Bacteriology

Question 1

4 common diagnostic techniques for bacteriology?

Answer 1

1. CULTURE
   o used to determine antimicrobrial resistance

a. sterile sites e.g. blood & CSF
b. non-sterile sites (usually have lots of commensal bacteria so difficult to identify)

2. SEROLOGY
   o used to determine the body’s response to an infection

e.g. doing blood at the beginning of an infection and at the end of it

3. MOLECULAR TECHNIQUES
   o detect resistance genes
4. ANTIMICROBIAL SUSCEPTIBILITY TESTING
   o used to test AB resistance (takes LONG)

Question 2

What can happen to commensal bacteria?

Answer 2

Can become pathogenic e.g. when have a cannula or catheter

Question 3

How do blood cultures work?

Answer 3

Broth at the bottom of the tube

1. Bacteria is placed in the tube
2. Incubated (dependent on bac)
3. If colour changes = indicates presence of bacterium

Question 4

What happens if the blood culture is positive?

Answer 4

Do gram-testing!

Gram-POSITIVE = skin & soft tissue

Gram-NEGATIVE = abdomen & urinary tract

Question 5

What are different types of agar you can use for gram-testing?

Answer 5

1. Non-selective agar plates
2. Chocolate Agar
   o COOKED blood
   o lets bacteria use the BLOOD NUTRIENTS to grow (as some bac. might NOT be able to lyse RBCs)
3. Macconkey Agar
   o grows gram -VE organisms

Question 6

Difference between gram +ve and -ve bacteria?

Answer 6

Gram +VE
o thicker peptidoglycan cell wall
o purple
o RETAINS dye

Gram -VE
 o thinner pepti. cell wall
 o have an OUTER MEMBRANE (so renders some AB ineffective as cannot get past it)
 o pink stain
 o LOSES dye

Question 7

Gram+ve cocci in lumps?

Answer 7

Most COMMON bacterium

e.g. Staphylococci - form CLUMPS (divide in 2 then daughter cells divide to form clump of 4)

Question 8

How can you differentiate between different Staphylococci?

Answer 8

COAGULASE TEST!! If:

o Coagulase POSITIVE
- it is STAPHYLOCOCCUS AUREUS

o Coagulase NEGATIVE

• common SKIN MICROBES
• tend to NOT cause infection unless there are some opportunistic circumstances e.g. central lines
• if found in blood culture, probably contaminants from taking blood

Question 9

Coagulase and Staphylococci?

Answer 9

STAPHYLOCOCCUS AUREUS!

Coagulase is a VIRULENCE FACTOR which helps it to cause infection!

Question 10

Coagulase?

Answer 10

a bacterial enzyme which brings about the coagulation of blood or plasma (fibrinogen to fibrin) and is produced by disease-causing forms of staphylococcus.

Question 11

What is different about coagulase NEGATIVE saphylococcus to coagulase POSITIVE?

Answer 11

o tend to NOT cause infection

• unless there are some opportunistic circumstances e.g. central lines
• can infect prosthetics e.g. in hip replacements
• if found in blood culture, probably contaminants from taking blood

Question 12

Gram+VE cocci in chains?

Answer 12

STREPTOCOCCI!

Question 13

How can Streptococci be separated?

Answer 13

On BLOOD agar!

Into 2 groups:

1. Alpha haemolysis
   - INCOMPLETE haemolysis
   - turns GREEN
   - e.g. streptococci pneumoniae
2. Beta haemolysis
   - COMPLETE haemolysis
   - turns CLEAR
   - e.g. Group A = streptococcus PYOGENES (skin/soft tissue infection)
   - e.g. Group B = streptococcus AGALACTIAE (sepsis in young)

Question 14

Gram-negative Bacilli?

Answer 14

e.g. E-coli!

Stain PINK as do NOT take up gram stain

Question 15

Possible causes of Diarrhoea?

Answer 15

1. Bacteria
   e. g. salmonella, shigella, campylobacter, E-coli, C.difficile, Cholrea
2. Parasites
   e. g. amoeba, diardia, cryptosporidium
3. Viruses

Question 16

Possible investigations for diarrhoea?

Answer 16

Stool sample!

Bacteria - culture on agar
- ONLY salmonella, shigella and campylobacter looked for routinely (as TOO MANY bacteria grow in faeces)

Parasites - concentration & special stain

Question 17

How did C.difficile get its name?

Answer 17

DIFFICULT to grow in culture

SO tend to detection of its TOXIN or TOXIN GENE as the marker (via. antigen & toxin gene PCR)

Question 18

Salmonella on XLD agar?

Answer 18

XLD agar is SPECIFIC for salmonella

Salmonella produces Hydrogen Sulfide so produces:
- BLACK CIRCLES

Does NOT ferment XLD so goes REDDISH (whilst normally bac. on this agar go YELLOW)

Question 19

Campylobacter on agar?

Answer 19

Takes LONG to grow (48hours)

Can survive at HIGH temperatures (42oC) so incubate at this to kill other bac. on stool sample

Can then put the remaining campylobacter on a selective agar dish

Question 20

Cholrea on TCBS agar

Answer 20

This agar is SPECIFIC for cholera

SO if try to grow cholera on other agar will NOT work

Question 21

PPV?

Answer 21

Positive predictive value

Question 22

What does PPV depend on?

Answer 22

Depends on pre-test probability of the sample being positive

i.e. the more likely a patient is to have a disease, the more likely a positive test represents a TRUE positive

Question 23

What does the PPV mean ultimately?

Answer 23

i.e. DO NOT test everybody for everything, send tests for things it is most likely to be

Question 24

How is ‘Sensitivity Testing’ carried out?

Answer 24

MIC - minimum inhibition concentraion

- the LOWEST amount of AB required to inhibit the growth of bacteria in vitro

Question 25

Is MIC useful on it own?

Answer 25

NO! - so people set BREAKPOINTS

Correlates MIC with clinical success when using the AB
o the bacterium with an MIC BELOW the breakpoint means there is a good chance of success with that AB
o a bacterium with an MIC ABOVE the breakpoint is RESISTANT

Breakpoint = a CHOSEN [AB]

Question 26

Disc Diffusion?

Answer 26

Use a SET [AB] in each disc
Incubate for 24hours

Zone size in interpreted using the breakpoints in an AB table