Staphylococcus aureus or (staph) or (S. aureus) :- is a type of bacteria
round-shaped bacterium that is a member of the Firmicutes, and it is a usual
member of the microbiota of the body, frequently found in the upper respiratory
tract and on the skin. It is often positive for catalase and nitrate reduction and is a
facultative (it can live in the presence or in the absence of oxygen).
It is a gram positive, non-motile bacterium.
It is called (staphylococcus) because it collects in the form of irregular balls that
resemble a cluster of grapes when viewed under a microscope , and is named
aureus because it appears in the form of yellow colonies when grown in blood
agar, and can fully analyze red blood cells.
8) Cells arrangement like clusters arrangement.
1) Gram-positive, singly, in pairs, or in a short chain of 3-4 bacteria.
2) Irregular clusters of cells.
3) Size of colonies from 1-2 mm in diameter.
4) Spherical colonies in clusters in two planes.
5) Cell wall- very thick peptidoglycan layer.
6) Non-Flagellated, Non-Motile and Non-Sporing.
7) They are capsulated.
Morphology of Staphylococcus aureus
The Staphylococcus aureus bacteria is found everywhere.
Because Staph bacteria are so hardy, they can survive on objects
such as pillowcases or towels long enough to spread to the person
who touches them.
Staph bacteria can survive in all environment like:
• Extremely high or low temperatures.
• Stomach acids.
So it can live in all kinds of environments.
Examples of the presence of bacteria S.aureas
Figure 4 / S.aureas on Skin. Figure 5/ S.aureas taken from urine.
Staphylococci can cause many forms of infection.
(1) S.aureus causes superficial skin lesions (boils, styes) and localized abscesses in other sites.
(2) S.aureus causes deep-seated infections, such as osteomyelitis and endocarditis and more
serious skin infections (furunculosis).
(3) S.aureus is a major cause of hospital acquired (nosocomial) infection of surgical wounds and,
with S epidermidis, causes infections associated with indwelling medical devices.
(4) S.aureus causes food poisoning by releasing enterotoxins into food.
(5) S.aureus causes toxic shock syndrome by release of superantigens into the blood stream.
(6) S.saprophiticus causes urinary tract infections, especially in girls.
(7) Other species of staphylococci (S.lugdunensis, S.haemolyticus, S.warneri, S.schleiferi,
S.intermedius) are infrequent pathogens.
S.aureus is notorious for causing boils, furuncles, styes, impetigo and other
superficial skin infections in humans (Figure 6). It may also cause more
serious infections, particularly in persons debilitated by chronic illness,
traumatic injury, burns or immunosuppression. These infections include
pneumonia, deep abscesses, osteomyelitis, endocarditis, phlebitis, mastitis
and meningitis, and are often associated with hospitalized patients rather
than healthy individuals in the community. S aureus and S.epidermidis are
common causes of infections associated with indwelling devices such as
joint prostheses, cardiovascular devices and artificial heart valves
Why S.aureus is dangerous?
Because the virulence factors of S. aureus where there are :-
α , β , γ and δ:- Toxins Destroy variety of cells (Polymorph).
Hyaluronidase :- Destroy hyaluronic acid (constituent of connective tissues)
Enterotoxins (A,B,C,D, & E) :- Food poisoning (Diarrhea, and Vomiting)
Polysaccharide A and Protein A :- Antiphagocytic and Adhesion.
Leucocytes :- Kills WBCS.
TSST:- Fever, hypotension, & skin rash followed by desquamation of skin .
Exofoliative toxin :- Desquamation of skin in case of exofoliative dermatitis in SSSS.
Coagulase :- Converting fibrinogen into fibrin.
Isolation and Identification
The presence of staphylococci in a lesion might first be suspected after examination of a direct
Gram stain. However, small numbers of bacteria in blood preclude microscopic examination and
require culturing first.
The organism is isolated by streaking material from the clinical specimen (or from a blood
culture) onto solid media such as blood agar, tryptic soy agar or heart infusion agar. Specimens
likely to be contaminated with other microorganisms can be plated on mannitol salt agar
containing 7.5% sodium chloride, which allows the halo-tolerant staphylococci to grow. Ideally
a Gram stain of the colony should be performed and tests made for catalase and coagulase
production, allowing the coagulase-positive S.aureus to be identified quickly.
Another very useful test for S.aureus is the production of thermostable
deoxyribonuclease. S.aureus can be confirmed by testing colonies for agglutination with
latex particles coated with immunoglobulin G and fibrinogen which bind protein A and
the clumping factor, respectively, on the bacterial cell surface. These are available from
commercial suppliers (e.g., Staphaurex). The most recent latex test (Pastaurex)
incorporates monoclonal antibodies to serotype 5 and 8 capsular polysaccharide in order
to reduce the number of false negatives. (Some recent clinical isolates of S.aureus lack
production of coagulase and/or clumping factor, which can make identification difficult.)
N. Test name Result Observance (notes)
1 Gram stain + Take crystal violet dye.
2 Endospore stain - It does not produce spores.
3 Capsule stain +
It produces a capsule to protect itself in all
environments, and this is a point of difference
between S.auruse and other types of staphlococci.
4 Mannitol fermented +
Its ability to ferment lactose, where the colonies
appear as golden instead of red (or pink) agar.
5 Hemolysis +
Its ability to fully analyze the blood and shows a
blank area around the colonies.
6 Coagulase + Clumping in 10 sec. or less in plasma.
7 Catalase +
It produces the enzyme catalase where bubbles
appear in the agar as a result of decomposition of
hydrogen peroxide, releasing water and oxygen.
8 DNAase +
DNA dissolution in the area where there are
Special tests for S.aureus bacteria
Infections acquired outside hospitals can usually be treated with
Hospital acquired infection is often caused by antibiotic resistant strains
and can only be treated with vancomycin.