Macrobdella decora : Old World Leech Gut Microbial Community Structure Conserved in a New World Leech

Abstract

Leeches are found in terrestrial, aquatic, and marine habitats on all continents. Sanguivorous leeches have been used in medicine for millennia. Modern scientific uses include studies of neurons, anticoagulants, and gut microbial symbioses. Hirudo verbana, the European medicinal leech, maintains a gut community dominated by two bacterial symbionts, Aeromonas veronii and Mucinivorans hirudinis, which sometimes account for as much as 97% of the total crop microbiota. The highly simplified gut anatomy and microbiome of H. verbana make it an excellent model organism for studying gut microbial dynamics. The North American medicinal leech, Macrobdella decora, is a hirudinid leech native to Canada and the northern U.S.A. In this study we show that M. decora symbiont communities are very similar to those in H. verbana. We performed an extensive study using field-caught M. decora and purchased H. verbana from two suppliers. Deep sequencing of the V4 region of the 16S rRNA gene allowed us to determine the core microbiome of M. decora as consisting of Bacteroides, Aeromonas, Proteocatella, and Butyricicoccus. The analysis revealed that the composition of the gut microbiome of the two leech species was significantly different at all taxonomic levels. The R2 value was highest at the genus and ASV levels and much lower at phylum, class, and order taxonomic levels. The gut and bladder microbial communities were distinct. We propose that M. decora is an alternative to H. verbana for studies of wild-caught animals and provide evidence for the conservation of digestive-tract and bladder symbionts in annelid models. IMPORTANCE Building evidence implicates the gut microbiome in critical animal functions such as regulating digestion, nutrition, immune regulation, and development. Simplified, phylogenetically diverse models for hypotheses testing are necessary because of the difficulty of assigning causative relationships in complex gut microbiomes. Previous research used Hirudo verbana as a tractable animal model of digestive-tract symbioses. Our data show that Macrobdella decora may work just as well without the drawback of being an endangered organism and with the added advantage of easy access to field-caught specimens. The similarity of the microbial community structure of species from two different continents reveals the highly-conserved nature of the microbial symbionts in sanguivorous leeches.

Marine vampires: Persistent, internal associations between bacteria and blood-feeding marine annelids and crustaceans

“…Blood-feeding invertebrate species had a significantly lower microbiome diversity (average Shannon index for each species was 0.5–1.5, at the 99% similarity 16S rRNA gene level; Supplementary Table S1 ) compared to non-blood-feeding invertebrates, swabs of fish prey species, and surrounding seawater (average Shannon index of 1.9–2.5; ANOVA p < 0.0001). In past studies, comparatively lower microbial diversity has been observed for other OBF taxa, including the freshwater leech genera Macrobdella and Hirudo ( Graf et al, 2006 ; McClure et al, 2021 ). One exception in this study was the copepod Lernanthropus with a slightly higher diversity (average Shannon index of 1.8 ± 0.4; Supplementary Table S1 ), comparable to the least diverse environmental sample.…”

“…For Branchellion , an undescribed gammaproteobacteria and betaproteobacteria co-occurred with Vibrio , each comprising ~15%–17% of recovered ribotypes ( Figure 2 ; Supplementary Table S3 ). The maintenance of a gut community dominated by only 2–3 bacterial taxa has been observed for both Macrobdella and numerous hirudinid leeches ( Graf et al, 2006 ; Kikuchi and Graf, 2007 ; Laufer et al, 2008 ; McClure et al, 2021 ). Many of the undescribed secondary bacterial ribotypes associated with the marine OBF taxa were dissimilar from known bacteria (< 90% 16S rRNA gene identity; Supplementary Figure S2 ), thus likely representing novel taxa.…”

“…Frontiers in Microbiology 06 frontiersin.org (Graf et al, 2006;Kikuchi and Graf, 2007;Laufer et al, 2008;McClure et al, 2021). Many of the undescribed secondary bacterial ribotypes associated with the marine OBF taxa were dissimilar from known bacteria (< 90% 16S rRNA gene identity; Supplementary Figure S2), thus likely representing novel taxa.…”

Draft Genome Sequence of Plesiomonas shigelloides MD22D9, Isolated from the Digestive Tract of Macrobdella decora

“… Plesiomonas shigelloides is a Gram-negative, facultative anaerobe belonging to the family Enterobacteriaceae and is commonly found in brackish and freshwater ecosystems ( 1 , 2 ). While P. shigelloides is a known opportunistic animal and human pathogen ( 2 ), its role in the medicinal leech is unknown ( 3 – 8 ). The gut microbiomes of sanguivorous leeches are hypothesized to be critical for maintaining host fitness and overcoming challenges of feeding exclusively on blood ( 9 , 10 ).…”

Dysbiosis of intestinal homeostasis contribute to Whitmania pigra edema disease

“…Compared with vertebrates, leeches and other annelids are regarded as the beginning of senior invertebrates, with a complex digestive system and circulatory system replaced by a haemocoelomic system (Mayr, 1969 ). Although reports have focused on the gut microbiota of several species of leech, including Hirudo verbena , Hirudo orientalis , Hirudo medicinalis , Hirudo nipponia Whitman , and Poecilobdella manillensis , little is known about the linkage of gut microbes with the tissue homeostasis and fitness of the host (Graf et al, 2006 ; Laufer et al, 2008 ; Maltz et al, 2014 ; McClure et al, 2021 ; Meng et al, 2022 ; Neupane et al, 2019 ; Shi et al, 2021 ; Whitaker et al, 2014 ; Worthen et al, 2006 ). Recently, Drosophila melanogaster and Caenorhabditis elegans have been cited as excellent models for investigating the genetic underpinnings of host–microbiota, which implies that the physiological regulation of gut microbes is equally important in primeval animals (Dierking & Pita, 2020 ; Lee & Lee, 2014 ).…”