Anaerobic growth and oxygen toxicity of Rhizopus cultures isolated from starters made by solid state fermentation

To explore the relationship between Rhizopus growth and its microaerophilic characteristics the present study was undertaken. A survey of 18 Rhizopus cultures growing in anaerobic or microaerobic condition found that 11 of the cultures grew quite well under microaerobic condition, but all the cultures grew poorly under absolutely anaerobic condition. Among the 18, 8 showed definitely banded mycelial growth below the surface of the medium in the tube. Six of 18 cultures--RT-1-8, RT-43, RT-46, A-1, C-1, and E-1--grew with much aerial hyphae when they were cultivated on media supplemented with different detoxifying agents under aerobic condition. These six strains are comparatively sensitive to oxygen toxicity (H2O2 or O2- or OH.) under aerobic condition, as many microaerophilic bacteria are. There were nine strains whose aerial hyphal growth was greatly enhanced by the presence of Aspergillus niger catalase. However, the growth of the majority of the cultures was not influenced by supplements of bovine liver catalase or bovine liver superoxide dismutase. In conclusion, some Rhizopus cultures from the starters exhibited both microaerophilic and aerobic growth. The phenomena of growth in low O2 condition, and the subsurface growth of mycelia in tubes, may provide a clue to exploitation of the microaerophile-like characteristics in some starter molds.