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Staphylococcus Aureus

Staphylococcus aureus is a Gram-positive, 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.

Staphylococcus aureus [staf I lō-kok is aw ree us] (staph), is a type of germ that about 30% of people carry in their noses. Most of the time, staph does not cause any harm; however, sometimes staph causes infections. In healthcare settings, these staph infections can be serious or fatal, including:

  • Bacteremia or sepsis when bacteria spread to the bloodstream.
  • Pneumonia, which most often affects people with underlying lung disease including those on mechanical ventilators.
  • Endocarditis (infection of the heart valves), which can lead to heart failure or stroke.
  • Osteomyelitis (bone infection), which can be caused by staph bacteria traveling in the bloodstream or put there by direct contact such as following trauma (puncture wound of foot or intravenous (IV) drug abuse).
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Who gets S. aureus infections?

Anyone can develop a S. aureus infection, although certain groups of people are more likely than others. This includes people with conditions such as: diabetes, cancer, vascular disease, eczema, lung disease, and people who inject drugs. Patients who are hospitalized in intensive care units (ICUs), patients who have undergone certain types of surgeries, and patients with medical devices inserted in their bodies, such as central lines and catheters, are at greater risk of a more serious S. aureus infection. People who often visit healthcare facilities and nursing home residents are also at an increased risk.

Symptoms of S. aureus infections?

S. aureus infections typically appear on the skin as a pocket of pus surrounded by red, painful skin, or cellulitis.

How is S. aureus treated?

Treatment depends on the type of infection caused by the bacteria. When antibiotics are prescribed, they are selected based on laboratory testing of the bacteria and may involve more than one type.

Staph bacteria are very adaptable, and many varieties have become resistant to one or more antibiotics. The rise of antibiotic-resistant strains of staph bacteria—often described as methicillin-resistant S. aureus (MRSA) strains—has led to the use of IV antibiotics, with the potential for more side effects.

How can you prevent S. aureus Infections?

To prevent staph infections, practice proper hand hygiene, keep infected areas covered and clean, and avoid sharing personal items like razors, towels, and needles.


Staphylococci grow in a carbon dioxide enriched atmosphere and also well grow aerobically. Most of the strains grows anaerobically, but less well. The Temperature range of S.Cocci is 10–42 ºC, whereas optimum temperature is 35–37 ºC. Colonies on solid media are smooth, glistening, round and raised.

1. Blood Agar & Choclate (Heated Blood) Agar

Staphylococcus Aureus produces occasionally white 1-2 mm in diameter colonies or yellow to cream after overnight incubation. Pigment is less clear in young colonies. Some strains are beta haemolytic in the presence of oxygen. Colonies are easily emulsified and slightly raised.

Staphylococcus Aureus on blood agar

2. Mannitol Salt Agar

Mannitol Salt Agar is a useful selective medium for S.Aureus. This agar also be used for screen the nasal carriers. S. Aureus ferments the mannitol and is able to grow on agar contaning 70 – 100 g/l sodium cholride. Mannitol Salt agar also be used for isolating Methicillin Resistant Staph Aureus (MRSA) which contain 75g/l sodium cholorid and 4mg/l methicillin.

Staphylococcus Aureus on Mannitol Salt Agar

3. Tryptic Soy Agar

Tryptic Soy Agar is a growth media for different bacteria. It is a non-selective media which provide enough nutrients to allow a wide variety of microorganisms. Staphylococcus show convex and circular colonies on this agar.

Staphylococcus Aureus on Tryptic Soy Agar

4. Biochemical Test

Staphylococcus Aureus is:

  • Catalase positive
  • Oxidase negative
  • Coagulase positive
  • Methyl red positive
  • Nitrate reduction positive
  • Beta hemolysis on Blood agar
  • Citrate positive
  • Motility negative
  • Urease positive
Biochemical Tests

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