Gram stain of Staphylococci
Gram Positive cocci arranged in cluster

Genus consists of at least 15 different species, of which three are of medical importance: S. aureus, S. epidermidis, S. saprophyticus.

The coagulase negative species S. epidermidis and S. saprophyticus and other less commonly isolated species are often referred to simply as coagulase-negative staphylococci (CoNS) without further identification.


Staphylococcus aureus:

Characteristics: Gram-positive coccus; cells in cluster; individual cells approximately 1 µm in diameter. Some strains produce capsules. Non-fastidious; capable of aerobic and anaerobic respiration.

Transmission: Normal habitat: humans; skin, especially nose and perineum. Spread is by contact and airborne routes. Organisms survive drying; tolerant of salt and nitrites.

Diseases: Boils, Skin sepsis; Post operative wound infection; Scalded skin syndrome; Catheter- associated infection; Food borne infection; Septicemia; endocarditis; Toxic shock syndrome; Osteomylitis; Pneumonia.


Staphylococcus epidermidis:

Characteristics: As for S. aureus.

Transmission: Normal habitat: skin (100 %). Spread by contact with self, other patients or hospital personnel. Almost all infection acquired in hospital, but may be endogenous. Survives drying; salt tolerant.

 Diseases: Opportunist pathogens associated with device-related sepsis due to production of slime (catheter, prosthetic valve, artificial joints, shunts): Urinary tract infection; Wound infection, Osteomylitis.


Staphylococcus saprophyticus

Characteristics: As for S. aureus.

Diseases: Urinary tract infection in previously healthy women.

Transmission: Normal habitat: skin, and genitourinary mucosa. Endogenous spread to urinary tract in colonized women.

Pathogenesis: Virulent factors unknown, but organism has the ability to colonize periurethral skin and mucosa.



Sample collection:

  1. Pus and sputum for microscopy and culture
  2. Blood for culture
  3. Faeces, vomit and remains of food if food poisoning is suspected.
  4. Anterior nasal swab to detect carriers.


Methods of identification of organism:

Staphylococcus species are non motile, non sporing and non capsulated.

  1. Gram staining: Gram positive cocci arranged in grape like clusters
  2. Culture: Culture media: Blood agar, Nutrient agar, MacConkey agar.
  3. Grow well aerobically,
  4. Biochemical reactions:
    • Coagulase test,
    • Catalase test,
    • Mannitol fermentation,
    • Phosphatase production,
    • DNAse production
Test S.aureus S. epidermidis  S. saprophyticus
Coagulase production +
Color of colony Golden yellow White colony Lemon yellow colony
Acid from mannitol +
Phosphatase + +
Hemolysis on Blood Agar + (β Type)
Growth on MacConkey agar Pink Colony
Novobiocin sensitivity S              S R


Coagulase Test:

Principle: Coagulase enzyme causes plasma to clot by converting fibrinogen to   fibrin.

Tube test: Detects free coagulase which converts fibrinogen to fibrin by activating a coagulase reacting factor present in plasma. It is seen by appearance of fibrin clot in the test tube.

Slide test: Detects bound coagulase which converts fibrinogen directly into fibrin without requiring coagulase reacting factor. It is seen as clumping of bacterial cells on the slide.


Catalase Test:

It is used to differentiate staphylococci from streptococci

Principle: Enzyme catalase produced by organism catalyses the H2O2 (Hydrogen peroxide) in to water and oxygen. Bubbles are seen in the test tube.


Antibiotic Sensitivity  Test:

MRSA (methicillin resistant Staphylococcus aureus) are resistence to penicillin, other beta- lactum antibiotics including the third generation cephalosporin and carbepenems. MRSA resistance is detected by disc diffusion test using cefoxitin disc.