What does a positive D test indicate?
A positive D test (see Figure) indicates the presence of macrolide-inducible resistance to clindamycin produced by an inducible methylase that alters the common ribosomal binding site for macrolides, clindamycin and the group B streptogrammins (quinupristin) (Woods, 2009) .
What is the disk diffusion test used for?
In diagnostic laboratories, the disk diffusion test is used to determine the susceptibility of clinical isolates of bacteria to different antibiotics. An effective antibiotic will produce a large zone of inhibition (disk C), while an ineffective antibiotic may not affect bacterial growth at all (disk A).
Why do we need to do the D test for MRSA isolates?
MRSA isolates with inducible clindamycin resistance (iCR) are resistant to erythromycin and sensitive to clindamycin on routine testing. D-test can help to determine whether clindamycin could be used as a therapeutic option2.
How do you test for zone of inhibition?
How Is a Zone of Inhibition Test Performed? In a typical Zone of Inhibition test, a petri dish containing a nutrient agar is streaked with the required bacteria culture. A sample of a product treated with an antimicrobial agent is cut into a piece (typically an inch by an inch) and placed onto the nutrient agar.
How do you interpret zone of inhibition?
If the observed zone of inhibition is greater than or equal to the size of the standard zone, the microorganism is considered to be sensitive to the antibiotic. Conversely, if the observed zone of inhibition is smaller than the standard size, the microorganism is considered to be resistant.
How do you measure zone of inhibition?
Take a ruler or caliper that measures in millimeters and place the “0” in the center of the antibiotic disk. Measure from the center of the disk to the edge of area with zero growth. Take your measurement in millimeters. This measures the radius of the zone of inhibition.
What does clindamycin resistance mean?
Inducible resistance to clindamycin was defined as blunting of the clear circular area of no growth around the clindamycin disk on the side adjacent to the erythromycin disk and was designated D-test positive.
What are the principles of disk diffusion susceptibility test?
PRINCIPLES OF THE PROCEDURE. In the disk-diffusion susceptibility test, disks containing known amounts of an antimicrobial agent are placed on the surface of an agar plate containing a nonselective medium that has been inoculated with a suspension of a strain of N. gonorrhoeae to give a confluent lawn of growth.
What is the D-Zone test used to diagnose?
The D-zone test is only standardized to detect inducible clindamycin resistance for Staphylococcus spp., S.pneumoniae, and beta-hemolytic Streptococcus.
What is the role of disk-diffusion and Etest susceptibility tests for gonorrhea?
However, when performed correctly, the disk-diffusion and Etest susceptibility tests can be used to identify isolates of N. gonorrhoeae that exhibit decreased susceptibility, intermediate resistance, and resistance to antimicrobial agents.
How do you incubate disk diffusion plates?
Invert the inoculated plates (lid side down), and incubate the plates at 35 C to 36 C in 5% CO 2 for 20 to 24 h. Examine the disk diffusion plates from the back, viewed against a black background and illuminated with reflected light.