Normal flora in Large intestine

Large intestine

In the human body, the colon, or large intestine, has the largest microbial population. It has been estimated that the number of microorganisms in stool specimens is about 1011 organisms per gram wet weight, which means about 25 percent of feces is made up of microorganisms. Over 300 different bacterial species have been isolated from human feces. It has been calculated that an adult excretes 3? 1013 (30 trillion) bacterial cells daily through defecation.

Various factors tend to remove microorganisms from the large intestine. One factor tends to remove microorganisms from the large intestine. One factor is the continual movement of intestinal contents through the channel o the intestine, due to peristalsis of the intestine. Desquamation of surface epithelial cells to which bacteria are attached is another factor. Mucus is a third factor just as this substance is important in the mechanical removal of microorganisms from the respiratory tract, it also plays a similar role in the intestine. However, the mucus in the intestine forms a discontinuous, meshlike layer rather than a continuous layer. Movement of intestinal contents causes adherence of microorganisms to the mucus, which subsequently rolls up into small masses that are eliminated in the feces, along with the attached microorganisms.

There are about 300 times as many anaerobic bacteria as facultatively anaerobic bacteria (the latter include Escherichia coli) in the large intestine. The anaerobic Gram-negative bacilli principally include species of Bacteroides (B. Fragilis, B.melaninogenicus, B.oralis) and Fusobacterium. The Gram-positive are mainly species of Bifidobacterium, Eubacterium, and Lactobacillus; Clostridium perfringens, a major causative agent of gas gangrene, is invariably present.

The facultatively anaerobic species found in the large intestine, which accounts for less than 1 percent of the total flora, belong to the genera Escherichia, Proteins, Klebsiella, and Enterobacter. Anaerobic streptococci are also common, and the yeast Candida albicans is present.

Some protozoa may also occur as harmless commensals in the large intestine, where they grow anaerobically by ingesting the bacteria that are present. For instance, a flagellated protozoan, Trichomonas hominids, inhabits the cecum (the dilated pouch at the beginning of the large intestine). In another example, amoebas belonging to the genera Entamoeba, Endolimax, and Iodamoeba are commensals of the colon. One species, Entamoeba histolytic, can live as a commensal but can also be pathogenic, causing amoebiasis.

Newborns lack an intestinal normal flora, but very soon after birth the infant;s intestine becomes colonized by microorganisms. It is interesting that the intestinal normal flora of the young breastfed infant consist mainly of bifidobacteria, which are Gram-positive anaerobic rods, whereas bottle-fed infants have lactobacilli, which are Gram-positive microaerophilic to anaerobic rods. With the eventual substitution of solid food and an adult type diet, a Gram-negative normal flora consisting mainly of Bacteroides species will predominate. This is probably because, without the high lactose content of milk for fermentation by the bifidobacteria and the lactobacilli to form organic acids, the stool becomes less acidic and favors the growth of the Gram-negative microorganisms.

Factors influencing the normal flora of the intestine

The composition of the normal flora of the intestine can be influenced by various factors, such as strong emotional stress and starvation. It should be noted that in the progress of intestinal flora undergoes considerable change. Alternation of the flora also occurs in persons receiving antibiotic treatment; here, microorganisms susceptible to the antibiotic being administered may be overgrown by antibiotic-resistant strains. Other factors that may affect or regulate the normal flora are diet, the bile acids secreted into the duodenum from the gallbladder, and the presence of antibodies secreted into the intestine.

Implantation of lactobacilli

As indicated in the preceding paragraph, prolonged therapy with certain antibiotics may eliminate many normal intestinal microorganisms, permitting antibiotic-resistant species to thrive. This, in turn, may cause gastrointestinal disturbance such as constipation or diarrhea. The oral administration of the harmless Gram-positive bacterium Lactobacillus acidophilus will alleviate the intestinal disorder in some instances. The principle is that ingestion disorder in some instances. The principle is that ingestion of large numbers of the lactobacilli may result in a replacement of undesirable intestinal organisms by harmless or beneficial organisms, a concept first proposed by the Russian bacteriologist Elie Metchnikoffin the early days of bacteriology. The implantation lactobacilli seem to depend on ingestion of a large number of the organisms and on supplying a suitable carbohydrate such as lactose that is not readily absorbed by the body but can be used easily by the organisms. Several commercial products containing lactobacilli are available for therapeutic use.

Genitourinary tract

In a healthy person, the kidney, ureters, and urinary bladder are free of microorganisms. The upper portion of the urethra, the canal that carries the urine to the outside of the body, is also bacteria-free; thus, in health the urine within the bladder is sterile. However, bacteria are commonly found in the lower portion of the urethra in both males and females. Gram-positive bacteria (Staphylococcus epidermis, Streptococcus faecal, and corynebacteria) are found frequently; Gram-negative cocci (Neisseria spp.) and Gram-negative facultatively anaerobic rods such as E. coli are occasionally present. The upper portion of the urethra, near the bladder, has no microorganisms, apparently because of some antibacterial effect exerted by the urethral mucous membranes and because of the mechanical removal of microorganisms by the frequent flushing of the urethral epithelium by urine. Urine acquires microorganisms as it passes from the bladder to the outside of the body through the lower portion of the urethra.

The adult female genital tract has a very complex normal flora. The character of this population changes with the menstrual cycle. The main residents of the adult vagina during the years of ovarian activity are the acid-tolerant lactobacilli; these break down glycogen produced by the vaginal epithelium, forming lactic acid in the process. As a result, the pH in the vagina is maintained at about 4.4 to 4.6. Microorganisms capable of multiplying at this low pH are found in the vagina and include enterococci, corynebacteria, the yeast Candida albicans, and large numbers of anaerobic bacteria. The accumulation of glycogen in the vaginal wall is due to the activity of the ovaries, the female reproductive organs that produce eggs and sex hormones. Thus glycogen is not present before puberty or after menopause. In its absence, the vaginal secretions are mildly alkaline and contain normal skin and colon microorganisms.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.