Pan-Genome Analysis Links the Hereditary Variation of Leptospirillum ferriphilum With Its Evolutionary Adaptation

Emergence of Raoultella ornithinolytica in human infections from different hospitals in Ecuador with OXA-48-producing resistance

Purpose

The purpose of this study was to highlight the clinical and molecular features of 13 Raoultella ornithinolytica strains isolated from clinical environments in Ecuador, and to perform comparative genomics with previously published genomes of Raoultella spp. As Raoultella is primarily found in environmental, clinical settings, we focused our work on identifying mechanisms of resistance that can provide this bacterium an advantage to establish and persist in hospital environments.

Methods

We analyzed 13 strains of Raoultella ornithinolytica isolated from patients with healthcare associated infections (HAI) in three hospitals in Quito and one in Santo Domingo de Los Tsáchilas, Ecuador, between November 2017 and April 2018. These isolates were subjected to phenotypic antimicrobial susceptibility testing, end-point polymerase chain reaction (PCR) to detect the presence of carbapenemases and whole-genome sequencing.

Results

Polymerase chain reaction revealed that seven isolates were positive isolates for blaOXA-48 and one for blaKPC-2 gene. Of the seven strains that presented the blaOXA-48 gene, six harbored it on an IncFII plasmid, one was inserted into the bacterial chromosome. The blaKPC gene was detected in an IncM2/IncR plasmid. From the bioinformatics analysis, nine genomes had the gene blaOXA-48, originating from Ecuador. Moreover, all R. ornithinolytica strains contained the ORN-1 gene, which confers resistance for β-lactams, such as penicillins and cephalosporins. Comparative genome analysis of the strains showed that the pangenome of R. ornithinolytica is considered an open pangenome, with 27.77% of core genes, which could be explained by the fact that the antibiotic resistance genes in the ancestral reconstruction are relatively new, suggesting that this genome is constantly incorporating new genes.

Conclusion

These results reveal the genome plasticity of R. ornithinolytica, particularly in acquiring antibiotic-resistance genes. The genomic surveillance and infectious control of these uncommon species are important since they may contribute to the burden of antimicrobial resistance and human health.

Trade-off in genome turnover events leading to adaptive evolution of Microcystis aeruginosa species complex

BACKGROUND

Numerous studies in the past have expanded our understanding of the genetic differences of global distributed cyanobacteria that originated around billions of years ago, however, unraveling how gene gain and loss drive the genetic evolution of cyanobacterial species, and the trade-off of these evolutionary forces are still the central but poorly understood issues.

RESULTS

To delineate the contribution of gene flow in mediating the hereditary differentiation and shaping the microbial evolution, a global genome-wide study of bloom-forming cyanobacterium, Microcystis aeruginosa species complex, provided robust evidence for genetic diversity, reflected by enormous variation in gene repertoire among various strains. Mathematical extrapolation showed an 'open' microbial pan-genome of M. aeruginosa species, since novel genes were predicted to be introduced after new genomes were sequenced. Identification of numerous horizontal gene transfer's signatures in genome regions of interest suggested that genome expansion via transformation and phage-mediated transduction across bacterial lineage as an evolutionary route may contribute to the differentiation of Microcystis functions (e.g., carbohydrate metabolism, amino acid metabolism, and energy metabolism). Meanwhile, the selective loss of some dispensable genes at the cost of metabolic versatility is as a mean of adaptive evolution that has the potential to increase the biological fitness.

CONCLUSIONS

Now that the recruitment of novel genes was accompanied by a parallel loss of some other ones, a trade-off in gene content may drive the divergent differentiation of M. aeruginosa genomes. Our study provides a genetic framework for the evolution of M. aeruginosa species and illustrates their possible evolutionary patterns.

Insertion sequence contributes to the evolution and environmental adaptation of Acidithiobacillus

BACKGROUND

The genus Acidithiobacillus has been widely concerned due to its superior survival and oxidation ability in acid mine drainage (AMD). However, the contribution of insertion sequence (IS) to their biological evolution and environmental adaptation is very limited. ISs are the simplest kinds of mobile genetic elements (MGEs), capable of interrupting genes, operons, or regulating the expression of genes through transposition activity. ISs could be classified into different families with their own members, possessing different copies.

