634 Chapter 18
evidence in humans suggest that the nature of the intestinal
microbiota contributes to obesity and perhaps to diabetes and
metabolic syndrome (chapter 19, section 19.4).
We obtain vitamins from foods and from the intestinal
microbiota, which provide us with vitamin K and with the B
vitamins (riboflavin, thiamin, biotin, pantothenic acid, and folic
acid) that are absorbed in the colon. In addition, the commensal
bacteria ferment (through anaerobic metabolism) some indi-
gestible molecules in the chyme that enters the large intestine.
Among the most important products of commensal bacterial
fermentatation are the short-chain fatty acids —acetate, propi-
onate, and butyrate. These can be obtained only by bacterial fer-
mentation of polysaccharides that resist digestion in the small
intestine and enter the colon. The short-chain fatty acids are
used for energy by the colonic epithelial cells and are absorbed
into the blood, accounting for about 10% of the calories in the
average Western diet.
In the small intestine, fluid absorption occurs primarily as
a result of osmosis following glucose-stimulated Na^1 transport,
but this cotransport carrier is absent from the colon. In the colon,
short-chain fatty acids produced by bacterial fermentation stimu-
late active Na^1 and Cl^2 absorption, promoting water absorption
by osmosis. The short-chain fatty acids thereby help retain calo-
ries, electrolytes, and water from the colon contents. Because
antibiotics reduce the population of the intestinal microbiota,
scientists believe that most cases of diarrhea in people taking
antibiotics are due to a reduced production of short-chain fatty
acids by the commensal bacteria. By this logic, adding a form
of starch that can arrive undigested to the colon should increase
fluid absorption (because of increased production of short-chain
fatty acids) and relieve diarrhea. This idea is currently being
tested as an improved solution for oral rehydration therapy
(chapter 6, section 6.3).
The intestine is lined by a simple columnar epithelium a mere
20 m m across that presents a huge surface area (about 200 m^2 )
to the 100 trillion commensal bacteria and potentially pathogenic
bacteria that could provoke inflammation. The intestine is pro-
tected from bacteria in its lumen by: (1) mucus from goblet cells
in the epithelium, which serves as the first line of defense against
intestinal bacteria; (2) antimicrobial peptides, secreted by Paneth
and other intestinal epithelial cells; and (3) secretion of IgA anti-
bodies by plasma cells in the lamina propria. The IgA antibodies
are transported through intestinal epithelial cells into the lumen,
where they can neutralize toxins (such as cholera toxin) and
reduce the ability of pathogenic bacteria (such as Salmonella and
Shigella ) to invade the mucosal barrier. Plasma cells that secrete
IgA may originate in the lamina propria, mesenteric lymph nodes,
Peyer’s patches (chapter 13; see fig. 13.38), and isolated lymphoid
follicles in the intestinal wall.
A normal intestinal microbiota helps to maintain a healthy
epithelial barrier that protects deeper tissues, limits inflamma-
tory damage, and promotes the repair of a damaged epithelium.
The intestinal microbiota is also needed to stimulate proper
development of the gut-associated lymphoid tissues ( GALT ),
which include the Peyer’s patches, crypt patches, and isolated
lymphoid follicles. Additionally, experiments in mice suggestThe microbiota are usually described as composed of
commensal bacteria; commensalism refers to a relationship
where one species benefits and the other is neither benefited
nor harmed. However, mutualism may better describe the rela-
tionship between the intestinal microbiota and their human
hosts; in mutualism, both species benefit. We provide the bac-
teria with nutrients and an anaerobic home in our large intes-
tine; they provide us with a variety of benefits.
Microbes from the mother invade an infant’s intestine dur-
ing birth (from the mother’s vagina during transvaginal births,
or from the mother’s skin during Caesarean births), and the
invasion continues during early infancy. Scientists believe that
this initial colonization affects the composition of the micro-
biota later in life. This explains why the composition of the
intestinal microbiota varies substantially between individuals
and shows more similarity among family members than among
unrelated people.
Scientists can accurately determine if a person is lean or
obese by examining their intestinal microbiota—those who are
obese have less diversity and a different relative abundance of
particular groups of microbes than those who are lean. Mice
raised germ-free (without an intestinal microbiota) and then
allowed to be colonized by the intestinal microbiota of obese
mice gained fat more rapidly than similar mice colonized by
the microbiota of lean mice. These observations and indirect
Figure 18.18 A radiograph of the large intestine.
The large intestine is seen after a barium enema has been
administered; the haustra are clearly visible.