Small Intestine

Introduction

The human body is not only made up of human cells, but is also comprised of bacterial cells. In fact, microorganisms are so abundant that there are about ten times as many bacteria as there are human cells; while there are 100 trillion human cells, there are 1000 trillion bacteria cells (Sears). There is an estimated 500 to 100,000 species of bacteria living in the human body (Sears). Microbes, another term for microorganisms, reside on the skin and mucus surfaces of human tissue, but not within tissues (textbookofbacteriology). They are mostly found on seven surfaces: the skin, eyes, mouth, nose, vagina, small intestine, and colon.

Most of the bacteria in the body have neither a beneficial or harmful effect and have been dubbed microbiota (Samuel). There are several that are beneficial (symbionts), while there are some that are harmful (parasites). Furthermore, the presence of useful bacteria provides a form of competitive exclusion and thus inhibits the growth of pathogenic bacteria (Salminen).

This article is concerned with those microbes residing in the small intestine of the digestive tract. Although most of the human flora (the term used for the bacteria living in the human body) is found in the colon, there are a good number in the small intestine. The following article will discuss: - the bacteria that contribute to the normal functioning of the human small intestine - the bacteria involved in diseases of the human small intestine - the bacteria that contribute to the normal functioning of the small intestine of other animals - the bacteria involved in diseases of the small intestine of other animals

Description of the niche

The small intestine is the site where most of the nutrient absorptions occurred. After food has been broken down in the stomach by strong hydrochloric acid, the pyloric sphincter opens and food get pushed into small intestine with peristalsis. There are three major structural parts of the small intestine, duodenum, jejunum, and ileum. Throughout these sections, finger-liked structure called villi increase surface area that helps the absorption of nutrients. The small intestine is very sensitive to change in temperature…?

Duodenum, a hollowed tube that situated between the stomach and the jejunum. It is mostly responsible for digesting chyme, food bolus that was created by the churning motion of the stomach. Since it doesn’t have the thick mucus lining like the stomach, the duodenum cannot tolerate the low pH introduced by the chyme. In order to neutralize the pH, liver secrets bile and bicarbonate into the duodenum and brings the pH of duodenum up to around 5 and 6 range, a much more tolerable pH for protein and enzymes to function. Enzymes such as lipase, trypsin and chymotrypsin are also secreted into the duodenum to help digestion.

The jejunum is situated right after the duodenum and it is about 10 feet long. (muhealth) Goblet cells are most numerous in jejunum, although they exist throughout small intestine. The primary functions of these cells are to secret mucus. Mucus provides protection against pH, stress, and microorganisms by trapping them (umdnj).

Ileum is the last section of the small intestine. Like jejunum, it is also around 10 feet long. Ileum is responsible for most of the food and liquid absorption, and the unabsorpted matter and waste products are passed into large intestine. (muhealth) One unique feature of the ileum is the dominance of Peyer’s patch, a form of lymphoid tissue. The main function of these Peyer’s patches is to provide immune leukocytes against foreign microorganisms (http://www.siumed.edu/~dking2/erg/giguide.htm).

Location

Not in the vagina niche

The Small Intestine (in order from stomach to the large intestine): Duodenum, Jejunum, and Ileum.¹

Subniches

Duodenum

The duodenum is the shortest section of the small intestine; it is 26cm long, on average.

Jejunum

The jejunum is 2.5m long, and is the site for absorption. The pH is 7-8 (slightly alkaline).

Physical conditions

pH

OVER 9000!

Influence by adjacent communities

Stomach

The stomach is located directly above the small intestine, and it is separated from the duodenum by the pyloric sphincter. As mentioned above, the duodenum has a neutral pH, which is needed because inside the stomach, gastric juice is secreted in order to provide the acidic environment needed to convert the inactive pepsinogen to the active pepsin. As the chyme (the semi-digested foodstuff) enters the duodenum, bile is secreted by the liver through the bile duct, to neutralize the acidic pH from the stomach.

Colon (large intestine)

The colon is connected directly to the other end of the small intestine.

Conditions under which the environment changes

Do any of the physical conditions change? Are there chemicals, other organisms, nutrients, etc. that might change the community of your niche.

Resident microbes

Currently no one :( Boo Radley

Which microbes are present?

