Bacterial diagnostics

Question 1

List four common diagnostic techniques in bacteriology

Answer 1

Culture of sterile and non sterile sites
Serology
Molecular techniques
Antimicrobial susceptibility testing

Question 2

Name two types of blood culture bottles for bacteriology

Answer 2

Anaerobic

Aerobic

Question 3

Describe how bacteria culture bottles show the presence of a pathogenic bacterium

Answer 3

Indicator in the blood culture bottle changes colour

This is detected by the machine

Question 4

How long does it take to flag up positive?

Answer 4

15-20hours

Question 5

What is the longest part of this process?

Answer 5

Antibiotic susceptibility testing (placing on agar plates and testing)

Question 6

What can give false negative results on agar plates?

Answer 6

When the patient has already been started on antibiotics, this may inhibit the growth of the bacterium before the antibiotic which has been applied for testing has even begun to work.

Question 7

How clinically useful are the antibiotics which inhibit the growth of bacterial growth on an agar plate?

Answer 7

Not all are useful; Clinically ineffective antibiotics may still inhibit the growth of bacteria on an agar plate

Question 8

What is therefore important when obtaining samples for bacterial culture?

Answer 8

Try to take it before any antibiotic has been given

Question 9

What is done to select antibiotics before the growth and how long does this take?

Answer 9

Gram staining - to select antibiotics 16-20 hrs

Question 10

What else can be done before the growth in order to further classify for the organism?

Answer 10

Look down a microscope - shape also gives you information about the bacterium

Question 11

What would clumps of coccus shaped, gram positive (purple) bacteria indicate?

Answer 11

Staphylococcus

Question 12

How can you distinguish between staphylococci?

Answer 12

Coagulase test

Question 13

Name coagulase positive staph

Answer 13

MRSA

Question 14

Where are coagulase negative staph found?

Answer 14

Staph negative staphylococci live on the skin (commensal)

Question 15

What would gram positive cocci in chains indicate?

Answer 15

Streptococci

Question 16

How are streptococci subdivided?

Answer 16

Haemolytic and non haemolytic

Question 17

How can you further subdivide haemolytic streptococci?

Answer 17

Alpha
Beta
Gamma

Question 18

Which of these haemolytic streptococci are pathogenic?

Answer 18

Beta haemolytic streptococci

Question 19

What are the main groups of beta haemolytic streptococci?

Answer 19

Group A
Group B
(also Group C and G - like group A, skin and soft tissue infections)

Question 20

What is another name for group A beta haemolytic streptococcus?

Answer 20

Streptococcus pyogenes

Question 21

What infections do group A b-HS (streptococcus pyogenes) cause?

Answer 21

Common cause of sore throat (quinsy)
Severe skin and soft tissue
Necrotising fasciitis
Rheumatic fever
Glomerular nephritis

Question 22

What is another name for group B beta haemolytic streptococcus?

Answer 22

Streptococcus agalactiae

Question 23

What infections does strep agalactiae cause?

Answer 23

Neonatal infections

Neonatal sepsis

Question 24

What do group C and G strep infections cause?

Answer 24

Like group A, skin and soft tissue infections

Cellulitis

Question 25

Which of the alpha haemolytic strep is most pathogenic?

Answer 25

Streptococcus pneumoniae

Question 26

Are gram positive or negative rods more resistant?

Answer 26

Gram negative rods are more resistant

Question 27

Which four bacteria are often looked for in stool if the doctor hasn't specified?

Answer 27

Salmonella Shigella Campylobacter E.coli O157

Question 28

What is the most common cause of infectious diarrhea/food poisoning?

Answer 28

Campylobacter

Question 29

Which bacteria is associated with diarrhea post antibiotic use?

Answer 29

C. difficile

Question 30

If the patient has been abroad, or is immunocompromised and has diarrhoea, which bacterium would be suspected?

Answer 30

Cryptosporidium

Question 31

If many people in nursing home have come down with diarrhoea simultaneously, what would be suspected?

Answer 31

Viral causes e.g. norovirus | Note: these do not cause vomiting

Question 32

Which parasites may cause diarrhoea?

Answer 32

Amoeba Giardia Cryptosporidium

Question 33

How would you test for clostridium difficile and why?

Answer 33

Routine toxin detection PCR for toxin gene Because it's difficult to grow (hence the name!)

Question 34

How is culturing of stool tailored to pathogenic bacteria?

Answer 34

Culture at higher temperatures | Agar plates containing antibiotics to kill off commensal bacteria

Question 35

Name a paediatric commensal of the gut

Answer 35

C.difficile !

Question 36

How are parasites tested for?

Answer 36

Concentration in stool is counted Special strains Note: these are NOT grown

Question 37

Which agar plate is used to detect salmonella?

Answer 37

XLD agar; the salmonella produce hydrogen sulphide and does not ferment the xylose therefore turns reddish whereas other bacteria turn yellow

Question 38

Which bacteria can tolerate high temps?

Answer 38

Campylobacter

Question 39

What colour does vibrio cholerae turn on an agar plate and what is the name of this agar plate?

Answer 39

TCBS agar | Turns yellow/green

Question 40

What does the positive predictive value of a test depend on?

Answer 40

The probability of positive pretest value i.e. the prevalence e.g. probability that a man is pregnant is low, therefore if the pregnancy test comes back positive, the PPV will be low

Question 41

What does MIC stand for and what does it mean?

Answer 41

Minimum inhibitory concentrations - the least amount of antibiotic needed to kill the bacteria

Question 42

How is MIC used to prescribe antibiotics and set standards?

Answer 42

MIC is compared with clinical outcome

Question 43

What is used instead of MIC?

Answer 43

Disc diffusion - produces zone of inhibition Note: zone size is NOT indicative of sensitivity

Question 44

Which bacterial diarrhoea is associated with systemic disease?

Answer 44

Typhoid | Amoebic abscess

Question 45

Infectious diarrhoea does not usually present for 4 months. List a differential for diarrhoea lasting this long.

Answer 45

Crohn's

Question 46

Which pathogens do not cause diarrhoea?

Answer 46

Helminths

Question 47

What do helminths cause instead?

Answer 47

Rashes Fever Eosinophilia

Question 48

Which toxin is commonly screened for in stool samples?

Answer 48

C difficile