Lactic acid bacteria are associated with fermented milk products were used to promote health benefits. The probiotics are helpful in balancing the microbial content in the gastrointestinal tract. They fight against pathogenic microorganisms, that may cause ailments. They are a fraction of the microorganisms that live in the human digestive tract. LAB found in the gut flora, and helps improve intestinal functions. Probiotics in the genus Lactobacillus can be found in foods or food supplements.  Although the bacteria is very beneficial to humans, it's a microorganism only .5-.8 micrometers big. They are straight rods also called bacilli.  Temperatures between 43-46 degrees Celsius  are most beneficial to Lactobacillus bulgaricus.   Lactic acid bacteria can survive in anaerobic conditions, because oxygen is not needed to produce their energy. They can tolerate aerobic  environments as well. Typically they are found in the human digestive tract, and yogurt products.    Lactobacillus bulgaricus, like most bacteria, reproduce asexually through binary fission. The DNA in the parent cell replicates, and attaches to different sides of the cell membrane.  Cytokinesis happens next splitting into two identical daughter cells. However, they don't produce endospores like other bacteria during unfavorable conditions to survive.  They are organotrophic and obtain their ATP by fermenting sugar, and not through the proton gradient. They are non-motile and lack catalase. Nutrient content is vital for the microbes growth.  If the environment is deficient in lactose, then it will have a hard time surviving and fermenting.  Other factors for survival include the water activity in milk.  For optimum growth the water activity needs to be around .98 aw, the lower the level is the more it will inhibit the growth. Since milk is fluid and has no barriers the bacteria can expand throughout and start fermenting if conditions are favorable.