Lecture 9 - The Lab

Question 1

which specimens are examined by microscopy on day 0?

Answer 1

only important specimens are examined on day 0 - if requested

Question 2

What does a culture show

Answer 2

type of organism and significance of growth 
antibiotic sensitivity so can determine class of antibiotics to use and route (oral or iv)

Question 3

define MIC and MBC

Answer 3

MIC- lowest conc of agent that inhibits growth

MBC- lowest conc of agent which is able to kill the bacteria

Question 4

Name 5 methods of antibacterial sensitivity testing

Answer 4

disc susceptibility (stokes and BSAC), agar breakpoint method, minimum inhibitory concentration (Tube or E-tests), automated methods (vitek), molecular methods (PCR)

Question 5

In disc susceptibility method of antibac testing - what indicates the susceptibility of the organism

Answer 5

size of the zone of inhibition around the disc

Question 6

Describe broth dilution AB sensitivity test (tube MIC)

Answer 6

tubes with same amount of bacteria but different concentrations of antibiotics to determine the MIC

Question 7

Which AB sensitivity test is semi-quantitive and how does it work?

Answer 7

Agar break point dilution. several plates each with diff concs of antibiotics to find the minimum conc that inhibits bacteria growth

Question 8

Describe E test

Answer 8

• E test has a strip with increasing levels of antibiotic - level at which the organism grows is minimum

Question 9

What affects the zone side in agar disc diffusion tests?

Answer 9

bacterial inoculum (heavy or light) and growth rate,
composition osmolality and pH of medium ,
antibiotic molecule and antibiotic formulation,
disc content age and condition,
enzymes secreted by bacteria into medium (beta lactimase)

Question 10

what factors are involved with choosing an antibiotic

Answer 10

site of infection, seriousness, likely organism, patient factors, cosy. toxicity and side effects, local/ national resistance rates, other underlying diseases, contraindications

Question 11

Which methods of AB sensitivity testing are qualitative and how do they work?

Answer 11

point source disc dilution and stokes

- Stokes has half control (known to be sensitive) and half organism testing

Question 12

Which methods of AB sensitivity testing are quantitative and what do they establish

Answer 12

MIC

Tube broth dilution and E tests

Question 13

Why is molecular testing of bacteria done? What further tests can be done if unsure?

Answer 13

• PCR, to see if organism carries the gene for resistance to methicillin (mecA gene codes for penicillin binding protein)
• microarray analysis - not done locally

Question 14

What is a serum antibiotic assay and when are they used? How are they done?

Answer 14

measures antibiotic levels in the blood. used when drugs have a narrow therapeutic window to prevent toxic/ therapeutic levels. e.g. vanc/gent/streptomycin (also used to check compliance e.g. TB)
- microbiological assays

Question 15

if ? serious infection name some steps that must be taken

Answer 15

1) allergies?
2) start treatment asap after diagnosis + follow local guides
3) record in drug chart
4) have all microbiological specimens been taken?
5) review 24h and change to oral or narrow spec if poss

Question 16

Name an automated method of bacteria detection?

Answer 16

VITEK