Probiotic candidates among dairy Lactobacilli and Streptococcus thermophiles strains for control of the oral pathogen Porphyromonas gingivalis.

INTRODUCTION: The gram-negative bacterium Porphyromonas gingivalis is a major causative agent of periodontitis in adults. It is also associated with disorders of the cardiovascular and endocrine systems, rheumatoid arthritis, pancreatic cancer, and Alzheimer's disease. Lactic acid bacteria (LAB) present in the oral cavity or introduced as probiotic preparations can support successful treatment of periodontitis due to their antagonism with the pathogen. AIM: The aim of this study was in vitro assessment of the antimicrobial activity of Lactobacillus spp. and Streptococcus thermophilus against P. gingivalis. MATERIALS AND METHODS: The antimicrobial effect of lactobacilli or S. thermophilus from the LBB Culture collection against P. gingivalis DSM 20709 was evaluated with the well diffusion assay on Wilkins Chalgren blood agar. Inhibition of the pathogen was evaluated by measuring the diameter of clear zones around the wells. RESULTS: Application of milk fermented with selected LAB resulted in а bacteriostatic effect. The most active culture was S. thermophilus 187/4, followed by L. delbr. ssp. bulgaricus (LBB.B1054, C3/2 and LBB.B120), L. helveticus LBB.H48/1 and L. rhamnosus I-1/13. The respective reconstituted freeze-dried preparations had a stronger inhibitory effect on the pathogen with the formation of clear bactericidal zones. The effect of milk acidified with lactic acid was apparent with minimal bactericidal zone observed at concentration of 0.1%. The effectiveness of the assay was confirmed with Elgydium and Eludril. CONCLUSIONS: P. gingivalis DSM 20709 was sensitive to the metabolites produced in fermented milk by selected strains of L. delbr. ssp. bulgaricus, L. helveticus, L. rhamnosus, and S. thermophilus. Reconstituted freeze dried fermented milk had а stronger inhibitory effect compared to fresh samples. Lactic acid produced by lactic acid bacteria was the key component for inhibition of the pathogen.

Selection and characterization of naturally occurring high acidification rate Streptococcus thermophilus strains.

Among Streptococcus thermophilus cultures, the principle component of yoghurt and cheese starters, a minority of strains forms the group of 'H'-strains which show an unusually high acidification rate, grow faster and coagulate milk 3-5 hours earlier than the typical S. thermophilus cultures. A large-scale screening study was performed to select 'H'-strains of S. thermophilus from more than 100 samples of home-made yoghurt, industrial yoghurt starters and single cultures, maintained in the LBB culture collection. Only four strains - LBB.TN1, LBB.M23, LBB.M34 and LBB.M60 - were isolated/selected due to their ability to form large yellowish colonies on milk agar, supplemented with beta-glycerophosphate and bromocresol purple. While in general S. thermophilus is described as a species with limited proteolytic capacity and in contrast to all other tested S. thermophilus cultures, the four selected strains invariably gave positive amplification product with the polymerase chain reaction when primers, specific for the membrane proteinase-coding gene prtS were used. The macrorestriction profiles of the genomic DNA of the four strains confirmed that they are non-isogenic and not related to each other. When grown in milk and compared to the control industrial strain LBB.A, the four strains showed a dramatically faster acidification, coagulating milk within four hours. The application of strain TN1 or M23 as adjunct culture to industrial yoghurt starter LBB.BY5-12 resulted in shortening the fermentation time with more than 30 min.

Isolation and characterization of Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus from plants in Bulgaria.

One of the traditional ways of preparation of yogurt starter in Bulgaria is placing a branch of a particular plant species into boiled sheep's milk maintained at about 45 degrees C, which is further incubated until a dense coagulum is obtained. To investigate the possible origin of the yogurt starter bacteria, Lactobacillus delbrueckii ssp. bulgaricus (L. bulgaricus) and Streptococcus thermophilus (S. thermophilus), the traditional way of yogurt-starter preparation was followed. Hundreds of plant samples were collected from four regions in Bulgaria and incubated in sterile skim milk. The two target bacteria at low frequencies from the plant samples collected were successfully isolated. Phenotypic and genotypic characteristics of these bacterial isolates revealed that they were identified as L. bulgaricus and S. thermophilus. Twenty isolates of L. bulgaricus and S. thermophilus, respectively, were selected from the isolated strains and further characterized with regard to their performance in yogurt production. Organoleptic and physical properties of yogurt prepared using the isolated strains from plants were not significantly different from those prepared using commercial yogurt-starter strains. It was therefore suggested that L. bulgaricus and S. thermophilus strains widely used for commercial yogurt production could have originated from plants in Bulgaria. To our knowledge, this is the first report on the isolation and characterization of L. bulgaricus and S. thermophilus strains from plants.