Growth-promoting factors for yeast cells ofParacoccidioides brasiliensis

Low-density seedings of yeast cells ofParacoccidioides brasiliensis give poor growth (as assessed by plating efficiency test) on conventional mycological agar media, and therefore growth-promoting factors for this fungus were sought. Water-extracts of yeast cells of sixP. brasiliensis isolates were all considerably effective in promoting the growth of low-density seedings ofP. brasiliensis isolates Pb-18 and Hachisuga, but had little effect on isolate Bt-4. Horse serum, at a concentration range of 2–4%, moderately or considerably promoted the growth of theseP. brasiliensis isolates. Combinations of the fungus cell extracts with horse serum were highly effective in promoting the growth of all of the fungal isolates. The fungus cell extracts showed siderophore (microbial iron carrier) activity. An iron-chelator, ethylenediaminetetraacetic acid, at a concentration of 100 μM also highly promoted the growth of the fungal isolates in the presence of horse serum, and ferric ion added to culture medium was considerably effective in the growth promotion. These results suggest that deficient utilization of external iron by the fungus cell is one of the growth-limiting processes for low-density seedings of yeast cells ofP. brasiliensis on conventional mycological agar media.