Refers to a spore-free, Gram-positive bacterium whose main product of fermented sugar is lactic acid. Most people don't exercise, and a few exercise with their hair. Bacteria are often arranged in chains. Homotypic lactic acid fermentation is called lactic acid fermentation, and heterotypic lactic acid fermentation is called lactic acid fermentation. Besides lactic acid, there are more substances such as acetic acid, ethanol and CO2. There are microaerobic bacteria and obligate anaerobic bacteria. According to spherical or rod-shaped cells, they can be divided into two categories, namely Streptococcus and Lactobacillus. Lactococcus family, the cells are spherical, usually in pairs or chains, with small colonies and slow growth on solid medium. Most of them are isoenzymes, such as streptococcus, which is an important flora closely related to human beings, and some bacteria are pathogenic bacteria of human beings and warm-blooded animals; Some are normal flora in human oral cavity and intestine; Some bacteria are commonly used in dairy products and plant fermented foods, and also in food industry, such as Streptococcus lactis. A few strains are heterotypic fermentation, such as Leuconostoc mesenteroides, which is an important strain for producing dextran (that is, plasma substitute) in pharmaceutical industry, but it is also a harmful bacteria in sugar industry, which often makes sugar juice sticky and cannot be processed. Lactobacillus family, rod-shaped, single-chain or chain-shaped, sometimes filamentous, producing false branches. According to the different products after using glucose, it can be divided into homotype fermentation group and heterotype fermentation group. Most kinds of lactose can be fermented without lactic acid, and the pH value can be reduced below 6.0 after fermentation. Lactobacillus is the most important species in this family, most of which are commonly used in industry, especially in food industry. It exists in dairy products, fermented plant foods such as pickles, sauerkraut, silage and human intestines, especially in infants. Thermofermentative bacteria are usually used in the industrial production of lactic acid. For example, the optimum growth temperature of Lactobacillus delbrueckii is 45℃, which is widely used in lactic acid manufacturing and calcium lactate manufacturing industries.