LEARNING MODULE 2: READING ASSESSMENT
1. List 4 variables that must be controlled with disk diffusion testing, including the standards
for each.
1. Media
Standards:
Formulation – Mueller-Hinton
Ca2+, Mg2+ content – 25 mg/L Ca2+, 12.5 mg/L Mg2+
Thymidine content – minimal or absent
pH – 7.2-7.4
Agar depth (disk diffusion) – 3-5 mm
2. Incubation
Atmosphere – Humidified ambient air
Temperature - 35°C
Length – Disk diffusion: 16-18 hours
Broth microdilution: 16-20 hours
24 hours - (sometimes fastidious bacteria) – staphylococci w/ oxacillin & Vancomycin
Enterococci w/ vancomycin & gentamicin HLAR
48 hours – enterococci w/ streptomycin HLAR
3. Antimicrobial agents
Disks – must contain FDA/CLSI-defined concentration of drug
• Proper storage
Solutions – proper placement on agar
• Reference standard powders used to prepare
• Proper storage
4. End point measurement
Disk diffusion – reflected light (apart from staphylococci w/ oxacillin & vancomycin, and for enterococci with
vancomycin) and plate held against black background
• Zones measured from back of plate
Transmitted light – staphylococci with oxacillin & vancomycin; enterococci w/ vancomycin
2. Describe each factor considered when trying to decide if susceptibility testing should be
performed.
Body site:
When faced with a decision regarding antibiotic susceptibility testing, it is crucial to consider the site on the
body that the organisms will be isolated from. Susceptibility tests are not usually done on organisms that are
taken from a site on the body that they are known to normally inhabit. Organisms typically undergo
susceptibility testing if they’re isolated from places they’re not known to normally colonize, or if the area
they’re found in is normally sterile. Normal flora and microbiota are essential and contribute to good health.
Therefore, susceptibility tests on such organisms could lead to the treatment of these harmless microorganisms,
possibly preventing treatment of true pathogens.
Quality of Specimen + Prescence of other bacteria:
1. List 4 variables that must be controlled with disk diffusion testing, including the standards
for each.
1. Media
Standards:
Formulation – Mueller-Hinton
Ca2+, Mg2+ content – 25 mg/L Ca2+, 12.5 mg/L Mg2+
Thymidine content – minimal or absent
pH – 7.2-7.4
Agar depth (disk diffusion) – 3-5 mm
2. Incubation
Atmosphere – Humidified ambient air
Temperature - 35°C
Length – Disk diffusion: 16-18 hours
Broth microdilution: 16-20 hours
24 hours - (sometimes fastidious bacteria) – staphylococci w/ oxacillin & Vancomycin
Enterococci w/ vancomycin & gentamicin HLAR
48 hours – enterococci w/ streptomycin HLAR
3. Antimicrobial agents
Disks – must contain FDA/CLSI-defined concentration of drug
• Proper storage
Solutions – proper placement on agar
• Reference standard powders used to prepare
• Proper storage
4. End point measurement
Disk diffusion – reflected light (apart from staphylococci w/ oxacillin & vancomycin, and for enterococci with
vancomycin) and plate held against black background
• Zones measured from back of plate
Transmitted light – staphylococci with oxacillin & vancomycin; enterococci w/ vancomycin
2. Describe each factor considered when trying to decide if susceptibility testing should be
performed.
Body site:
When faced with a decision regarding antibiotic susceptibility testing, it is crucial to consider the site on the
body that the organisms will be isolated from. Susceptibility tests are not usually done on organisms that are
taken from a site on the body that they are known to normally inhabit. Organisms typically undergo
susceptibility testing if they’re isolated from places they’re not known to normally colonize, or if the area
they’re found in is normally sterile. Normal flora and microbiota are essential and contribute to good health.
Therefore, susceptibility tests on such organisms could lead to the treatment of these harmless microorganisms,
possibly preventing treatment of true pathogens.
Quality of Specimen + Prescence of other bacteria:
