Which test is most appropriate for presumptive identification of Peptostreptococcus anaerobius?

• A. SPS disk
• B. Colistin disk
• C. Kanamycin disk
• D. Vancomycin disk

Answer: (A)

The main idea is using a selective susceptibility test to distinguish an anaerobic gram-positive coccus. Peptostreptococcus anaerobius is known to be inhibited by sodium polyanetholesulfonate (SPS). When an SPS disk is placed on a lawn of the organism, a zone of inhibition around the disk supports presumptive identification as P. anaerobius because this susceptibility pattern is characteristic, while other common anaerobic cocci don’t show the same clear SPS-sensitive reaction.

The other antibiotic disks target broader groups or different resistance patterns and won’t provide the same specific, rapid cue for identifying this organism. So the SPS disk is the best choice for presumptive ID of Peptostreptococcus anaerobius.