Diagnostic Bacteriology Flashcards
What are the 5 main types of infection?
Bacteria Viruses Fungi Protozoa Helminth worms
What are the three types of bacteriological tests and what does each test identify?
Microscopy (Gram stain) - Which type of bacteria?
Culture - Which species of bacteria?
Sensitivities - Which treatment should be used?
What colour are gram negative bacteria and what does it consist of?
Pink
2 outer membranes with thin peptidoglycan layer in the middle of them
What colour are gram positive bacteria and what does it consist of?
Purple
1 outer membrane with thick peptidoglycan around it
Which part of a bacterial cell envelope “holds” the Gram stain?
Peptidoglycan
In what order are the gram stain substances applied?
Crystal violet
Iodine (mordant)
Alcohol (decolorisation)
Safranin (counterstain)
What is the purpose of the alcohol washes?
Dehydrates the peptidoglycan area resulting in it becoming tighter
What makes gram positive purple?
Alcohol causes peptidoglycan layer to thicken and tighten hence the crystal and iodine complexes can’t be washed out hence it remains stained with crystal violet and appears purple
Why does gram negative bacteria stain pink?
Alcohol washes out crystal violet and iodine as there is only a thin peptidoglycan layer
Safranin then stains bacteria pink
What shape does coccus describe?
Sphere
What shape does bacillus describe?
Round-ended cyclinder
How would you describe the shape of coccobacilli?
Oval-like
Describe the shape of diplococci
Spherical in pairs
What is hemolytic activity useful in differentiating between?
Gram positive → Staphylococcus, Streptococcus, Enterococcus
What does the hemolytic activity test for?
Capacity of microorganism to produce hemolysins (enzymes that damage red blood cells)
List and describe the results of the haemolysis test
Gamma - no haemolysis
Alpha - partial (opaque zone)
Beta - complete haemolysis (transparent greenish zone)
What is the major cause of haemolysis in new born babies?
Group B Streptococcus or Streptococcus agalactiae is the commonest cause of meningitis in babies under 3 months
Outline how to complete the haemolytic activity test
- Pick a bacterial colony from an agar plate
- Streak out the bacterial colony onto medium containing 5% sheep red blood cells
- Incubate plate overnight and assess activity next day
- What we assess are the affect of the bacteria on the red blood cells
What type of agar is used for the lactose fermentation test?
MacConkey agar - containing bile salts, crystal violet and lactose
What is the lactose fermentation used for?
Differentiating gram negative bacteria (more specifically, between species of Bacilli)
What does the lactose fermentation test measure?
Difference in pH to determine whether bacteria is fermenting or non-fermenting
What are the results for a lactose-fermenter?
Agar plate shows pink colonies after being incubated overnight
What are the results for a lactose non-fermenter?
Colourless (yellow) colonies on agar plate
What does the catalase test determine?
Determines whether a bacteria produces catalase
