Fungus and bacteria live in water and their source of food is hydrocarbons which is found in diesel fuel. The fungus and bacteria multiply throughout the system alarmingly quickly contaminating everything in its path. The first thing will be the fuel filter that it contaminates, leaving a film like substance and in a short space of time will end up reducing the effectiveness of the all important first line of defence, the fuel filter.                                                                                                               

There are different types of bacteria which are all different colours, they can be brown, green or black, but one thing about all of them they will all be detrimental and affect your diesel fuel system whether a fuel pump, injector or a heating system.

Once the fungus and bacteria enters your fuel system the only way to eradicate them out of the system completely would be to strip, clean and drain your system by flushing it with a biocide to eliminate all the growth.