That is not correct, the symptoms you describe, by themselves, do not indicate a bacterial infection. The change in poo texture, color, etc. can be associated to the change in eating habits, so the question that needs to be answered is why this fish has had such a drastic change in eating habits. Besides possible illness, water conditions/fluctuations, stress, and many other things can contribute to change of eating habits.
Can you please provide information about your tank? How long has it been set up? What size tank, how many fish, what species of fish (how many of each species) and sizes of fish? What type of filtration? How often are you performing water changes? How much water each time? How often do you gravel vac? Can you please post test results (using a liquid test kit) for ammonia, nitrite, nitrate, and pH? What type of decor is in the tank? How much decor? Live plants? Feeding schedule? Foods offered? After you feed, how long until ALL food is consumed by the fish? Is food reaching the bottom/substrate when you feed? Water temp?
These questions will get us started, but anything else you can offer for information about this tank and the fish inside of it will only help lead to a proper diagnosis of the problem and a safe, healthy solution.