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.
LABORATORY DIAGNOSIS:
Sample collection:
- Pus and sputum for microscopy and culture
- Blood for culture
- Faeces, vomit and remains of food if food poisoning is suspected.
- Anterior nasal swab to detect carriers.
Methods of identification of organism:
Staphylococcus species are non motile, non sporing and non capsulated.
- Gram staining: Gram positive cocci arranged in grape like clusters
- Culture: Culture media: Blood agar, Nutrient agar, MacConkey agar.
- Grow well aerobically,
- 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.
