Dietary fat absorption relies on the presence of the primary bile salts cholate and chenodeoxycholate. Both primary bile salts are synthesised in the liver with cholate making up around 80% of the total. Conjugation of bile salts to glycine or taurine lowers the pKa and results in an ionised form that is secreted to the small intestine via the gall bladder. Here the primary bile salts aid the absorption of dietary fat by forming an emulsion and decreasing the fat droplet size, before being reabsorbed via a specific transporter system in the distal ileum. They are then recycled by the liver and thus remain in the enterohepatic circulation. Bile salts that are not reabsorbed in the terminal ileum pass to the colon where bacterial degradation and metabolism result in the formation of the secondary bile salts. Deoxycholate and lithocholate, the secondary bile salts formed from cholate and chenodeoxycholate respectively, can then be partly reabsorbed and re-enter the enterohepatic circulation.
The maintenance of the bile salt pool is regulated such that any loss in the faecal matter is compensated for by synthesis in the liver. Because bile salt synthesis requires the use of cholesterol as a substrate, bile salt turnover can contribute to the regulation of cholesterol homeostasis in the body. This may have implications for whole body cholesterol homeostasis through modulation of the cholesterol concentrations in plasma. Bile salt synthesis varies from around 250 mg per day up to nearly 900 mg per day, with the variation resulting from both genetic and environmental factors. Diet may play a role in regulating the synthesis of new bile salts, although exact mechanisms are not fully understood. For example, high fat diets may effect the primary bile salt synthesis rates by inducing an enhanced capacity for primary bile acid re-absorption in the terminal ileum1.
As one of the functions of the liver, it plays a central role in the synthesis, secretion and metabolism of bile. Bile is a fluid that may vary in color from dark green to yellowish brown. stores and concentrates the bile.
The bile contains bile salts, inorganic salts, fats and bilirubin. This hepatic secretion aids in the digestion and absorption of dietary fats across the wall of small intestine. It is also a major excretory route for lipid-soluble waste products.
Functions of the liver also include the synthesis of various important compounds in addition to plasma proteins. Synthesis and secretion of transferrin, globulins and other such compounds also falls under the domain of liver.
One method of use is as follows (based on Okrend3 ) :
Prepare a 1 in 10 dilution of the sample in EC Broth (Reduced Bile Salts) CM0990, supplemented with novobiocin as directed. Blend in a Stomacher for 2 minutes. Incubate for either 6 hours at 37°C on a rotary shaker at 100rpm or static for 24 hours at 35°C.
EC Broth (Reduced Bile Salts) can be used for the enrichment of Escherichia coli O157 directly from food and environmental samples, especially for use with frozen food samples, where the bacterium may be sub-lethally damaged1. Novobiocin may be added to provide a more selective primary enrichment medium.
The formulation includes a buffered lactose broth and reduced levels of Bile Salts No. 3 based on a recommendation of Szabo et al.2. The presence of the lower level of bile salts inhibits the growth of spore formers and enterococci, but allows the growth of E. coli O157 at 37°C.
When using the reduced temperature of 37°C suggested by Okrend, supplementation with Novobiocin Supplement SR0181 at a final concentration of 20mg per litre improves reduction in background flora with no reduction in growth of Escherichia coli O1573.
Novobiocin is primarily bacteriostatic, but in higher concentrations it may be bactericidal against more sensitive species. It principally inhibits synthesis of DNA, although RNA synthesis is also inhibited to a lesser extent. Its action on DNA is specifically related to the selective and competitive inhibition of the binding of ATP to gyrase B substrates. The gyrase enzyme forms negative supercoils from closed circular double-stranded DNA, and is inhibited by novobiocin at low concentrations. However, novobiocin-resistant Escherichia coli cells contain a DNA gyrase which is unaffected by the drug. Novobiocin also inhibits protein and cell wall synthesis.
EC Broth (Reduced Bile Salts) and Novobiocin Supplement SR0181 is a USDA/FSIS4 and AIFST1 recommended enrichment medium for the detection of Escherichia coli O157:H7 and O157:NM (NonMotile) from meat and poultry products.
Dissolve 18.3g in 500ml of distilled water. Sterilise by autoclaving at 121°C for 15 minutes. Cool the EC Broth (Reduced Bile Salts) to 50°C and aseptically add the contents of 1 vial of Novobiocin Supplement SR0181, reconsituted as directed. Dispense into final sterile containers.
Dietary fibres from fruits, vegetables and cereals have been shown to have cholesterol lowering properties2. In particular, soluble fibre such as pectin from fruit appears to be beneficial. The exact mechanism of action is unknown but evidence suggests that different fibres have slightly different effects. It is likely that soluble fibres are able to increase excretion of bile acids in the faecal matter through binding, alter the ratios of the primary to secondary bile acids, increase faecal cholesterol and fatty acid excretion, as well as having minor indirect effects, such as the replacement of cholesterol containing foods by low cholesterol high fibre alternatives. In addition, some vegetables such garlic and other allium plants, may decrease cholesterol by reduction of hepatic cholesterol synthesis via inhibition of the enzyme hydroxymethylglutaryl-CoA (HMG-CoA) reductase. Tocotrienols, the unsaturated forms of vitamin E, may also inhibit HMG-CoA reductase.
Dietary fiber appears to deter the synthesis and absorption of secondary bile acids which are unhealthy bile metabolites that form in the intestinal tract.