Medical Bacteriology MBIO 460 Lecture 7 Dr. Turki Dawoud 2 nd Semester 1436/1437 H

Normal Microbial Flora of the Gastrointestinal Tract  The human gastrointestinal tract consists of the stomach, small intestine, and large intestine (Figure 28.8).  The gastrointestinal tract is responsible for digestion of food, absorption of nutrients, and the production of nutrients by the indigenous microbial flora.  Starting with the stomach, the digestive tract is a column of nutrients mixed with microorganisms.  The nutrients move one way through the column, encountering ever-changing populations of microorganisms.  Overall, about to microbial cells are present in the entire gastrointestinal tract.

The Stomach  Because stomach fluids are highly acidic (about pH 2), the stomach is a chemical barrier to the entry of microorganisms into the gastrointestinal tract.  However, microorganisms do populate this seemingly hostile environment.  Studies using 16S rRNA sequences obtained from human stomach biopsies indicate that the stomach microbial population consists of several different phyla and a large number of bacterial taxa.  Individuals clearly have very different populations, but all contain species of Gram- positive bacteria, Proteobacteria, Bacteroidetes,Actinobacteria, and Fusobacteria (Figure 28.8).  Helicobacter pylori, the most common single organism found, colonizes the stomach wall in many, but not all, individuals and can cause ulcers in susceptible hosts. Some of the bacteria that populate the stomach consist of organisms found in the oral cavity, introduced with the passage of food.

 Distal to the stomach, the intestinal tract consists of the small intestine and the large intestine, each of which is divided into different anatomical structures.  The composition of the intestinal flora in humans varies considerably and is somewhat dependent on diet.  For example, persons who consume a considerable amount of meat show higher numbers of Bacteroides and lower numbers of coliforms and lactic acid bacteria than do individuals with a vegetarian diet.  Representative microorganisms found in the gastrointestinal tract are shown in Figure 28.8.

The Small Intestine  The small intestine is separated into two parts, the duodenum and the ileum, with the jejunum connecting them.  The duodenum, adjacent to the stomach, is fairly acidic and resembles the stomach in its microbial flora.  From the duodenum to the ileum, the pH gradually becomes less acidic, and bacterial numbers increase.  In the lower ileum, cell numbers of 10 5 –10 7 /gram of intestinal contents are common, even though the environment becomes progressively more anoxic.  Fusiform anaerobic bacteria are typically present here, attached to the intestinal wall at one end (Figure 28.9). Figure 28.9 Scanning electron micrographs of the microbial community on the surface of the epithelial cells in the mouse ileum. (a) An overview at low magnification. Note the long, filamentous fusiform bacteria lying on the surface. (b) Higher magnification, showing several filaments attached at a single depression. Note that the attachment is at the end of the filaments only. Individual cells are 10–15 µm long

The Large Intestine  The ileum empties into the cecum, the connecting portion of the large intestine.  The colon makes up the rest of the large intestine.  In the colon, bacteria are present in enormous numbers. The colon is a fermentation vessel, and many bacteria live here, using nutrients derived from the digestion of food (Table 28.1).  Facultative aerobes such as Escherichia coli are present but in smaller numbers than other bacteria; total counts of facultative aerobes are less than 10 7 /gram of intestinal contents.  The facultative aerobes consume any remaining oxygen, making the large intestine strictly anoxic.  This condition promotes growth of obligate anaerobes, including species of Clostridium and Bacteroides.  The total number of obligate anaerobes in the colon is enormous. Bacterial counts of to cells/gram in distal gut and fecal contents are normal, with Bacteroidetes and gram-positive species accounting for greater than 99% of all bacteria.

 The methanogen Methanobrevibacter smithii can also be present in significant numbers.

Functions and Products of Intestinal Flora  Intestinal microorganisms carry out a wide variety of essential metabolic reactions that produce various compounds (Table 28.2).  The composition of the intestinal flora and the diet influence the type and amount of compounds produced.  Among these products are vitamins B 12 and K. These essential vitamins are not synthesized by humans, but are made by the intestinal microflora and absorbed from the gut.  Steroids, produced in the liver and released into the intestine from the gall bladder as bile acids, are modified in the intestine by the microbial flora; modified and now activated steroid compounds are then absorbed from the gut.

 Other products generated by the activities of fermentative bacteria and methanogens include gas (flatus) and the odor-producing substances listed in Table  Normal adults expel several hundred milliliters of gas, of which about half is N 2 from swallowed air, from the intestines every day.  Some foods metabolized by fermentative bacteria in the intestines result in the production of hydrogen (H 2 ) and carbon dioxide (CO 2 ).  Methanogens, found in the intestines of over one-third of normal adults, convert H 2 and CO 2 produced by fermentative bacteria to methane (CH 4 ).  The methanogens in the rumen of cattle produce significant amounts of methane, up to a quarter of the total global production

 During the passage of food through the gastrointestinal tract, water is absorbed from the digested material, which gradually becomes more concentrated and is converted to feces.  Bacteria make up about one-third of the weight of fecal matter.  Organisms living in the lumen of the large intestine are continuously displaced downward by the flow of material, and bacteria that are lost are continuously replaced by new growth.  Thus, the large intestine resembles a chemostat in its action. The time needed for passage of material through the complete gastrointestinal tract is about 24 h in humans; the growth rate of bacteria in the lumen is one to two doublings per day.  The total number of bacterial cells shed per day in human feces is on the order of

Changing the Normal Flora  When an antibiotic is taken orally, it inhibits the growth of the normal flora as well as pathogens, leading to the loss of antibiotic-susceptible bacteria in the intestinal tract.  This is often signaled by loose feces or diarrhea.  In the absence of the full complement of normal flora, opportunistic microorganisms such as antibiotic-resistant Staphylococcus, Proteus, Clostridium difficile, or the yeast Candida albicans can become established.  The retention of opportunistic pathogens can lead to a harmful alteration in digestive function or even to disease. For example, antibiotic treatment allows less susceptible microorganisms such as C. difficile to flourish, causing infection and colitis.  After antibiotic therapy, however, the normal intestinal flora is typically reestablished quite quickly in adults.  More rapid re-colonization of the gut by desired species can be accomplished by administration of probiotics, live cultures of intestinal bacteria that, when administered to a host, may confer a health benefit. This is because a rapid recolonization of the gut may reestablish a competitive local flora and provide desirable microbial metabolic products

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