Isolation, identification and characterization of yoghurt cultures from traditionally poduced yoghurts and natural plant samples

The present Ph D thesis outlines the isolation and identification of lactic acid bacteria, particularly yoghurt bacteria Lactobacillus delbrueckii and Streptococcus thermophilus from natural environments such as traditionally produced homemade yogurts, plants collected from the wild environment, rainwater washed plants and from dew drops in spring time. For this aim, a total of 100 yoghurt samples were selected for their organoleptic properties from villages in a relatively extended geographical area in Marmara and Aegean regions. One hundred samples from each of plants, rainwater washed plants and dew drops were also collected from the same regions. Thereafter, these last samples were tested for their milk clotting properties after inoculation into pasteurized milk. 17 of plants samples, 20 of rainwater samples and 18 of dew drops samples were able to coagulate milk after incubation process. In the samples where the clotting formed, the number of lactic acid bacteria reached to desired level after 12 h incubation (107-108 cfu / g), and typical yoghurt consistency and taste were observed in some samples. Even, some of the samples exhibited acceptable quality level by sensorial analyses. Isolated bacteria were then identified by basic biochemical tests and matrix- assisted lazer ionization time of flight, mass spectrometry (MALDITOF-MS) analysis and 45 isolates were confirmed as L. delbrueckii ssp. 27 of isolates were from traditionally produced yoghurts, 3 from plants, 8 from rainwater washed plants and 7 from drew drops. Moreover, 66 strains of S.thermophilus were identified by the same technique, of which, 42 were from naturally produced yogurts, four from plants, eight from rainwater washed plants and 12 from dew drops. Some technologically important properties of strains such as acidification, proteolysis, bacteriolysis as well as antibiotic resistance profiles were assessed as a part of the study. Acidification the strains was measured as the change in pH in buffered medium and reconstituted skim milk after incubation for 24 hours. Statistically important difference was observed between lactobacillus and streptococcus strains in term of their acidification at 2. h of incubation (p < 0.01).Proteolytic activity and lactose utilization of the strains were determined on FSD Agar (Fast Slow DifferentialAgar). Strong [17 lactobacillus (38 %), 9 streptecoccus (14 %)], moderate [12 lactobacillus (26 %), 26 streptecoccus (39%)] and weak [16 lactobacillus (37%), 31 streptecoccus (47%)] level of proteolytic activity were noted for strains. Higher bacteriolytic property exhibiting strains are not preferred in yogurt production because of the retardation of acidification which is the main step in yoghurt production. Bacteriolysis which is a strain dependent character was measured turbidimetrically in potassium phosphate buffer. The rate of bacteriolysis as measured at 2. h of incubation was found to vary between 3 and 35% and its extend varied between 18 and 95% after 24 h incubation for L. delbrueckii. For S. thermophilus, the rate varied between 3-42%, and the extend varied between16 and 76%. This provides evidence that lactobacilli have higher bacteriolysis compared to streptococcus. Antibiotic resistance profile of strains was determined by the disk diffusion method. L. delbrueckii strains were susceptible to quinupristin / dalfopristin, tetracycline and chloramphenicol and 21 strains were shoved resistance to only one antibiotic. On the other hand, eight strains were sensitive to all tested antibiotics. One strain showing multi-drug resistance profile was determined to be resistant to 7 different antibiotics. As for the streptococcus strains, eight of which were sensitive to all tested antibiotics, and three were resistant to only one antibiotic. The resistance to gentamicin (45 strains), streptomycin (43 strains) and erythromycin (40 strains) was very common among the strains. Interestingly, multiple antibiotic resistances were also quite common and 31 strains (47%) showed resistance to 6 or more antibiotics.As a result, L. delbrueckii and S. thermophilus have been isolated from a range of natural sources such as traditionally produced yogurts, plants, rainwater washed plants and dew drops. Some technologically important characters of the strains were also determined. The probable use of these strains in dairy technology as starter requires some additional tests and manufacturing trials.