2016601106Department of Agricultural Microbiology
Normal microflora and its groups
• The term “normal microbial flora” denotes the population of
microorganisms that inhabit the skin and mucous membranes of
healthy normal persons.
• The skin and mucous membranes always harbor a variety of
microorganisms that can be arranged into two groups:
The resident flora consists of relatively fixed types of
microorganisms regularly found in a given area at a given age;
If disturbed, it promptly re-establishes itself.
1. Resident flora 2.Transient flora
• The transient flora consists of non-pathogenic or potentially
pathogenic microorganisms that inhabit the skin or mucous
membranes for hours, days, or weeks.
• It is derived from the environment, does not produce disease, and
does not establish itself permanently on the surface.
• Generally of little significance so long as the normal resident flora
• However, if the resident flora is disturbed, transient microorganisms
may colonize, proliferate, and produce disease.
• Normal flora are microorganisms that are frequently found in a
particular site in normal healthy individual, they are mostly bacteria
and it doesn't cause any illness.
• Normal flora is divided into different types:
Commensals Natural relationship with host
Residents Present for a long time
Transients Present for a short time
Carrier state This type is different from the other –
Ex: Streptococcus pneumoniae
The most comprehensive analysis - 27 distinct body sites and
revealed the presence of 22 bacterial phyla, with most sequences
(92.3%) related to just four phyla:
Common group of Microbiota in Human body
• Members of the normal flora form part of the host and include:
Facultative pathogens and True pathogens
Malassezia species - skin
Major group of microbes
Sterile organs in human body
Internal organs except alimentary tract are STERILE at health.
Sterility maintained by:
1. Local defense mechanisms
2. Chemical substances in serum & tissues. Ex: Antibodies.
3. Phagocytic activity of polymorphomononucleocytes (PMN)
Areas of the body with normal flora:
• Respiratory tract
• Gastrointestinal tract
• Genital tract
• External auditory meatus
Newborn is sterile from normal flora in uterus.
After birth, it will be
exposed to many sources of
normal flora Ex: mother’s
genital tract and skin
Largest organ – 2 m2 of area
Inhabits 102 - 104 organisms/sq. cm
Unfavourable habitat for microorganisms –
1. has many areas subject to periodic drying
2. acidic pH
3. high sodium chloride concentration
Most skin microbes are associated with glands
eccrine glands—dispersed sweat glands
apocrine glands—sweat glands activated during puberty
(underarm, genital area, etc.)
sebaceous glands—with each hair follicle
Gland secretions contain water, amino acids, urea, salts, and
fatty acids that can serve as nutrients
Staphylococcus epidermidis- found in regions of high moisture
including urethra and
Yeast (Candida sp.)
S. epidermidis: Major inhabitant making up more than 90% of the
S. aureus: Nose, perineum, vulvar skin
Occurrence in nasal passages varies with age being greatest in
newborns, less in adults
Micrococci, Diphtheroids, Propionibacterium
Eg. P. acnes = children younger than 10 years are rarely colonized
Variety of bacteria: low numbers present
1. High moisture
2. Blinking mechanically removes bacteria
3. Lachrymal secretions include lysozyme – peptidoglycan break
Predominant organisms of the eyes are:
Non hemolytic streptococci
The nasopharynx of the infant is sterile at birth but in 2-3
days time it acquires the flora.
The nasopharynx is a natural habitat of the common
pathogenic bacteria causing infection of the nose, throat, bronchi
The flora of nose harbours
• Haemophilus, and
• Moraxella lacunata
Modes of protection:
1. continuous stream of flowing
mucus produced by ciliated cells
2. phagocytic action of macrophages
3. production of lysozyme in mucus
Major sections of the respiratory tract:
Upper: nasopharynx, oral cavity, throat –
This is colonized by a wide assortment of microorganisms
(streptococci, staphylococci, Gram-negative cocci) including
pathogens (Staphylococcus aureus, Streptococcus pyrogenes, S.
pneumoniae, Corynebacterium diphtheriae)
Lower: trachea, lungs – nearly sterile
Ecology and developmental stages
1. Birth: sterile mouth within 4-12 hours
2. Neonate (Streptococcus salivarius,
staphylococci, Neisseriae, Moraxella
3. Teeth appear (Streptococcus mutans,
4. Gingival crevice area (Anaerobic species,
5. Puberty (Bacteroides, spirochetes)
108 bacteria/mL of saliva;
potentially >700 species
Mycobacterium smegmatis a harmless commensal is found in the
secretions (smegma) of both males and females genitalia.
Gardnerella vaginalis, bacteroides and alpha streptococci have
been found in penile urethra.
The microbiology of the vagina is especially interesting:
1. Pre-puberty: no glycogen, High pH
2. After puberty: glycogen is used by microbes as food, fermented to
lactic acid (especially, Lactobacillus acidophilus) and pH is acidic.
3. After menopause: returns to the glycogen-free, alkaline
environment lacking lactic acid bacteria.
due to the actions of circulating estrogens
Lactobacillus acidophilus and a vaginal
squamous epithelial cell.
Streptococcus mutans, the main
bacterium implicated in dental caries
Propionibacterim acnesStaphylococcus epidermidis
Neisseria species - throat.Clostridium difficile - large intestine
Enterococcus faecalis ‐ intestineBifidobacterium bifidus
Escherichia coli Gram stain and colonies
on EMB agar