RESULTS

In this study, the distribution and evolution of ISs, as well as the functions of the genes around ISs in 36 Acidithiobacillus genomes, were analyzed. The results showed that 248 members belonging to 23 IS families with a total of 10,652 copies were identified within the target genomes. The IS families and copy numbers among each species were significantly different, indicating that the IS distribution of Acidithiobacillus were not even. A. ferrooxidans had 166 IS members, which may develop more gene transposition strategies compared with other Acidithiobacillus spp. What's more, A. thiooxidans harbored the most IS copies, suggesting that their ISs were the most active and more likely to transpose. The ISs clustered in the phylogenetic tree approximately according to the family, which were mostly different from the evolutionary trends of their host genomes. Thus, it was suggested that the recent activity of ISs of Acidithiobacillus was not only determined by their genetic characteristics, but related with the environmental pressure. In addition, many ISs especially Tn3 and IS110 families were inserted around the regions whose functions were As/Hg/Cu/Co/Zn/Cd translocation and sulfur oxidation, implying that ISs could improve the adaptive capacities of Acidithiobacillus to the extremely acidic environment by enhancing their resistance to heavy metals and utilization of sulfur.

CONCLUSIONS

This study provided the genomic evidence for the contribution of IS to evolution and adaptation of Acidithiobacillus, opening novel sights into the genome plasticity of those acidophiles.

A comprehensive genomic analysis provides insights on the high environmental adaptability of Acinetobacter strains

Acinetobacter is ubiquitous, and it has a high species diversity and a complex evolutionary pattern. To elucidate the mechanism of its high ability to adapt to various environment, 312 genomes of Acinetobacter strains were analyzed using the phylogenomic and comparative genomics methods. It was revealed that the Acinetobacter genus has an open pan-genome and strong genome plasticity. The pan-genome consists of 47,500 genes, with 818 shared by all the genomes of Acinetobacter, while 22,291 are unique genes. Although Acinetobacter strains do not have a complete glycolytic pathway to directly utilize glucose as carbon source, most of them harbored the n-alkane-degrading genes alkB/alkM (97.1% of tested strains) and almA (96.7% of tested strains), which were responsible for medium-and long-chain n-alkane terminal oxidation reaction, respectively. Most Acinetobacter strains also have catA (93.3% of tested strains) and benAB (92.0% of tested strains) genes that can degrade the aromatic compounds catechol and benzoic acid, respectively. These abilities enable the Acinetobacter strains to easily obtain carbon and energy sources from their environment for survival. The Acinetobacter strains can manage osmotic pressure by accumulating potassium and compatible solutes, including betaine, mannitol, trehalose, glutamic acid, and proline. They respond to oxidative stress by synthesizing superoxide dismutase, catalase, disulfide isomerase, and methionine sulfoxide reductase that repair the damage caused by reactive oxygen species. In addition, most Acinetobacter strains contain many efflux pump genes and resistance genes to manage antibiotic stress and can synthesize a variety of secondary metabolites, including arylpolyene, β-lactone and siderophores among others, to adapt to their environment. These genes enable Acinetobacter strains to survive extreme stresses. The genome of each Acinetobacter strain contained different numbers of prophages (0-12) and genomic islands (GIs) (6-70), and genes related to antibiotic resistance were found in the GIs. The phylogenetic analysis revealed that the alkM and almA genes have a similar evolutionary position with the core genome, indicating that they may have been acquired by vertical gene transfer from their ancestor, while catA, benA, benB and the antibiotic resistance genes could have been acquired by horizontal gene transfer from the other organisms.

