In vitro thermal tolerance of a hypervirulent lineage of Batrachochytrium dendrobatidis: Growth arrestment by elevated temperature and recovery following thermal treatment

ABSTRACT

Chytridiomycosis, an emerging infectious disease caused by Batrachochytrium dendrobatidis (Bd), poses a serious threat to amphibians. The thermal optimum of Bd is lower than that of most amphibians, providing an opportunity to cure infected individuals with elevated temperature. However, this approach presupposes detailed knowledge about the thermal tolerance of the fungus. To determine the temperature that may effectively reduce infection burdens in vivo, detailed in vitro studies are needed to characterize thermal tolerance of the fungus without complexities introduced by the species-specific characteristics of hosts’ immune systems. The aim of our study was to evaluate the thermal tolerance of a hypervirulent isolate of Bd, considering the limits of its thermal tolerance and its capacity to rebound following heat treatment. We incubated Bd cell cultures at five different temperatures (21, 25.5, 27, 29, or 30.5 C) for one of six exposure durations (3, 4, 5, 6, 7, or 8 days) and subsequently counted the number of zoospores to assess the temperature dependence of Bd growth. We observed intensive Bd growth at 21 C. At 25.5 C, the number of zoospores also increased over time, but the curve plateaued at about half of the maximum values observed in the lower temperature treatment. At temperatures of 27 C and above, the fungus showed no measurable growth. However, when we moved the cultures back to 21 C after the elevated temperature treatments, we observed recovery of Bd growth in all cultures previously treated at 27 C. At 29 C, a treatment duration of 8 days was necessary to prevent recovery of Bd growth, and at 30.5 C a treatment duration of 5 days was needed to achieve the same result, revealing that these moderately elevated temperatures applied for only a few days have merely a fungistatic rather than a fungicidal effect under in vitro conditions.

KEYWORDS:

• Amphibia
• disinfection
• growth inhibition
• pathogen
• temperature dependence
• thermal optimum mismatch

ACKNOWLEDGMENTS

We thank T. W. J. Garner for providing the Bd isolate IA042. We are grateful to M. Z. Németh for help during zoospore counting and M. Szederkényi for assistance.

DISCLOSURE STATEMENT

No potential conflict of interest was reported by the author(s).

SUPPLEMENTARY MATERIAL

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Additional information

Funding

The research was supported by the European Commission under an FP7 Marie Curie Career Integration Grant (FP7-PEOPLE-2013-CIG: 631722); and the Hungarian Academy of Sciences (MTA) under the “Lendület” program (LP2012-24/2012) and the National Research, Development and Innovation Office (NKFIH) of Hungary (grant no. Nemzeti Kutatási Fejlesztési és Innovációs Hivatal K-124375). A.K. and U.J. were supported by the Ministry for Innovation and Technology, Hungary (ITM), under the ÚNKP-19-3 and ÚNKP 21-4 New National Excellence Program; A.H. was supported by MTA under a János Bolyai Scholarship (ÚNKP-19-4) and by the Ministry of Human Capacities, Hungary (EMMI), under the ÚNKP Bolyai+ Scholarship (ÚNKP-20-5).