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The energy sources of yoghurt bacteria (Streptococcus thermophilus and Lactobacillus bulgaricus) were examined with a focus on probable impact of sucrose on their galactose uptake. Yoghurt bacteria were isolated from samples of yoghurt which were purchased from different outlets and kept under refrigeration conditions throughout the period of isolation using nutrient agar (NA) and potato dextrose agar (PDA). After obtaining pure cultures of the isolates which were placed on NA and PDA slants, cultural characteristics and biochemical tests were done on them for identification purposes. Their abilities to ferment glucose, lactose fructose, maltose, sucrose and galactose as carbon source were examined. The effect of sucrose on uptake of galactose by the isolates was also examined. The four strains of L. bulgaricus (LBI, LB2, LB3 and LB4) and one strain of S. thermophilus (ST5) obtained had similar characteristics typical of lactic acid bacteria. Sugar fermentation by the isolates differed from one strain to the other. The extent of sugar fermentation by the isolates also varied depending on the types of sugar employed as carbon source. Glucose and lactose were better when used for growth by all the isolates when compared to other sugars. All the isolates had weak fermentation of galactose when compared to other sugars. All isolates however, had better fermentation of galactose in the presence of sucrose, with LB2 having the best and LB4 the least fermentation of galactose in the presence of sucrose. Appropriate amount of sucrose could thus be employed as possible enhancer for galactose uptake by galactose non-fermentive strains of yoghurt bacteria.
Key words: Lactobacillus bulgaricus, Streptococcus thermophilus, lactic acid bacteria, galactose, yoghurt.