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Detection of β-Gal activity using assays based on two chromogenic substrates: ONPG (i) and X-Gal (ii). Organisms used include the positive control E. coli ATCC 25922 (A), S. aureus (B), and S. pseudintermedius (C). A false-positive result for S. aureus is observed using the ONPG-based assay; the same strain is negative for β-Gal activity on X-Gal. X-Gal plates consisted of tryptic soy agar supplemented with 17.5 μg of X-Gal.
