As the stomach lining secretes mucus in order to protect the cells from the low (acidic) pH in the stomach lining. I believe that eventually mucus will be digested in the stomach but it takes a while, don't forget that the stomach lining is continually being renewed. One of the main constituents of mucus is mucin (a glycoprotein*) and pepsin is present in the stomach which is a protease** enzyme that non-selectively will break down proteins. To my knowledge the intestine will not break down these molecules as it lacks the enzymes and the conditions, it is not acidic, in fact as far as I remember it is in mildly alkaline conditions, this might be on account of the bile though. Some fat emulsification happens in the intestines via bile, water and nutrient absorption directly into the bloodstream, also there is a lot of bacteria, a large part of which has never been cultured in a laboratory. The flora of the human gut is quite a murky area in terms of research, much of the bacteria will be well-known bacteria species like Escherichia coli, commonly known as E.coli but many other species are unknown. There are some rather great limitations to culturing these in petri dishes, mainly because we do not know what the species actually feed on and what cultures will support them.

Some bacteria varieties contain various features that enable survival in the very hostile, acidic conditions of the stomach. Survival mechanisms might be anything from neutralising their surroundings, to having acid resistant capsules, to creating spores (like Bacillus anthracis, commonly known as anthrax.) It really depends on the species. Stomach ulceration, for instance, can be caused by bacterial infection.

*Glycoprotein is a combination of a protein (a polymer formed of amino acids) bound to a glycan molecule, which is an oligosaccharide, ie:, polymeric chain of sugars.
**protease = any enzyme that digests proteins into peptides and/or amino acids.
***Sporulation is a survival mechanism, some bacteria will produce spores under stress conditions. Spores are typically much more resistant to environmental extremes and survive longer than the bacterium.

(If anyone is curious as to why the stomach does not digest its own cells and how the cells produce pepsin and HCl, see here: [www.scientificamerican.com] )



Edited 3 time(s). Last edit at 01/29/2013 10:08AM by istirith.