Normal flora, or commensal microorganisms, live throughout the human body including the skin, eyes, ears, respiratory tract, oral cavity, gastrointestinal tract, and urogenital tract. In each site, certain bacteria predominate as the normal residents. While generally harmless, normal flora can opportunistically cause infection if they spread to normally sterile sites or if the host's immune system is compromised. Normal flora play an important role in protecting against pathogens, aiding nutrient absorption, and stimulating immune system development.

1. Normal Flora

3. Symbiotic relationships between
Organisms
• Neutralism
Two organisms living together, and neither is affected by that.
• Mutualism
Two organisms living together, and both benefit from that.
• Commensalism:
Two organisms living together, one is benefited and the other
is not been affected.
• Parasitism:
Two organisms living together, one is benefited ‘’called
parasite’’ and the other is harmed ‘’called host’’.
• Synergism:
Sometimes, two (or more) microorganism may work
together “team up” to produce a disease that neither could
cause by itself.

4. Benefits
The overall beneficial effects of microbes are summarized below:
1. The normal flora synthesize and excrete vitamins in excess of their own
needs, which can be absorbed as nutrients by their host. For example, in
humans, enteric bacteria secrete Vitamin K and Vitamin B12, and lactic acid
bacteria produce certain B-vitamins. Germ-free animals may be deficient in
Vitamin K to the extent that it is necessary to supplement their diets.
2. The normal flora prevent colonization by pathogens by competing for
attachment sites or for essential nutrients. This is thought to be their most
important beneficial effect, which has been demonstrated in the oral cavity,
the intestine, the skin, and the vaginal epithelium. In some experiments,
germ-free animals can be infected by 10 Salmonella bacteria, while the
infectious dose for conventional animals is near 106 cells.
3. The normal flora may antagonize other bacteria through the production of
substances which inhibit or kill nonindigenous species. The intestinal bacteria
produce a variety of substances ranging from relatively nonspecific fatty acids
and peroxides to highly specific bacteriocins, which inhibit or kill other
bacteria.

5. 4. The normal flora stimulate the development of certain immune tissues,
i.e., the caecum and certain lymphatic tissues (Peyer's patches) in the GI tract.
The caecum of germ-free animals is enlarged, thin-walled, and fluid-filled,
compared to that organ in conventional animals. Also, based on the ability to
undergo immunological stimulation, the intestinal lymphatic tissues of germ-
free animals are poorly-developed compared to conventional animals.
5. The normal flora stimulate the production of natural antibodies. Since the
normal flora behave as antigens in an animal, they induce an immunological
response, in particular, an antibody-mediated immune (AMI) response. Low
levels of antibodies produced against components of the normal flora are
known to cross react with certain related pathogens, and thereby prevent
infection or invasion. Antibodies produced against antigenic components of
the normal flora are sometimes referred to as "natural" antibodies, and such
antibodies are lacking in germ-free animals.

6. Harmful Effects of the Normal Flora
Harmful effects of the normal flora, some of which are observed in studies with
germ-free animals, can be put in the following categories. All but the last two are
fairly insignificant.
1. Bacterial synergism between a member of the normal flora and a potential
pathogen. This means that one organism is helping another to grow or survive. There
are examples of a member of the normal flora supplying a vitamin or some other
growth factor that a pathogen needs in order to grow. This is called cross-
feeding between microbes. Another example of synergism occurs during treatment
of "staph-protected infections" when a penicillin-resistant staphylococcus that is a
component of the normal flora shares its drug resistance with pathogens that are
otherwise susceptible to the drug.

7. 2. Competition for nutrients Bacteria in the gastrointestinal tract must absorb
some of the host's nutrients for their own needs. However, in general, they
transform them into other metabolisable compounds, but some nutrient(s) may
be lost to the host. Germ-free animals are known to grow more rapidly and
efficiently than conventional animals. One explanation for incorporating
antibiotics into the food of swine, cows and poultry is that the animal grows
faster and can therefore be marketed earlier. Unfortunately, this practice
contributes to the development and spread of bacterial antibiotic resistance
within the farm animals, as well as humans.
3. Induction of a low grade toxemia Minute amounts of bacterial toxins (e.g.
endotoxin) may be found in the circulation. Of course, it is these small
amounts of bacterial antigen that stimulate the formation of natural antibodies.

8. 4. The normal flora may be agents of disease. Members of the normal flora
may cause endogenous disease if they reach a site or tissue where they
cannot be restricted or tolerated by the host defenses. Many of the normal
flora are potential pathogens, and if they gain access to a compromised tissue
from which they can invade, disease may result.
5. Transfer to susceptible hosts Some pathogens of humans that are
members of the normal flora may also rely on their host for transfer to other
individuals where they can produce disease. This includes the pathogens that
colonize the upper respiratory tract such as Neisseria meningitidis,
Streptococcus pneumoniae, Haemophilus influenzae and Staphylococcus
aureus, and potential pathogens such as E. coli, Salmonella or Clostridium in
the gastrointestinal tract.

