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Staphylococcus aureus

lab diagnosis of Staphylococcus aureus

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Staphylococcus aureus

  1. 1. Staphylococcus aureus Prepared by Samira Fattah Assis. Lec. College of health sciences-HMU Lab 2
  2. 2. • The Staphylococcus genus includes at least 40 species. • The three main species of clinically importance are: • S. aureus: pathogenic and commensally found in nose (nares). • S. epidermidis: is a commensal of the skin, but can cause severe infections in immune-suppressed patients. • S. saprophyticus: is part of the normal vaginal flora, is predominantly implicated in genitourinary tract infections.
  3. 3. LABORATORY DIAGNOSIS: Specimen collection • Pus from pyogenic lesions. • blood from septicaemia. • Cerebrospinal fluid from meningitis. • sputum from respiratory infection • suspected food, vomit or faeces from food poisoning. • Mid-stream urine in urinary tract infection. • Anterior nasal swab from suspected carriers.
  4. 4. microscopic examination 1-Gram stain gram positive. 2-Morphology cocci (spherical). 3-Arrengment single cell or pairs or in short chain but appear predominantly in grape-like clusters.
  5. 5. culture characteristic colony morphology on many types of agars: -On nutrient agar Staphylococcus aureus colonies are: large, circular, smooth , shiny surface and are pigmented (golden-yellow).
  6. 6. -On blood agar • S. aureus β-hemolysis /clear zone around the colonies.
  7. 7. • S.epidermidis -White-creamy colonies -no hemolysis of red blood cells.
  8. 8. • S. Saprophyticus - white-yellow colony -no haemolysis of red blood cells
  9. 9. -On mannitol salt agar - It is a differential medium for mannitol fermentors. -The media contain mannitol and the indicator phenol red and 7- 10% sodium chlorid. -Staphylococcus aureus produce yellow colonies with yellow zones(mannitol fermentor). -Other Staphylococci produce small pink or red colonies with no colour change to the medium(non-mannitol fermentor)
  10. 10. Biochemical tests - Catalase test: • Is used to differentiate between staphylococci (catalase +ve) and streptococci(catalase –ve).
  11. 11. -DNase TEST •Deoxyribonucleic Acid enables the detection of DNase that depolymerize DNA. •A zone of clearing around the spot or streak indicates DNase activity.
  12. 12. -Coagulase test is used to differentiate Staphylococcus aureus from coagulase-negative staphylococci. fibrinogen fibrin (clot formation) coagulase
  13. 13. slide coagulase test • Procedure – Place a drop of sterile water on a slide and emulsify a colony – Add a drop of rabbit plasma to the suspension – Observe • Agglutination = Positive (S. aureus) • No agglutination= Negative (other staphylococcci) tube Coagulase Test • Causes a clot to form when bacterial cells are incubated with plasma. • Procedure • Inoculate rabbit plasma with organism and incubate at 35-37 0 C • Observe at 30 minutes for the presence of a clot • Continue for up to 24 hours, if needed.
  14. 14. -Novobiocin Susceptibility Test • This test is used to differentiate coagulase-negative staphylococci. Staphylococcus saprophyticus Staphylococcus epidermidis
  15. 15. API Test -API STAPH Combination of standard biochemical tests and fermentation tests which are the reference tests for the identification of staphylococci.
  16. 16. Rapid diagnostic tests • This techniques include Real-time PCR which is increasingly being employed in clinical laboratories.

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