Normal Flora
THE NORMAL FLORA
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In a healthy animal, the internal tissues, e.g. blood, brain, muscle, etc., are normally free of microorganisms.
However, the surface tissues, i.e., skin and mucous membranes, are constantly in contact with environmental
organisms and become readily colonized by various microbial species.
The mixture of organisms regularly found at any anatomical site is referred to as the normal flora, except by
researchers in the field who prefer the term "indigenous microbiota".
The normal flora of humans consists of a few eucaryotic fungi and protists, but bacteria are the most numerous
and obvious microbial components of the normal flora.
Significance of the Normal Flora
The normal flora influences the anatomy, physiology, susceptibility to pathogens, and morbidity of the host.
Types of normal flora
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Skin Flora
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Oral and Upper
Respiratory Tract
Flora
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Gastrointestinal
Tract Flora
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The varied environment of the skin results in locally dense or sparse populations,
with Gram-positive organisms (e.g., staphylococci, micrococci, diphtheroids)
usually predominating.
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A varied microbial flora is found in the oral cavity, and streptococcal anaerobes
inhabit the gingival crevice.
The pharynx can be a point of entry and initial colonization
for Neisseria, Bordetella,Corynebacterium, and Streptococcus spp.
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Urogenital Flora
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Organisms in the stomach are usually transient, and
their populations are kept low (103 to 106/g of contents) by acidity. Helicobacter
pylori is a potential stomach pathogen that
apparently plays a role in the formation of certain ulcer types.
In normal hosts the duodenal flora is sparse (0 to 103/g of contents). The ileum
contains a moderately mixed flora (106 to 108/g of contents).
The flora of the large bowel is dense (109 to 1011/g of contents) and is composed
predominantly of anaerobes.
These organisms participate in bile acid conversion and in vitamin K and ammonia
production in the large bowel.
They can also cause intestinal abscesses and peritonitis.
The vaginal flora changes with the age of the individual, the vaginal pH, and
hormone levels.
Transient organisms (e.g., Candida spp.) frequently cause vaginitis.
The distal urethra contains a sparse mixed flora; these organisms are present in
urine specimens (104/ml) unless a clean-catch, midstream specimen is obtained.
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Conjunctival Flora
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The conjunctiva harbors few or no organisms.
Haemophilus and Staphylococcus are among the genera most often detected.
Interaction Between the Normal Microbiota and the Host
Host Infection
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Many elements of the normal flora may act as opportunistic pathogens, especially in hosts rendered susceptible
by rheumatic heart disease, immunosuppression, radiation therapy, chemotherapy, perforated mucous
membranes, etc.
The flora of the gingival crevice causes dental caries in about 80 percent of the population.
SYMBIOSIS: “Living together”.
1. Commensalism: One organism benefits, the other is not affected (+/0).
• Many microbes live off secretions and dead cells and do not benefit or harm
host.
2. Mutualism: Both organisms benefit from living together (+/+).
• E. coli synthesizes vitamin K and some B vitamins.
3. Parasitism: One organism benefits, the other is harmed (+/-).
• Most disease causing bacteria.
FACTORS AFFECTING NORMAL FLORA
Main factor :
- nature of the local environment.
Determined by:
 pH
 Temperature
 Redox potential
 Oxygen
 Water
 Nutrient levels
Other factor :
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Peristalsis
Saliva
Lysozyme secretion
Secretion of immunoglobulins
e.g:
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infant begins to contact organisms as it moves through the birth canal.
A Gram-positive population (bifidobacteria arid lactobacilli) predominates in the gastrointestinal tract early in
life if the infant is breast-fed.
This bacterial population is reduced and displaced somewhat by a Gram-negative flora (Enterobacteriaceae)
when the baby is bottle-fed.
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The type of liquid diet provided to the infant is the principal instrument of this flora control; immunoglobulins
and, perhaps, other elements in breast milk may also be important.
ORAL FLORA
1. Gram Positive organisms:
• Bulk of oral bacteria
• Rods (bacilli), cocci or irregular shape (pleomorphic)
• Oxygen tolerance varies from aerobes to strict anaerobes
• Most are fermentative
• Cell wall has thick peptidoglycan layer (penicillin has effect by interferingproduction of this layer)
Three important genera:
• Actinomyces, facultative anaerobe
• Lactobacillus, produce lactic acid, facultative anaerobe, role in dentine cariesrather than enamel caries
• Streptococcus facultative anaerobic cocci, produce lactic acid someimplicated in caries
Streptococci:
• Isolated from all sights of the mouth
• Large proportion of resident microflora
• Majority α-haemolytic
Strep mutans:
• Associated with caries
• Associated with bacterial endocarditis
Strep salivarius:
• Colonise mucosal surfaces especially the tongue
Strep angiosus:
• Isolated dental plaque & mucosal surfaces
• Seen in maxillofacial infections, brain, liver etc
Strep mitis:
• Opportunistic pathogens e.g. endocarditis
Distribution of Streptococci in the oral Cavity
Species
Cheek
S.mutans
S. mitior
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S. salivarius
Actinomyces:
• Short pleomorphic rods with branching
• Major proportion of plaque
• Increase in gingivitis
• Associated with root caries
Tongue
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Saliva
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tooth
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2. Gram Negative organisms
 Many Gram-negative bacteria found in the mouth, especially in established/subgingival plaque
 Cocci, rods, filamantous rods, spindle shaped or spiral shaped
 Range of oxygen tolerance but most important strict or facultative anaerobes
 Some fermentative, produce acids which other organisms use acids as an energy source, others produce
enzymes which break down tissue
 Cell wall different to Gram positive with a thin peptidoglycan layer, has B-lactamase which breaks down
penicillin, also has LPS/endotoxin
• Porphyromonas: P. gingivalismajor periodontal pathogen
• Prevotella: P. intermedia a periodontal pathogen
• Fusobacterium: F. nucleatumperiodontal pathogen
• Actinobacillus/Aggregatibacter: A.actinomycetemcomitansassociated withaggressive periodontitis
•Treponema: group important in acute periodontal conditions i.e ANUG
• Neisseria
• Veillonella
BENEFITS OF NORMAL FLORA
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The normal flora protect us from disease by
1. Competing with invaders for space and nutrients
2. Producing compounds (bacteriocins) which kill other bacteria
3. Lowering the pH so that other bacteria can't grow
HAZARD OF FLORA
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If the normal flora escape from their normal location, they can cause disease. For
example, Escherichia coli, commonly found in the intestine, can cause urinary tract
infections if introduced into the bladder.
Immunosuppression can allow otherwise harmless bacteria to cause disease. AIDS,
some cancer treatments and transplant rejection drugs all suppress the immune
system and allow the normal flora to cause occasionally serious disease.
may cause endogenous disease if they reach a site or tissue where they cannot be
restricted or tolerated by the host defenses