Value of the O-nitrophenyl-beta-D-galactopyranoside test to differentiate among the aerobic actinomycetes

A comparative study to determine beta-D-galactosidase activity among 171 strains of aerobic actinomycetes (including mycobacteria and rhodococci) was performed by using two growth media and four O-nitrophenyl-beta-D-galactopyranoside (ONPG) substrates. The ONPG test was found to be a valuable screening test to differentiate between the ONPG-positive Nocardia spp. and the rapidly growing ONPG-negative mycobacteria and rhodococci. However, ONPG results varied significantly depending on the growth medium and test substrate used.