L17 - Role of the Laboratory in Supporting Good Antimicrobial Practice

Question 1

What does the rational use of antibiotics require?

Answer 1

A clinical AND a bacteriological diagnosis - for which the causative organism must be isolated and tested

Question 2

What is the objective of chemotherapy?

Answer 2

To aid the body in combating a specific infection by inhibiting/destroying the causative micro-organisms

Question 3

What information is important for the microbiological consultation?

Answer 3

Clinical details
Underlying condition
Symptoms
Pyrexia
Travel history
Occupation
Operation
Prev. antibiotics

Question 4

What three lab tests are most important?

Answer 4

Microscopy
Culture
Antibiotic susceptibility testing

Question 5

Define the minimum inhibitory concentration

Answer 5

Lowest concentration of an agent that inhibits growth after overnight innoculation

Question 6

Define the minimum bacterocidal concentration

Answer 6

Lowest concentration of an agent which is able to kill the bacterial strain

Question 7

What is the aim of systemic therapy?

Answer 7

To maintain the MIC continuously/with short intermissions until the infection has been overcome

Question 8

How do we perform antibacterial susceptibility tests?

Answer 8

Disc susceptibility tests
Agar breakpoint method
MIC
Automated methods
Molecular methods

Question 9

Describe Disc Susceptibility Tests

Answer 9

Inoculate test organism evenly across agar
Apply antibiotic filter overnight
Circular zone of growth appears
Size of zone of inhibition indicates susceptibility of organism

Question 10

What factors can affect the zone size in the Disc Susceptibility test?

Answer 10

Bacterial inoculum/growth rate
Medium
Antibiotic molecule
Antibiotic formulation
Disc
Enzymes

Question 11

Describe Agar Breakpoint testing

Answer 11

Uses published ‘breakpoint’ conc
Antibiotic conc set to clinical breakpoint and a spot of organism added
Growth = Resistance
No growth = Susceptibility

Question 12

Describe MIC testing

Answer 12

Tube MIC - Series of antibiotic doubling dilutions in tubes w/ liquid media. Subculture tubes w/ no growth to determine MBC

Etest (Gradient MIC) - Exponential gradient of animicrobial agent on one side of plastic strip in doubling dilutions. Level of growth corresponds to MIC

Question 13

Describe automated testing

Answer 13

Automated antibiotic susceptibility testing

Often requires expert interpretation

Question 14

Describe molecular methods

Answer 14

Application of genotypic methods to detect resistance genes
Single/multiplex/real time PCR
DNA seq
Hybridisation based techniques

Question 15

Describe microarray analysis

Answer 15

Can detect SNPs that differentiate between narrow/broad spectrum B-lactamases
Assay time = 6 hours
Requires purified DNA

Question 16

What is the purpose of the serum antibiotic assay?

Answer 16

To check for compliance
To demonstrate bactericidal activity
To manage therapy with antibiotics that have a narrow therapeutic range

Question 17

How can antibiotic assays be performed? - Microbiological

Answer 17

On large plates
Concentration proportional to zone
Plot calibration curve w/ standards

Question 18

How can antibiotic assays be performed? - Non-microbiological

Answer 18

Enzymatic
Immunoassay
HPLC

Question 19

What bacterial factors determine antibiotic therapy?

Answer 19

Pathogenicity
Virulence
Invasiveness
Adherence
Toxin production
Transmissibility
Growth requirements

Question 20

What host factors determine antibiotic therapy?

Answer 20

State of health/disability
Nutritional state
Immune status
Underlying illness
Implants/foreign bodies
Portal of entry
Normal flora

Question 21

What factors are involved in choosing antibiotics?

Answer 21

Site of infection
Seriousness
Likely organism
Patient factors/circumstances
Cost
Toxicity/Side effects
Local/national resistance rates
Underlying disease
Contraindications