The effects of temperature, pH, and salinity on the growth and dimorphism of Penicillium marneffei

Penicillium marneffei is an important thermal dimorphic fungus that is endemic to Southeast Asia and China and causes penicilliosis, an AIDS- defining disease. Dimorphic switching is considered an important growth characteristic associated with its pathogenicity. In recent years, the molecular mechanisms underlying both dimorphic switching and monomorphic growth have been studied. However, little is known about the physical and chemical factors that impact either dimorphic switching or monomorphic growth of this organism. Further, the natural history of the disease is unclear. Our experiments focus upon the effects of temperature, pH and salinity on both growth phases of P. marneffei. We compared 11 isolates of P. marneffei and found that all could grow at a wide temperature range ( 8.0 - 39.88 degrees C), but growth was dramatically inhibited at 40 degrees C. The morphological switch from hyphae to yeast growth was initiated at 32 degrees C. However, the sensitivity to elevated temperatures during this transition varied among isolates. Both hyphae and yeast growth forms grew much better at acidic ( pH 5, 6) and neutral pH than at alkaline conditions. While similar sensitivities were observed at high concentrations of NaCl and CaCl2, in general, yeast cells displayed a greater sensitivity to both compounds. Our data demonstrate that isolate differences occur in growth patterns. Importantly, the growth requirements defined in our study may shed light on the environmental conditions that favor its survival, a subject that is not completely resolved in the current literature.