Improvement of mannitol lysine crystal violet brilliant green agar for theselective isolation of H2S-positive Salmonella

Mannitol lysine crystal violet brilliant green agar (MLCB) is widely used i n Japan for Salmonella isolation because the medium has been commercially a vailable. Colonies of Salmonella on MLCB appear colorless with black center s due to H2S gas production; however, most Citrobacter freundii also produc e H2S gas. In order to distinguish H2S-positive Salmonella from C. freundii we have improved MLCB. To MLCB was added 1% lactose (L-MLCB). The relation for pH and black center colony formation was examined. The pH of MLCB and L-MLCB inoculated with Salmonella species was slightly acid after 7 h, but the pH of L-MLCB inoculated with C. freundii did not become acid for 24 h. The colony of C. freundii did not have a black center because the productio n of acid from lactose lowers the pH below 10 where it is needed for H2S to react with iron to produce black pigments. Of 99 Salmonella strains includ ing 13 serotypes tested, all strains had the same colony morphologies on ML CB and L-MLCB. When MLCB and L-MLCB were evaluated with 36 C. freundii stra ins isolated from foods, only colonies on MLCB had black centers. We conclu de that L-MLCB is useful for detection of nonlactose-fermenting, H2S-positi ve Salmonella in food samples.