Normal Flora
Definition

Normal flora is the mixture of microorganisms (bacteria and fungi) that are
regularly found at any anatomical site of human body

They are found in areas like: skin, conjunctiva, respiratory tract, mouth , ears,
urogenetal tract and alimentary tract.
Properties of Normal flora:
–
Acquired rapidly during & after birth
–
Reflects age of person
–
Changes continuously throughout life
–
Reflects nutrition of person
–
Reflects genetics of person
–
90% is S. epidermidis; S. aureus, may be in moist areas
–
Reflects environment of person
–
Reflects sex of person
Sterile tissues:
In a healthy human, the internal tissues such as blood, brain, muscle, cerbrospinal fluid (CSF) are
normally free of microorganisms.
Advantages of the normal flora:
1. They constitute a protective host defense mechanism by occupying ecological niches.
2. They produce vitamin B and vitamin K in intestine.
3. The oral flora contributes to immunity by inducing low levels of circulating and secretary
antibodies that may cross react with pathogens.
4. The oral bacteria flora exerts microbial antagonism against non-indigenous species by
production of inhibitory fatty acids, peroxides, bacteriocins, etc
1
5. The normal flora may antagonize other bacteria through the production of substances
which inhibit or kill non-indigenous species.
Disadvantages of the normal flora:
1. They can cause disease in the following:
a) When individuals become immunocompromised or debilitated.
b) When they change their usual anatomic location.
2. The oral flora of humans may harm their host since some of these bacteria are pathogens
or opportunistic pathogens
It has been calculated that the normal flora human body about 1012 bacteria on the skin, 1010 in
the mouth, and 1014 in the gastrointestinal tract.
1. Normal Flora of the Skin
The most important sites are: axilla, perineum, toe webs, hands, face, trunk, upper arms and legs
The majority of skin microorganisms are found in the most superficial layers of the epidermis
and the upper parts of the hair follicles.
Important bacteria:
•
Staphylococcus. epidermidis
–
•
•
Staphylococcus aureus
–
Nose, perineum, vulvar skin
–
Occurrence in nasal passages varies with age being greatest in newborns, less in
adults
Micrococci, Diphtheroids, Propionibacterium
–
•
•
Major inhabitant making up more than 90% of the flora
Eg. Propionibacterium acne in children younger than 10 years are rarely
colonized with it
Neither profuse sweating nor washing significantly modifies normal skin flora
–
Soap or disinfectant (hexachlorophene) diminish microbial population
–
But normal flora rapidly replenished from sebaceous and sweat glands
Pathogenic organisms are eliminated by normal flora
2
2. Normal flora of the conjunctiva
Pathogens which do infect the conjunctiva are,
Staphylococcus epidermidis
Corynebacterium sp.
Propoinibacteriumacnes)
Staphylococcus aureus
Viridans streptococci
Neisseria sp.
Haemophilus influenzae
•
There are many variety of bacteria and are in low numbers present
•
They need high moisture
•
Blinking mechanically removes bacteria from conjunctiva
•
Lachrymal secretions include lysozyme kills the bacteria in eye
3. Normal flora of the respiratory tract
The nares (nostrils) contains,
1.
2.
3.
4.
5.
6.
Staphylococcus epidermidis
Corynebacteria
Staphylococcus aureus
Neisseria sp.
Haemophilus sp
Streptococcus pneumoniae
4. Normal flora of the gastrointestinal tract (GIT)
In humans, the GIT flora is influenced by: age, diet, cultural conditions and the use of antibiotics
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: Bifidobacteria account for more than 90% of the total intestinal bacteria. Other
bacteria includes, Enterobacteriaceae, Enterococci, Bacteroides, Staphylococci, Lactobacilli and
Clostridia
3
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: Enterococci,
Bacteroides, Staphylococci, Lactobacilli and Clostridia
5. Normal flora of the gastrointestinal tract (GIT)
In the upper GIT of adult humans mainly acid-tolerant lactobacilli present e.g. Helicobacter
pylori
In the small intestine, Lactobacilli, Enterococcus faecalis , Coliforms and Bacteroides
Gnotobiology
Gnotobiology is the study of gnotobiotic animals
Gnotobiotic animals are “germfree” (axenic)
–
Fetus is sterile
•
Cesarean sections to obtain fetus
•
Fetus growing in sterile isolator
•
Not anatomically or physiologically normal
•
–
Poorly developed lymphoid system, thin intestinal wall, enlarged
cecum, low antibody titers
–
Die of intestinal atonia ( motility problem)
–
Require vitamin K and B complexes
–
No dental caries or plaque
More susceptible to pathogens
Roles of normal flora:
1. Normal flora may be source of opportunistic infections
- e.g. in-patients with impaired defense mechanisms
2. They causes Immunostimulation
a) They produce antibodies which may contribute to host defenses.
b) Some of these antibodies may cross react with normal tissue
components.
4
3. They give protection from external invaders
i. Because of the normal flora occupy body’s epithelial surfaces, they are
able to prevent other bacteria from establishing themselves by blocking
receptors (attachment), competing for essential nutrients or producing
anti-bacteria substances e.g. Fatty acids, peroxides , Bacteriocins.
4. They provide nutrition
i. Some of the normal intestinal flora e.g. E. coli & Bacteroids produce
Vitamin K in the gut which is available for use by host.
5. It plays a role in the production of Carcinogens
i. Some normal flora may modify, through their enzymes, some chemicals in
our diets into carcinogens
ii. e.g.
Artificial sweeteners may be enzymatically modified into bladder
carcinogens.
6. Predominant and important flora of various body sites in normal health.
7. They stimulate the development of certain tissues:
i.
Caecum and lymphatic tissues (Peyer’s patches) in GI tract and influence
immunology of gut-associated lymphatics
ii. Upper urinary tract (kidneys, ureters, bladder) usually sterile
iii. Male anterior urethra Same as skin +enteric+enterococcus
Vagina: complex microbiota
At birth Same as mother (PH 5)
Neonate Same as skin+enteric+ strept (PH 7)
At puberty Lactobacillus+same as skin+anaerobes+strep (PH 5)
At menopause: return to prepuberty flora
Bacterial-human relationships

Normal flora sometimes may cause opportunistic infections and pathogenic infections.

Dental plaque is caused by Streptococcus sanguis, Streptococcus mutans

Dental caries: It is the destruction of enamel, dentin or cementum of teeth

Streptococcus mutans bacteria in plaque produce lactic acid. Lactic acid demineralizes
enamel and causes destruction.

Some normal floras become opportunistic pathogens. Ex: Staphylococcus aureus,
Streptococcus mutans, Enterococcus faecalis, Streptococcus pneumoniae, Pseudomonas
aeruginosa, etc.
5

Bacterium at one site may be commensal, but might be pathogenic at another site

Commensal in gastrointestinal tract (E.coli) might be pathogenic in lung or urinary tract
Probiotics/Prebiotics
•

Probiotic – It is the oral administration of living organisms to promote health
–
Mechanism speculative: competition with other bacteria; stimulation of
nonspecific immunity
–
Species specific: adherence and growth (tropism)
Prebiotic
– It is the Non-digestible food that stimulates growth or activity of GI microbiota,
especially bifidobacteria and lactobacillus bacteria (both of which are
noninflammatory)
6