RT Journal Article SR Electronic T1 Cell-to-cell spread of microsporidia causes C. elegans organs to form syncytia JF bioRxiv FD Cold Spring Harbor Laboratory SP 053181 DO 10.1101/053181 A1 Keir M. Balla A1 Robert J. Luallen A1 Malina A. Bakowski A1 Emily R. Troemel YR 2016 UL http://biorxiv.org/content/early/2016/05/13/053181.abstract AB The growth of pathogens is dictated by their interactions with the host environment. Many obligate intracellular pathogens undergo several cellular decisions as they progress through their life cycles inside of host cells. We studied this process for several species of microsporidia in the genus Nematocida in their co-evolved animal host Caenorhabditis elegans. We found that microsporidia can restructure multicellular host tissues into a single contiguous multinucleate cell. In particular, we found that all three Nematocida species we studied were able to spread across the cells of C. elegans tissues before forming spores, with two species causing syncytial formation in the intestine, and one species causing syncytial formation in the muscle. We also found that the decision to switch from replication to differentiation in N. parisii was altered by the density of infection, suggesting that environmental cues influence the dynamics of the pathogen life cycle. These findings show how microsporidia can maximize the use of host space for growth, and that environmental cues in the host can regulate a developmental switch in the pathogen.