The tempeh fungus Rhizopus oligosporus

Tempeh has been produced in Indonesia for many centuries. However, it was the Dutch scientist Prinsen Geerligs, who in 1895 identified the tempeh mould for the first time (Shurtleff & Aoyagi, 2001). Many different moulds are found in tempeh, but species within the zygomycete genus Rhizopus dominate (Steinkraus et al., 1983). Rhizopus (Mucorales, Mucoraceae, Zygomycota) includes three species groups: R. oryzae group, R. stolonifer group, and R. microsporus group (Schipper & Stalpers, 1984), with species from the latter group dominating in tempeh.

Among them, R. oligosporus (Fig. 8) is the most preferred species in tempeh fermentation (Sharma & Sarbhoy, 1984), due to properties such as rapid growth at high temperature (30-42°C), an inability to ferment sucrose, high proteolytic and lipolytic activities and production of strong antioxidants (Steinkraus et al., 1983). R. oligosporus is considered as a domesticated form of R. microsporus (Samson, 1985), which can produce toxic secondary metabolites such as rhizoxin, and rhizonins A and B (Jennessen et al., 2005). Rhizoxin can inhibit mitosis and thus the cell cycle, and have been suggested as a potential anti-tumour agent (Tsuruo et al., 1986; Takahashi et al., 1987). Rhizoxin also causes rice seedling blight disease (Goh et al., 1978), which was recently reported to be produced by an endosymbiotic bacterium (Burkholderia spp.) within Rhizopus spp. (Partida-Martinez & Hertweck, 2005). Rhizonins A and B have a strong hepatotoxic activity (Wilson et al., 1984). However, R. oligosporus does not produce any of these metabolites in different laboratory and natural substrates, not even under prolonged incubation conditions conducive to the formation of these metabolites (Jennessen et al., 2005), nor during barley tempeh fermentation (Feng, X-M and Nielsen, K. F., unpublished). R. oligosporus might have lost the rhizoxin-producing endosymbiotic bacteria and the ability to produce rhizonins during centuries of domestic cultivation in rich substrates such as soybeans.

Due to the great similarity in morphology between R. oligosporus and R. microsporus, additional differential characteristics are required to make sure that the correct strains are used for tempeh fermentation. The most recent study based on sporangiospore shape, size and ornamentation patterns under low-temperature scanning electron microscopy (SEM) demonstrated that the R. microsporus group includes three subgroups: 1) R. rhizopodiformis, R. chinensis, R. azygosporus and R. oligosporus, 2) R. caespitosus, R. schipperae and R. homothallicus, 3) R. microsporus (Jennessen et al, manuscript). R. oligosporus has the greatest number of large size irregular spores within the R. microsporus group and a proportion of more than 10 % irregular spores can be used as a marker to distinguish R.

oligosporus from other strains within the R. microsporus group (Jennessen et al, manuscript).

Rhizopus Oligosporus

Mycelium Sporangophore Sporangiophore with Rhizoid with spores

Fig. 8. Morphology of R. oligosporus (Photo: Inger Ohlsson).

Mycelium Sporangophore Sporangiophore with Rhizoid with spores

Fig. 8. Morphology of R. oligosporus (Photo: Inger Ohlsson).

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