AUTHOR=Liu Xiao-Ling , Zhao Heng , Wang Yi-Xin , Liu Xin-Ye , Jiang Yang , Tao Meng-Fei , Liu Xiao-Yong 

TITLE=Detecting and characterizing new endofungal bacteria in new hosts: Pandoraea sputorum and Mycetohabitans endofungorum in Rhizopus arrhizus

JOURNAL=Frontiers in Microbiology

VOLUME=15

YEAR=2024

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2024.1346252

DOI=10.3389/fmicb.2024.1346252

ISSN=1664-302X

ABSTRACT=<p>The fungus <italic>Rhizopus arrhizus</italic> (=<italic>R. oryzae</italic>) is commonly saprotrophic, exhibiting a nature of decomposing organic matter. Additionally, it serves as a crucial starter in food fermentation and can act as a pathogen causing mucormycosis in humans and animals. In this study, two distinct endofungal bacteria (EFBs), associated with individual strains of <italic>R. arrhizus</italic>, were identified using live/dead staining, fluorescence <italic>in situ</italic> hybridization, transmission electron microscopy, and 16S rDNA sequencing. The roles of these bacteria were elucidated through antibiotic treatment, pure cultivation, and comparative genomics. The bacterial endosymbionts, <italic>Pandoraea sputorum</italic> EFB03792 and <italic>Mycetohabitans endofungorum</italic> EFB03829, were purified from the host fungal strains <italic>R. arrhizus</italic> XY03792 and XY03829, respectively. Notably, this study marks the first report of <italic>Pandoraea</italic> as an EFB genus. Compared to its free-living counterparts, <italic>P. sputorum</italic> EFB03792 exhibited 28 specific virulence factor-related genes, six specific CE10 family genes, and 74 genes associated with type III secretion system (T3SS), emphasizing its pivotal role in invasion and colonization. Furthermore, this study introduces <italic>R. arrhizus</italic> as a new host for EFB <italic>M. endofungorum</italic>, with EFB contributing to host sporulation. Despite a visibly reduced genome, <italic>M. endofungorum</italic> EFB03829 displayed a substantial number of virulence factor-related genes, CE10 family genes, T3SS genes, mobile elements, and significant gene rearrangement. While EFBs have been previously identified in <italic>R. arrhizus</italic>, their toxin-producing potential in food fermentation has not been explored until this study. The discovery of these two new EFBs highlights their potential for toxin production within <italic>R. arrhizus</italic>, laying the groundwork for identifying suitable <italic>R. arrhizus</italic> strains for fermentation processes.</p>