“During evolution, due to the low energy gain by fermentation and to the harsh life condition in an ‘oxidant’ world (like the one in which we now live), LAB have been compelled to specialise their metabolism rather toward stress defence than to acquire strategic biosynthetic abilities. Therefore, they developed symbiont/parasite relationships with plants and animals which can supply the vitamins, proteins, and amino acids. It is worth noting that LAB had acquired the ability to recognise several sugars, such as for instance xylose, cellobiose, ribose, arabinose, glucose, and fructose, before they developed the ability to ferment lactose to lactate, which was made possible only after mammal’s expansion on Earth. Therefore, they firstly colonised fruits and vegetable ecological niches, and later cheese, wine, and especially milk, which constitute their election habitat being rich in lactose.”
Enrica Pessione, “Lactic-Acid Bacteria Contribution to Gut Microbiota Complexity: Lights and Shadows, 2012.