Staphylococcus aureus: Introduction, Pathogenecity, Lab Diagnosis and Treatment

Staphylococcus aureus on blood agar

Introduction of Staphylococcus aureus

Scientific classification of Staphylococcus aureus is given as:

  • Domain: Bacteria
  • Phylum: Firmicutes
  • Class: Bacilli
  • Order: Bacillales
  • Family: Staphylococceae
  • Genus: Staphylococcus
  • Species: aureus

Staphyle means bunch and kokkos mean berry.

It was isolated by Pasteur (1880) from a pus sample.

The pathogenic role of Staphylococcus was described by Sir Alexander Ogston and who was a surgeon from Scotland.

Other than Staphylococcus cluster forming cocci are-

  1. Micrococcus
  2. Gafkey
  3. Sarcina
  4. Peptococcus
  5. Aerococcus

Staphylococcus aureus

Definition

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

Habitat

Normal flora of the skin, upper respiratory tract, and feces of humans, animals, and birds too.

Morphology

  • Round or spherical arranged in clusters.
  • 0.8-1.0 μm in size
  • Non-motile, non-sporing, usually non encapsulate while some strains are encapsulated. The capsule is of two types
  • Microcapsule <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 inconsistency

On blood agar

Beta hemolytic

Pigmentation

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. Phenolphthalein 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.

Resistance 

The 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 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.

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 / Staphylococcal Scalded Syndrome)
  • Group 3: 6/47 (Enterotoxin producer)

Serotype

There are 30 serotypes based on protein A antigen.

Enzyme and toxins

Toxins

  • Haemolysin: alpha, beta, gamma
  • 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

Enzymes

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

Phosphatase

DNAse

Staphylokinase

Hyaluronidase

Lipase

Protease

Pathogenicity of  Staphylococcus aureus

Staphylococcus aureus can cause the following diseases-

  1. Abscess
  2. Conjunctivitis
  3. Corneal ulcer
  4. Septicemia
  5. Endocarditis
  6. Pneumonia
  7. Mastitis: It is an inflammation of the breast.
  8. Empyema: It is an 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 the site of infection and the nature of the 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 the cluster

Culture

Media -for routine Nutrient agar and blood agar

for Selective

  • 7-10% salt agar
  • Mannitol salt agar

  • Tellurite glycine agar
  • Phenolphthalein 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 inconsistency

On blood agar

Beta hemolytic

Biochemical tests

 

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 test

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

Types

  1. 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.
  2. 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.

Treatment

Following antibiotics are available for antibiotics sensitivity test (AST)-

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

Keynotes

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

Further Readings

  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 Handbook, 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 Textbook 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.  Textbook 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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