Peptone glucose fluconazole agar, a selective medium for rapid and enhanced isolation of Aspergillus fumigatus from aqueous suspensions and sputum seeded with Candida albicans

Abstract

We report here the effect of fluconazole (5 μg/ml) incorporated in peptone glucose agar (PGA) on isolation of Aspergillus fumigatus from aqueous suspensions and sputum specimens seeded with predetermined conidial inoculum and graded concentrations of Candida albicans yeast cells. The recovery of A. fumigatus from aqueous suspensions containing C. albicans was markedly higher on peptone glucose fluconazole agar (PGFA) than on PGA. This was evident from the manifold higher count of A. fumigatus colonies emerging on the former medium at 4 days of incubation at 28°C. Also, growth of A. fumigatus on PGFA appeared at 3 days of incubation with some of its colonies already exhibiting pigmentation due to sporulation. In strong contrast, A. fumigatus showed no visible growth at this stage on PGA, barring some incipient growth rarely seen when the mixed inoculum of A. fumigatus and C. albicans had the lowest tested population density ratio of 1 : 10. The data further showed that the ability of fluconazole to yield enhanced recovery of A. fumigatus was limited by the population density of C. albicans in the mixed aqueous suspensions, i.e. the higher the C. albicans population vis-à -vis that of A. fumigatus, the greater the inhibition of the latter and lesser the effectiveness of fluconazole to yield its enhanced isolation through suppression of C. albicans. The greater efficacy of PGFA than that of PGA was further supported by the higher recovery on the former medium of A. fumigatus from sputum specimens experimentally seeded with the two antagonistic fungi. The superiority of PGFA to PGA for isolation of A. fumigatus is attributed to the higher susceptibility of C. albicans than that of A. fumigatus to the inhibitory effect of fluconazole. Based upon these observations, use of PGFA is suggested as a selective medium for rapid and enhanced isolation of A. fumigatus from sputum specimens heavily colonized by C. albicans.