Why E.coli ferments lactose while s.typhi does not?

Question

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Why E.coli ferments lactose while s.typhi does not?

Answer 1

E. coli ferments lactose because it possesses the lac operon, a set of genes that encode enzymes required for the uptake and metabolism of lactose. These enzymes include β-galactosidase, which breaks down lactose into glucose and galactose that E. coli can use as an energy source. This ability allows E. coli to ferment lactose and produce acid and gas as byproducts.

On the other hand, Salmonella typhi lacks a functional lac operon or the necessary enzymes to metabolize lactose. As a result, it cannot ferment lactose and instead relies on other sugars or substrates for energy.

In summary:

1. E. coli has the genetic machinery (lac operon) to utilize lactose as an energy source and ferment it.
2. S. typhi lacks this machinery, so it does not ferment lactose.

Answer 2

Look around the presence or absence of the enzyme LDH Lactose Dehydrogenase in E. coli versus S. typhi. S. typhi is likely to miss a key enzyme for the use of lactose as source of carbon.