Name two QC strains commonly used for disk diffusion quality-control.

Name two QC strains commonly used for disk diffusion quality-control.

• A. Escherichia coli ATCC 25922 and Staphylococcus aureus ATCC 25923
• B. Pseudomonas aeruginosa ATCC 27853 and Klebsiella pneumoniae ATCC 700603
• C. Enterococcus faecalis ATCC 29212 and Streptococcus pneumoniae ATCC 6303
• D. Staphylococcus aureus ATCC 29213 and Escherichia coli ATCC 35218

Answer: (A)

Disk diffusion quality control relies on two standard reference strains that give well-defined, reproducible responses across many antibiotics. Using these strains ensures the test setup—media, inoculum density, disc potency, and incubation conditions—produces zone diameters that match published QC ranges. The two strains commonly used are Escherichia coli ATCC 25922 for Gram-negative coverage and Staphylococcus aureus ATCC 25923 for Gram-positive coverage. They provide a reliable benchmark: if the observed inhibition zones fall within the established ranges, the assay is functioning properly; if not, the procedure needs adjustment. Other strains may be used in specific contexts or for particular antibiotic panels, but they do not serve as the universal disk diffusion QC pair the way these two do.

Disk diffusion quality control relies on two standard reference strains that give well-defined, reproducible responses across many antibiotics. Using these strains ensures the test setup—media, inoculum density, disc potency, and incubation conditions—produces zone diameters that match published QC ranges. The two strains commonly used are Escherichia coli ATCC 25922 for Gram-negative coverage and Staphylococcus aureus ATCC 25923 for Gram-positive coverage. They provide a reliable benchmark: if the observed inhibition zones fall within the established ranges, the assay is functioning properly; if not, the procedure needs adjustment.

Other strains may be used in specific contexts or for particular antibiotic panels, but they do not serve as the universal disk diffusion QC pair the way these two do.