 1 was determined to be 46.15 mg/mL. The proven in vitro functionality of the selected lactobacilli make them suitable for development of target probiotics with specific beneficial effects expected in vivo. Further investigations on produced postbiotics and safety have to be completed before they can be considered as scientifically proven probiotic strains.