9. What are Normal Flora?
• Normal flora are microorganisms (bacteria, fungi,
protozoa, and viruses), mostly bacteria that
continuously inhabited the human body
(Resident Normal Flora).
• Under normal conditions in a healthy human they
are harmless and may even be beneficial.
• Also called Commensals i.e. organisms that dine
together or Microflora.

10. Stuff about Normal Flora
• A fetus is sterile when born (No Normal Flora), then newborn
start having the normal flora from its mother, air, food and the
environment.
• Our internal organs are sterile like the spleen, liver, pancreas,
bladder, CSF, and blood unless during infection.
• Normal flora differ from one human to another depending
on age, diet, and geographic habitat.
• When the number of resident normal flora is greatly reduced,
opportunistic microbes can easily cause infections in these
areas e.g. Candida albicans that cause candidiasis.

11. Why Should We Know About Normal
Flora?
We all should know about the types and distribution of
normal flora in our bodies because:
1. It gives us better understanding of the possible
infections that result from injury to a specific body
site.
2. As well as the possible sources and significance of
microorganisms isolated from the site of an infection.

12. Transient Normal Flora
• Normal flora that are temporarily living on and within
humans.
• The transient microbes living in the external environment are
attracted to moist, warm body areas.
Why are these microbes temporary??
1. They may be washed from external areas by bathing.
2. They may not be able to compete with resident normal flora.
3. They may be killed by substances produced by the resident
normal flora.
4. They may not survive in the acidic or alkaline pH of the site.
5. They may be flushed away by bodily secretions like tears,
sweat, oil, urine, feces,..).

13. Where Can We find Normal Flora?
• The human body serves as a microbial ecosystem with a wide variety of
environments ranging from the skin to mucous membranes and the
digestive tract. Due to the microbial environmental diversity in and around
the human body, the microbial population varies depending on location.
This confers a wide range of bacteria and other microbes inhabiting the
human body. The human ear serves as a unique environment with its own
microbiome due to its distinct anatomy.
1. Skin
2. Eyes and Ears
3. Respiratory Tract
4. Oral Cavity (Mouth)
5. Gastrointestinal Tract
6. Urogenital Tract

14. Skin (pH 5.5)
• Microbial number on skin is more where there is increased
amount of moisture, higher body temperature, and greater
concentrations of skin surface lipids.
• The axilla, perineum, and toe webs are more frequently
colonized by gram-negative bacilli than are drier areas of the
skin.
• The 3 types of predominant resident microorganisms of the
skin are:
1. Staphylococcus epidermidis 90% (Gram +, cocci, opportunistic bacteria;
location…almost all over the body, illnesses; )
2. Staphylococcus aureus (Gram+, round-shaped bacterium ,
illnesses….boils, cellulitis, folliculitis, scalded skin, abscess etc)
3. Anaerobic bacteria (old name; Propionibacterium acnes and new name;
Cutibacterium acnes is a Gram +, relatively slow-growing, typically
aerotolerant anaerobic)
4. Candidia spp.

15. Eyes (pH normally neutral i.e. 7.0 to 7.3)
• Pseudomonas aeruginosa is found to be the primary
bacterium when wearing contact lens, typically soft contacts,
that can cause corneal ulcer. Pseudomonas aeruginosa is a
gram-negative, aerobic rod-shaped bacterium which
optimally grows at 42 degrees celcius. Although classified as
aerobic, it can adapt and live in anaerobic environments.
Being an opportunist, this bacterium greatly affects the eye
when the defensive mechanisms of the eye are weakened.
Otherwise, it does not ordinarily cause disease. It is, however,
a very durable bacterium, some of which includes minimal
food requirement, a protective outer coating, and resistance
to many antibiotics
• Eye contain lysozymes which kill infection causing bacteria

16. Ear
• The human ear is divided into three compartments: the outer
ear, middle ear and internal ear.
• The outer ear is exposed to the environment and is covered
in skin. Earwax is produced in the outer ear in order to clean
and lubricate the skin of the outer ear. Earwax contains a
mixture of hydrocarbons, fatty acids, and cholesterol
as well as a mix of antimicrobial proteins.
• Earwax is produced by a combination of sebaceous and
apocrine glands in the outer third portion of the outer ear.
• The middle ear, or tympanic cavity, is an air-filled cavity
contain a set of three ossicles: the malleus, incus and stapes.
• The inner ear contains the organs and nerves that are
involved in hearing and balance.

