Lecture 10: Laboratory Diagnosis of Bacterial Infections (II) PDF

# Lecture 10 ## Laboratory Diagnosis of Bacterial Infections (II) - Dr. Rasha Mokhtar Elnagar - M.B.B.C, MSc, PhD, MD Medical Microbiology and Immunology - Associate Professor of Medical Microbiology & Immunology - Consultant Microbiology & Immunology ## 3. Selective Media - Contain selective subst...

# Lecture 10 ## Laboratory Diagnosis of Bacterial Infections (II) - Dr. Rasha Mokhtar Elnagar - M.B.B.C, MSc, PhD, MD Medical Microbiology and Immunology - Associate Professor of Medical Microbiology & Immunology - Consultant Microbiology & Immunology ## 3. Selective Media - Contain selective substance to allow the growth of specific bacteria and inhibit the others. ### Examples: - Thayer Martin media: antibacterial and antifungal (for pathogenic _Neisseria_ spp.). - Löwenstein-Jensen media: malachite green dye (For _Mycobacterium_ spp.). - Mannitol salt agar: high salt contents (For _Staphylococcus aureus_). - MacConkey agar: bile salts (for _Enterobacteriaceae_). ## 4. Differential (Indicator Media) Media - Usually contain an indicator which changes its color as a result of the metabolic activities of particular organisms. ### Examples: - MacConkey agar (differentiate lactose fermenters from Non-lactose fermenters). - Mannitol salt agar (_Staphylococcus aureus_). ## Bacterial Colonies Identification - **Shape:** colonial morphology (size, texture, color, hemolysis, smell...) helps in bacterial identification. - **Color:** The colonies may be colorless or pigmented due to production of endo-pigments or exo-pigments. - _Staph. aureus_ produce golden yellow endo-pigment. - _Serratia marcescens:_ Red endo-pigment. - _Pseudomonas aeruginosa_ produce a Bluish green exo-pigment in the surrounding media. - **Biochemical tests:** catalase, coagulase, urease...... ## Antibiotics Sensitivity Test - Tests done to check the effectiveness of antibiotics against the detected bacteria to select the best antibiotics treatment ### Methods of Antibiotics Sensitivity Test: - Kirby-Bauer method (Disc-diffusion method). - E-test (Antibiotic diffusion). - Broth dilution method. ## The Broth Dilution Method: (Quantitative) - Used to determine the minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC). - **Minimal inhibitory concentration (MIC):** is the lowest concentration of antibiotic that prevents visible growth after overnight incubation. - **Minimal Bactericidal Concentration (MBC):** The lowest concentration of antibiotic that kill bacteria after overnight incubation. ## 3. Antigen Detection - Detection of bacterial parts (Antigens) in the specimens - Rapid and easy to perform. - Culture is still needed for antibiotics sensitivity. ### Examples: - _S. pyogenes_ antigens in tonsillar swab - _H. pylori_ antigens in stool sample. - _N. meningitidis_, _H. infleunzae_, _S. pneumoniae_ capsular antigens in CSF. ## 4. Molecular Analysis - DNA is unique for each organism so bacteria can be identified by detection of their DNA. ### Example: Polymerase Chain Reaction (PCR) ### Advantages: - Rapid - Sensitive - Specific ## Steps of PCR: - **Denaturation:** separation of the 2 DNA strands by heat. - **Annealing:** binding of the specific primers. - **Extension** of the primers to make new strands (by the polymerase enzyme). - The cycles are repeated several times. - The end result is many DNA copies that can be detected by special methods. ## Case Study A 57-year-old man complains of dysuria and profuse purulent urethral discharge. A presumptive diagnosis of gonococcal urethritis is made. Which one of the following media is most suitable selective media for identifying _Neisseria gonorrhoeae_ in urethral discharge? - A. Sheep blood agar - B. Chocolate agar - C. MacConkey agar - **D. Thayer-Martin medium** ## Reference - Lippincott Illustrated Microbiology 4th edition, Chapters: 4.