The neurotropic black yeast Exophiala dermatitidis has a possible origin in the tropical rain forest

Abstract

The black yeast Exophiala dermatitidis is known as a rare etiologic agent of neurotropic infections in humans, occurring particularly in East and Southeast Asia. In search of its natural habitat, a large sampling was undertaken in temperate as well as in tropical climates. Sampling sites were selected on the basis of the origins of previously isolated strains, and on the basis of physiological properties of the species, which also determined a selective isolation protocol. The species was absent from outdoor environments in the temperate climate, but present at low abundance in comparable habitats in the tropics. Positive outdoor sites particularly included faeces of frugivorous birds and bats, in urban as well as in natural areas. Tropical fruits were found E. dermatitidis positive at low incidence. Of the human-made environments sampled, railway ties contaminated by human faeces and oily debris in the tropics were massively positive, while the known abundance of the fungus in steam baths was confirmed. On the basis of the species' oligotrophy, thermotolerance, acidotolerance, moderate osmotolerance, melanization and capsular yeast cells a natural life cycle in association with frugivorous animals in foci in the tropical rain forest, involving passage of living cells through the intestinal tract was hypothesized. The human-dominated environment may have become contaminated by ingestion of wild berries carrying fungal propagules.

Keywords: Black yeasts; Exophiala dermatitidis; frugivorous animals; human faeces; intestinal colonization; neurotropism.

Fig 1.

Map of Thailand showing the sampling sites.

Fig 2.

A. Temple complex at Chachaosao province, Thailand, with colony of flying foxes positive for E. dermatitidis; B. Roosting flying foxes; C. Faeces of flying foxes; D. Cage in the zoo of Chonburi province, Thailand, with indoor feeding area (E) positive for two genotypes of E. dermatitidis (dH 13132, dH 13133, dH 13134); F. Mountain at Nakornsawan province, Thailand, with numerous caves harboring insectivorous bats and surrounded by agricultural land; G. Collecting bat faeces in cave at Chiengmai province, Thailand, with bat faeces (insert); H: Hala-Bala Wildlife Sanctuary, Narathiwat province, Thailand, with hornbill nesting sites positive for dH 13183; I. Public hot sping in Chiangmai province, Thailand, positive for dH 13145; J. Public toilet in Chiangmai province, Thailand; K. Squat toilet in public toilet; L. Railway ties in Prachinburi province, Thailand, with collection sites (a) outside ties contaminated with petroleum oil, and (b) between ties (arrows).