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 means berry.

It was isolated by Pateur (1880) from pus sample.

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

Other than Staphylococcus cluster forming cocci are-

Micrococcus

Gafkey

Sarcina

Peptococcus

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 human and animal.

Habitat

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

Morphology

Round or spherical arranged in clusters.

0.8-1.0 μm

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

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. 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.

Resistance 

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)

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

Hyluronidase

Lipase

Prptease

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)

Faeces and remains of food  (food poisoning)

Gram stain: Gram positive cocci in cluster

Culture

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.

Types

  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.

Treatment

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.

 

 

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