Question

bacteria present on uncleaned teeth convert the sugar in food into a substance X
which causes tooth decay

Answer 1

You may not realize it, but your mouth is teeming with bacteria. And  when you eat, they eat, too, gorging themselves on the food debris that's left on your teeth. Bacteria convert sugars and starches from food into acid. When acid combines with leftover food debris and  saliva, it forms a sticky substance called plaque.