Host Matters: Medicinal Leech Digestive-Tract Symbionts and Their Pathogenic Potential

Abstract

Digestive-tract microbiota exert tremendous influence over host health. Host-symbiont model systems are studied to investigate how symbioses are initiated and maintained, as well as to identify host processes affected by resident microbiota. The medicinal leech, Hirudo verbana, is an excellent model to address such questions owing to a microbiome that is consistently dominated by two species, Aeromonas veronii and Mucinivorans hirudinis, both of which are cultivable and have sequenced genomes. This review outlines current knowledge about the dynamics of the H. verbana microbiome. We discuss in depth the factors required for A. veronii colonization and proliferation in the leech crop and summarize the current understanding of interactions between A. veronii and its annelid host. Lastly, we discuss leech usage in modern medicine and highlight how leech-therapy associated infections, often attributable to Aeromonas spp., are of growing clinical concern due in part to an increased prevalence of fluoroquinolone resistant strains.

Keywords: Aeromonas; Hirudo; bacteroidetes; beneficial bacteria; digestive-tract symbiosis; leech therapy; mucinivorans.

FIGURE 1

Hirudo verbana Digestive Tract. Schematic of the leech digestive tract (modified from Nelson and Graf, 2012 and Maltz et al., 2014). The ingested blood meal is stored in the crop where it forms a highly viscous intraluminal fluid (ILF) consisting of densely packed erythrocytes (dark circles surrounded by autofluorescence, examples indicated with arrow heads in insets). Fluorescence in situ hybridization micrographs of the leech crop describe (A) thick layers of mucus (red arrows) near the crop epithelium (dashed line) that develop after feeding and (B) circulating hemocytes (blue arrows) within the ILF that contain bacterial cells (green arrows). DAPI (blue), sWGA (red), and EUB338 (green). Scale bars = 10 μm.