diagnostic bacterialogy

Question 1

• What colour are gram negative bacteria and what does it consist of?
• What colour are gram positive bacteria and what does it consist of?
• Which part of a bacterial cell envelope “holds” the Gram stain?

Answer 1

Pink
two thin outer membrane with peptidoglycan in middle

Purple
one thin outer membrane with peptidoglycan around it

peptidoglycan

Question 2

• In what order are the gram stain substances applied?
• What is the purpose of the alcohol washes?
• What makes gram positive purple?
• Why does gram negative bacteria stain pink?

Answer 2

Crystal violet, iodine, alcohol, safranin

dehydrates peptidoglycan so it is tighter

tighter peptidoglycan so crystal iodine complexes cant be washed out, crystal violet stains it purple

alcohol washes out crystal violet
safranin stains it pink

Question 3

• What shape does coccus describe?
• What shape does bacillus describe?
• How would you describe the shape of coccobacilli?
• Describe the shape of diplococci

Answer 3

sphere

round ended cylinder

oval like

spherical in pairs

Question 4

• What is hemolytic activity useful in differentiating between?
• What does the hemolytic activity test for?
• List and describe the results of the haemolysis test

Answer 4

differentiates types of streptococcus, staphylococcus, enterococcus bacteria

a bacteria’s ability to produce haemolysins

Gamma - no haemolysis 

Alpha - partial (opaque zone)

Beta - complete haemolysis (transparent greenish zone)

Question 5

• Outline how to complete the hemolytic activity test

- What is the major cause of haemolysis in new born babies?

Answer 5

1. Pick a bacterial colony from an agar plate
   
   2. Streak out the bacterial colony onto medium containing 5% sheep red blood cells
   3. Incubate plate overnight and assess activity next day
   4. What we assess are the affect of the bacteria on the red blood cells
   Group B Streptococcus or Streptococcus agalactiae is the commonest cause of meningitis in babies under 3 months

Question 6

• What type of agar is used for the lactose fermentation test?
• What is the lactose fermentation used for?
• What does the lactose fermentation test measure?

Answer 6

MacConkey agar - containing bile salts, crystal violet and lactose

Differentiates between gram negative bacteria types, specifically bacilli

bacteria produces acid during lactose fermentation
the test detects a change in PH

Question 7

• What does the catalase test determine?
• What does catalase break down?
• What does the catalase test discriminate between?
• What are the results for catalase positive and catalase negative?

Answer 7

whether a bacteria produces catalase

hydrogen peroxide

staphylococci: positive
streptococcus: negative

oxygen bubbles
no oxygen bubbles

Question 8

• What is coagulase?
• What can coagulase be used to discriminate between?
• Describe the results of coagulase positive and negative
• Outline the steps for the coagulase test

Answer 8

enzyme used to cross link fibrinogen

S aureus from other types of staphylococci

positive- clumps
negative- no clumps

add bacteria onto slide
add plasma
incubate for 15seconds and then rotate
check for clumps

Question 9

• What are the results for a lactose-fermenter?
• What are the results for a lactose non-fermenter?
• What happens when there is no growth

Answer 9

pink agar

yellow agar

gram positive bacteria present as bile salts inhibit their growth