antimicrobials

Question 1

antibiotic susceptibility test purpose

Answer 1

1. for clinical isolates(
   - result:Suspectible,Intermediate or Resistant
   - to determine susceptibility to antibiotics to guide antibiotic therapy for patients
   - to monitor antimicrobial resistance patterns
2. For a new drug

Question 2

describe the breakpoint

Answer 2

term:the concentration at which MIC below = sensitive and MIC above= resistant

A breakpoint is a chosen concentration (mg/L) of an antibiotic which defines whether a species of bacteria is susceptible or resistant to the antibiotic. If the MIC is less than or equal to the susceptibility breakpoint the bacteria is considered susceptible to the antibiotic.

Question 3

describe >3 methods used for antimicrobial susceptibility testing

Answer 3

1. Disc diffusion
2. Broth dilution(macro and micro)
3. Agar dilution
4. antibiotic gradient diffusion(E-test)

Question 4

empirical therapy

Answer 4

• choice of antibiotic made before bacterial cultures are available
• based on assessment of likely pathogens and predicted antimicrobial susceptibilities of the pathogens

Question 5

what if a patient is acutely ill with a bacterial infection?

Answer 5

1. use “empirical therapy”

2. often tazocin

Question 6

AST parameters

Answer 6

1. inoculum conc(too high and susceptibility maybe underestimated
2. optimal media:media components may interfere with antibiotic
3. antimicrobial content: especiallly discs
4. growth and inoculum condition: co2 incubaiton lowers medium pH
5. Depth of agar in plates- affects diffusion of antibiotic

Question 7

culture media (common)

Answer 7

Mueller Hinton Agar

Question 8

fastidious organisms culture media(eg.Neisseria gonorrhoeae

Answer 8

Neisseria gonorrhoeae:blood or hemoglobin and several amino acids and vitamins to grow

Question 9

Disc diffusion principle

Answer 9

antibiotic diffuses from disc through agar and inhibits bacterial growth

Question 10

disc diffusion advantage

Answer 10

easy,reproductible,cheap,
can customise which ediscs to use
can identify a subset of isolates for further AST by other methods

Question 11

Broth and agar dilution aim

Answer 11

to determine the lowest concentration of antimicrobial required to kill or inhibit the growth of a bacterium

Question 12

MIC term

Answer 12

minimal inhibitory concentration=lowest concentration of antimicrobial prevents growth of a particular pathogen

Question 13

MBC term

Answer 13

minimal bactericidal concentration=lowest concentration of antimicrobial required to kill or inhibit the growth of a bacterium

Question 14

broth dilution method

Answer 14

1. two-fold dilutions of antibiotic prepared in a liquid growth medium then dispensed into tube( tube inoculated with a standardised bacterial suspension)
2. after incubation, tubes examined for visible growth
3. lowest concentration with no visible growth is the MIC

Question 15

other AST methods

Answer 15

1. automated systems
2. genotypic methods:PCR,gene-specific probes,DNA microarrays
3. flow cytometry