The function of the Lactobacillus bulgaricus and Streptococcus thermophilus  cultures is to ferment lactose to produce lactic acid. The increase in lactic acid decreases pH and causes the milk to clot, or form the soft gel that is characteristic of yogurt. 

In a lactose free milk. All the lactose was transformed in glucose et galactose using lactase. Is it possible for those bacterias to ferment glucose et galactose instead of lactose ? Or is this based on other cbiological reactions ?