Genotype to ecotype in niche environments: adaptation of Arthrobacter to carbon availability and environmental conditions

Niche environmental conditions influence both the structure and function of microbial communities and the cellular function of individual strains. The terrestrial subsurface is a dynamic and diverse environment that exhibits specific biogeochemical conditions associated with depth, resulting in distinct environmental niches. Here, we present the characterization of seven distinct strains belonging to the genus Arthrobacter isolated from varying depths of a single sediment core and associated groundwater from an adjacent well. We characterized genotype and phenotype of each isolate to connect specific cellular functions and metabolisms to ecotype. Arthrobacter isolates from each ecotype demonstrated functional and genomic capacities specific to their biogeochemical conditions of origin, including laboratory-demonstrated characterization of salinity tolerance and optimal pH, and genes for utilization of carbohydrates and other carbon substrates. Analysis of the Arthrobacter pangenome revealed that it is notably open with a volatile accessory genome compared to previous pangenome studies on other genera, suggesting a high potential for adaptability to environmental niches.

Bacillus pumilus Group Comparative Genomics: Toward Pangenome Features, Diversity, and Marine Environmental Adaptation

Background

Members of the Bacillus pumilus group (abbreviated as the Bp group) are quite diverse and ubiquitous in marine environments, but little is known about correlation with their terrestrial counterparts. In this study, 16 marine strains that we had isolated before were sequenced and comparative genome analyses were performed with a total of 52 Bp group strains. The analyses included 20 marine isolates (which included the 16 new strains) and 32 terrestrial isolates, and their evolutionary relationships, differentiation, and environmental adaptation.

Results

Phylogenomic analysis revealed that the marine Bp group strains were grouped into three species: B. pumilus, B. altitudinis and B. safensis. All the three share a common ancestor. However, members of B. altitudinis were observed to cluster independently, separating from the other two, thus diverging from the others. Consistent with the universal nature of genes involved in the functioning of the translational machinery, the genes related to translation were enriched in the core genome. Functional genomic analyses revealed that the marine-derived and the terrestrial strains showed differences in certain hypothetical proteins, transcriptional regulators, K⁺ transporter (TrK) and ABC transporters. However, species differences showed the precedence of environmental adaptation discrepancies. In each species, land specific genes were found with possible functions that likely facilitate survival in diverse terrestrial niches, while marine bacteria were enriched with genes of unknown functions and those related to transcription, phage defense, DNA recombination and repair.

Conclusion

Our results indicated that the Bp isolates show distinct genomic features even as they share a common core. The marine and land isolates did not evolve independently; the transition between marine and non-marine habitats might have occurred multiple times. The lineage exhibited a priority effect over the niche in driving their dispersal. Certain intra-species niche specific genes could be related to a strains adaptation to its respective marine or terrestrial environment(s). In summary, this report describes the systematic evolution of 52 Bp group strains and will facilitate future studies toward understanding their ecological role and adaptation to marine and/or terrestrial environments.

Description of Candidatus Mesopelagibacter carboxydoxydans and Candidatus Anoxipelagibacter denitrificans: Nitrate-reducing SAR11 genera that dominate mesopelagic and anoxic marine zones

The diverse and ubiquitous members of the SAR11 lineage (Alphaproteobacteria) represent up to 30-40% of the surface and mesopelagic oceanic microbial communities. However, the molecular and ecological mechanisms that differentiate closely related, yet distinct, SAR11 members that often co-occur under similar environmental conditions remain speculative. Recently, two mesopelagic and oxygen minimum zone (OMZ)-associated subclades of SAR11 (Ic and IIa.A) were described using single-cell amplified genomes (SAGs) linked to nitrate reduction in OMZs. In this current study, the collection of genomes belonging to these two subclades was expanded with thirteen new metagenome-assembled genomes (MAGs), thus providing a more detailed phylogenetic and functional characterization of these subclades. Gene content-based predictions of metabolic functions revealed similarities in central carbon metabolism between subclades Ic and IIa.A and surface SAR11 clades, with small variations in central pathways. These variations included more versatile sulfur assimilation pathways, as well as a previously predicted capacity for nitrate reduction that conferred unique versatility on mesopelagic-adapted clades compared to their surface counterparts. Finally, consistent with previously reported abundances of carbon monoxide (CO) in surface and mesopelagic waters, subclades Ia (surface) and Ic (mesopelagic) have the genetic potential to oxidize carbon monoxide (CO), presumably taking advantage of this abundant compound as an electron donor. Based on genomic analyses, environmental distribution and metabolic reconstruction, we propose two new SAR11 genera, Ca. Mesopelagibacter carboxydoxydans (subclade Ic) and Ca. Anoxipelagibacter denitrificans (subclade IIa.A), which represent members of the mesopelagic and OMZ-adapted SAR11 clades.

