Major nitpick: Dopamine isn't the "feel good" chemical.
Endorphins are more accurately "feel good" chemicals.
Dopamine is involved in motivation through reward reinforcement, movement, and cognition in the human nervous system.
Another nitpick: L-DOPA isn't a building block of dopamine. It's dopamine with an extra molecule attached to it that makes it "inactive." This molecule gets snapped off by the AADC enzyme, which "activates" it. L-DOPA is the precursor to dopamine.
Third nitpick: We can understand how these gut flora affect ingestion of L-DOPA, and neurotransmitter precursors, by looking at how it's used in medical treatments. Most notably, Parkinson's patients. Suffice to say, it really makes no fucking difference, and orally-administered L-DOPA (with a dehydroxylaze inhibitor) is effective.
Whatever L-DOPA is stolen by gut-flora, can be augmented with a higher dosage. If for some strange reason, the converted L-DOPA -> Dopamine gets released into the digestive tract, and absorbed by the body, it's readily broken down in the blood. L-DOPA converts to dopamine in the blood-stream anyway, so it makes absolutely no difference.
I wanted this paper to be novel, but after reading it, it's not.
Gut flora have AADC-like enzymes, cool?