In vitro studies of aculeacin A, a new antifungal antibiotic.

Aculeacin A is a cyclopeptide-containing long-chain fatty acid, representing a new class of antibiotics. It has a relatively narrow antifungal spectrum in vitro and is highly active against some groups of yeasts. Of 31 strains of Candida ad Torulopsis tested, the majority were susceptible to aculeacin A at 0.31 micrograms per ml or less. On the other hand, the antibiotic was scarcely active or inactive against other yeasts, such as Cryptococcus neoformans, and all filamentous and dimorphic fungi tested. A distinct inoculum effect has been observed in vitro with a number of strains of C. albicans; minimum growth-inhibitory concentrations (MIC) have tended to increase with increased incubation time. MIC values were also increased in the presence of serum. Aculeacin A is fungicidal for growing cells of C. albicans. It was most lethal against sensitive yeasts at 0.08 to 0.31 micrograms per ml, and increases in the concentrations of the drug above this range reduced, rather than increased, its lethal effect.