A new technique for isolating pathogenic fungi from mixed flora inocula

The development of differential and selective media for the isolation of fungi pathogenic for man is briefly reviewed. A new primary culture technique for medical mycology is presented. This involves the use of an appropriate paper carrier impregnated with suitable concentrations of cycloheximide and chloramphenicol. The carrier, when properly placed on the surface of an agar medium inoculated with a mixed-flora specimen, favors the isolation of a wide spectrum of pathogenic and opportunistic fungi. Evaluation systems for the technique included artificial mixtures of bacteria and fungi, pooled clinical materials artificially seeded with fungi, and routine clinical laboratory specimens. Test fungi were grouped according to a selection pattern based on the extent of their outgrowth within the bacterial inhibition zone (BIZ) established by the carrier on a battery of media. Two main advantages of the antibiotics-impregnated carrier system over the antibiotics-fortified medium system are: (1) the diffusion gradient of cycloheximide allows for the selective outgrowth throughout the BIZ of pathogenic fungi resistant to it, and allows the more susceptible fungal pathogens and opportunists to grow out up to their levels of tolerance to the antibiotic within the BIZ; (2) the system is economically advantageous.