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September 22, 2019  |  

A large-scale comparative metagenomic study reveals the functional interactions in six bloom-forming Microcystis-epibiont communities.

Cyanobacterial blooms are worldwide issues of societal concern and scientific interest. Lake Taihu and Lake Dianchi, two of the largest lakes in China, have been suffering from annual Microcystis-based blooms over the past two decades. These two eutrophic lakes differ in both nutrient load and environmental parameters, where Microcystis microbiota consisting of different Microcystis morphospecies and associated bacteria (epibionts) have dominated. We conducted a comprehensive metagenomic study that analyzed species diversity, community structure, functional components, metabolic pathways and networks to investigate functional interactions among the members of six Microcystis-epibiont communities in these two lakes. Our integrated metagenomic pipeline consisted of efficient assembly, binning, annotation, and quality assurance methods that ensured high-quality genome reconstruction. This study provides a total of 68 reconstructed genomes including six complete Microcystis genomes and 28 high quality bacterial genomes of epibionts belonging to 14 distinct taxa. This metagenomic dataset constitutes the largest reference genome catalog available for genome-centric studies of the Microcystis microbiome. Epibiont community composition appears to be dynamic rather than fixed, and the functional profiles of communities were related to the environment of origin. This study demonstrates mutualistic interactions between Microcystis and epibionts at genetic and metabolic levels. Metabolic pathway reconstruction provided evidence for functional complementation in nitrogen and sulfur cycles, fatty acid catabolism, vitamin synthesis, and aromatic compound degradation among community members. Thus, bacterial social interactions within Microcystis-epibiont communities not only shape species composition, but also stabilize the communities functional profiles. These interactions appear to play an important role in environmental adaptation of Microcystis colonies.

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September 22, 2019  |  

Comparative genomics of smut pathogens: Insights from orphans and positively selected genes into host specialization.

Host specialization is a key evolutionary process for the diversification and emergence of new pathogens. However, the molecular determinants of host range are poorly understood. Smut fungi are biotrophic pathogens that have distinct and narrow host ranges based on largely unknown genetic determinants. Hence, we aimed to expand comparative genomics analyses of smut fungi by including more species infecting different hosts and to define orphans and positively selected genes to gain further insights into the genetics basis of host specialization. We analyzed nine lineages of smut fungi isolated from eight crop and non-crop hosts: maize, barley, sugarcane, wheat, oats, Zizania latifolia (Manchurian rice), Echinochloa colona (a wild grass), and Persicaria sp. (a wild dicot plant). We assembled two new genomes: Ustilago hordei (strain Uhor01) isolated from oats and U. tritici (strain CBS 119.19) isolated from wheat. The smut genomes were of small sizes, ranging from 18.38 to 24.63 Mb. U. hordei species experienced genome expansions due to the proliferation of transposable elements and the amount of these elements varied among the two strains. Phylogenetic analysis confirmed that Ustilago is not a monophyletic genus and, furthermore, detected misclassification of the U. tritici specimen. The comparison between smut pathogens of crop and non-crop hosts did not reveal distinct signatures, suggesting that host domestication did not play a dominant role in shaping the evolution of smuts. We found that host specialization in smut fungi likely has a complex genetic basis: different functional categories were enriched in orphans and lineage-specific selected genes. The diversification and gain/loss of effector genes are probably the most important determinants of host specificity.

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September 22, 2019  |  

Genome sequence, assembly and characterization of two Metschnikowia fructicola strains used as biocontrol agents of postharvest diseases.

The yeast Metschnikowia fructicola was reported as an efficient biological control agent of postharvest diseases of fruits and vegetables, and it is the bases of the commercial formulated product “Shemer.” Several mechanisms of action by which M. fructicola inhibits postharvest pathogens were suggested including iron-binding compounds, induction of defense signaling genes, production of fungal cell wall degrading enzymes and relatively high amounts of superoxide anions. We assembled the whole genome sequence of two strains of M. fructicola using PacBio and Illumina shotgun sequencing technologies. Using the PacBio, a high-quality draft genome consisting of 93 contigs, with an estimated genome size of approximately 26 Mb, was obtained. Comparative analysis of M. fructicola proteins with the other three available closely related genomes revealed a shared core of homologous proteins coded by 5,776 genes. Comparing the genomes of the two M. fructicola strains using a SNP calling approach resulted in the identification of 564,302 homologous SNPs with 2,004 predicted high impact mutations. The size of the genome is exceptionally high when compared with those of available closely related organisms, and the high rate of homology among M. fructicola genes points toward a recent whole-genome duplication event as the cause of this large genome. Based on the assembled genome, sequences were annotated with a gene description and gene ontology (GO term) and clustered in functional groups. Analysis of CAZymes family genes revealed 1,145 putative genes, and transcriptomic analysis of CAZyme expression levels in M. fructicola during its interaction with either grapefruit peel tissue or Penicillium digitatum revealed a high level of CAZyme gene expression when the yeast was placed in wounded fruit tissue.

