The Physiology of the Medicinal Leech and its Digestive-Tract Microbiome.

摘要

Most animal digestive tracts are colonized by complex microbial communities making it difficult to study the molecular interactions underlying the symbiosis. Due to its simple gut microbiome, the medicinal leech serves as a naturally occurring model for studying digestive-tract associations. Two symbionts, Aeromonas veronii and a Rikenella- like bacterium, dominate the microbial community within the crop, the largest compartment of the leech digestive tract. The goal of this work was to use high-throughput sequencing of RNA transcripts to identify genes expressed by the host and the dominant members of the crop microbiome during the course of the symbiosis. By identifying expressed genes one is able to form testable hypotheses about the physiologies of both host and symbiont.;A temporal analysis of host and symbiont transcriptomes revealed the symbionts' nutrient sources. The most abundant, yet uncultured Rikenella- like bacterium foraged host mucin, despite the recent influx of a nutrient-rich blood meal. These data were used to culture the dominant symbiont on a mucin-based medium, which further facilitated its characterization including genome sequencing. Data also revealed that A. veronii utilized sialic acid, fatty acids and arginine in succession.;The temporal transcriptomes indicated that A. veronii experienced extracytoplasmic stress during all time points examined. Interestingly, the host transcriptome data suggested that the potential source of this stress could be antimicrobial peptides and lipopolysaccharide binding/bactericidal permeability increasing proteins. Several genes encoding these proteins are down-regulated after feeding, a time when the symbionts are increasing in cell number. Preliminary data suggest that the signal triggering this down regulation is related to ingestion of a blood meal. However, further experiments are needed to identify the mechanism underlying the down-regulation of the innate immunity genes.;Results of this work have enhanced the utility of the medicinal leech as a model organism because it has resulted in the isolation and genome sequencing of the Rikenella-like bacterium, construction of an expressed sequence tag library for Hirudo verbana and provided insight into physiologies of the symbiotic partners. Finally, discoveries from this study may be important in other host-microbe associations and may serve as hypotheses that can be tested in more complex systems.