Elucidating the structure, and composition of bacterial symbionts in the gut regions of wood-feeding termite, Coptotermes formosanus and their functional profile towards lignocellulolytic systems

Abstract

The wood-feeding termite, Coptotermes formosanus, presents an efficient lignocellulolytic system, offering a distinctive model for the exploration of host-microbial symbiosis towards lignocellulose degradation. Despite decades of investigation, understanding the diversity, community structure, and functional profiles of bacterial symbionts within specific gut regions, particularly the foregut and midgut of C. formosanus, remains largely elusive. In light of this knowledge gap, our efforts focused on elucidating the diversity, community composition and functions of symbiotic bacteria inhabiting the foregut, midgut, and hindgut of C. formosanus via metagenomics. The termite harbored a diverse community of bacterial symbionts encompassing 352 genera and 26 known phyla, exhibiting an uneven distribution across gut regions. Notably, the hindgut displayed a higher relative abundance of phyla such as Bacteroidetes (56.9%) and Spirochetes (23.3%). In contrast, the foregut and midgut were predominantly occupied by Proteobacteria (28.9%) and Firmicutes (21.2%) after Bacteroidetes. The foregut harbored unique phyla like Candidate phylum_TM6 and Armatimonadetes. At the family level, Porphyromonadaceae (28.1, 40.6, and 53.5% abundance in foregut, midgut, and hindgut, respectively) and Spirochaetaceae (foregut = 9%, midgut = 16%, hindgut = 21.6%) emerged as dominant families in the termite's gut regions. Enriched operational taxonomic units (OTUs) were most abundant in the foregut (28), followed by the hindgut (14), while the midgut exhibited enrichment of only two OTUs. Furthermore, the functional analyses revealed distinct influences of bacterial symbionts on various metabolic pathways, particularly carbohydrate and energy metabolisms of the host. Overall, these results underscore significant variations in the structure of the bacterial community among different gut regions of C. formosanus, suggesting unique functional roles of specific bacteria, thereby inspiring further investigations to resolve the crosstalk between host and microbiomes in individual gut-regions of the termite.

Keywords: bacterial diversity; gut-regions; high-throughput sequencing; microhabitats; symbiotic functions; termites.

Figure 1

A schematic overview of the methodologies and algorithms used for the processing of the metagenomic data related to the gut microbial communities of C. formosanus.

Figure 2

Summary of the overall processing of taxon tags for species annotation to operation taxonomic units (OTU).

Figure 3

Venn diagram depicting the number of unique and shared operational taxonomic units (OTUs) between the gut-regions of wood-feeding lower termite, C. formosanus.

Figure 4

Comparison of alpha diversity (A: Sob index, B: Shannon index, C: phylogenetic diversity index, and D: ACE index) of the bacterial communities of C. formosanus between Foregut, midgut and hindgut regions (Tukey’s HSD test, *p ≤ 0.05 and **p ≤ 0.01).

Figure 5

Similarity between the foregut, midgut and hindgut bacterial communities harbored by C. formosanus. Analysis of similarity (ANOSIM) box plot comparing the phylogenetic distances of the bacterial communities between the gut-regions of C. formosanus based on unweighted UniFrac distance.

Figure 6

Similarity in the composition of bacterial community in three gut-regions of the termite, C. formosanus. (A) Representation of the UPGMA-based clustering dendrogram showing the distance matrix information between the samples while (B,C) depict principal coordinate analysis and nonmetric multidimensional scaling analyses of the bacterial communities in the gut system of the termite.

Figure 7

Taxonomic profiles of gut bacterial communities in the gut system of C. formosanus. A Cluster dendrogram calculated from the Bray–Curtis dissimilarity in bacterial community structure based on 16S rRNA gene amplicon sequencing, together with the distribution of OTUs into bacterial phyla and heat map representation of relative abundance of dominant OTUs from 4 major bacterial phyla in gut-regions of C. formosanus. ANOSIM R was equal to 0.877 with p < 0.05.

Figure 8

Region-wise distribution and abundance of the top 10 bacterial families in the gut system of wood-feeding termite, C. formosanus.

Figure 9

Genera-wise distribution and relative abundance of the bacteria in the gut-regions of the termite.

Figure 10

(A) Ternary plot demonstrating the enriched OTU particular to the gut region of the termite. (B) Phylogenetic tree revealing the taxonomic classification and relatedness of the enriched OTUs drawn on the basis of metagenomic sequences of the bacterial community in the gut-regions of the C. formosanus. Only nodes with bootstrap values based on 1,000 replicates >90% (●) and >50% (ο) are marked.

Figure 11

Putative functional profile of the bacterial symbionts in the gut of C. formosanus. (A) Metabolic pathway information of the bacterial symbionts based on KEGG prediction and KO abundance. (B) Heatmap of the relative abundance of modules related to carbohydrate metabolism identified in prokaryotic microbiomes of C. formosanus. *p ≤ 0.05 indicates statistical significance of particular functions of bacteria between gut-regions based on Tukey HSD test. The abbreviations FG, MG, and HG denote the 3 sample replicates each of the foregut, midgut, and hindgut regions of the termite under consideration.

Figure 12

A proposed model for region-wise diversity and abundance of bacterial symbionts in the gut system of wood-feeding termite, Coptotermes formosanus, where they perform a variety of functions for the host in a collaborative fashion.