The book "Antibiotic Susceptibility of Lactic Acid Bacteria from Gut of Healthy Individuals" is the M.Phil Thesis/Dissertation... Lactic Acid Bacteria (LAB) are Gram positive, non-sporulating and catalase negative rods or cocci that ferment various carbohydrates mainly to lactate and acetate. LAB are heterogenous group of bacteria, it comprises 20 genera within phylum Firmicutes. The genera Aerococcus, Carnobacterium, Enterococcus, Lactobacillus, Lactococcus, Leuconostoc, Oenococcus, Pediococcus, Streptococcus, Tetragenococcus, Vagococcus and Weissella are considered as the principal LAB. A feature common to all LAB is their ability to ferment hexose to produce lactic acid as the main end product. LAB re fermenting organisms, they lack cytochromes, electron transport system and Kreb's cycle. LAB are divided into two groups, homofermentative such as Lactococcus and Streptococcus and heterofermentative such as Leuconostoc and Weisella sp, based on the end products of glucose metabolism. The group comprises cocci (Streptococcus, Pediococcus and Leuconostoc) and rods (Lactobacillus and Bifidobacterium), which are either entirely (homofermentative) or around 50 % (heterofermentative) and lactate producers. Various amino acids, vitamins and minerals are essential for their growth, LAB preferably grow in carbohydrate rich environment. Accordingly, they are commonly associated with nutritious environments like foods, decaying material and the mucosal surfaces of the gastrointestinal and urogenital tract, where they enhance the host protection against pathogens. They can also be found in plants or material of plant origin, fodder, fermented food (yogurt, cheese, olives, pickles, etc.) and also in the oral cavities. Broadly, LAB can be referred as a pioneering organism that colonises the digestive tract in great numbers. LAB was considered among the first living organisms to be present on the earth. They are assumed to have appeared on the transition period approximately three billion years ago from anerobiosis to aerobiosis. Since they bear all the required enzymes involved in respiration pathways, they seem to have adapted to both the aerobic and anaerobic conditions. However, during evolution they failed to acquire heme, component of cytochromes, so they require an external supplement of hemine to shift their respiratory metabolism. Therefore, LAB lacks an ability to get functional heme enzymes such as peroxidises and catalases, the proteins involved in hydroxyl scavenging, which results in impaired resistance to Oxygen toxicity. Nevertheless, LAB can tolerate oxygen and are known as microareophilic organisms.