Lactic acid is a versatile chemical with wide application in many industries. It can be produced by the fermentation of different sugars by various lactobacilli and investigations on lactic acid production from different substrates and by different strains are still in progress. The present study aimed to study lactic acid production from lactose by Lactiplantibacillus plantarum AC 11S and to choose a mathematical model describing in the best way the experimental data obtained. The influence of initial substrate concentration was investigated, and optimal pH and temperature were determined. An unstructured mathematical model was developed comprising equations for bacterial growth, substrate consumption, and product formation. The model was solved with different terms for specific growth rates considering substrate and/or product inhibition. The best bacterial growth and lactic acid production were achieved at pH = 6.5 and 30 °C. Production of lactic acid was mainly growth-associated, and at initial substrate concentration over 15 g/L, a considerable product inhibition was observed. The parameters of different models were determined and compared. The modified Gompertz equation gave the best fit when solving only the equation for biomass growth at different initial substrate concentrations. Solving the entire set of differential equations for bacterial growth, substrate consumption, and product formation, the best results were obtained when using a variant of the logistic equation for biomass growth. This variant included a term for product inhibition and described in the best way all experimental data. Solving the model for different biomass concentrations showed that an increase in biomass led to a shorter lag phase and the stationary phase was reached faster. The results obtained, optimum conditions and the kinetic model, are good bases for studying pH-controlled fermentation, as well as a continuous process.

Filamentous moulds are the main spoilage microorganisms, responsible for significant economic losses and several healthy risks in human food chain. The lactic acid bacteria (LAB), especially lactobacilli could be a natural antagonist of these dangerous organisms. In Bulgaria, a very limited data exists on the antifungal activity of LAB microbiota of fermented dairy products. In the present study, four active strains were isolated from traditional fermented curd/yogurt-like product "katak", produced in Bulgaria from centuries. The new isolates KR3, KR4, KR51 and KR53 were identified by API 50 CH biochemical test and different molecular methods (species-specific PCR, RAPD-PCR and 16S rDNA sequence analysis) as Lactobacillus brevis. According to our knowledge, this is the first data on the molecular characterization of the Lactobacillus microbiota of "katak". A broad spectrum of antifungal activity of the four L. brevis KR strains against test-cultures representatives of carcinogenic, toxigenic, deteriorative and allergenic fungi from the genera Aspergillus, Fusarium, Penicillium and Trichoderma was estimated. Strains L. brevis KR3, KR4 and KR51 completely suppress the growth of Penicillium claviforme, Aspergillus awamori and Aspergillus niger. With regard to Aspergillus flavus and Trichoderma viride, a lower and strain-specific inhibitory activity was observed. The antifungal activity of our new L. brevis isolates seems to be a promising advantage of these four strains, suggesting their potential applications in different food technologies as bio-preservative agents against moulds.

Background: Recently, various lactic acid bacteria (LAB) and their post-metabolites, have shown many positive effects on human and animal welfare. They appear to be beneficial in different disorders and pathological conditions, including in a broad-spectrum of infectious diseases. Aim: To estimate in vitro the anti-herpes simplex activity of 11 postbiotic samples (lysates or cell-free supernatants - CFS), produced during the fermentation of six candidate-probiotic Lactobacillus strains, isolated from Bulgarian fermented milk products. Materials and methods: In vitro protocols for assessment of different LAB samples on the Herpes simplex virus type 1 (HSV-1) replication, adsorption and virucidal effects were applied using MDBK cells. Results: Four of the studied LAB samples expressed a statistically significant inhibition of the replication of HSV-1. The highest selective index (79.75) was calculated for the post-metabolites of Lactiplantibacillus plantarum, followed by a high molecular fraction of cell-derived fragments of Limosilactobacillus fermentum culture (S6) (SI = 34.63), CFS from late exponential L. plantarum (SI = 28.26) and neutralized CFS from L. fermentum (SI = 28.11). Pronounced virucidal activities of the postbiotics S1, S11 (L. fermentum), S3 (L. plantarum) and S6 (L. fermentum) were recorded, too. The inhibitory effect of the majority of the samples on the stage of adsorption of the virus to MDBK cells was remarkable. In addition, almost all of the postbiotics exerted a protective effect on healthy cells and significantly reduced viral yield at subsequent infection. Conclusion: Pre-selected Lactobacillus strains demonstrated strain-specific effects against HSV-1. These postbiotics influence different stages of viral infection in cell cultures and their promising characteristics are currently evaluated.

Lactobacillus 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: Lactiplantibacillus plantarum L6 and F53, Ligilactobacillus salivarius 1, and Lactobacillus helveticus 611. A broad spectrum of antibacterial and antifungal activity was determined. The strain-specific inhibition of Staphylococcus aureus, Streptococcus mutans, Escherichia coli, Pseudomonas aeruginosa, 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 L. salivarius 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.