Production of 2-aminophenoxazin-3-one by Staphylococcus aureus causes false-positive results in ß-galactosidase assays.

Staphylococcus aureus can be distinguished from similar coagulase-positive staphylococci by its absence of ß-galactosidase activity. This is commonly tested using o-nitrophenyl-ß-D-galactopyranoside (ONPG) as the substrate. Unexpectedly, 111 and 58 of 123 isolates displayed apparent ß-galactosidase activity in the ONPG assay and on the Vitek 2 system, respectively. Compositional analysis showed that the yellow coloration of the positive ONPG assay resulted from production of 2-aminophenoxazin-3-one. Alternative ß-galactosidase substrates like X-Gal (5-bromo-4-chloro-3-indolyl-ß-D-galactopyranoside) should be used for testing staphylococci.