Background Salmonella and Shigella species are pathogens of great medical and public health importance. However, laboratory identification of these organisms is time consuming. Using current standard laboratory algorithms, the vast majority of organisms submitted for serological typing with Salmonella-specific and Shigella-specific antisera are not clinically significant.

Aims To assess the addition of the O-nitrophenyl-β-d-galactopyranoside (ONPG) test to the standard screening protocol for identification of Salmonella and Shigella species, and to describe a revised algorithm for the identification of these pathogens.

Methods 71 non-lactose-fermenting isolates that were urease negative, indole negative, and produced acid and gas in triple sugar iron agar, with no H₂S production in the agar, were tested for β-galactosidase activity using the ONPG test. The test results were read at half-hourly intervals over a 4 h incubation period in O₂ at 37°C.

Results 42 isolates (59.2%) were found to be Hafnia alvei, of which 36 strains (85.7%) were ONPG positive. All 18 of the Enterobacter species (25.4%) were ONPG positive. Overall, about 79% of the ONPG-positive isolates were positive at the end of the first half-hour of incubation. The two pathogenic isolates obtained during this study were both identified as Salmonella enterica serovar Paratyphi A, and they were ONPG negative.

Conclusions Incorporation of a 30 min ONPG test for non-lactose-fermenting organisms that are indole negative, urease negative, producers of acid and gas from glucose, oxidase negative and non-H₂S gas producers eliminates the need for further serological testing of 79% of isolates, substantially improving the efficiency of the identification protocol.