Blood agar showing beta hemolytic colony: Introduction, Possible organism and its details

Blood agar


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.

Staphylococcus aureus


Spherical, non motile, gram positive, cluster forming. On nutrient agar , growth is opaque and golden yellow or white color. Catalase test positivecoagulase test positive, oxidase negative , aerobic or facultative anaerobe. Parasite of human and animal.


Normal flora of skin, upper respiratory tract  and feces of human, animal and birds too.


  • Round or spherical arranged in clusters.
  • 0.8-1.0 μm in size
  • Non motile, non sporing, usually non encapsulate while some strains are encapsulate. Capsule is of two types
  • Micro capsule <200 nm
  • Macro capsule >200 nm and is responsible for slime layer

Cultural characteristics

On nutrient agar

  • Smooth, circular, often yellow pigmented colonies and non diffusible.
  • 1-2 mm in diameter
  • Butyrous in consistency

On blood agar

Beta haemolytic


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.

Selective media for Staphylococcus

  1. 7-10% salt agar
  2. Mannitol salt agar
  3. Tellurite glycine agar
  4. Phenolphthalien phosphate agar
  5. Polymyxin B agar (75 μg/ml)

Note: Staphylococcus aureus grow well on this medium but coagulase negative Staphylococci (CoNS) can not grow.


Thermal death point of Staphylococcus aureus  is 60°C for 30 minutes.

It can survive in dried pus for 2-3 months.

Cell wall

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.

Phage types

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.

Enzyme and toxins


  • Haemolysin: alpha, betagamma
  • Leucodin
  • Enterotoxin A-F
  • Type A and B are responsible for food poisoning.
  • 25 µg of toxin B can cause food poisoning.
  • Epidermolytic  toxin: It is responsible for Staphylococcal Scalded Syndrome (SSS) or Ritter’s disease.
  • Toxic Shock Syndrome Toxin (TSST): Type -F


Coagulase : It is of two types bound and free coagulase.







Pathogenecity of  Staphylococcus aureus

Staphylococcus aureus can cause following diseases-

  1. Abscess
  2. Conjunctivitis
  3. Corneal ulcer
  4. Septicemia
  5. Endocarditis
  6. Pneumonia
  7. Mastitis: It is an inflammation of breast.
  8. Empyemia : It is accumulation  of pus in the body cavity.
  9. Food poisoning
  10. Staphylococcal Scalded Syndrome
  11. Toxic Shock Syndrome (TSS)-enterotoxin F
  12. Septic arthritis
  13. Meningitis
  14. Osteomyelitis

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)

Blood (septicemia)

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

for Selective

  • 7-10% salt agar
  • Mannitol salt agar

  • Tellurite glycine agar
  • Phenolphthalien phosphate agar
  • Polymyxin B agar (75 μg/ml)

Colony characteristics

on nutrient agar

Smooth, circular, often yellow pigmented colonies and non diffusible.

-1-2 mm in diameter

Butyrous in consistency

On blood agar

Beta haemolytic

Biochemical tests


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 test

Coagulase brings about the clotting of plasma which is similar to thrombin -catalytic conversion of fibrinogen  into fibrin.


  1. 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.
  2. 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)-

  • Clindamycin
  • Erythromycin
  • Cefoxitin
  • Chloramphenicol
  • Ciprofloxacin
  • Gentamycin
  • Ofloxacin
  • Cotrimoxazole
  • Doxycycline
  • Vancomycin
  • Teicoplanin
  • Linezolid
  • Nitrofurantoin

Key Notes

  1. Nitrofurantoin is only applicable in case of urinary tract infection replacing chloramphenicol.
  2. To treat Methicillin-resistant Staphylococcus aureus (MRSA), vancomycin is recommended where as to treat Vancomycin-resistant Staphylococcus aureus (VRSA), linezolid is preferred.


  1. Bailey & Scott’s Diagnostic Microbiology. Editors: Bettey A. Forbes, Daniel F. Sahm & Alice S. Weissfeld, 12th ed 2007, Publisher Elsevier.
  2. Clinical Microbiology Procedure Hand book, Chief in editor H.D. Isenberg, Albert Einstein College of Medicine, New York, Publisher ASM (American Society for Microbiology), Washington DC.
  3. Colour Atlas and Text book of Diagnostic Microbiology. Editors: Koneman E.W., Allen D.D., Dowell V.R. Jr and Sommers H.M.
  4. Jawetz, Melnick and Adelberg’s Medical Microbiology. Editors: Geo. F. Brook, Janet S. Butel & Stephen A. Morse, 21st ed 1998, Publisher Appleton & Lance, Co Stamford Connecticut.
  5. Mackie and Mc Cartney Practical Medical Microbiology. Editors: J.G. Colle, A.G. Fraser, B.P. Marmion, A. Simmous, 4th ed, Publisher Churchill Living Stone, New York, Melborne, Sans Franscisco 1996.
  6.  Manual of Clinical Microbiology. Editors: P.R. Murray, E. J. Baron, M. A. Pfaller, F. C. Tenover and R. H. Yolken, 7th ed 2005, Publisher ASM, USA
  7.  Text book of Diagnostic Microbiology. Editors: Connie R. Mahon, Donald G. Lehman & George Manuselis, 3rd edition2007, Publisher Elsevier.
  8. Topley & Wilsons Principle of Bacteriology, Virology and immunology. Editors: M.T. Parker & L.H. Collier, 8th ed 1990, Publisher Edward Arnold publication, London.
  9. District Laboratory Practice in  Tropical Countries  –  Part-2-   Monica Cheesebrough-   2nd Edn Update
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