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September 22, 2019  |  

Comparative genomics of the wheat fungal pathogen Pyrenophora tritici-repentis reveals chromosomal variations and genome plasticity.

Pyrenophora tritici-repentis (Ptr) is a necrotrophic fungal pathogen that causes the major wheat disease, tan spot. We set out to provide essential genomics-based resources in order to better understand the pathogenicity mechanisms of this important pathogen.Here, we present eight new Ptr isolate genomes, assembled and annotated; representing races 1, 2 and 5, and a new race. We report a high quality Ptr reference genome, sequenced by PacBio technology with Illumina paired-end data support and optical mapping. An estimated 98% of the genome coverage was mapped to 10 chromosomal groups, using a two-enzyme hybrid approach. The final reference genome was 40.9 Mb and contained a total of 13,797 annotated genes, supported by transcriptomic and proteogenomics data sets.Whole genome comparative analysis revealed major chromosomal segmental rearrangements and fusions, highlighting intraspecific genome plasticity in this species. Furthermore, the Ptr race classification was not supported at the whole genome level, as phylogenetic analysis did not cluster the ToxA producing isolates. This expansion of available Ptr genomics resources will directly facilitate research aimed at controlling tan spot disease.

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September 22, 2019  |  

Evolution of sequence type 4821 clonal complex meningococcal strains in China from prequinolone to quinolone era, 1972-2013.

The expansion of hypervirulent sequence type 4821 clonal complex (CC4821) lineage Neisseria meningitidis bacteria has led to a shift in meningococcal disease epidemiology in China, from serogroup A (MenA) to MenC. Knowledge of the evolution and genetic origin of the emergent MenC strains is limited. In this study, we subjected 76 CC4821 isolates collected across China during 1972-1977 and 2005-2013 to phylogenetic analysis, traditional genotyping, or both. We show that successive recombination events within genes encoding surface antigens and acquisition of quinolone resistance mutations possibly played a role in the emergence of CC4821 as an epidemic clone in China. MenC and MenB CC4821 strains have spread across China and have been detected in several countries in different continents. Capsular switches involving serogroups B and C occurred among epidemic strains, raising concerns regarding possible increases in MenB disease, given that vaccines in use in China do not protect against MenB.

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September 22, 2019  |  

The genome sequence of “Candidatus Fokinia solitaria”: Insights on reductive evolution in Rickettsiales.

Candidatus Fokinia solitaria is an obligate intracellular endosymbiont of a unicellular eukaryote, a ciliate of the genus Paramecium. Here, we present the genome sequence of this bacterium and subsequent analysis. Phylogenomic analysis confirmed the previously reported positioning of the symbiont within the “Candidatus Midichloriaceae” family (order Rickettsiales), as well as its high sequence divergence from other members of the family, indicative of fast sequence evolution. Consistently with this high evolutionary rate, a comparative genomic analysis revealed that the genome of this symbiont is the smallest of the Rickettsiales to date. The reduced genome does not present flagellar genes, nor the pathway for the biosynthesis of lipopolysaccharides (present in all the other so far sequenced members of the family “Candidatus Midichloriaceae”) or genes for the Krebs cycle (present, although not always complete, in Rickettsiales). These results indicate an evolutionary trend toward a stronger dependence on the host, in comparison with other members of the family. Two alternative scenarios are compatible with our results; “Candidatus Fokinia solitaria” could be either a recently evolved, vertically transmitted mutualist, or a parasite with a high host-specificity.

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September 22, 2019  |  

Gene presence-absence polymorphism in castrating anther-smut fungi: Recent gene Gains and Phylogeographic Structure.

