7-2: Kirby Bauer Disk Diffusion susceptibility

to establish antimicrobial susceptibility

what is this test used for

in mm

how are zones of inhibition measured

R - resistant

I - intermediate

S - sensitive

state what R, I, and S mean on the reference or look up table

Mueller-Hinton plates which are nutrient-rich agar plates (high in protein)

what plates does the Kirby-Bauer test require

- all plates must be shallow (4 mm agar depth) to limit diffusion in non-horizontal vectors and lookup tables are calculated for experimentation on 4 mm agar depths

- nutrient richness is important as lack of growth can be attributed to action of the antibiotic on the disk and not to lack of nutrition

describe the parameters of the Mueller-Hinton plates

slows growth of bacteria

define bacteriostatic

kills bacteria

define bacteriocidal

blocks 23S rRNA peptidyl transferase activity and thus is a translation blocker

poor uptake of drug or inactivation of drug

describe the mechanism of the antibiotic and resistance mechanism:

chloramephenicol

blocks prokaryotic DNA gyrase (which relieves hypertension during DNA replication) and thus is a DNA replication blocker

altered target or poor uptake of drug

describe the mechanism of the antibiotic and resistance mechanism:

cirpofloxacin

blocks tetrapeptide synthesis and thus is a cell wall blocker

altered target, poor uptake of drug, or production of beta-lactamases

describe the mechanism of the antibiotic and resistance mechanism:

penicillin

scrambles the translational fidelity (genetic code is scrambled); GGG codon specifies glycine but streptomycin GGG specifies something other than glycine

altered target

describe the mechanism of the antibiotic and resistance mechanism:

streptomycin

blocks charged aminoacyl tRNAs entry into A site of ribosome and thus blocks translation

efflux mechanism

describe the mechanism of the antibiotic and resistance mechanism:

tetracycline

inhibits purine and pyrimidine synthesis; is also a sulfa drug that blocks the PABA to vitamin B9 anabolic pathway

altered target

describe the mechanism of the antibiotic and resistance mechanism:

trimethoprim