Genome-Wide Analysis Reveals Genetic Potential for Aromatic Compounds Biodegradation of Sphingopyxis

Members of genus Sphingopyxis are frequently found in diverse eco-environments worldwide and have been traditionally considered to play vital roles in the degradation of aromatic compounds. Over recent decades, many aromatic-degrading Sphingopyxis strains have been isolated and recorded, but little is known about their genetic nature related to aromatic compounds biodegradation. In this study, bacterial genomes of 19 Sphingopyxis strains were used for comparative analyses. Phylogeny showed an ambiguous relatedness between bacterial strains and their habitat specificity, while clustering based on Cluster of Orthologous Groups suggested the potential link of functional profile with substrate-specific traits. Pan-genome analysis revealed that 19 individuals were predicted to share 1,066 orthologous genes, indicating a high genetic homogeneity among Sphingopyxis strains. Notably, KEGG Automatic Annotation Server results suggested that most genes pertaining aromatic compounds biodegradation were predicted to be involved in benzoate, phenylalanine, and aminobenzoate metabolism. Among them, β-ketoadipate biodegradation might be the main pathway in Sphingopyxis strains. Further inspection showed that a number of mobile genetic elements varied in Sphingopyxis genomes, and plasmid-mediated gene transfer coupled with prophage- and transposon-mediated rearrangements might play prominent roles in the evolution of bacterial genomes. Collectively, our findings presented that Sphingopyxis isolates might be the promising candidates for biodegradation of aromatic compounds in pollution sites.

Insights into ecological roles and potential evolution of Mlr-dependent microcystin-degrading bacteria

Over decades many studies have focused on the biodegradation of microcystins (MCs), and some Mlr-dependent MC-degrading bacteria were recorded, but the ecological functions, metabolic traits, and potential evolution of these organisms remain poorly understood. In this study, 16S rRNA-based phylogeny unraveled a wide range of genetic diversity across bacterial lineage, accompanied by re-evaluation of taxonomic placement of some MC-degrading species. Genome-wide comparison showed that considerable genes unique in individual organisms were identified, suggesting genetic differentiation among these Mlr-dependent MC-degrading bacteria. Notably, analyses of metabolic profiles first revealed the presence of functional genes involved in phenylacetate biodegradation in the specialized genomic regions, and mlr gene cluster was located around the neighborhood. The identification of transposable elements further indicated that these genomic regions might undergo horizontal gene transfer events to recruit novel functionalities, suggesting an adaptive force driving genome evolution of these organisms. In short, phylogenetic and genetic content analyses of Mlr-dependent MC-degraders shed light on their metabolic potential, ecological roles, and bacterial evolution, and expand the understanding of ecological status of MCs biodegradation.

Whole genome sequence analysis reveals high genetic variation of newly isolated Acidithiobacillus ferrooxidans IO-2C

