Lab 8: Approach to Identification API System and Antimicrobial Susceptibility Testing
Microorganism Identification
Recall how many tests are done to identify Gram Negative Rods (below).
Wouldn’t it be convenient if there was a one-step system for all of these tests?
Rapid Identification Systems: API 20E
• The API 20E system is a multiple test system allowing the determination of
20 different biochemical tests simultaneously.
• Most commonly used to identify gram-negative bacteria like E.coli,
Salmonella, and Shigella.
See pgs 105-111
Antimicrobial Susceptibility Testing:
The Kirby-Bauer Method
• Antimicrobial is a general term for something that inhibits or kills
microbes.
• An antibiotic is a type of antimicrobial that is made by microbes to kill or
inhibit other microbes.
• Very few antibiotics are effective against all types of bacteria (most are
effective against either gram positive OR gram negative bacteria).
Antimicrobial Susceptibility Testing:
The Kirby-Bauer Method
• Antibiotics are produced by metabolic reactions of bacteria and
fungi.
• Most antibiotics produced come from 2 bacterial strains,
Streptomyces and Bacillus, and the fungi Penicillium.
Penicillium
Antimicrobial Susceptibility Testing:
The Kirby-Bauer Method
• This lab technique asks the question “Which of these drugs stops the
spread of this particular strain of bacteria?”
• The procedure is designed to evaluate the susceptibility of a pathogen to
assorted antibiotics.
• Disk diffusion test uses a Petri plate of agar (Mueller-Hinton) inoculated
with bacteria over entire surface. Paper disks with standard concentration
of antibiotic is placed on the agar.
• Zone of inhibition: inhibition of bacterial growth around the antibiotic disk.
Zone of inhibition
Antimicrobial Susceptibility Testing:
The Kirby-Bauer Method
Minimum inhibitory concentration (MIC): the concentration of antibiotic at the
edge of the disk zone of inhibition. MIC is determined by comparing the zone
of inhibition to MIC standards in a table (pg 116).
Susceptible: zone size indicates antibiotic is effective for treatment.
Intermediate: zone size indicates antibiotic is moderately effective for treatment.
Resistant: zone size indicates antibiotic is ineffective for treatment.
Antibiotic Susceptibility Test: Part 1
Pages 114-116
Antibiotic Susceptibility Test: Part 1
Antibiotic Susceptibility Test: Part 1
Assignments for this week
Lab Reports:
• API: PRE-LAB questions 1-3
• Kirby Bauer Lab: chart and questions 1-4
Lab Smart (reminder):
• Identification of Unknown Bacteria
Available Oct 21st, Due Nov 17th
Antibiotic Susceptibility Test: Part 2
Antibiotic Susceptibility Test: Part 2
Antibiotic Susceptibility Test: Part 2