An intestinally secreted host factor limits bacterial colonization but promotes microsporidia invasion of C. elegans

Microsporidia are ubiquitous obligate intracellular pathogens of animals. These parasites often infect hosts through an oral route, but little is known about the function of host intestinal proteins that facilitate microsporidia invasion. To identify such factors necessary for infection by Nematocida parisii, a natural microsporidian pathogen of Caenorhabditis elegans, we performed a forward genetic screen to identify mutant animals that have a Fitness Advantage with Nematocida (Fawn). We isolated four fawn mutants that are resistant to Nematocida infection and contain mutations in T14E8.4, which we renamed aaim-1 (Antibacterial and Aids invasion by Microsporidia). Expression of AAIM-1 in the intestine of aaim-1 animals restores N. parisii infectivity and this rescue of infectivity is dependent upon AAIM-1 secretion. N. parisii spores in aaim-1 animals are improperly oriented in the intestinal lumen, leading to reduced levels of parasite invasion. Conversely, aaim-1 mutants display both increased colonization and susceptibility to the bacterial pathogen Pseudomonas aeruginosa and overexpression of AAIM-1 reduces P. aeruginosa colonization. Competitive fitness assays show that aaim-1 mutants are favoured in the presence of N. parisii but disadvantaged on P. aeruginosa compared to wild type animals. Together, this work demonstrates how microsporidia exploits an antibacterial immune protein to promote host invasion. Our results also highlight the evolutionary trade-offs that exist to optimizing host defense against multiple classes of pathogens.