Practical Microbiology PDF

CARDIOVASCULAR BLOCK MICROBIOLOGY & IMMUNOLOGY DEPARTMENT  Rheumatic fever Cause: It's an autoimmune disease occur 2-4 weeks after infection with group A beta hemolytic streptococci (Strept pyogenes) Laboratory diagnosis: Confirmation of preceding GAS infection Sample: Throat swab...

CARDIOVASCULAR BLOCK MICROBIOLOGY & IMMUNOLOGY DEPARTMENT  Rheumatic fever Cause: It's an autoimmune disease occur 2-4 weeks after infection with group A beta hemolytic streptococci (Strept pyogenes) Laboratory diagnosis: Confirmation of preceding GAS infection Sample: Throat swab Blood for serology 1. Examination of Gram stained smear: Strept. pyogenes are G+ve cocci arranged in chains 2. Culture of throat swab: On blood agar Colonies producing complete (β- hemolysis) Streptococcus pyogenes  Colonies on blood agar are further identified by B.R as catalase test: Strept pyogenes Catalase negative 3. Rapid antigen detection test (RADT) For detection of GAS antigen in throat swabs. NOTE Negative result of throat culture and RADT do NOT rule out the diagnosis of Rheumatic Fever. 4. STREPTOCOCCAL ANTIBODY TESTS: Anti streptolysin O and Antideoxyribonuclease B are the frequently used antibody assays. Antistreptolysin O test (ASOT): It determines the titer of antibodies against streptolysin O toxin produced by Streptococci. It depends on the fact that Streptolysin O has the ability to cause lyses of R.B.Cs if it is present in free form. Procedure: ▪ Serial dilutions of patient’s serum are mixed with streptolysin O toxin and 5% RBCs suspension. ▪ The maximum dilution that gives no hemolysis (opaque) is called the ASO Titer. ▪ Titer ≥200 Todd’s unit is significant. 5. PCR based diagnosis assay: Is used for identifying Stept. Pyogenes exotoxin B gene. 6. Acute phase reactants (ESR, CRP): A positive CRP And a high ESR are helpful in the diagnosis and follow up of cases of RF.  Infective Endocarditis Most common causative organisms: Streptococcus viridans Staphylococcus aureus Staphylococcus epidermidis Enterococcus faecalis Less common pathogens: Streptococcus gallolyticus Gram-negative HACEK group Haemophylus Aggregatibacter Cardiobacterium Eikenella Kingella Coxiella burnetii Candida species Lab diagnosis: 1. Blood culture 2. Subculture on solid media and organism identification by: ❑Gram stain ❑Biochemical reactions 3- Antibiotic Susceptibility testing of isolated organism 4- Serology 5- PCR  Blood culture: “Blood Culture” is the standard test to diagnose etiology of infective endocarditis. Three "blood culture bottles" containing specific liquid media for aerobic and anaerobic bacteria and for fungi. Blood culture bottles are incubated for 5 days at 35°C and inspected daily to detect turbidity. Subculture from the blood culture is performed on different agar plates to obtain colonies of the pathogen.  Blood agar Beta (complete hemolysis) Alpha (incomplete) hemolysis (green coloration) Staph. aureus or strept. pyogenes Strept. viridans Mannitol salt agar: Staph. Aureus Yellow colonies surrounded by yellow zone Staph epidermidis Pink colonies Bile esculin agar Enterococcus faecalis Black colonies Sabouraud dextrose agar: Candida species Creamy colonies Further identification of isolated organism can be done by: a. Gram stain Gram positive cocci arranged in clusters (grape like) Staphylococci Gram positive cocci arranged in chains Streptococci or Enterococci Gram +ve round or oval cells Candida species b. Biochemical reactions Catalase test Positive Negative Staphylococci Streptococci Coagulase test Staph. aureus Staph epidermidis Serology: to detect antibodies in patient serum. PCR: best method for detection Culture Negative Endocarditis (CNE) Antibiotic Susceptibility test: To tailor the antimicrobial therapy according to the susceptibility profile of the causative microorganism.