Beta-galactosidase and lactose fermentation in the identification of enterobacteria including salmonellae

Abstract

114 strains of non-lactose fermenters and 127 lactose fermenters on MacConkey''s agar were compared in the 5% and 1% lactose tests and in [beta]-galacto-sidase production, using ortho-nitro-phenyl-[beta]-D-galactopyranoside (ONPG) as a test substance. The superiority of the ONPG test in the number of positive results and its rapidity is shown. In general, late or non-lactose fermenting strains of genera, usually lactose-positive, yield a rapidly positive ONPG reaction. 41 wild strains of Salmonella, Proteus, Providencia, and Pseudomonas aeruginosa were found negative in all three tests. Of 1075 stock strains of Salmonella examined in the ONPG test, all were negative except nine; four of these were lactose-positive strains. For practical purposes, Salmonella strains in Great Britain may be regarded as ONPG negative. Among 100 stock strains of Arizona there was considerable variation of behavior in the ONPG test and in the 5% and 1% lactose tests. Most strains of Arizona can be considered to yield a positive ONPG test but a minority give a negative result. The test is recommended for routine use in the differentiation of Salmonella from other enterobacteria and for use in bacterial identification. The 5% lactose fermentation test in parallel is suggested when the ONPG test is used for isolating routine pathogens, because organisms such as Shigella sonnei, S. dysenteriae 1, and Pasteurella psuedotuberculosis are ONPG positive.