Lactobacilli Count, Morphology Analysis and Biochemical Identification of Lactic Acid Bacteria Isolates from Top Yogurt Brands

Lactic acid bacteria (LAB) are pronounced probiotics. They strongly improve the human immune system and produce beneficial for the human health vitamins. Lactobacilli show a strong bactericidal and antifungal activity. Due to the synthesis of many enzymes involved in the degradation of toxic substances, yogurt is known as a food favorable to longevity. Six top brands of 3.6% cow's milk yogurt available on the Bulgarian market were examined for viable lactobacilli count, and both morphological and biochemical analysis of LAB isolates was carried out. Lactobacilli were enumerated in 1 g of yogurt via the Koch method. API 50-CH kit with 50 different metabolites was applied to examine the biochemical activity of LAB isolates and their identification. The morphology of LAB isolates was studied through light microscopy and Scanning Electron Microscopy (SEM). Our results revealed that the number of lactobacilli per g in all studied yogurt samples conformed to the Bulgarian and EU standards: in Elena yogurt - 0,7x106, in Vereia yogurt – 2.6x106, in Na baba yogurt 3.4x106, in Rodopeia yogurt - 4x106, in Parshevitza yogurt - 2.8x107, in LB yogurt 7.1x107. Slightly under the standard was the number of lactobacilli in Elena yogurt. 12 LAB isolates were cultured and identified as Lactobacillus delbrueckii ssp. bulgaricus, being positive in the degradation of only four out of totally 50 metabolites: D-glucose, D-fructose, D-mannose and D-lactose. In three out of six samples, the score of affiliation reached 99% while for the remaining three samples it was lower. The morphology analysis of the isolates via light microscopy and SEM showed characteristics typical of lactobacilli. The examined six top yogurt brands available on the Bulgarian market are safe for consumption and contain Lactobacillus delbruekii. subsp. bulgaricus at the standard concentrations recommended by the EU.