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Overview :
The emergence of methicillin-resistant
Staphylococcus aureus (MRSA) is a serious problem in outpatients, especially
children, for whom parenteral antibiotics may seem inappropriate. Some patients
can be treated with clindamycin, but inducible resistance may occur in some strains
that appear susceptible in broth susceptibility tests. The D-zone disk
diffusion test can help identify strains with inducible resistance to
clindamycin.
Inducible resistance to
clindamycin is erm-mediated and tends to be associated with hospital-acquired
MRSA infections.
Test elements:
(1) 2 microgram clindamycin
antibiotic disk.
(2) 15 microgram erythromycin
antibiotic disk.
(3) Petri dish with sheep blood
agar for disk diffusion test.
(4) pure isolate of
Staphylococcus aureus
Testing:
(1) Suspend the isolate in TSB to a 0.5 McFarland standard and then
streak the Petri dish of sheep blood agar.
(2) The clindamycin disk is placed at a standard distance (usually 15 mm,
up to 26 mm) from the erythromycin disk.
(3) The Petri dish is then incubated at 33-35°C for 16-18 hours.
(4) Measure the zone of inhibition around each disk
Interpretation:
• A clear zone around an antibiotic disk after incubation indicates
antibiotic effect on the isolate.
• Growth of the bacteria around the antibiotic disk indicates resistance.
• A patient with a strain of Staphyloccus aurues with inducible
resistance to clindamycin may fail to respond to clindamycin, although some
patients will.
• A patient with a strain of Staphylococcus aureus without inducible
resistance to clindamycin can be expected to respond to clindamycin therapy.
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Zone Diameter Around Erythromycin Disk
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Zone Diameter Around Clindamycin Disk
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Interpretation
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<=
13 mm
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<=
14 mm
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resistance
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14
- 22 mm
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15
- 20 mm
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intermediate
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>=
23 mm
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>=
21 mm
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susceptible
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Appearance of Growth
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Interpretation
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no or small zone of inhibition
around clindamycin disk
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resistance to clindamycin
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uniform circular zone of
inhibition around the clindamycin disk, including zone nearest to the
erythromycin disk
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susceptible to clindamycin with
no evidence of inducible resistance (negative for inducible resistance)
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zone of inhibition around the
clindamycin disc, with zone flattened in quadrant nearest to the erythromycin
disk
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susceptible to clindamycin in
vitro but showing inducible resistance (positive for inducible resistance
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where:
• The name "D zone" is based on the similarity between the zone
of inhibition seen with an inducible strain and the letter D with its flattened
side.
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