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Journal of clinical microbiology

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


PMID 22972831

Abstract

Staphylococcus aureus can be distinguished from similar coagulase-positive staphylococci by its absence of β-galactosidase activity. This is commonly tested using o-nitrophenyl-β-D-galactopyranoside (ONPG) as the substrate. Unexpectedly, 111 and 58 of 123 isolates displayed apparent β-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 β-galactosidase substrates like X-Gal (5-bromo-4-chloro-3-indolyl-β-D-galactopyranoside) should be used for testing staphylococci.