Diagnostic Bacteriology

Question 1

What do gram stains show us?

Answer 1

1. whether gram positive or negative
2. shape of bacteria ( rod shaped or spherical)

Question 2

What colour does gram positive bacteria stain?

Answer 2

Purple

Question 3

What is the cell wall of a gram positive bacteria?

Answer 3

Thick peptidoglycan cell wall

Question 4

What colour does gram negative bacteria stain?

Answer 4

Pink

Question 5

What is the cell wall of gram negative bacteria like?

Answer 5

two outer membranes with a thin peptidoglycan cell wall between it

Question 6

What part of the bacterial cell wall “holds” the stain?

Answer 6

The peptidoglycan

Question 7

In what order are the gram stain substances applied?

Answer 7

Crystal Violet
Iodine
Alcohol
Safranin

Question 8

what does iodine do in gram staining technique?

Answer 8

it fixes crystal violet into the cell walls

Question 9

what does alcohol do in the gram staining technique?

Answer 9

dehydrates the peptidoglycan layer resulting in it becoming tightened

Question 10

How do gram negative bacteria stain pink?

Answer 10

After the alcohol has washed all the crystal violet out, safranin is added which stains any remaining bacteria pink

Question 11

What does the shape coccus describe?

Answer 11

Sphere

Question 12

What does the term bacillus describe?

Answer 12

The rounded oval shape

Question 13

What shape can coccus bacteria sometimes form?

Answer 13

Groups / clusters like grapes

Question 14

Describe the shape of diplococci

Answer 14

Spherical in pairs

Question 15

What does staphylcocci look like

Answer 15

Purple, spherical in grape like clusters

Question 16

What does streptococci look like?

Answer 16

Purple, spherical in long chains

Question 17

What does the hemolytic test do?

Answer 17

Used for differentiating between gram positive bacteria like staphylococcus, streptococcus and enterococcus

We test to see if bacteria can produce haemolysins (enzymes that damage RBCs)

Question 18

What is gamma hemolysis?

Answer 18

No hemolysis therefore no yellow zone

Question 19

What is beta hemolysis?

Answer 19

Full hemolysis therefore fully transparent / yellow zone

Question 20

What is alpha hemolysis?

Answer 20

Partial hemolysis, therefore more opaque yellow / green area

Question 21

What medium is the hemolytic test conducted on?

Answer 21

medium containing sheep red blood cells

Question 22

What is the hemolytic test useful for differetiating between?

Answer 22

Staph, Strep and enterococcus

Question 23

What type of hemolysis producing greening?

Answer 23

Alpha

Question 24

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

Answer 24

Group B Streptococcus or Streptococcus agalactiae is the commonest cause of meningitis in babies under 3 months

Question 25

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

Answer 25

MacConkey Agar

Question 26

What does the MacConkey Agar contain?

Answer 26

bile salts, crystal violet and lactose

Question 27

What colour does lactase positive bacteria show as?

Answer 27

Pink - this is because they are lactose fermenting which produces lactic acid, lowers the pH and therefore forms pink colonies

Question 28

What colour does lactase negative bacteria appear as?

Answer 28

Yellow/colourless colonies

Question 29

Which type of gram (negative or positive) is the lactose fermentation test used to differentiate between?

Answer 29

Negative

Question 30

What test is used to differentiate initially between types of gram positive bacteria?

Answer 30

Catalase Test

Question 31

What will be seen in catalase positive bacteria?

Answer 31

Bubbles of oxygen

Question 32

What will be seen in catalase negative bacteria?

Answer 32

No bubbles

Question 33

What does catalase do?

Answer 33

Breaks down hydrogen peroxide

Question 34

What test is used to differentiate between different catalase positive bacteria?

Answer 34

Coagulase test

Question 35

What will be seen in coagulase positive bacteria?

Answer 35

Clumps

Question 36

What will be seen in coagulase negative bacteria?

Answer 36

No clumps

Question 37

How is the coagulase test conducted?

Answer 37

Add bacteria and plasma together

Question 38

How do clumps form in the coagulase test?

Answer 38

Coagulase catalyses prothrombin to thrombin which causes fibrinogen to be cross linked, resulting in fibrin which forms a clot

Question 39

How does the antibiotic susceptibility test show resistance?

Answer 39

There will be no zone of inhibition

Question 40

What does a larger zone of inhibition suggest?

Answer 40

A more successful antibiotic as it has killed more bacteria

Question 41

Which bacteria is gram positie, catalase positive and coagulase positive?

Answer 41

Staphylococcus aureus