17. Ear (continues….)
• The outer ear is exposed to the outside oxygen-filled
environment, the majority of the bacterial flora on the auricle
and in the external auditory canal is made up of aerobic
species. The outer ear is home to a diverse set of microbes
including bacteria, viruses, and fungi.
• The skin of the external auditory canal and auricle is predominantly
occupied by Gram-positive over Gram-negative bacteria.
• The main Gram-positive bacterial species are Staphylococcus
auricularis, S. capitis, S. epidermidis, S. warnen, Turicella
otitidis, Alloiococcus otitis, Micrococcus luteus.
• Gram-negative species inhabit the external auditory structure to a much
lesser extent with Pseudomonas aeruginosa and Moraxella osloensis.
• Candida albicans, and Penicillium species form the majority of fungal
isolates of the ear integument

18. SKIN
• Aerobic Bacteria:
- Present in the outer layer of skin.
- Staphylococcus epidermidis (accounts 90%) +
Staphylococcus aureus.
• Anaerobic Bacteria: (More than Aerobic bacteria)
- Present in the deeper skin layers, hair follicles, and
sweat & sebaceous glands.
- Propionibacterium acnes.
• Skin normal flora are generally
harmless but it might cause
bloodstream infections if skin
was penetrated.

19. Normal Flora
in the Skin

20. EYES
• The conjunctiva of the eye has
primarily S. epidermidis, followed
by S. aureus, C. diphtheroids, and
S. pneumoniae.
• Some skin normal flora are also present but at fewer
amounts.
• Tears (Lysozyme enzyme), mucus, and oil will protect
the conjunctiva of the eye from colonization by more
bacteria.

21. Normal Flora
in the Eye

22. EARS
• The middle ear and inner
ear: are usually sterile.
• The outer ear and the
auditory canal: contain the
same normal flora of the
skin.
• When the person coughs,
sneezes, or blows his nose,
these microbes may move
into the middle ear where
they cause infection.

23. Respiratory Tract
• Upper Respiratory Tract:
- Nose and throat have
Many microorganisms. Some
are normal flora, some are
opportunistic, and others are
carried like C. diphtheroids.
- Nasopharynx: Streptococcus pneumoniae
In immune compromised or elderly it might cause acute
bacterial pneumonia.
• Lower Respiratory Tract:
Is usually sterile because the mucous membranes of the
lungs remove any microbes.

24. Oral Cavity (Mouth)
• They have both aerobic
and anaerobic bacteria.
The most common ones are:
C. diphtheroides, S. aureus,
and S. epidermidis.
• Also yeasts, molds,
protozoa, and viruses can be living in the mouth.
Teeth and Gengiva: S. mutans
• Poor dental hygiene help bacteria to grow and cause
dental caries, gingivitis,...
• After dental surgeries, there might be a risk of
bloodstream infection that might cause endocarditis.

25. Gastrointestinal Tract
• Stomach:
Only few bacteria are present in the
stomach due to gastric enzymes and
acidic pH.
• Small intestine:
Only few normal flora are present in
the upper part of small intestine because
bile kills them. Lower parts have more no.
of normal flora.
• Large intestine:
- Has more bacteria than any other part of the body. 99% of normal flora in the
large intestine are anaerobic Bacteroides spp.
- Also many fungi, protozoa, and viruses can live there.
- Many of the normal flora are opportunistic i.e. if they move to other areas e.g.
E.coli cause urinary infection.

26. Normal Flora
in the GIT

27. Urogenital Tract
Urinary Tract
• Kidneys, Ureters and Urinary Bladder: are sterile.
• Lower Urethra and external opening: bacteria, yeast,
and viruses. Has the same bacteria present on the skin.
Genital Organs
• Male and female genitals: are sterile except vagina.
• Vagina: Lactobacillus spp. keeps the pH acidic to protect
the vagina from opportunistic infections e.g. fungal
vaginitis (Candida albicans) or bacterial vaginosis
(Bacteroides spp., Gardnerella vaginalis).

28. Normal Flora
in the Vagina

29. Beneficial Functions of Normal Flora
1. Protect our organs and systems that are in direct
contact with the external environment from invading
pathogens. Some normal flora produce substances
that kills pathogens and others compete for with them
for nutrients.
2. In newborns, normal flora stimulates the development
of immune system.
3. Normal flora of the gut provides important nutrients
such as Vitamin K which aid in digestion and
absorption of nutrients.

30. Harmful Effect of Normal Flora
1. When the normal flora are displaced from their normal site
of the body e.g. bloodstream infections by S. epidermidis.
2. When potential pathogens gain a competitive advantage
due to diminished populations of harmless competitors e.g.
C. difficile growing in the gut after antibiotic therapy.
3. When harmless, commonly ingested food substances are
converted into carcinogenic derivatives by bacteria in the
colon e.g. sweetener cyclamate.
4. When individuals are immunocompromised, normal flora
can overgrow and become pathogenic.