In the picture you can see a typical E.coli fermenting lactose (Strain C) and other bacterium (Strain D) that appears to be lactose negative (after 24 hours). Both of them are E.coli and both of them are lactose positive. E.coli isolates that look like Strain D are usually so called "late lactose fermenters" (or delayed lactose fermenters).
The delayed fermenters have the potential to ferment lactose (they produce the intracellular enzyme necessary to metabolize lactose) but they lack the β-galactosidase-permease enzyme.

These organisms over a period of time(48 hours to several days) produce some mutant cells which possess β-galactosidase-permease along with nonlactose fermenting cells and late lactose fermentation will be observed.

The ability to ferment lactose can be predicted by demonstrating the presence (or absence) of β-galactosidase activity when employing the organic compound o-nitrophenyl-β-D-galactopyranoside (ONPG).

General description of testing on Endo agar with biochemical slope is here.



Cultivation 24 hours, 37°C in an aerobic atmosphere.