General Microbiology — SCT I [Vol. 4] Flashcards
What does selective toxicity mean?
Selective inhibition of the growth of the microorganism without damage of the host
What is the mechanism of action of lysozyme? Where is lysozyme found in the host?
Destroys bacterial cell wall peptidoglycan by hydrolysing the glocosyl bond between NAM (N-acetylmuramicacid) and NAG (N-acetylglucosamine).
It occurs in tear, saliva, respiratory secretions
What roles can be attributed to specific antibodies in antibacterial immunity? Specify at least 3!
- Neutralisation of exotoxins and enzymes
- Complement-dependent bacteriolysis
- Opsonisation (helping phagocytosis)
- Interfering with attachment of bacteria to muscosal surfaces
Which type of hypersensitivity is involved in the tuberculin test?
Late type (Type IV)
Which Gram negative organelles or structures are associated with the O, H, and K antigens respectively?
O: LPS
H: Flagella
K: Capsule
How can one identify the different surface antigens of Gram negative bacteria?
With slide agglutination tests using antibodies of known specificity
Starting from a fixed smear, specify the main steps of Gram stain in the appropriate order!
- Crystal violet
- Lugol solution
- Differentation - ethanol wash
- Counterstaining - safranin or fuchsin
Wash with tap water after each step
Starting from a fixed smear, specify the main steps of the acid-fast stain (Ziehl-Neelsen) in the appropriate order!
- Stain with carbol-fuchsin (with heating)
- Differentation - wash with acid-ethanol
- Counterstaining with methylene-blue
Wash with tap water after each step
Describe the principle of acid fast staining procedures!
Myobacteria contain Mycolic Acids in high amounts, that can be stained by hot carbol-fuchsin
But, they resist decoloursiation with acid-alcohol so they remain red
All other cells are decolorised by acid-alcohol and then stained blue by counterstaining (methylene blue)
What is the difference between agglutination and precipitation?
Specific binding of the antigen with the antibody directly results in a reaction visible by the naked eye
Agglutination: particulate antigen (RBC, bacteria)
Precipitation: antigen is in solution before the reaction
What does titer mean in serological tests?
Highest dilution of the serum sample that still gives a positive reaction in the test
How can one differentiate between past and current infections in serological tests?
Either IgM class specific antibodies
or a significant (at least 4-fold) rise in the titer in the case of paired serum specimens
Specify the main steps of an ELISA test in which we detect antibodies from patients’ serum samples using known antigens!
- Binding antigen to plastic surface
- Adding diluted serum sample
- Adding conjugate
- Adding substrate of the enzyme
- Reading colour reaction
Wash with buffer after steps 1-3
How can one determine the minimal inhibitory concentration (MIC) of an antibiotic to a bacterial isolate?
Inoculate bacterial into a series of test tubes containing 2-fold diluition series of the drug.
After overnight incubation, the lowest concentration of drug that prevents visible growth of the organism is MIC.
Describe the principle of the Kirby-Bauer (disk diffusion) method used to determine antibiotic sensitivity!
Disks impregnated with different antibiotics are placed on the surface of appropriate agar media, which was previously inoculated with the bacterium
After overnight incubation, antibiotics diffusing from the disks may cause zones of inhibition around the disks
Size of the zone should be compared to standard values to determine antibiotic sensitivity
