Genome-wide evidence of positive selection in Bacteroides fragilis

Comparative Evolutionary Genomics Reveals Genetic Diversity and Differentiation in Bacteroides fragilis

BACKGROUND/OBJECTIVES

Bacteroides fragilis is the pathogenic anaerobe most commonly isolated from intra-abdominal infections, abscesses, and blood. Despite its clinical importance, research on its pan-genome-scale evolution is still limited.

METHODS

Herein, we analyzed the pan-genome architecture of 374 B. fragilis strains to explore their intra-species genomic diversity and evolutionary patterns.

RESULTS

Our analysis revealed an open pan-genome with a high proportion of accessory genomes, indicating high genetic variability. Accessory genome genes were substantially enriched in the functions of "Replication, Recombination, and Repair" suggesting their roles in gene transfer and divergence. Phylogenomic analysis divided B. fragilis into two distinct clades: divisions I and II, differing in gene content, antimicrobial resistance genes, and mobile genetic elements. Division II revealed higher Tajima's D values, suggesting that it separated after B. fragilis's recent species diversification. The extreme shift in the distribution of gene-wise Hudson's fixation index (Fst) values for each division suggested that several genes are highly differentiated or evolved between the two clades. Average nucleotide identity and 16S rRNA analyses showed that B. fragilis division II represents a distinct species, Bacteroides hominis. Additionally, a considerable depletion of recombination in genes with Fst values > 0.99 was noted, suggesting that the highest Fst genes with little recombination are the basis for differentiation between divisions.

CONCLUSIONS

Overall, this study enhances the understanding of B. fragilis's genomic diversity, evolutionary dynamics, and potential role in pathogenesis, shedding light on its adaptation and diversification.

Genome-wide analyses reveal genes subject to positive selection in Toxoplasma gondii

Toxoplasma gondii is an important protozoan pathogen that infects many wild and domestic animals and causes infections in immunocompromised humans. However, there has been little investigation of the molecular evolutionary trajectories of this pathogenic protozoa using comparative genomics data. Here, we employed a comparative evolutionary genomics approach to identify genes that are under site- and lineage-specific positive selection in nine strains of T. gondii, including two closely related species, Neospora caninum and Hammondia hammondi. Based on the analyses of five coccidian core genomes, 4.5% of the 5788 core genome genes showed strong signals for positive selection in the site model. In addition, the branch-site model analyses in the nine T. gondii core genomes indicated that 2 to 20 genes underwent significant positive selection along each lineage leading to T. gondii strains. Many of the protein products encoded by the positively selected genes are secretory or surface proteins that have previously been implicated in host pathogenesis. The adaptive changes in these positively selected genes might be related to dynamic interactions between the host immune systems and might play a crucial role in the infection and pathogenic processes of T. gondii.

Genomic analysis of Leptospira interrogans serovar Paidjan and Dadas isolates from carrier dogs and comparative genomic analysis to detect genes under positive selection

BACKGROUND

Leptospirosis is an emerging infectious disease worldwide that can cause high morbidity and mortality rates in humans and animals. The causative spirochetes have reservoirs in mammalian hosts, but there has been limited analysis of the genomes of isolates recovered from animals. The aims of this study were to characterize genomic features of two Leptospira interrogans strains recently isolated from asymptomatic dogs in Thailand (strains CUDO5 and CDUO8), and to perform comparative genome analyses with other strains. Molecular adaptive evolution in L. interrogans as signaled by positive selection also was analyzed.

RESULTS

Whole genome sequence analysis revealed that strains CUDO5 and CUDO8 had genome sizes of approximately 4.9 Mbp with 35.1% GC contents. Using monoclonal antibodies, strains CUDO5 and CUDO8 were identified as serovars Paidjan and Dadas, respectively. These strains harbored genes known to be associated with acute and chronic infections. Using Single Nucleotide Polymorphisms phylogeny (SNPs) with 97 L. interrogans strains, CUDO5 and CUDO8 had closest genetic relatedness with each other. Nevertheless, the serovar determinant region (rfb locus) showed variations in the genes encoding sugar biosynthesis. Amongst 13 representative L. interrogans strains examined for molecular adaptive evolution through positive selection under the site-model of Phylogenetic Analysis of Maximum Likelihood, genes responsible for iron acquisition (tlyA and hbpA), motility (fliN2, flgK, and flhB) and thermal adaptation (lpxD1) were under increased selective pressure.

CONCLUSIONS

L. interrogans serovar Paidjan strain CUDO5 and serovar Dadas strain CUDO8 had close genetic relatedness as analyzed by SNPs phylogeny. They contained genes with established roles in acute and chronic leptospirosis. The rfb locus in both serovars showed gene variation associated with sugar biosynthesis. Positive selection analysis indicated that genes encoding factors involved in motility, temperature adaptation, and iron acquisition were under strong positive selection in L. interrogans. These may be associated with adaptation in the early stages of infection.

