Staphylococcus aureus

• Staphylococcus aureus is a gram-positive bacterium belonging to the family Staphylococcaceae.

• S. aureus is an opportunistic pathogen, commonly found on the skin and mucous membranes of human.

• Staphylococcus aureus is gram-positive cocci bacteria.

Morphology

• Gram-positive bacteria.

• It appears in clusters resembling grapes when viewed under a microscope.

• Size: 1 μm in diameter.

• Motility: They are Non-motile

• Non-spore-forming

• Capsulated: Non-capsulated.

• Facultative Anaerobic

Antigenic structure

Capsule:

• Some strain of staph. Aureus contain capsule

• Capsule is composed of polysaccharides

1. Peptidoglycan:

• It provide rigidity to the cell

2. Protein A:

• It is a cell wall components of most strain of staph. Aureus

3. Teichoic acid:

• Major antigenic determinant of all strain of staphylococcus aureus

• It is associated with the peptidoglycan in insoluble form

• It is absent in staphylococcus epidermidis

TOXIN AND ENZYMES

TOXIN:

1. HAEMOLYSIN-hemolysin are alpha, beta, gamma and delta hemolysin produced by streptococci

2. Panton-Valentine Leucocidin (PVL) : it is composed of two components slow and fast

3. Enterotoxin :it is responsible for food poisoning nausea vomiting diarrhoeaoccurring with contaminated food

4. Exfoliative toxin :This toxin are produced by some strain of staph. aureus

• two types of exfoliative toxin A and B have been described toxin

• A is heat stable while toxin B is heat labile

5. Toxic shock syndrome toxin (TSST) : it is characterized by fever, vomiting, hypertension, diarrhoea

Enzyme:

• Produce a number of enzymes such as coagulase, phosphatases and deoxyribonuclease

Pathogenesis:

• Staphylococcus Aureus is an important pyogenic organism.

• Thick creamy pus is formed in staphyl. Infection.

➢ Source of infection:

• Contaminated food, patient with septic lesion, infected blood or body fluid, etc.

➢ Mode of transmission:

• Staphy. aureus are most often transmitted by direct or indirect content, inhalation, ingestion & inhalation.

➢ Route of transmission:

• Respiratory tract, GI tract, etc.

➢ Incubation period:

• About 2-6 hours in food born staphylococcus aureus.

• Disease may be classified as

❖ Cutaneous infection: It Include postules, boils, abcesses, burn & wound.

❖ Deep infection: It Include Osteomyelitis, tonsilitis, pharyngitis, sinusitis, meningitis.

❖ Food poisoning: It may follow 2-6 hours after ingestion contaminated food.

❖ Nosocomial infection: It refers hospital acquired disease.

❖ Skin exoflative disease: commonly seen in children. Example – staphylococcus scalded skin syndrome (SSSS).

❖ Toxic shock syndrome: women’s are commonly affect.

Lab diagnosis

• Specimen:

  • Pus

  • Wound swabs

  • Sputum

  • Blood

  • Urine, or other body fluids.

• Microscopy:

  • After gram stain Gram-positive cocci in clusters seen.

• Culture:

  • Grow on culture media such as nutrient agar, MacConkey agar, blood agar, milk agar & mannitol salt agar.

  • Incubate at 37*C for overnight.

  • For growing staph. Aureus used media are

    • Nutrient Agar

    • Blood Agar

    • MacConkey agar

    • Selective media

    • Mannitol salt Agar

    • Milk Agar &

    • Liquid media

• Biochemical reaction:

  • Coagulase test – positive

  • Phosphatase test – positive

  • Catalase test – positive

  • Oxidase test – negative

• Antibiotic susceptibility testing:

  • Susceptibility is determined by various method such as disk-diffusion.

TREATMENT

• Some antibiotics which are effective for the treatment are -

  • Benzyl penicillin

  • Vancomycin

  • Linezolid

PROPHYLAXIS

• Isolation and treatment

• Sterilization of instrument

• Stop misuse of antibiotic

• Detection of Carriers among Hospital staff their isolation and treatment