Blood agar showing beta hemolytic colony: Introduction, Possible organism and its details
5% sheep blood agar is with beta (β) hemolytic colony of Staphylococcus aureus as shown above image. Colonies of Streptococcus agalactiae , Streptococcus pyogenes are also beta hemolytic on blood agar but they are pin point where as S. aureus pin head and catalase test positive. Streptococci are catalase test negative.
Golden yellow and increased in the presence of CO2 and also at room temperature. Pigmentation can be induced by culturing bacteria into 30% milk agar, potato and 1% glycerol monoacetate or phosphate agar.
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 specific affinity for Fc portion of the IgG molecule (except Ig3) leaving the Fab region free to combine with its specific antigen resulting in agglutination known as coagglutination. Peptidoglycan of cell activates complement and induces release of inflammatory cytokines. Similarly teichoic acid of 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 / Staphyococcal Scalded Syndrome)
Group 3: 6/47 (Enterotoxin producer)
There are 30 serotypes based on protein A antigen.
Staphylococcus aureus can cause following diseases-
Mastitis: It is an inflammation of breast.
Empyemia : It is 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 according to site of infection and the nature of 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 cluster
Media -for routine Nutrient agar and blood agar
7-10% salt agar
Mannitol salt agar
Tellurite glycine agar
Phenolphthalien 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
Butyrous in consistency
On blood agar
Catalse test: Positive
Oxidation and fermentation (OF) test: Fermentative
Coagulase test: Positive
DNAse test : Positive
From these features 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 coagulse : This is an extracelluar enzyme of bacteria secreted into the medium. It is thrombin like substance which can nealy fibrinogen to fibrin. Tube coagulase test is performedn 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 link between the bacteria. This cause the clumping of Staphylococci. Hence , bound coagulase is also known as clumping factor. Slide coagulate 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.