_E. coli_ has a high level of metabolic flexibility, allowing it to survive in many different growth conditions. As a facultative anaerobe, it is capable of using different bacterial electron transport complexes (such as Ndh and Cyd) to allow ATP synthesis in a range of conditions. Some electron transport complexes are more resistant to reducing conditions, for instance.
Being able to grow successfully in a wide range of conditions increases the chances of successful infection in pathogenic bacteria, due to the ability to use different electron acceptors to maintain a redox balance. This increases the likelihood of an electron acceptor being present, allowing the electron transport chain to occur (which is where the most ATP is synthesised in respiration).
_E. coli_ is able to express different electron transport complexes for specific conditions, ensuring it is using the most efficient complexes possible.