Acidithiobacillus ferrooxidans, a chemolithoautotrophic bacterium, is well known for its mineral oxidizing properties. The current study combines experimental and whole genome sequencing approaches to investigate an iron oxidizing, extreme acidophilic bacterium, A. ferrooxidans isolate (IO-2C) from an acid seep area near Carlos, TX, USA. Strain IO-2C was capable of oxidizing iron i.e. iron sulphate and iron ammonium sulphate yielding shwertmannite and jarosite minerals. Further, the bacterium's genome was sequenced, assembled and annotated to study its general features, structure and functions. To determine genetic heterogeneity, it was compared with the genomes of other published A. ferrooxidans strains. Pan-genome analysis displayed low gene conservation and significant genetic diversity in A. ferrooxidans species comprising of 6926 protein coding sequences with 23.04% (1596) core genes, 46.13% (3195) unique and 30.82% (2135) accessory genes. Variant analysis showed >75,000 variants, 287 of them with a predicted high impact, in A. ferrooxidans IO-2C genome compared to the reference strain, resulting in abandonment of some important functional key genes. The genome contains numerous functional genes for iron and sulphur metabolism, nitrogen fixation, secondary metabolites, degradation of aromatic compounds, and multidrug and heavy metal resistance. This study demonstrated the bio-oxidation of iron by newly isolated A. ferrooxidans IO-2C under acidic conditions, which was further supported by genomic analysis. Genomic analysis of this strain provided valuable information about the complement of genes responsible for the utilization of iron and tolerance of other metals.

Phylogeny, Divergent Evolution, and Speciation of Sulfur-Oxidizing Acidithiobacillus Populations

Background

Habitats colonized by acidophiles as an ideal physical barrier may induce genetic exchange of microbial members within the common communities, but little is known about how species in extremely acidic environments diverge and evolve.

Results

Using the acidophilic sulfur-oxidizer Acidithiobacillus as a case study, taxonomic reclassifications of many isolates provides novel insights into their phylogenetic lineage. Whole-genome-based comparisons were attempted to investigate the intra- and inter-species divergence. Recent studies clarified that functional and structural specificities of bacterial strains might provide opportunities for adaptive evolution responding to local environmental conditions. Acidophilic microorganisms play a key role in the acidification of natural waters and thus the formation of extremely acidic environments, and the feedbacks of the latter might confer the distinct evolutionary patterns of Acidithiobacillus spp. Varied horizontal gene transfer events occurred in different bacterial strains, probably resulting in the expansion of Acidithiobacillus genomes. Gene loss as another evolutionary force might cause the adaptive phenotypic diversity. A conceptual model for potential community-dependent evolutionary adaptation was thus proposed to illustrate the observed genome differentiation.

Conclusions

Collectively, the findings shed light on the phylogeny and divergent evolution of Acidithiobacillus strains, and provided a useful reference for evolutionary studies of other extremophiles.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5827-6) contains supplementary material, which is available to authorized users.

In Silico Genome-Wide Analysis Reveals the Potential Links Between Core Genome of Acidithiobacillus thiooxidans and Its Autotrophic Lifestyle

The coinage “pan-genome” was first introduced dating back to 2005, and was used to elaborate the entire gene repertoire of any given species. Core genome consists of genes shared by all bacterial strains studied and is considered to encode essential functions associated with species’ basic biology and phenotypes, yet its relatedness with bacterial lifestyle of the species remains elusive. We performed the pan-genome analysis of sulfur-oxidizing acidophile Acidithiobacillus thiooxidans as a case study to highlight species’ core genome and its relevance with autotrophic lifestyle of bacterial species. The mathematical modeling based on bacterial genomes of A. thiooxidans species, including a novel strain ZBY isolated from Zambian copper mine plus eight other recognized strains, was attempted to extrapolate the expansion of its pan-genome, suggesting that A. thiooxidans pan-genome is closed. Further investigation revealed a common set of genes, many of which were assigned to metabolic profiles, notably with respect to energy metabolism, amino acid metabolism, and carbohydrate metabolism. The predicted metabolic profiles of A. thiooxidans were characterized by the fixation of inorganic carbon, assimilation of nitrogen compounds, and aerobic oxidation of various sulfur species. Notably, several hydrogenase (H₂ase)-like genes dispersed in core genome might represent the novel classes due to the potential functional disparities, despite being closely related homologous genes that code for H₂ase. Overall, the findings shed light on the distinguishing features of A. thiooxidans genomes on a global scale, and extend the understanding of its conserved core genome pertaining to autotrophic lifestyle.