Which of the following is not a standard weekly disk-diffusion quality-control organism?

• A. Staphylococcus epidermidis (ATCC 25833)
• B. Pseudomonas aeruginosa (ATCC 27853)
• C. Escherichia coli (ATCC 25922)
• D. Enterococcus faecalis (ATCC 29212)

Answer: (A)

Disk-diffusion quality control relies on standardized reference strains that have well-established, published zone-diameter ranges across many antibiotics. These QC strains are used to confirm that the diffusion conditions, media, and incubation are working properly before testing patient samples. For this method, the commonly accepted weekly QC organisms include the Gram-negative Escherichia coli ATCC 25922 and Pseudomonas aeruginosa ATCC 27853, as well as a Gram-positive organism such as Enterococcus faecalis ATCC 29212 (and Staphylococcus aureus ATCC 25923 as the usual Staphylococcus species QC). The listed Staphylococcus epidermidis ATCC 25833 is not part of the standard weekly disk-diffusion QC panel, so it would not be used as the routine reference strain to validate results. Using the established strains ensures that the expected inhibition zones are matched to the documented criteria, keeping interpretations accurate.