Rhizopus oligosporus is a fungus of the family Mucoraceae that is a widely used starter culture for the home production of tempeh. The spores produce fluffy, white mycelia, binding the beans together to create an edible "cake" of partly fermented soybeans.
Rhizopus oligosporus produces an antibiotic that inhibits gram-positive bacteria, including the potentially harmful Staphylococcus aureus and the beneficial Bacillus subtilis (present in nattō), even after the Rhizopus is consumed. This supports anecdotal evidence that those who eat tempeh regularly have fewer intestinal infections.
One strain of this fungus used in tempeh production was shown to contain bacterial endosymbionts of the genus Burkholderia, which were shown to produce toxic rhizoxins. Whether other strains used for tempeh production contain these bacteria has not been established.
Rhizopus oligosporus is more properly known as Rhizopus microsporus var. oligosporus.