Diagnosing bacteria

Question 1

Briefly, how do you do a gram stain

Answer 1

Fix bacteria by heating
Add crystal violet and wash off
Add iodine and wash off
Add gram's alcohol
Add a counter stain (Fuschule)

Question 2

How do you innoculate a sample

Answer 2

Swab the bacteria
Sterilise the swab in a bunsen burner
Scrape it again but so it slightly overlaps where you scraped before - repeaat 5 times

Question 3

How can you distinguish S.aureus from other staphylococci?

Answer 3

Gold colonies on blood age

Coagulase test - S.aureus is positive

Question 4

What is the outcome of the coagulase test? What does it show?

Answer 4

Positive - clot formed
Negative - remains liquid

If positive, then the staphylococci is likely pathogenic

Question 5

With a gram stain, what distinguishes staphylococci and streptococci?

Answer 5

Staphy are gram positive in clusters

Strep are gram positive in chains

Question 6

Which gram positive bacteria can using an optochoin disc help identify?

Answer 6

S.pneumoniae is optochin sensitive, while other alpha haemolytic streptococci are not

Question 7

Describe the appearnace of alpha, beta and gamma haemolysis on blood agar

Answer 7

Beta - blood is completely broken down, there is a solid distinction between white and red
Alpha - partially broken down, there is more yellow/green
Gamma - no change, all red

Question 8

What kind of bacteria do we do a haemolysis test on?

Answer 8

Gram positive, streptococcus

Question 9

Give an example of each streptococci which does alpha, beta and gamma haemolysis

Answer 9

Strep pyogenes
Strep intermedius
Strep mutans

Question 10

How do you distinguish pathogens from normal flora?

Answer 10

You can alter the environment (oxygen, temperature, nutrients, media) to get certain pathogens to grow

Can also use a pH indicator

Question 11

Where are gram negative bacilli typically found?

Answer 11

In the GI tract

Question 12

What three steps do we do to identify gram negative bacteria?

Answer 12

Gram stain
Culture on MacConkey agar
Subclassify nonlactose fermenters with oxidase test

Question 13

What are the 2 outputs of the MacConkey test?

Answer 13

Pink/red - lactose fermenters (they make lactic acid making pH indicator red
Yellow - nonlactose fermenters

Question 14

What is the oxidase test? What bacteria is it used on?

Answer 14

Looks for certain cytochrome oxidase enzymes.

It is done on non-lactose fermenting gram negative bacteria

Question 15

How can we identify streptococci from other gram-positive bacteria?

Answer 15

Streptococci are gram-positive cocci in CHAINS

Question 16

Name 2 gram-negative cocci and 4 gram-negative bacilli

Answer 16

Gram negative cocci - Neisseria, Moraxella

Gram negative bacilli - E.coli, Pseudomonas, Salmonella, Shigella

Question 17

Are streptococci and staphylocci aerobic or anaerobic?

Answer 17

Aerobic

Question 18

Name 2 gram positive bacilli

Answer 18

Crynebacteria and listeria

Question 19

Which gram positive bacteria are classically in chains and which are in clusters?

Answer 19

Streptococci - chains

Staphylocci - clusters