Question

Identfication lab problem. Introduced carbohydrate metabolism tests were glucose,lactose,raffinose,sucrose fermentations, mannitol fermentation, Macconkey agar, citric acid utilization, MR/VP test, and esculin hydrolysis.

Q) Two different strains of E. coli are practically identical to each other except one can ferment sugar "X" and the other cannot.  
How could the E. coli strain that can ferment sugar "X" been generated evolutionarily at the molecular level, and how could the E. coli strain that cannot ferment sugar "X" been generated evolutionarily at the molecular level?

Answer 1

Fermentation is a proces used by cells to generate energy where a suitable substrate is metabolised to make ATP by substrate level phosphorylation.There are many fermentation pathways known inbacteria. they differ in type of sustrate used, the key enzyme, energy yeilds, and end in the end products made. E.coli is metabolically versatile microbe that can ferment sugars besides growing aerobically or anaerobically by respiration.

In this regard, E.coli long term evolution experiment (LTE) led by Richard Lenski will explain the right way of fermentation of sugars by E.coli.

E. coli is normally unable to grow aerobically on citrate due to the inability to express a citrate transporter when oxygen is present. However, E. coli has a complete citric acid cycle, and therefore metabolizes citrate as an intermediate during aerobic growth on other substances, including glucose. Most E. coli can grow anaerobically on citrate via fermentation, if a co-substrate such as glucose is available to provide reducing power.The anaerobic growth is possible due to the expression of a transmembrane citrate-succinate antiporter gene, This gene is co-regulated with other genes involved in citrate fermentation found on the cit operon, which is turned on only when oxygen is absent.