A starter culture is a preparation of selected bacteria, yeasts, or moulds added to a food to begin and direct fermentation. Yoghurt relies on Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus; cheese on various Lactococcus and Lactobacillus strains; salami on Lactobacillus sakei and Staphylococcus starters; sourdough on wild yeasts with lactic acid bacteria. Using defined cultures gives consistent acidity, flavour, texture, and safety, because the chosen microbes outcompete spoilage organisms and pathogens by producing acid, and sometimes bacteriocins, quickly. Cultures are supplied freeze-dried or frozen and must be stored cold. Poor culture management — wrong temperature, antibiotic residues in milk — is a common cause of failed fermentations.