Answer this though some bacteria cause disease and illness in humans, the majority are harmless. some bacteria are used to aid digestion, used in preparation of certain foods, used to battle infections or used by industry. bacteria have many important roles in nature. all but one is a role of bacteria out in the world around us. a) some bacteria increase the soil fertility. b) bacteria carry out 60% of total nitrogen fixation in natural ecosystems. c) bacteria are responsible for converting sunlight into usable chemical energy. d) bacteria decompose organic matter, by decay and putrefaction, to keep our earth clean.