Among older adults, age-related changes in the stool microbiome differ by HIV-1 serostatus

Background

HIV-1 infection and physiological aging are independently linked to elevated systemic inflammation and changes in enteric microbial communities (dysbiosis). However, knowledge of the direct effect of HIV infection on the aging microbiome and potential links to systemic inflammation is lacking.

Methods

In a cross-sectional study of older people living with HIV (PLWH) (median age 61.5 years, N = 14) and uninfected controls (median 58 years, n = 22) we compared stool microbiota, levels of microbial metabolites (short-chain fatty acid levels, SCFA) and systemic inflammatory biomarkers by HIV serostatus and age.

Findings

HIV and age were independently associated with distinct changes in the stool microbiome. For example, abundances of Enterobacter and Paraprevotella were higher and Eggerthella and Roseburia lower among PLWH compared to uninfected controls. Age-related microbiome changes also differed by HIV serostatus. Some bacteria with inflammatory potential (e.g. Escherichia) increased with age among PLWH, but not controls. Stool SCFA levels were similar between the two groups yet patterns of associations between individual microbial taxa and SCFA levels differed. Abundance of various genera including Escherichia and Bifidobacterium positively associated with inflammatory biomarkers (e.g. soluble Tumor Necrosis Factor Receptors) among PLWH, but not among controls.

Interpretation

The age effect on the gut microbiome and associations between microbiota and microbial metabolites or systemic inflammation differed based on HIV serostatus, raising important implications for the impact of therapeutic interventions, dependent on HIV serostatus or age.

A new collagenase enzyme of the marine sponge pathogen Pseudoalteromonas agarivorans NW4327 is uniquely linked with a TonB dependent receptor

The primary pathogen of the Great Barrier Reef sponge Rhopaloeides odorabile, recently identified as a novel strain (NW4327) of Pseudoalteromonas agarivorans, produced collagenase which degraded R. odorabile skeletal fibers. We now report the collagenase of P. agarivorans as a metalloprotease which required Ca²⁺ and Zn²⁺ as cofactors. The collagenase was a TonB dependent receptor (TBDR) having a carboxypeptidase regulatory like domain (CRLD) in the N-terminal along with an outer membrane (OM) channel superfamily domain. The genes for TBDR sub-components and collagenase formed one unified entity in the genome of P. agarivorans NW4327. This association of a collagenase with a TBDR distinguished it from all known functional collagenases till date and for the first time, established the enzymatic capability of TBDRs. Predicted TBDR model demonstrated only 15% identity with ferripyoverdin receptor and the CRLD displayed merely 24% identity with carboxypeptidase catalytic chain. Presence of signal peptide, lack of transmembrane helices, absence of N-terminal in the cytoplasmic side, extracellular localization and recovery from the culture supernatant implicated that the TBDR was secreted. Stronger binding of the collagenase with marine sponge type IV collagen than type I collagen, revealed through molecular docking, indicated higher specificity of the enzyme towards type IV collagen.

Bacteroides fragilis in biopsies of patients with major abscesses and diabetic foot infections: direct molecular versus culture-based detection

Direct determination by pathogen-specific real-time PCR assay for Bacteroides fragilis was compared to culture in major abscess and diabetic foot infection biopsy samples. Real-time PCR resulted in an increased detection rate of 12% for B. fragilis and could improve the detection of B. fragilis in clinical samples.

Molecular Evolution of the TET Gene Family in Mammals

Ten-eleven translocation (TET) proteins, a family of Fe²⁺- and 2-oxoglutarate-dependent dioxygenases, are involved in DNA demethylation. They also help regulate various cellular functions. Three TET paralogs have been identified (TET1, TET2, and TET3) in humans. This study focuses on the evolution of mammalian TET genes. Distinct patterns in TET1 and TET2 vs. TET3 were revealed by codon-based tests of positive selection. Results indicate that TET1 and TET2 genes have experienced positive selection more frequently than TET3 gene, and that the majority of codon sites evolved under strong negative selection. These findings imply that the selective pressure on TET3 may have been relaxed in several lineages during the course of evolution. Our analysis of convergent amino acid substitutions also supports the different evolutionary dynamics among TET gene subfamily members. All of the five amino acid sites that are inferred to have evolved under positive selection in the catalytic domain of TET2 are localized at the protein’s outer surface. The adaptive changes of these positively selected amino acid sites could be associated with dynamic interactions between other TET-interacting proteins, and positive selection thus appears to shift the regulatory scheme of TET enzyme function.