Diagnosis of Bacterial Infections

Question 1

How do gram positive bacteria retain the crystal violet and iodine stains?

Answer 1

They have thicker peptidoglycan walls

Question 2

What colour are gram positive bacteria?

Answer 2

Purple

Question 3

What colour are gram negative bacteria?

Answer 3

Pink

Question 4

What else can gram staining tell you?

Answer 4

Size, shape, arrangement

Question 5

Describe how to perform a gram stain

Answer 5

Smear bacteria, heat fix the slide, flood with crystal violet, wash with water, flood with iodine, decolourise with Gram’s alcohol, wash with water, flood with counterstain, wash with water, blot dry

Question 6

What shape do gram positive bacteria tend to be?

Answer 6

Cocci

Question 7

What shape do gram negative bacteria tend to be?

Answer 7

Bacilli

Question 8

What arrangements do gram positive cocci tend to be in?

Answer 8

Chains or clusters

Question 9

What type of bacteria are gram positive cocci in clusters?

Answer 9

Staphylococcus

Question 10

What type of bacteria are gram positive cocci in chains?

Answer 10

Streptococcus

Question 11

What is blood agar?

Answer 11

A good medium for many types of bacteria

Question 12

What is chocolate agar?

Answer 12

Blood agar heated at 80 degrees for 5 minutes to release nutrients

Question 13

What is CLED agar?

Answer 13

Often used to differentiate microorganisms in urine and can differentiate lactose and non-lactose fermenting gram negative bacilli

Question 14

What is MacConkey agar?

Answer 14

Agar primarily for growth of gram negative bacteria and contains lactose

Question 15

What is gonococcus agar?

Answer 15

Contains growth factors that promote growth of N. gonorrhoea and other neisseriae

Question 16

What is XLD agar?

Answer 16

Used to isolate shigella (red) and salmonella (red with black centres)

Question 17

What is Sabouraud’s agar?

Answer 17

Used to culture fungi

Question 18

Describe the appearance of S.aureus on blood agar

Answer 18

Creamy/yellow (gold)

Question 19

Does S.aureus give a positive or negative coagulase test?

Answer 19

Positive

Question 20

What are white staphylococcus colonies that are coagulase negative grouped as?

Answer 20

Coagulase negative staphylococcus

Question 21

Give an example of a coagulase negative staphylococcus

Answer 21

S.epidermidis

Question 22

What do bacteria use coagulase for?

Answer 22

A defence mechanism by clotting areas of plasma around them, allowing them to avoid phagocytes

Question 23

What is beta haemolysis?

Answer 23

Clearing of agar around colonies

Question 24

What is alpha haemolysis?

Answer 24

Greening of agar around colonies

Question 25

How do you differentiate S.pneumoniae from other alpha haemolytic streptococci?

Answer 25

Optochin test. Pneumococci are sensitive but other streptococci are resistant

Question 26

How are beta haemolytic streptococci differentated?

Answer 26

Lancefield grouping using surface antigens

Question 27

What test is used to put beta haemolytic streptococci into their Lancefield groups?

Answer 27

Latex agglutination test

Question 28

Give an example of streptococci that are alpha haemolytic and resistant to optochin

Answer 28

Viridians streptococci

Question 29

What sites are normally sterile?

Answer 29

Blood, CSF, pleural fluid, peritoneal cavity, joints, urinary tract, lower respiratory tract

Question 30

What sites are not normally sterile?

Answer 30

Mouth, skin, vagina, urethra, large intestine

Question 31

How can pathogens be differentiated from normal flora?

Answer 31

By altering culture conditions like atmosphere, temperature, nutrients, media or a pH indicator

Question 32

Where are gram negative bacteria usually cause infection?

Answer 32

In the GI tract or in when normal flora enter a usually sterile site

Question 33

What colour do lactose fermenting gram negative bacteria stain MacConkey agar?

Answer 33

Pink

Question 34

Give an example of lactose fermenting gram negative bacilli

Answer 34

E.coli

Question 35

What colour do non-lactose fermenting gram negative bacteria stain MacConkey agar?

Answer 35

Yellow/colourless

Question 36

What addition test can you perform on non-lactose fermenting gram negative bacilli?

Answer 36

Oxidase test

Question 37

What does a positive oxidase test look like?

Answer 37

The disc turns blue

Question 38

Give some examples of non-lactose fermenting gram negative bacteria that give a negative oxidase test

Answer 38

Proteus, Shigella, salmonella

Question 39

Give an example of non-lactose fermenting gram negative bacteria that gives a positive oxidase test

Answer 39

Pseudomonas species

Question 40

What is a complication of a clostridium difficile infection?

Answer 40

Pseudomembranous colitis

Question 41

What drugs can treat S.aureus?

Answer 41

Cefotaxime, clarithromycin, flucloxacillin, vancomycin

Question 42

What bacteria is gram negative diplococcus?

Answer 42

Neisseria meningitidis