The ortho-nitrophenol (ONPG) test and acid from lactose in Gram-negative genera

The results are given of the ortho-nitrophenol test for beta-galactosidase production (ONPG test) on 588 strains of 123 aerobic species of bacteria, representing 30 genera. Apart from some strains of Erwinia herbicola (synonym Chromobacterium typhiflavum) and of Yersinia spp, these strains were not members of the Enterobacteriaceae, in which family the ONPG test is widely used and well documented. The strains were also tested for acid production from 1, 5, and 10% lactose and the findings are discussed in relation to the ONPG test.