Diagnosis of Staphylococcus aureus (Updated )

Lab diasgnosis of Staphylococcus aureus

1. Staphylococcus aureus Prepared by Dr. Samira Fattah PhD in Medical Bacteriology College of Health Sciences-HMU Lab 2

2. Laboratory Diagnosis Specimen collection Pus from pyogenic lesions. Cerebrospinal fluid from meningitis. Sputum from respiratory infection Suspected food, vomit or feces from food poisoning. Mid-stream urine in urinary tract infection. Anterior nasal swab from suspected carriers.

3. 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.

4. Culture Characteristic On nutrient agar Staphylococcus aureus colonies are: large, circular, smooth , shiny surface and are pigmented (golden-yellow).

5. Culture Characteristic On blood agar S. aureus β - hemolysis /clear zone around the colonies .

6. Culture Characteristic S. epidermidis White-creamy colonies No hemolysis of red blood cells .

7. Culture Characteristic S. Saprophyticus White-yellow colony No hemolysis of red blood cells

8.   On Mannitol Salt Agar - It is a differential medium for mannitol fermenters. - contain mannitol and the indicator phenol red and 7- 10% sodium chlorid . -Staphylococcus   aureus  produce yellow colonies with yellow zones(mannitol fermenter). -Other  Staphylococci  produce small pink or red colonies with no color change to the medium(non - mannitol fermenter) Culture Characteristic

9. DNase TEST Deoxyribonucleic Acid enables the detection of DNase that depolymerize DNA. A zone of clearing around the spot or streak indicates DNase activity. Biochemical tests

10. Biochemical tests Catalase test: Used to differentiate between staphylococci (catalase + ve ) and streptococci(catalase – ve ).

11. Biochemical tests Coagulase test Used to differentiate  Staphylococcus aureus  from coagulase negative staphylococci. fibrinogen fibrin ( clot formation ) Coagulase enzyme

12. 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.

13. - Novobiocin Susceptibility Test This test is used to differentiate coagulase-negative staphylococci. If the zone of inhibition is bigger than 16mm, the organism is susceptible to the antibiotic. Novobiocin resistance is indicated by a zone of inhibition less than or equal to 16mm.

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

15. Rapid diagnostic tests Detection of ID genes by PCR like 16srRNA and nuc gene( nuclease gene)

16. resistant sensitive