Tuesday, September 22, 2015

Normal flora of the human body (Part two)

normal flora of the human body

Welcome once again to the review of the Human normal flora. Like we stated at the end of part one about the resident flora having both advantages and disadvantages, now let us take a look at them before proceeding to further discussions.


1.      They prevent or suppress the entry of the pathogens.
2.      These synthesize the vitamins especially Vitamin K and several B Group vitamins.
3.      The normal flora evokes the Antibodies production. These Antibodies cross react with pathogens having related or shared antigens, thus raising the immune status of the host against the invading pathogen.
4.      Colonies produced by some organisms of normal flora have a harmful effect on the pathogens.
5.      Endotoxins liberated by normal flora may help the defence mechanism of the body.


1.      They become pathogenic when the immunity is lowered.
2.      They may act as pathogens in different issue (other than their normal habitat) e.g. normal flora of intestine may cause urinary tract infection (UTI).
3.      Normal flora may cause confusion in diagnosis due to their ubiquitous presence in the body and their resemblance to some of the pathogens.

Transient flora

It consists of both non-pathogenic and potentially pathogenic bacteria that inhabit the body surface or mucous membranes for a limited period. They can be removed from the body surface by mechanical means, for example Pneumococcus and Meningococcus can be removed from nasopharynx of the human beings from time to time. Members of the normal flora form part of the host and include: saprophytes, commensals, facultative pathogens and true pathogens.

Normal flora of the skin

Skin is constantly exposed to and is in contact with the environment, the skin is particularly apt to contain transient microorganisms. The predominant resident microorganisms of the skin are aerobic and anaerobic diphtheroid bacilli (eg, corynebacterium, propionibacterium); nonhemolytic aerobic and anaerobic staphylococci (Staphylococcus epidermidis, occasionally S. aureus, and peptostreptococcus species); gram-positive, aerobic, spore-forming bacilli that
are ubiquitous in air, water, and soil; alphahemolytic streptococci (viridans streptococci) and enterococci (enterococcus species); and gram-negative coliform bacilli and acinetobacter.

Low pH, fatty acids in sebaceous secretions and presence of lysozymes are important factors for eliminating non-resident microorganisms from the skin. Normal skin inhabits 102 - 104 organisms/sq. cm.
Microorganisms present on the skin surface are:
Staphylococcus epidermidis and Diphtheroids are the most common. Less common are
Peptococcus, Strept.viridens, Enterococcus, Micrococcus, Escherichia coli, Candida,

Normal flora of Conjuctiva

The conjunctiva is relatively free from bacteria due to the presence of lysozyme in the tears which flushes the bacteria. Predominant organisms of the eyes are:
Moraxella spp, Diphtheroids, Straph epidermidis, Non hemolytic streptococci

Normal flora of Nose and Nasopharnyx

The nasopharynx of the infant is sterile at birth but in 2-3 days time it acquires the flora carried by the mother and attendants. The nasopharynx is a natural habitat of the common pathogenic bacteria causing infection of the nose, throat, bronchi and lungs.
The flora of nose harbours:
Haemophilus, and
Moraxella lacunata



A Medical Microbiologist, Public health worker, blogger and artcle writer, and of course a fan of Manchester United football club