The skin of a piece of fruit is the first barrier to the saprophytes that decompose it.
The microorganisms that decompose fruit are collectively called saprophytes. They include bacteria, fungi, molds, algae and protozoa. Different saprophytes operate best under different conditions of light, moisture, temperature and the presence of oxygen. As a particular set of saprophytes finishes its role in breaking down fruit, it often prepares the fruit for the next wave. This is a process called succession, in which the organisms that begin the process of decomposition are rarely those that finish it.
Bacteria
Bacteria are usually the first line to attack a piece of fruit. The skin of the fruit can protect it from decomposition for several days after it is off the tree. Once a cut is made in the fruit, bacteria immediately invade the opening. Aided by moisture, the bacteria secrete enzymes that help break down cellular walls. This makes it easier for micro-organisms to digest the pieces. In early stages, the fruit may just appear a bit mushy and with diminished flavor.
Fungi
Fungi attack organic matter and begin a process called fragmentation. Fragmentation further breaks the fruit down. By the time fungi have taken firm hold, the fruit is at a stage that you would consider rotten, though not necessarily putrid. In some plants, fungi usually attack first and are followed by bacteria, which work best in a moist, oxygen-rich environment. Fungi are often mistaken for mold in the early stages of decomposition.
Slime Molds
Slime molds feed on bacteria. They move about and leave tiny traces of slime in their path. They produce tiny fruiting bodies which, at advanced stages of decay, can resemble miniature mushrooms, particularly when magnified. These further break down the fruit. By the time slime molds are present, the fruit has become putrid.
Protozoans
Protozoans are the simplest form of animal life. They act much like bacteria in decomposition, but are present in much smaller numbers. Usually, they do not enter into the process until it is fairly advanced and feed on the saprophytes that have proceeded them as well as the fruit.
Physical Decomposers
The worms and insects that largely finish the process of decomposition after rot has set in are called physical decomposers. These include mites, millipedes, snails, slugs, beetles, ants, flies and worms. They chew through the largely decomposed fruit leaving behind carbon dioxide and mineral forms of nutrients such as nitrogen, which can then be absorbed by living plants to create new fruit, leaves and growth.
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