Beta hemolytic colony of Staphylococcus aureus on Blood agar and this organism related informations
Beta hemolytic colony of Staphylococcus aureus on Blood Agar
Beta hemolytic colony of Staphylococcus aureus on blood agar as shown above image and it has following features-On 5% defibrinated sheep blood agar, colonies are circular, convex, smooth, shining, opaque, emulsified easily. On blood agar plates, colonies of Staphylococcus aureus are frequently surrounded by a clear zone of hemolysis i.e. beta-hemolysis. The golden appearance of colonies of some strains is the etymological root of the bacteria’s name; aureus meaning “golden” in Latin and observed clearly on nutrient agar.
S. aureus may occur as a commensal on human skin. It also occurs in the nose frequently ( about a third of the population) and throat less commonly. The occurrence of this organism under these circumstances does not always indicate infection and therefore does not always require treatment.
Gram-positive cocci in the cluster
Uniform turbidity in peptone water
Pigmented colonies ( golden yellow), pinhead size, circular, convex, smooth, shining, opaque, emulsified easily on nutrient agar.
5 % sheep blood agar:-In blood agar beta-hemolytic colony
It ferments with acid production only in glucose, lactose, sucrose, and mannitol.
Catalase test: Positive
Coagulase test: Positive ( both slide and tube)
Sodium chloride tolerance (7.5%)
Egg yolk medium lipolytic activity: Opalescence around the colony
DNase test: Positive
The thermal death point of Staphylococcus aureus is 60°C for 30 minutes.
It can survive in dried pus for 2-3 months.
Protein -A: It has a specific affinity for the Fc portion of the IgG molecule (except Ig3) leaving the Fab region free to combine with its specific antigen resulting in agglutination known as co-agglutination. The peptidoglycan of the cell activates complement and induces the release of inflammatory cytokines. Similarly teichoic acid of the cell wall facilitates adhesion of the cocci to the host cell surface.
With the use of 28 phages, several hundred phage types have been identified among them important phage types are-
Group 1: 8052A/79 (Hospital strains)
Group 2: 3B/3C/55 (Impetigo / Staphylococcal Scalded Syndrome)
Group 3: 6/47 (Enterotoxin producer)
There are 30 serotypes based on protein A antigen.
Staphylococcus aureus can cause the following diseases-
Mastitis: It is an inflammation of the breast.
Empyema: It is an accumulation of pus in the body cavity.
Staphylococcal Scalded Syndrome
Toxic Shock Syndrome (TSS)-enterotoxin F
Laboratory Diagnosis of Staphylococcus aureus
Samples/ specimens collection
It depends on the site of infection and the nature of the lesion. e.g.
Pus (Suppurative lesion)
CSF ( meningitis)
Sputum( respiratory infection)
Nasal swab (detection of carriers)
Feces and remains of food (food poisoning)
Gram stain: Gram-positive cocci in the cluster
Media -for routine Nutrient agar and blood agar
7-10% salt agar
Mannitol salt agar
Tellurite glycine agar
Phenolphthalein phosphate agar
Polymyxin B agar (75 μg/ml)
on nutrient agar
Smooth, circular, often yellow-pigmented colonies and non-diffusible.
-1-2 mm in diameter
On blood agar
Catalase test: Positive
Oxidation and fermentation (OF) test: Fermentative
Coagulase test: Positive
DNAse test: Positive
From these features, the organism is identified as Staphylococcus aureus.
Coagulase brings about the clotting of plasma which is similar to thrombin -catalytic conversion of fibrinogen into fibrin.
Free coagulase: This is an extracellular enzyme of bacteria secreted into the medium. It is a thrombin-like substance that can change fibrinogen to fibrin. A tube coagulase test is performed for its detection.
Bound coagulase: This is closely bound to the cell wall. On its surface, it has receptors for fibrinogen so that fibrin forms links between the bacteria. This causes the clumping of Staphylococci. Hence, bound coagulase is also known as the clumping factor. Slide coagulase test is done for its detection.
Following antibiotics are available for antibiotics sensitivity test (AST)-
Nitrofurantoin is only applicable in case of urinary tract infection replacing chloramphenicol.