Susceptibility of Zygomycetes to human serum

Seven members of the class Zygomycetes were tested for serum susceptibility. Five of the isolates were inhibited to different degrees by pooled fresh human serum. Heating the serum at 56 degrees C for 30 min to inactivate complement did not abolish the inhibitory activity in the serum. Members of the genera Cunninghamella and Absidia appeared most susceptable to the inhibitory effects of serum while members of the genus Rhizopus were most resistant. One isolate of R. arrhizus appeared to have enhanced growth in the presence of human serum. Our findings suggest certain species of the class Zygomycetes are strongly inhibited by serum, and this may account for the rarity of human infections by these species.