Transcript Antibiotic susceptibility testing powerpoint

Antimicrobial
Susceptibility Testing
(AST)
Purpose
Offer guidance to physician
in selecting effective
antibacterial therapy for a
pathogen
Factors to Consider When Determining
Whether Testing is Needed
• Body site of infection
Susceptibility not performed on bacteria
isolated from body site where there are
normal flora
Ex. Susceptibility for E. coli is NOT
performed when isolated from stool, but IS
performed when isolated from blood
Factors to Consider When Determining
Whether Testing is Needed
• Presence of other bacteria in specimen
• Ex. Two or more organisms grown in a urine
specimen
• Health status of patient
• Immunocompromised patients
• Allergies to usual antimicrobials
Selecting Antimicrobial Agents for
Testing
Primary objective
• Use the least toxic
• Refrain from more costly agents
• Avoid broader-spectrum agents
Selecting Antimicrobial Agents for
Testing
• There are approximately 50 antibacterial
agents
• Each laboratory should have a battery of
antibiotics ordinarily used for testing
• Drugs to use decided by medical staff,
pharmacists, and medical technologists
Selecting Antimicrobial Agents for
Testing
• There are approximately 50 antibacterial
agents
• Each laboratory should have a battery of
antibiotics ordinarily used for testing
• Drugs to use decided by medical staff,
pharmacists, and medical technologists
Selecting Antimicrobial Agents for
Testing
• Generally, labs choose 10-15 antibiotics to
test susceptibility for Gram positive
organisms and another 10-15 for Gram
negative organisms
• Too many choices can confuse physicians and
be too expensive
Drug
Ampicillin
Enterococcus
Staphylococcus spp.
X
Cefazolin
X
Clindamycin
Erythromycin
Linezolid
X
X
Oxacillin
Penicillin G
X
X
Rifampin
X
X
Streptomycin-2000
X
Tetracycline
X
Trimeth/ Sulfa
Vancomycin
X
X
X
X
X
Disk Diffusion/ Kirby- Bauer
• Procedure
• Use a well-isolated, 18-24 hour old
organism
• Inoculate agar by swabbing in
three different directions “Lawn of
growth”
•
Place filter paper disks
impregnated with antimicrobial
agents on the agar
• Invert and incubate for 16-18
hours at35 oC in non-CO2
Disk Diffusion/ Kirby-Bauer
• During incubation, drug
diffuses into agar
• Depending on the
organism and drug,
areas of no growth
form a zone of
inhibition
• Zones are measured to
determine whether the
organism is susceptible,
intermediate, or
resistant to the drug
Virtual Labs
• learn.chm.msu.edu/vibl/content/antimicrobial.html
• vlab.amrita.edu/?sub=3&brch=73&sim=1628&cnt=1