We found an emergence of 8 strains of S. sonnei with negative phenotype for o-nitrophenyl-β-d-galactopyranoside (ONPG) in late August 2011.

A negative reaction in ONPG hydrolysis reduces the likelihood of correct species identification as S. sonnei by Vitek and API 20E methods.

The ONPG-negative S. sonnei with distinct pulsed-field gel electrophoresis pattern lacked lac operon (lacZYI gene cluster) and mhp operon (mhpABR gene cluster), compared with the genome of ONPG-positive S. sonnei and the reference Ss046.