Antimicrobial activity and antibiotic susceptibility were tested for 23 Lactobacillus and three Bifidobacterium strains isolated from different ecological niches. Agar-well diffusion method was used to test the antagonistic effect (against Staphylococcus aureus, Escherichia coli, Bacillus cereus and Candida albicans) of acid and neutralized (pH 5.5) lyophilized concentrated supernatants (cell-free supernatant; CFS) and whey (cell-free whey fractions; CFW) from de Man–Rogosa–Sharpe/trypticase-phytone-yeast broth and skim milk. Acid CFS and CFW showed high acidification rate-dependent bacterial inhibition; five strains were active against C. albicans. Neutralized CFS/CFW assays showed six strains active against S. aureus (L. acidophilus L-1, L. brevis 1, L. fermentum 1, B. animalis subsp. lactis L-3), E. coli (L. bulgaricus 6) or B. cereus (L. plantarum 24-4В). Inhibition of two pathogens with neutralized CFS (L. bulgaricus 6, L. helveticus 3, L. plantarum 24-2L, L. fermentum 1)/CFW (L. plantarum 24-5D, L. plantarum 24-4В) was detected. Some strains maintained activity after pH neutralization, indicating presence of active substances. The antibiotics minimum inhibitory concentrations (MICs) were determined by the Epsilometer test method. All strains were susceptible to ampicillin, gentamicin, erythromycin and tetracycline. Four lactobacilli were resistant to one antibiotic (L. rhamnosus Lio 1 to streptomycin) or two antibiotics (L. acidophilus L-1 and L. brevis 1 to kanamycin and clindamycin; L. casei L-4 to clindamycin and chloramphenicol). Vancomycin MICs > 256 μg/mL indicated intrinsic resistance for all heterofermentative lactobacilli. The antimicrobially active strains do not cause concerns about antibiotic resistance transfer and could be used as natural biopreservatives in food and therapeutic formulations.

Probiotics possibly affect local and systemic immune reactions and maintain the intestinal immune homeostasis in healthy individuals and patients with diseases such as irritable bowel syndrome (IBS). In this single-center, blinded trial, we enrolled 40 individuals (20 patients with IBS and 20 healthy individuals) whose blood and fecal samples were collected before and after a 21-day administration of a product comprising Lactobacillus spp., larch arabinogalactan, and colostrum. The percentage of HLA-DR+ natural killer (NK) cells was higher in healthy individuals (p = 0.03) than in patients with IBS after product supplementation. In the fecal samples of patients with IBS, we observed a decline in IL-6, IFN-γ, TNF-α, and secretory IgA levels and, simultaneously, an increase in IL-10 and IL-17A levels after supplementation, although non-significant, whereas, in healthy individuals, we observed a significant decline in IL-6 and IFN-γ levels after supplementation (p < 0.001). Nevertheless, we observed a clinical improvement of symptoms in 65–75% of patients with IBS and the complete resolution of the initial symptoms in five of the 20 patients. We also observed a possible prophylactic effect by the inducing system antiviral impact accompanied by a trend for local immune tolerance in the gut in healthy individuals, where it is the desirable state.

Lactobacilli are considered important probiotics for the prevention of some infections. In this study, the antifungal effect of both cells and cell-free supernatants of twenty-three strains of lactobacilli were investigated against Candida albicans by co-aggregation, agar diffusion assay, agar spot assay and co-culture assay. In all cases, a fungistatic effect was recorded. In the agar diffusion assay and agar spot assay, an effect was established primarily for heterofermentative species via the production of lactic acid. The anti-Candida effect was higher with microbial suspension than with cultural supernatants in the co-culture assay. A strain-specific reduction in the yeast growth up to 28.9% in MRS broth and up to 17.1% in BHI broth was observed. Cells of Limosilactobacillus fermentum LLF-01 and Limosilactobacillus reuteri LLR-K67 showed the highest activity in both model systems. For all strains, a lower reduction up to 9.7% was recorded with cultural supernatants. L. fermentum LLF-01 showed the highest ability of co-aggregation (64.8%) with C. albicans, followed by Lactobacillus acidophilus LLA-01, Lactobacillus gasseri LLG-V74, Lactobacillus delbrueckii subsp. bulgaricus LLB-02 and two strains of Lactobacillus delbrueckii subsp. lactis LLL-14 and LLL-F18. The present study showed the potential of several strains of lactobacilli to affect the population of C. albicans in vitro. The combination of cultures with proved anti-Candida and co-aggregation activity in a probiotic formula may have a positive effect for the prevention of yeast overgrowth in the gut and hence for the suppression of candidiasis.

Lactic acid bacteria have long been considered as protective microflora, able to displace and kill different pathogens. Twenty vaginal strains, pre-selected as probiotics, were additionally characterized as effective antagonists against human pathogens. Four clinical reference strains- Staphylococcus aureus ATCC 25923, Staphylococcus aureus (MRSA) ATCC 39592, Pseudomonas aeruginosa ATCC 27853 and E. coli ATCC 25922 were used as test-cultures in initial screening procedure. The active strains belonging to the species L. fermentum, L. gasseri and L. salivarius were tested against 10 antibiotic-resistant and problematic clinical isolates. In vitro the growth inhibition of two Acinetobacter baumannii and two Pseudomonas aeruginosa out-patient strains was observed. The active antimicrobials were the lactic acid and hydrogen peroxid produced during the cultivation of ten vaginal strains, in combination with termo and proteinase- sensitive bacteriocin-like metabolites. Our results provide a promising base for further characterization of active compounds synthesized from Bulgarian vaginal Lactobacillus strains.