Microorganisms play several important roles in decomposition, digestion, and nutrient cycling. In decomposition, bacteria break down dead organisms and waste into compounds like ammonia. In digestion, certain bacteria in the intestines and yogurt help break down plant matter like cellulose and protect against harmful microbes. Microbes also fix nitrogen from the air or waste into forms plants can use as nitrates or through symbiotic relationships in roots. Many industrial products also rely on microbial fermentation like milk, yogurt, cheese, vinegar and vitamins.