Staphylococcus aureus

lab diagnosis of Staphylococcus aureus

1. Staphylococcus aureus Prepared by Samira Fattah Assis. Lec. College of health sciences-HMU Lab 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. 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. 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. 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. - On blood agar S. aureus β - hemolysis /clear zone around the colonies .

7. S.epidermidis - White-creamy colonies -no hemolysis of red blood cells .

8. S. Saprophyticus - white-yellow colony -no haemolysis of red blood cells

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. Biochemical tests - Catalase test: Is used to differentiate between staphylococci ( catalase + ve ) and streptococci( catalase – ve ).

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. - Coagulase test is used to differentiate  Staphylococcus aureus  from coagulase -negative staphylococci. fibrinogen fibrin ( clot formation ) coagulase

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 C Observe at 30 minutes for the presence of a clot Continue for up to 24 hours, if needed.

14. - Novobiocin Susceptibility Test This test is used to differentiate coagulase -negative staphylococci. Staphylococcus saprophyticus Staphylococcus epidermidis  

15. API Test - API STAPH Combination of standard biochemical tests and fermentation tests which are the reference tests for the identification of staphylococci.

16. Rapid diagnostic tests This techniques include Real-time PCR which is increasingly being employed in clinical laboratories.