In sourdough starters you will find both yeast and lactic acid bacteria. There are three types of lactic acid bacteria, based on the kind of acids that they produce. The LAB ( Lactic acid Bacteria) ferment carbohydrates and produce carbon dioxide and acids.
These produce organic acids that flavour the bread and facilitate the biochemical processes that contributes to the enhanced nutritional profiles of sourdough bread.
Homofermentative LAB (homo meaning ‘all the same’) refers to the end product of fermentation, which is only, or ‘all’ lactic acid, they also produce carbon dioxide (CO2) as by-product . Lactic acid is the flavour that you might recognise as tasting tangy like yogurt.
For more on bacteria in sourdough see Heterofermentative lactobacilli & Facultative Heterofermentative lactic acid bacteria.