Gene presence-absence polymorphisms segregating within species are a significant source of genetic variation but have been little investigated to date in natural populations. In plant pathogens, the gain or loss of genes encoding proteins interacting directly with the host, such as secreted proteins, probably plays an important role in coevolution and local adaptation. We investigated gene presence-absence polymorphism in populations of two closely related species of castrating anther-smut fungi, Microbotryum lychnidis-dioicae (MvSl) and M. silenes-dioicae (MvSd), from across Europe, on the basis of Illumina genome sequencing data and high-quality genome references. We observed presence-absence polymorphism for 186 autosomal genes (2% of all genes) in MvSl, and only 51 autosomal genes in MvSd. Distinct genes displayed presence-absence polymorphism in the two species. Genes displaying presence-absence polymorphism were frequently located in subtelomeric and centromeric regions and close to repetitive elements, and comparison with outgroups indicated that most were present in a single species, being recently acquired through duplications in multiple-gene families. Gene presence-absence polymorphism in MvSl showed a phylogeographic structure corresponding to clusters detected based on SNPs. In addition, gene absence alleles were rare within species and skewed toward low-frequency variants. These findings are consistent with a deleterious or neutral effect for most gene presence-absence polymorphism. Some of the observed gene loss and gain events may however be adaptive, as suggested by the putative functions of the corresponding encoded proteins (e.g., secreted proteins) or their localization within previously identified selective sweeps. The adaptive roles in plant and anther-smut fungi interactions of candidate genes however need to be experimentally tested in future studies.

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September 22, 2019  |  

Multi-omics approach identifies novel pathogen-derived prognostic biomarkers in patients with Pseudomonas aeruginosa bloodstream infection

Pseudomonas aeruginosa is a human pathogen that causes health-care associated blood stream infections (BSI). Although P. aeruginosa BSI are associated with high mortality rates, the clinical relevance of pathogen-derived prognostic biomarker to identify patients at risk for unfavorable outcome remains largely unexplored. We found novel pathogen-derived prognostic biomarker candidates by applying a multi-omics approach on a multicenter sepsis patient cohort. Multi-level Cox regression was used to investigate the relation between patient characteristics and pathogen features (2298 accessory genes, 1078 core protein levels, 107 parsimony-informative variations in reported virulence factors) with 30-day mortality. Our analysis revealed that presence of the helP gene encoding a putative DEAD-box helicase was independently associated with a fatal outcome (hazard ratio 2.01, p = 0.05). helP is located within a region related to the pathogenicity island PAPI-1 in close proximity to a pil gene cluster, which has been associated with horizontal gene transfer. Besides helP, elevated protein levels of the bacterial flagellum protein FliL (hazard ratio 3.44, p < 0.001) and of a bacterioferritin-like protein (hazard ratio 1.74, p = 0.003) increased the risk of death, while high protein levels of a putative aminotransferase were associated with an improved outcome (hazard ratio 0.12, p < 0.001). The prognostic potential of biomarker candidates and clinical factors was confirmed with different machine learning approaches using training and hold-out datasets. The helP genotype appeared the most attractive biomarker for clinical risk stratification due to its relevant predictive power and ease of detection.

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September 22, 2019  |  

Comparative genomics of bdelloid rotifers: Insights from desiccating and nondesiccating species.

Bdelloid rotifers are a class of microscopic invertebrates that have existed for millions of years apparently without sex or meiosis. They inhabit a variety of temporary and permanent freshwater habitats globally, and many species are remarkably tolerant of desiccation. Bdelloids offer an opportunity to better understand the evolution of sex and recombination, but previous work has emphasised desiccation as the cause of several unusual genomic features in this group. Here, we present high-quality whole-genome sequences of 3 bdelloid species: Rotaria macrura and R. magnacalcarata, which are both desiccation intolerant, and Adineta ricciae, which is desiccation tolerant. In combination with the published assembly of A. vaga, which is also desiccation tolerant, we apply a comparative genomics approach to evaluate the potential effects of desiccation tolerance and asexuality on genome evolution in bdelloids. We find that ancestral tetraploidy is conserved among all 4 bdelloid species, but homologous divergence in obligately aquatic Rotaria genomes is unexpectedly low. This finding is contrary to current models regarding the role of desiccation in shaping bdelloid genomes. In addition, we find that homologous regions in A. ricciae are largely collinear and do not form palindromic repeats as observed in the published A. vaga assembly. Consequently, several features interpreted as genomic evidence for long-term ameiotic evolution are not general to all bdelloid species, even within the same genus. Finally, we substantiate previous findings of high levels of horizontally transferred nonmetazoan genes in both desiccating and nondesiccating bdelloid species and show that this unusual feature is not shared by other animal phyla, even those with desiccation-tolerant representatives. These comparisons call into question the proposed role of desiccation in mediating horizontal genetic transfer.

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September 22, 2019  |  

Genomic structural variations affecting virulence during clonal expansion of Pseudomonas syringae pv. actinidiae biovar 3 in Europe.

