Susceptibility Testing for the Corynebacterium

In general, CLSI recommends performing susceptibility testing only on organisms considered part of the normal microbiota of human skin and mucous membranes when the isolate is recovered from a sterile site.

• The primary recommended antimicrobials for testing are erythromycin, gentamicin, penicillin, and vancomycin.

Per the M45 CLSI guideline³⁷, "Methods for Antimicrobial Dilution and Disk Susceptibility of Testing of Infrequently Isolated or Fastidious Bacteria":

• CLSI M45 guidelines do not include any instructions or interpretations for the use of disk diffusion, only for the use of MICs (Etest or MIC) using approved methods; approved MIC interpretations are available for Corynebacterium and other "coryneform" organisms using Susceptible, Intermediate, and Resistant only.
• If an organism/antimicrobial combination has an interpretation only for the "susceptible" category, this indicates that there have been rare (or no known) resistant strains.
• For isolates that have a higher MIC level that is suggestive of "nonsusceptible" (meaning, categories of Intermediate or Resistant) the organism identification should be confirmed as well as the susceptibility testing repeated. If the results are consistent with initial testing, then the isolate should be saved and sent to a referral laboratory to confirm this unusual result.

Corynebacterium isolates may need media supplemented with blood for adequate growth; still, some lipophilic isolates may not grow well enough to be tested with this standardized method. If isolates demonstrate susceptible results at 24 hours of incubation for β-lactam drugs, the panel or plate must be reincubated and results read at 48 hours. Other antimicrobial agents may be reported at 24 hours if there is adequate growth.