human normal flora- part 2 (gastrointestinal track)

The bacterial flora of the gastrointestinal (GI) tract of animals has been studied more extensively than that of any other site. The composition differs between various animal species, and within an animal species. In humans, there are differences in the composition of the flora which are influenced by age, diet, cultural conditions, and the use of antibiotics.  The latter greatly perturbs the composition of the intestinal flora.

The prevalence of bacteria in different parts of the GI tract appears to be dependent on several factors, such as pH, peristalsis, redox potential, bacterial adhesion, bacterial cooperation, mucin secretion, nutrient availability, diet, and bacterial antagonism.

Because of the low pH of the stomach and the relatively swift peristalsis through the stomach and the small bowel, the stomach and the upper two-thirds of the small intestine (duodenum and jejunum) contain only low numbers of microorganisms, which range from 103 to 104 bacteria/mL of the gastric or intestinal contents, mainly acid tolerant lactobacilli and streptococci. In the distal small intestine (ileum), the microflora begin to resemble those of the colon, with around 107–108 bacteria/mL of the intestinal contents. It is estimated that these gut flora have around a hundred times as many genes in aggregate as there are in the human genome.

At birth the entire intestinal tract is sterile, but bacteria enter with the first feed. The initial colonizing bacteria vary with the food source of the infant. In breast-fed infants, bifidobacteria account for more than 90% of the total intestinal bacteria.Enterobacteriaceae and enterococci are regularly present, but in low proportions, while bacteroides, staphylococci, lactobacilli and clostridia are practically absent. In bottle-fed infants, bifidobacteria are not predominant. When breast-fed infants are switched to a diet of cow's milk or solid food, bifidobacteria are progressively joined by enterics, bacteroides, enterococci lactobacilli and clostridia.  Apparently, human milk contains a growth factor that enriches for growth of bifidobacteria, and these bacteria play an important role in preventing colonization of the infant intestinal tract by non indigenous or pathogenic species. 

The four dominant phylum in the human gut are Firmicutes, Bacteroidetes, Actinobacteria, and Proteobacteria. Most bacteria belong to the  genera Bacteroides, Clostridium, Fusobacterium, Eubacterium, Ruminococcus, Peptococcus,Peptostreptococcus, and Bifidobacterium. Other genera, such as Escherichia and Lactobacillus, are present to a lesser extent. Species from the genus Bacteroides alone constitute about 30% of all bacteria in the gut, suggesting that this genus is especially important in the functioning of the host.

 NORMAL FLORA IN TABULAR FORM

Binomial name	Location
Actinomyces spp	Amygdala
Ruminococcus spp	Cecum, large intestine
Providencia spp	Feces
Morganella morganii	Feces
Anaerobiospirillum spp	Feces
Streptococcus anginosus	General distribution
Propionibacterium acnes	General distribution
Plesiomonas shigelloides	General distribution
Escherichia coli	General distribution
Enterococcus faecium	General distribution
Enterococcus faecalis	General distribution
Enterobacter cloacae	General distribution
Campylobacter coli	General distribution
Bacteroides fragilis	General distribution
Eikenella corrodens	General distribution, mouth
Bacterionema matruchotii	Gingiva
Methanobrevibacter smithii	Intestines
Yersinia enterocolitica	Large intestine
Sarcina spp	Large intestine
Ruminococcus bromii	Large intestine
Propionibacterium spp	Large intestine
Clostridium difficile	Large intestine
Campylobacter spp	Large intestine
Butyriviberio fibrosolvens	Large intestine
Bacillus spp	Large intestine
Acinetobacter calcoaceticus	Large intestine
Acidaminococcus fermentans	Large intestine
Bifidobacterium spp	Large intestine, feces
Vibrio spp	Large intestine, Small intestine (Ileon)
Pseudomonas aeruginosa	Large intestine, small intestine (Ileon)
Mycoplasma spp	Large intestine, small intestine (Ileon)
Mycobacteria spp	Large intestine, small intestine (Ileon)
Flavobacterium spp	Large intestine, small intestine (Ileon)
Clostridium spp	Large intestine, small intestine (Ileon)
Alcaligenes faecalis	Large intestine, small intestine (Ileon)
Aeromonas spp	Large intestine, small intestine (Ileon)
Achromobacter spp	Large intestine, small intestine (Ileon)
Wolinella succinogenes	Mouth
Vibrio sputorum	Mouth
Treponema refringens	Mouth
Treponema denticola	Mouth
Torulopsis glabrata	Mouth
Staphylococcus epidermidis	Mouth
Rothia dentocariosa	Mouth
Porphyromonas gingivalis	Mouth
Neisseria spp	Mouth
Mycoplasma spp	Mouth
Leptotrichia buccalis	Mouth
Haemophilus paraphrophilus	Mouth
Haemophilus parainfluenzae	Mouth
Fusobacterium nucleatum	Mouth
Eubacterium spp	Mouth
Corynebacterium spp	Mouth
Capnocytophaga spp	Mouth
Candida albicans	Mouth
Campylobacter upsaliensis	Mouth
Campylobacter sputorum	Mouth
Buchnera aphidicola	Mouth
Bacteroides intermedius	Mouth
Bacteroides gingivalis	Mouth
Arachnia propionica	Mouth
Aggregatibacter actinomycetemcomitans	Mouth
Actinomyces viscosus	Mouth
Actinomyces naeslundii	Mouth
Peptostreptococcus spp	Mouth, amygdala
Micrococcus spp	Mouth, Amygdala
Fusobacterium spp	Mouth, amygdala
Enterococcus spp	Mouth, amygdala
Bacteroides spp	Mouth, amygdala
Veillonella spp	Mouth, amygdala, large Intestine
Bacteroides melaninogenicus	Mouth, feces
Peptococcus spp	Mouth, large intestine
Streptococcus viridans	Mouth, large intestine, small intestine (Ileon)
Staphylococcus aureus	Mouth, large intestine, small intestine (Ileon)
Lactobacillus spp	Mouth, Saliva
Streptococcus pneumoniae	Nasopharynx
Bacteroides pneumosintes	Pharynx
Neisseria sicca	Saliva, sputum
Mycobacterium chelonae	Sputum
Gordonia Bacterium spp	Sputum
Citrobacter freundii	Sputum
Clostridium sordellii	Stomach, small intestine, large intestine, rectus, anus channel
Streptococcus sobrinus	Teeth: Dental plaque
Streptococcus oralis	Teeth: Dental plaque
Streptococcus mutans	Teeth: Dental plaque