Pseudomonas syringae pv. actinidiae (Psa) biovar 3 caused pandemic bacterial canker of Actinidia chinensis and Actinidia deliciosa since 2008. In Europe, the disease spread rapidly in the kiwifruit cultivation areas from a single introduction. In this study, we investigated the genomic diversity of Psa biovar 3 strains during the primary clonal expansion in Europe using single molecule real-time (SMRT), Illumina and Sanger sequencing technologies. We recorded evidences of frequent mobilization and loss of transposon Tn6212, large chromosome inversions, and ectopic integration of IS sequences (remarkably ISPsy31, ISPsy36, and ISPsy37). While no phenotype change associated with Tn6212 mobilization could be detected, strains CRAFRU 12.29 and CRAFRU 12.50 did not elicit the hypersensitivity response (HR) on tobacco and eggplant leaves and were limited in their growth in kiwifruit leaves due to insertion of ISPsy31 and ISPsy36 in the hrpS and hrpR genes, respectively, interrupting the hrp cluster. Both strains had been isolated from symptomatic plants, suggesting coexistence of variant strains with reduced virulence together with virulent strains in mixed populations. The structural differences caused by rearrangements of self-genetic elements within European and New Zealand strains were comparable in number and type to those occurring among the European strains, in contrast with the significant difference in terms of nucleotide polymorphisms. We hypothesize a relaxation, during clonal expansion, of the selection limiting the accumulation of deleterious mutations associated with genome structural variation due to transposition of mobile elements. This consideration may be relevant when evaluating strategies to be adopted for epidemics management.

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September 22, 2019  |  

Recombination of plasmids in a carbapenem-resistant NDM-5-producing clinical Escherichia coli isolate.

To investigate the genetic features of five plasmids recovered from an NDM-5-producing clinical Escherichia coli strain, BJ114, and to characterize the plasmid recombination event that occurred during the conjugation process.The genetic profiles of the five plasmids were determined by PCR, conjugation, S1-PFGE, Southern hybridization and WGS analysis. Plasmid sequences were analysed with various bioinformatic tools.Complete sequences of five plasmids were obtained. Two small plasmids, pBJ114-141 and pBJ114-46, were speculated to have recombined into a large fusion plasmid, pBJ114T-190. When conjugated to other E. coli strains, some of the fusion plasmids were able to be resolved into the original two single plasmids. A non-conjugative plasmid, pBJ114-96, exhibited a high degree of sequence identity with the phage P7-like plasmid as well as an mcr-1-bearing plasmid. Another plasmid, pBJ114-78, was found to contain multidrug resistance genes and various mobile elements.The fusion plasmid recoverable from the transconjugant was found to be generated as a result of a recombination event that occurred upon interaction between a blaNDM-5-carrying plasmid and another plasmid present in the parental strain. Such recombination events presumably play a potential role in the dissemination of the blaNDM genes among different plasmids and pathogenic bacterial strains.

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September 22, 2019  |  

Genomic comparison between members of the Salinibacteraceae family, and description of a new species of Salinibacter (Salinibacter altiplanensis sp. nov.) isolated from high altitude hypersaline environments of the Argentinian Altiplano.

The application of tandem MALDI-TOF MS screening with 16S rRNA gene sequencing of selected isolates has been demonstrated to be an excellent approach for retrieving novelty from large-scale culturing. The application of such methodologies in different hypersaline samples allowed the isolation of the culture-recalcitrant Salinibacter ruber second phylotype (EHB-2) for the first time, as well as a new species recently isolated from the Argentinian Altiplano hypersaline lakes. In this study, the genome sequences of the different species of the phylum Rhodothermaeota were compared and the genetic repertoire along the evolutionary gradient was analyzed together with each intraspecific variability. Altogether, the results indicated an open pan-genome for the family Salinibacteraceae, as well as the codification of relevant traits such as diverse rhodopsin genes, CRISPR-Cas systems and spacers, and one T6SS secretion system that could give ecological advantages to an EHB-2 isolate. For the new Salinibacter species, we propose the name Salinibacter altiplanensis sp. nov. (the designated type strain is AN15T=CECT 9105T=IBRC-M 11031T). Copyright © 2018 Elsevier GmbH. All rights reserved.

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September 22, 2019  |  

Spread of plasmid-encoded NDM-1 and GES-5 carbapenemases among extensively drug-resistant and pandrug-resistant clinical Enterobacteriaceae in Durban, South Africa.

