that would be in the field of synthetic biology. although genetically engineering ecoli up produce more serotonin might be harder than using artificial selection to make happy ecoli.

about being better at digesting or making proteins, same thing, except that they already have natural selection pushing then to suggest as efficiently as possible.

in fact if you made a super strain, it’ll eventually die out as it’s replaced with the natural ecoli (or lose those traits) because they are less optimised than your original ecoli