Characterizing in vitro spherule morphogenesis of multiple strains of both species of Coccidioides

Abstract

The disease San Joaquin Valley Fever (coccidioidomycosis) is caused by the inhalation of Coccidioides arthroconidia. In vivo, arthroconidia transform into pathogenic structures termed spherules. Exposure to the host milieu triggers spherule development; however, the molecular mechanisms responsible for the morphological shift are not well characterized. This study compared the morphogenesis of five strains of both species of Coccidioides in two media types to improve the in vitro model of dimorphism that can be easily reproduced, and is amenable to tissue culture. We also sought to establish a modern record of the morphological switch among commonly used lab strains through a detailed account of growth under various conditions. Spherules from five strains were grown in standard (Converse) and experimental media (RPMI-sph). Strain behavior was quantified by median spherule size and spherule concentration, beginning 3 days after inoculation and followed for 10 days of growth. There were significant differences observed among Coccidioides immitis and C. posadasii strains, as well as differences between the in vitro systems.

Population Structure and Genetic Diversity among Isolates of Coccidioides posadasii in Venezuela and Surrounding Regions

“…Besides showing a highly differentiated genome, C. posadasii grows more slowly than C. immitis at high concentrations of salt (22). Additionally, spherules of C. posadasii appear to develop asynchronously compared to C. immitis (23). There are also broad differences in the geographic distribution of the two species.…”

Proper Care and Feeding of Coccidioides: A Laboratorian's Guide to Cultivating the Dimorphic Stages of C. immitis and C. posadasii

“…Commonly used and cited laboratory strains are C. posadasii C735 and Silveira (NR-48944) and C. immitis RS (NR-48942) and 2006 (NR-48934). We also recommend the attenuated strain C. posadasii cts2/ ard1/ cts3 , which can be handled in a biosafety level 2 (BSL-2) environment and has been extensively characterized in vitro and in vivo (Hung, del Pilar Jimenez-Alzate, et al, 2014;Hung, Zhang, et al, 2018;Hung, Castro-Lopez, & Cole, 2014;Hung, Gonzalez, Wuthrich, Klein, & Cole, 2011;Mead, Roe, et al, 2020;Mead, Teixeira, Galgiani, & Barker, 2019;Xue et al, 2009). Isolates arrive as a 1-ml glycerol stock that should be stored at -80°C until use.…”

Early Events in Coccidioidomycosis

“…1). As Mead and others wrote in 2018, "the molecular mechanisms that initiate the morphological switch from a saprobic to a parasitic phase are not understood" (29). The obstacles to understanding these mechanisms include (i) applying in vitro models to in vivo processes, (ii) limitations of applying animal models (primarily murine) to the human disease process (14), (iii) difficulties in studying the early infective process in humans (30), and (iv) the risk of attempting to ascribe biological processes studied in other fungi to Coccidioides.…”