Whole-genome sequence analyses revealed the presence of blaNDM-1 (n = 31), blaGES-5 (n = 8), blaOXA-232 (n = 1), or blaNDM-5 (n = 1) in extensively drug-resistant and pandrug-resistant Enterobacteriaceae organisms isolated from in-patients in 10 private hospitals (2012 to 2013) in Durban, South Africa. Two novel NDM-1-encoding plasmids from Klebsiella pneumoniae were circularized by PacBio sequencing. In p19-10_01 [IncFIB(K); 223.434 bp], blaNDM-1 was part of a Tn1548-like structure (16.276 bp) delineated by IS26 The multireplicon plasmid p18-43_01 [IncR_1/IncFIB(pB171)/IncFII(Yp); 212.326 bp] shared an 80-kb region with p19-10_01, not including the blaNDM-1-containing region. The two plasmids were used as references for tracing NDM-1-encoding plasmids in the other genome assemblies. The p19-10_01 sequence was detected in K. pneumoniae (n = 7) only, whereas p18-43_01 was tracked to K. pneumoniae (n = 4), Klebsiella michiganensis (n = 1), Serratia marcescens (n = 11), Enterobacter spp. (n = 7), and Citrobacter freundii (n = 1), revealing horizontal spread of this blaNDM-1-bearing plasmid structure. Global phylogeny showed clustering of the K. pneumoniae (18/20) isolates together with closely related carbapenemase-negative ST101 isolates from other geographical origins. The South African isolates were divided into three phylogenetic subbranches, where each group had distinct resistance and replicon profiles, carrying either p19-10_01, p18-10_01, or pCHE-A1 (8,201 bp). The latter plasmid carried blaGES-5 and aacA4 within an integron mobilization unit. Our findings imply independent plasmid acquisition followed by local dissemination. Additionally, we detected blaOXA-232 carried by pPKPN4 in K. pneumoniae (ST14) and blaNDM-5 contained by a pNDM-MGR194-like genetic structure in Escherichia coli (ST167), adding even more complexity to the multilayer molecular mechanisms behind nosocomial spread of carbapenem-resistant Enterobacteriaceae in Durban, South Africa. Copyright © 2018 American Society for Microbiology.

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September 22, 2019  |  

An improved medium for colistin susceptibility testing.

The plasmid-located colistin resistance gene mcr-1 confers low-level resistance to colistin, a last-line antibiotic against multidrug-resistant Gram-negative bacteria. Current CLSI-EUCAST recommendations require the use of a broth microdilution (BMD) method with cation-adjusted Mueller-Hinton (CA-MH) medium for colistin susceptibility testing, but approximately 15% of all MCR-1 producers are classified as sensitive in that broth. Here we report on an improved calcium-enhanced Mueller-Hinton (CE-MH) medium that permits simple and reliable determination of mcr-1-containing Enterobacteriaceae Colistin susceptibility testing was performed for 50 mcr-1-containing Escherichia coli and Klebsiella pneumoniae isolates, 7 intrinsically polymyxin-resistant species, K. pneumoniae and E. coli isolates with acquired resistance to polymyxins due to mgrB and pmrB mutations, respectively, and 32 mcr-1-negative, colistin-susceptible isolates of Acinetobacter baumannii, Citrobacter freundii, Enterobacter cloacae, E. coli, K. pneumoniae, and Salmonella enterica serovar Typhimurium. A comparison of the colistin MICs determined in CA-MH medium and those obtained in CE-MH medium was performed using both the BMD and strip-based susceptibility test formats. We validated the data using an isogenic IncX4 plasmid lacking mcr-1 Use of the CE-MH broth provides clear separation between resistant and susceptible isolates in both BMD and gradient diffusion assays; this is true for both mcr-1-containing Enterobacteriaceae isolates and those exhibiting either intrinsic or acquired colistin resistance. CE-MH medium is simple to prepare and overcomes current problems associated with BMD and strip-based colistin susceptibility testing, and use of the medium is easy to implement in routine diagnostic laboratories, even in resource-poor settings. Copyright © 2018 American Society for Microbiology.

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September 22, 2019  |  

Mutant phenotypes for thousands of bacterial genes of unknown function.

One-third of all protein-coding genes from bacterial genomes cannot be annotated with a function. Here, to investigate the functions of these genes, we present genome-wide mutant fitness data from 32 diverse bacteria across dozens of growth conditions. We identified mutant phenotypes for 11,779 protein-coding genes that had not been annotated with a specific function. Many genes could be associated with a specific condition because the gene affected fitness only in that condition, or with another gene in the same bacterium because they had similar mutant phenotypes. Of the poorly annotated genes, 2,316 had associations that have high confidence because they are conserved in other bacteria. By combining these conserved associations with comparative genomics, we identified putative DNA repair proteins; in addition, we propose specific functions for poorly annotated enzymes and transporters and for uncharacterized protein families. Our study demonstrates the scalability of microbial genetics and its utility for improving gene annotations.

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