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

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

Answer: (A)

Testing for susceptibility to a sodium polyanethol sulfonate (SPS) disk is useful because Peptostreptococcus anaerobius, a gram-positive anaerobic coccus, is characteristically inhibited by SPS. On an anaerobic culture plate, a clear zone of inhibition around the SPS disk supports presumptive identification as this organism, since other common test disks (colistin, kanamycin, vancomycin) don’t provide a distinctive, reliable pattern for this microbe. Colistin and kanamycin mainly differentiate Gram-negative bacteria or non–GP cocci, and vancomycin susceptibility is not unique to Peptostreptococcus species, so they’re less informative for this purpose. In practice, the SPS disk test offers a practical, organism-specific cue, with confirmation by additional biochemical or molecular methods as needed.