From The Aquarium Wiki
Heterotroph bacteria can be either gram positive or the gram negative type. Some are strictly aerobic, many species are facultative anaerobic  (they can switch mode from zero-low to high levels of oxygen).
When in the prescent of oxygen they convert ammonia to nitrite but they do this very inefficiently (about one million times less efficient) and are not true nitrifying bacteria (few heterotroph species can convert nitrites into nitrates. But only in zero levels of oxygen). Also in low-zero levels of oxygen they actively convert nitrogen waste back into nitrite and ammonia!
Heterotrophic bacteria are usually considered to be organic sludge degraders. That is, they breakdown the mulm. They are mostly from the genera Bacillus and Pseudomonas.