Unformatted text preview: ase. The increase in pH is due to pancreatic
secretions. Pancreatic secretions (pH 10) raise the intestinal pH to about pH 8, which
relieves the acid stress on the resident organisms. Bile is one type of pancreatic secretion. FOOD-305 It is an organic substance with detergent like action used in the digestion of dietary fats.
In general, Gram-negative bacteria are more resistant to bile than Gram-positive bacteria
because of the protective nature of their their LPS- based outer membrane which has
lowered fluidity and thus lowered permeability compared to the cytoplasmic membrane
(Bile does not penetrate the outer membrane through porins). However, any organism can
exhibit increased resistance to bile if it possesses a gene encoding an extracellular enzyme
that catalyzes the hydrolysis of bile.
In the lower part of the small intestine just above the large intestine, bacterial
populations reach 105-107 organisms /gm of intestinal contents. Large intestine The large intestine has a slightly acidic pH (pH 5-7) and a lower concentration of bile than
the small intestine. It is a generally anaerobic environment populated by both obligately
anaerobic and facultatively (an)aerobic bacteria in a ratio of about 1,000 obligate organisms
to 1 facultative organism. It is the facultative (an)aerobes which render the environment
anaerobic by consuming the small amount of O2 that diffuses from the intestinal wall
into the lumen of the intestine.
The majority of the bacteria that live in the large intestine require a fermentable
carbohydrate for growth, so it has generally been assumed that carbohydrate metabolism is
necessary for colonization by most species. In the large intestine, carbohydrates are
derived from partially digested food and from host secretions. One organism, Bacteroides
thetaiotaomicron (a member of the “Other” box in Figure 1-11) provides us with a
great benefit. Much of this organism's genome is dedicated to taking many of the complex
carbohydrates we eat and degrading them into products that can be absorbed by the body.
We humans actually absorb 15-20% of our daily caloric intake in this way. The mucus
layer covering epithelial tissues is also recognized as an important source of carbohydrates
in the intestine for the bacteria that live there (Mucus is a complex gel of glycoproteins
and glycolipids). For example, E. coli will grow rapidly in mucus but the organism
grows very poorly in feces itself. Thus, E. coli doesn't grow on what we eat but on what
In the large intestine, prokaryotes are present in enormous numbers; 109-1011
organisms /gm wet weight of feces are normal (the growth rate of bacteria in the lumen is i;
is about two doublings per day). Not only are prokaryotes abundant, they are also
diverse. Over 400 different species have been isolated from human feces. Although we
have considerable knowledge of the gastrointestinal microflora, there remains much to
learn. For example, many of the species that inhabit the large intestine have never
been cultured in the labor...
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This note was uploaded on 10/25/2013 for the course MICB 201 taught by Professor Davidturner during the Fall '12 term at UBC.
- Fall '12