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 Staphylococcus aureus 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.
|
Zone Diameter Around Erythromycin Disk
|
Zone Diameter Around Clindamycin Disk
|
Interpretation
|
|
<= 13 mm
|
<= 14 mm
|
resistance
|
|
14 - 22 mm
|
15 - 20 mm
|
intermediate
|
|
>= 23 mm
|
>= 21 mm
|
susceptible
|
|
Appearance of Growth
|
Interpretation
|
|
no or small zone of inhibition around clindamycin disk
|
resistance to clindamycin
|
|
uniform circular zone of inhibition around the clindamycin disk, including zone nearest to the erythromycin disk
|
susceptible to clindamycin with no evidence of inducible resistance (negative for inducible resistance)
|
|
zone of inhibition around the clindamycin disc, with zone flattened in quadrant nearest to the erythromycin disk
|
susceptible to clindamycin in vitro but showing inducible resistance (positive for inducible resistance
|
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.
