Bacteria are responsible for most of the decomposition and heat generation in compost. They are the most nutritionally diverse group of compost organisms, using a broad range of enzymes to chemically break down a variety of organic materials. Bacteria don't have to be added to the compost pile. They are present virtually everywhere and enter the pile on every piece of organic matter.
Many types of bacteria are at work in the compost pile.
Each type thrives on special conditions and different types of organic waste. Once the compost cools down, mesophilic bacteria again predominate. The numbers and types of mesophilic microbes that recolonize compost as it matures depend on what spores and organisms are present in the compost as well as in the immediate environment. In general, the longer the curing or maturation phase, the more diverse the microbial community it supports.