'D-zone' clue key to clindamycin-resistant MRSA: test inducible resistance

OB/GYN News, Nov 1, 2003 by Robert Finn

ASPEN, COLO. -- Physicians should make sure their microbiology lab is testing for both inducible and constitutive resistance to clindamydn when they send samples for susceptibility testing, Dr. Carol J. Baker said at a conference on pediatric infectious diseases sponsored by Children's Hospital, Denver.

With the rise in methicillin-resistant Staphylococcus aureus (MRSA), susceptibility testing has become routine. But unless the lab takes special care, the report may say that the patient's S. aureus is susceptible to clindamycin when actually it is not,

In constitutive resistance, the microbe is. clearly resistant to both erythromycin and clindamycin, and the lab will be able to pick this up using any susceptibility test, she explained at the meeting, which was also sponsored by the University of Colorado.

The problem is with inducible resistance. During ordinary testing, the microbe appears to be resistant to erythromycin and susceptible to clindamycin, but once the patient is exposed to clindamycin, he or she quickly develops resistance.

"It's very clinically important," said Dr. Baker, head of the section of pediatric infectious diseases at Baylor College of Medicine, Houston: "You need to know about this or your patients are going to have problems."

Fortunately, it's quite simple and inexpensive to test for inducible resistance. The lab merely has to place erythromycin near clindamycin on the culture dish. Normally there will be a clear circular area around the clindamycin sample, which indicates susceptibility, and no such area around erythromycin, which indicates resistance.

Sometimes, however, the circular area around clindamycin will have a flat or blunted edge where that sample is adjacent to the erythromycin sample, and that's proof of inducible resistance to clindamycin.

The flat edge on one side of the circular area around the clindamycin sample looks something like the letter D, and Dr. Baker refers to this as the "D-zone."