Bifidobacterium longum is a Gram-positive, catalase-negative, rod-shaped bacterium present in the human gastrointestinal tract and one of the 32 species that belong to the genus Bifidobacterium. It is a micro-aerotolerant anaerobe and considered to be one of the earliest colonizers of the gastrointestinal tract of infants. When grown on general anaerobic medium, B. longum forms white, glossy colonies with a convex shape. While B. longum is not significantly present in the adult gastrointestinal tract, it is considered part of the gut flora and its production of lactic acid is believed to prevent growth of pathogenic organisms. B. longum is non-pathogenic and is often added to food products for its beneficial probiotic health effects. 
In 2002, three previously distinct species of Bifidobacterium, B. infantis, B. longum, and B. suis, were unified into a single species named B. longum with the biotypes infantis, longum, and suis respectively. This occurred as the three species had extensive DNA similarity including a 16s rRNA gene sequence similarity greater than 97%. In addition, the three original species were phenotypically difficult to distinguish due to different carbohydrate fermentation patterns among strains of the same species. As probiotic activity varies among strains of B. longum, there exists an interest in the exact classification of new strains, although this is made difficult by the high gene similarity between the three biotypes. Currently, strain identification is done through polymerase chain reaction (PCR) on the subtly different 16s rRNA gene sequences.
B. longum colonizes the human gastrointestinal tract where it, along with other Bifidobacterium, represents up to 90% of the bacteria of an infant’s gastrointestinal tract. This number gradually drops to 3% in an adult’s gastrointestinal tract as other enteric bacteria such as Bacteroides and Eubacterium begin to dominate. Some strains of B. longum were found to have high tolerance for gastric acid and bile, suggesting that these strains would be able to survive the gastrointestinal tract to colonize the lower small and large intestines. The persistence of B. longum in the gut is attributed to the glycoprotein-binding fimbriae structures and bacterial polysaccharides, the latter of which possess strong electrostatic charges that aid in the adhesion of B. longum to intestinal endothelial cells. This adhesion is also enhanced by the fatty acids in the lipoteichoic acid of the B. longum cell wall.
B. longum is considered to be a scavenger, possessing multiple catabolic pathways to utilize a large variety of nutrients in order to increase its competitiveness among the gut flora. Up to 19 types of permease exist to transport various carbohydrates with 13 being ATP-binding cassette transporters. B. longum has several glycosyl hydrolases to metabolise complex oligosaccharides for carbon and energy. This is necessary as mono and disaccharides have usually been consumed by the time they reach the lower gastrointestinal tract where B. longum reside. In addition, B. longum can uniquely ferment galactomannan-rich natural gum using glucosaminidases and alpha-mannosidases that participate in the fermentation of glucosamine and mannose respectively. The high number genes associated with oligosaccharide metabolism is a result of gene duplication and horizontal gene transfer, indicating that B. longum is under selective pressure to increase its capability to compete for various substrates in the gastrointestinal tract. Furthermore, B. longum possesses hydrolases, deaminases, and dehydratases in order to ferment amino acids. B. longum also have bile salt hydrolases to hydrolyze bile salts into amino acids and bile acids. The function of this is not clear, although it is suggested that B. longum could use the amino acids products or better tolerate bile salts.
Probiotic Health Benefits
As an important organism involved in the maintenance of the human gastrointestinal tract, B. longum is commonly used as a probiotic in various dairy products. Its presence has been associated with many health benefits including improving lactose tolerance and preventing diarrhea, food allergies, and colonization by pathogens. Some strains of B. longum were demonstrated to have an antioxidative effect by inhibiting linoleic acid peroxidation, a process that results in the creation of lipid hydroperoxides that decompose into highly reactive radicals associated with aging and age-related diseases. Additionally, B. longum can scavenge free radicals, lowering a person’s chance of atherosclerosis and stroke. The ability of B. longum to remove cholesterol from its environment by incorporating cholesterol into its membrane is thought to lower the serum cholesterol level in humans. B. longum may also bind and suppress resorption of bile acids, which also lowers serum cholesterol levels as bile salt replacement requires utilization of cholesterol within the body. B. longum supplementation was shown to significantly suppress tumor volume and incidence, although the exact mechanism is not clear. Since high colonic pH is thought to promote colorectal cancer, it is postulated that B. longum can inhibit colorectal cancer by producing bile acid and cholesterol metabolites that lower the intestinal pH.
As an anaerobe, B. longum is able to localize and proliferate in the hypoxic regions of solid tumors when injected intravenously. It is proposed that by introducing genes into B. longum that generate anti-tumor enzymes, B. longum will be able to act as the vector in cancer gene therapy. B. longum is ideal as it is tumor-specific, non-pathogenic, and easily killed by antibiotics unlike other potential anaerobic vectors such as Salmonella or Clostridium.
Immune system regulation
Several studies indicate that B. longum has a positive effect on modulating the immune system. A strain of B. longum has been shown to reduce the symptoms of Japanese cedar pollinosis, while other strains have been shown to reduce the symptoms of influenza infection and fever in elderly persons. As well, the use of B. longum was shown to shorten the duration and minimize the severity of symptoms associated with the common cold with a similar effect to that of neuraminidase inhibitors for influenza.
Sensitive skin treatment
Pancreatic necrosis if left untreated has an almost 100 percent fatality rate due to bacterial translocation. Bifidobacterium longum subspecies infantis has been found to have a wide spectrum of coverage against pathogenic organisms that translocate from the gastrointestinal tract thereby demonstrating therapeutic benefit in the management of pancreatic necrosis. The addition of other probiotic strains reduces pro-inflammatory cytokines and further suppressed bacterial overgrowth in the small intestine leading to a reduction in bacterial translocation.
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