The correct answer is Bacteria. Hard and semi hard cheeses manufacture requires that the fresh curd may be matured for several months or even years. Throughout ripening, the microflora biodiversity will usually decrease in cheese cores and only a small number of LAB species resist to the harsh conditions of hard cheese. The living bacteria, as well as the enzymes from the bacteria dying off throughout the ripening process, play a key role in the final taste and aroma development of hard cheeses.