STAPHYLOCOCCUS AUREUS

 

Habitat:  Staphylococci are associated with skin, skin glands and mucous membranes of almost all the warm blooded animals.  When we talk about mucous membranes we are talking about the vestibule of the mouth, the vaginal canal, anal canal, the conjunctival sac of the eyes and the inner part of the nose. 

This bacterium is also widely present in the environment. Nearly one-third of the human population supports the colonization of Staphylococcus aureus and are designated as carriers.  This makes it one of the closest organism in causing sepsis and hospital acquired infections.

Transmission:  Nasal carriage of S. aureus occurs in 40-50% of humans. Major habitat in man is anterior nares or on skin elsewhere.  Hospitalised patients as well as medical and paramedical staff show higher incidence of carriage of Staph. aureus.  Several coagulase negative species of staphylococci are present on the skin as commensal normal flora.  

Morphology:  These are non motile, non spore forming, Gram positive cocci which measure around 0.7 to 1.2 μm in diameter. These are characteristically grouped in irregular clusters that resemble bunches of grapes.   This appearance is because of incomplete separation of cells after successive cell divisions which take place in perpendicular planes. Instead of residual attachment along the plane of division, the attachment point is usually eccentric to the plane of division. This results in irregular aggregates of cocci. The clusters are usually seen in growth obtained on solid media or in pathological materials.  When cultivated in liquid medium, staphylococci usually form short chains.  Single cocci, pairs or tetrads are also seen in liquid cultures.  

Growth and growth factors:  These are easy to grow organisms. They prefer aerobic environment but can also grow in the absence of oxygen; range of temperature for growth is 6-44°C (optimum 37°C) and the range of pH is 4.2-9.3 (optimum 7). The organisms grow easily in simple liquid media especially nutrient broth and peptone water. In both these media the growth becomes visible as uniform turbidity.  

Susceptibility to Physical and Chemical Agents:  Staphylococci are extremely hardy organisms and can survive in adverse environment for a very long time. Some strains can even withstand temperature of 60^oC for 30 minutes.  As compared to other bacteria these are more resistant to the action of disinfectants.  Staph. aureus is very sensitive to aniline dyes and a concentration of 1:500,000 of crystal violet can inhibit their growth.  Most strains can grow in the presence of 10% NaCl.  Fatty acids inhibit the growth of staphylococci.  

Biochemical Properties: There is no consistent pattern of biochemical reactions for Staph. aureus.  The single most important test to differentiate Staph. aureus from Staph. epidermidis is the production of coagulase. Approximately 97% of staphylococci isolated from pathological lesions elaborate this enzyme.  For remaining three percent, one has to resort to testing of DNAse to label it as Staph. aureus. They ferment a number of sugars producing acid but no gas. Fermentation of mannitol is of particular use since it is positive only for Staph. aureus strains.

RELATED;

1.  STREPTOCOCCUS  

2.  VIDEO DEMONSTRATION BACTERIAL CELL WALL

3.  NOSOCOMIAL INFECTIONS

4.  NORMAL FLORA OF THE HUMAN BODY

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