diagnostic bacteriology Flashcards
What library diagnostic tests are used and what samples can be taken?
Laboratory investigations
○ Microbiological tests
○ Immunological tests
○ Molecular biology tests
Samples
○ Saliva, blood. Urine, faeces
○ Sputum, cerebrospinal fluid
○ Tissue biopsies, pus
Why is diagnosis important?
Improves effectiveness of treatment - Avoid long term complications
Identify and prevent outbreaks
Prevent transmission of virus/bacteria
Prevent further cases or an outbreak
Prevent overuse or mis-use of treatments - Reduce use of antibiotics when unnecessary
What is the gram stain test based on and what does it identify?
• Gram-negative bacteria - 2 outer membranes with thin peptidoglycan
Stays purple
• Gram-positive bacteria - 1 outer membrane with thick peptidoglycan
Appears pink
Explain the process of gram staining
- use heat to fix bacteria to surface of slide
- Apply crystal violet solution to the bacteria, crystal violet molecules enter cell wall
- Iodine is applied, leading to the formation of crystal violet and iodine complexes that are larger in size
- Series of washes with alcohol, alcohol dehydrates the peptidoglycan layer resulting in it becoming tightened
In gram positive bacteria the peptidoglycan later is thick complexes are unable to wash out so stay stained purple
gram negative bacteria- a thinner peptidoglycan layer the complexes are washed out
Counter stain with safranin and this stains all bacteria pink so gram negative stains pink
What are the two classifications of bacterial shape?
Spheres - coccus
Round ended cylinders - bacillus
Can also have oval shaped - coccobacilli
What are the four growth tests done to diagnose bacteria?
And what gram bacteria are they used for?
+ve
Hemolytic activity - whether they produce hemolysins - enzymes to damage RBCs
Catalase - determines whether bacteria produces catalase (breaks down H2O2)
coagulase test - discriminate gram positive bacteria - staphylococci (produce catalase) and streptococci (do not produce catalase).
-ve
Lactose fermentation - tests the capacity of a microorganism to ferment lactose
method to test for haemolytic activity?
• Method
1. Pick out a colony from an agar plate 2. Strike out the colony on an agar plate that contains five percent sheep red blood cells 3. Incubate the plate overnight and assess hemolytic activity the next day 4. See whether the red blood cells have been lysed
method to test for fermentation?
- Pick a bacterial colony off an agar plate
2. Streak out the colony onto a Conkey agar
that contains an indicator, at neutral agar should be
red
3. Incubate overnight and assess lactose fermentation
the day after
4. If lactose fermenter bacteria, lactic acid will lower
ph and the colonies will be pink if not the colonies
appear colourless
method of coagulate test|?
- Bacteria applied to a glass slide
- Plasma applied on top
- Short incubation with gentle rotation
- Coagulase positive forms clumps whereas negative will not
Method of catalase test?
- Bacteria is applied to a glass slide
- Hydrogen peroxide is applied to bacteria
- Short incubation
- Observe whether bubbles are produced
the two methods to test for antibiotic resistance?
Zone of inhibition
E-test to measure minimum inhibitory concentration