The bacterial environment of the jejunum, which is the middle section of the small intestine, consists mainly of gram-positive Streptococci and Lactobacilli [2]. Research done on several healthy volunteers showed the presence of Streptococci, Lactobacilli, Staphylococci, and fungi in the intestinal tract [2]. In the ileocaecal valve, Bacteroides and coliform bacteria are the dominant bacteria present there, in addition to anaerobic Lactobacilli [2]. In the ileum, the last section of the small intestine, the bacterial presence is varied, due to the “backwash contamination of the colon” [2].

Do the microbes that are present interact with each other?

Describe any negative (competition) or positive (symbiosis) behavior

Do the microbes change their environment?

Do they alter pH, attach to surfaces, secrete anything, etc. etc.

Do the microbes carry out any metabolism that affects their environment?

The microbes of the small intestine carry out a number of functions such as protective, structural, and metabolic functions. ²

Protective functions

• The natural flora of the small intestine protect the host by taking up space inside the small intestine. The presence of natural flora inside the small intestine prevents pathogens from obtaining a foothold.

• The natural flora of the small intestine provides competition for nutrients making it more difficult for new pathogens to grow.

• The natural flora of the small intestine produces anti-bacterial products to eliminate competition from other pathogens.

Structural functions

• The structural function of natural flora in the small intestine makes up part of the intestinal barrier

• The natural flora of the small intestine helps in the natural development of the immune system.

Metabolic functions

• The natural flora of the small intestine protect the host by metabolizing carcinogens in dietary foods.

• The natural flora of the small intestine provide the host with synthesized vitamins, such as biotin and folate.

Microbial diseases of the small intestine

Peptic Ulcers from Helicobacter Pylori (http://digestive.niddk.nih.gov/ddiseases/pubs/hpylori/)

Small intestinal bacterial overgrowth (SIBO) (http://www.medicinenet.com/small_intestinal_bacterial_overgrowth/article.htm)

Crohn's Disease (http://www.asm.org/microbe/index.asp?bid=59965)

Salmonellosis (http://www.ncbi.nlm.nih.gov/pubmed/18589722?ordinalpos=15&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum)

Irritable Bowel Syndrome (http://www.ncbi.nlm.nih.gov/pubmed/18700692?ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum)

Necrotizing Entercolitis (NEC) (http://www.ncbi.nlm.nih.gov/pubmed/18446181?ordinalpos=18&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum)

Current Research

Enter summaries of the most recent research. You may find it more appropriate to include this as a subsection under several of your other sections rather than separately here at the end. You should include at least FOUR topics of research and summarize each in terms of the question being asked, the results so far, and the topics for future study. (more will be expected from larger groups than from smaller groups)

References

¹ Bentley-Hibbert, Dr. Stuart. Small Intestine [The small intestine is the portion of the digestive system most responsible for absorption of nutrients from food into the bloodstream. The pyloric sphincter governs the passage of partly digested food from the stomach into the duodenum. This short first]. Digital image. MedlinePlus Medical Encyclopedia. 25 Oct. 2006. ADAM. 26 Aug. 2008 <http://www.nlm.nih.gov/medlineplus/ency/imagepages/19221.htm>.

² O'Hara, Ann M., and Fergus Shanahan. "The gut flora as a forgotten organ." European Molecular Biology Organization 7 (2006): 688-93.

JANET'S REFERENCES:

http://www.textbookofbacteriology.net/normalflora.html

Samuel Baron MD; Charles Patrick. Davis (1996). "Bacteriology": Chapter 6. Normal Flora. University of Texas Medical Branch at Galveston.

Sears CL (2005 Oct;11). "A dynamic partnership: celebrating our gut flora". Anaerobe (5):247-51: 247–251. Academic Press.

Salminen S, Gueimonde M, Isolauri E (2005). "Probiotics that modify disease risk". J Nutr 135 (5): 1294 – 8. PMID 15867327.

KEVIN'S REFERENCES:

[1] Baker, S.J. "Tropical Sprue." British Medical Bulletin 28 (1972): 87-91.

[2] Glynn, Judith. "Tropical sprue--its aetiology and pathogenesis." Journal of the Royal Society of Medicine 79 (1986): 599-606.

[3] Westgaard, Henrik. "Tropical Sprue." Current Treatment Options in Gastroenterology 7 (2004): 7-11.

Edited by [Ellis Chiu, Janet Kwok, Kevin Lee, Susan Lee, Martin Tang, Adnan Vahora], students of Rachel Larsen