Normal flora of Mouth and Gastrointestinal tract

Mouth

The abundant moisture and constant presence of dissolved food and small food particles would seem to make the mouth an ideal environment for bacterial growth. However, the continuous flow of saliva through the mouth causes a mechanical flushing action that removes many microorganisms, which are swallowed and destroyed by the hydrochloric acid of the stomach. Desquamation of epithelial cells is a second mechanical factor that removes microorganisms from the oral cavity. Consequently, it is not surprising that many of the microbes that constitute the normal flora of the mouth resist such mechanical removal by being able to adhere firmly to various surfaces of the oral cavity.

Acquisition of mouth flora:

At birth the oral cavity is essentially a sterile, warm, and moist cavity containing a variety of nutritional substances. The saliva is composed of water, amino acids, proteins, lipids, carbohydrates, and inorganic compounds. Together they provide a rich environment for microbial growth. (Saliva itself generally contains transient microbes from other sites of the oral cavity, particularly from the upper surface of the tongue; it usually has a microbial population of about 108 bacteria per millimeter.)

The normal flora of a newborn is established within a few days after birth. The predominant bacterial species belong to the genera Streptococcus, Neisseria, Veillone Ua, Actinomyces, and Lactobacillus; yeasts are also present. The numbers and kinds of microbial species found are related to the infants diet and to associations with other people and objects such as toward and feeding bottles. The only species consistently recovered from the oral cavity, starting on the second day after birth, is Streptococcus salivarius. This species has an affinity for epithelial tissues and appears in large numbers on the upper surface of the tongue.

Normal microbial flora of the teeth

Until eruption of the teeth, most microorganisms in the mouth are aerobes and facultative anaerobes. As the first teeth now provide an anaerobic environment, and species of Gram-negative anaerobic become more evident.

The teeth themselves can become areas for microbial adherence. Streptococcus mutants are associated with the tooth surface and appear to be the major causative agent of dental caries, or tooth decay. Streptococcus mutants produce glucan, a sticky polymer of glucose that acts like a cement and binds the bacterial cells together and to the tooth surface. This glucan is formed only in the presence of the disaccharide sucrose (the type of sugar found in confections), through a process catalyzed by an enzyme located on the surface of the cocci. The enzyme splits the sucrose molecules into their component monosaccharides, glucose and fructose. The enzyme then links the glucose molecules together to form the glucan, while the fructose molecules are fermented by the Streptococci into lactic acid. Lactic acid can etch the surface of the teeth, enhancing microbial adherence. Although S. mutants initiate dental caries, other bacteria such as Lactobacillus and Actinomyces species can contribute to caries as secondary invaders. The aggregation of bacteria and organic matter on the surface of the teeth is termed dental plaque. Dental plaque contains a very high number of bacteria, about 108 cells per milligram.

The normal flora of the healthy human gingiva (gum) consists mainly of Gram-positive bacteria such as Stretococcus sanguis, a facultative organisms, and species of Actinomyces, an anaerobic genus. In diseased gums, there is a shift in the type of flora. Gum diseases range from gingivitis, an inflammation that results in redness and sometimes bleeding of the gums, to periodontitis, inflammation in which the cement between the gum and enamel of the teeth comes apart, resulting in pockets that become filled with anaerobic bacteria. In these diseases, the bacterial flora is predominantly. Gram-negative and consists mainly of species of Porphyromonas, PrevoteUa, Bacteroides, and Fusobacterium, which are nonmotile bacteria such as Treponema denticola, and WolineUa and Selenomonas species. Some Gram-positive anaerobes such as Eubacterium are also present. One organsism that is suspected as a cuse of periodontitis is Porphyromonas gingivalis; however, periodontitis is probably not caused by a single organism buy rather by the combined action of a group of organisms.

In addition to bacteria, certain commensalism protozoa may inhabit the oral cavity. For instance, the flagellated Trichomonas Tenax may occur in gum margins and in the plaque and cavities of the teeth. Its presence is usually associated with poor oral hygiene.

Gastrointestinal tract

Some of the highest concentrations of normal flora are found in the human gastrointestinal tract, which includes the esophagus, stomach, small intestine, and large intestine (colon).

Stomach:

Although the stomach constantly receives numerous transient bacteria from the oral cavity, the fluid contents of a healthy stomach generally contain fewer than 10 bacteria per millimeter. This scarcity is due to the bactericidal effect of the stomach’s hydrochloric acid and digestive enzymes. The few organisms found are mainly lactobacilli and yeasts such as Candida species. Following the ingestion of food the number of bacteria in the stomach increases (103 to 106 organisms per gram of content), but it soon falls as the gastric juice is secreted and the pH of the stomach drops.

Small intestine

The upper portion of the small intestine is called the duodenum. Few bacteria (usually<103 per milliliter of fluid) survive in the duodenum, because of the combined influence of a strongly acidic environment in the stomach and the inhibitory action of bile from the gallbladder. Of those present, the majority are Gram-positive cocci and bacilli. The second part of the small intestine is the jejunum; here species of entero cocci, lactobacilli, and corynebacteria occasionally occur. The yeast Candida albicans may also be found in this part of the small intestine. The third and last portion of the small intestine is the ileum, where the flora begins to resemble that of that of the heavily populated large intestine. In the ileum, anaerobic bacteria such as Bacteroides species and facultative anaerobes such as Escherichia coli grow in large numbers.

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.