July 19, 2019  |  

Evolution of hypervirulence by a MRSA clone through acquisition of a transposable element.

Staphylococcus aureus has evolved as a pathogen that causes a range of diseases in humans. There are two dominant modes of evolution thought to explain most of the virulence differences between strains. First, virulence genes may be acquired from other organisms. Second, mutations may cause changes in the regulation and expression of genes. Here we describe an evolutionary event in which transposition of an IS element has a direct impact on virulence gene regulation resulting in hypervirulence. Whole-genome analysis of a methicillin-resistant S. aureus (MRSA) strain USA500 revealed acquisition of a transposable element (IS256) that is absent from close relatives of this strain. Of the multiple copies of IS256 found in the USA500 genome, one was inserted in the promoter sequence of repressor of toxins (Rot), a master transcriptional regulator responsible for the expression of virulence factors in S. aureus. We show that insertion into the rot promoter by IS256 results in the derepression of cytotoxin expression and increased virulence. Taken together, this work provides new insight into evolutionary strategies by which S. aureus is able to modify its virulence properties and demonstrates a novel mechanism by which horizontal gene transfer directly impacts virulence through altering toxin regulation. © 2014 John Wiley & Sons Ltd.


July 19, 2019  |  

Genome modification in Enterococcus faecalis OG1RF assessed by bisulfite sequencing and Single-Molecule Real-Time Sequencing.

Enterococcus faecalis is a Gram-positive bacterium that natively colonizes the human gastrointestinal tract and opportunistically causes life-threatening infections. Multidrug-resistant (MDR) E. faecalis strains have emerged, reducing treatment options for these infections. MDR E. faecalis strains have large genomes containing mobile genetic elements (MGEs) that harbor genes for antibiotic resistance and virulence determinants. Bacteria commonly possess genome defense mechanisms to block MGE acquisition, and we hypothesize that these mechanisms have been compromised in MDR E. faecalis. In restriction-modification (R-M) defense, the bacterial genome is methylated at cytosine (C) or adenine (A) residues by a methyltransferase (MTase), such that nonself DNA can be distinguished from self DNA. A cognate restriction endonuclease digests improperly modified nonself DNA. Little is known about R-M in E. faecalis. Here, we use genome resequencing to identify DNA modifications occurring in the oral isolate OG1RF. OG1RF has one of the smallest E. faecalis genomes sequenced to date and possesses few MGEs. Single-molecule real-time (SMRT) and bisulfite sequencing revealed that OG1RF has global 5-methylcytosine (m5C) methylation at 5′-GCWGC-3′ motifs. A type II R-M system confers the m5C modification, and disruption of this system impacts OG1RF electrotransformability and conjugative transfer of an antibiotic resistance plasmid. A second DNA MTase was poorly expressed under laboratory conditions but conferred global N(4)-methylcytosine (m4C) methylation at 5′-CCGG-3′ motifs when expressed in Escherichia coli. Based on our results, we conclude that R-M can act as a barrier to MGE acquisition and likely influences antibiotic resistance gene dissemination in the E. faecalis species.The horizontal transfer of antibiotic resistance genes among bacteria is a critical public health concern. Enterococcus faecalis is an opportunistic pathogen that causes life-threatening infections in humans. Multidrug resistance acquired by horizontal gene transfer limits treatment options for these infections. In this study, we used innovative DNA sequencing methodologies to investigate how a model strain of E. faecalis discriminates its own DNA from foreign DNA, i.e., self versus nonself discrimination. We also assess the role of an E. faecalis genome modification system in modulating conjugative transfer of an antibiotic resistance plasmid. These results are significant because they demonstrate that differential genome modification impacts horizontal gene transfer frequencies in E. faecalis. Copyright © 2015, American Society for Microbiology. All Rights Reserved.


July 19, 2019  |  

Complete bypass of restriction systems for major Staphylococcus aureus lineages.

Staphylococcus aureus is a prominent global nosocomial and community-acquired bacterial pathogen. A strong restriction barrier presents a major hurdle for the introduction of recombinant DNA into clinical isolates of S. aureus. Here, we describe the construction and characterization of the IMXXB series of Escherichia coli strains that mimic the type I adenine methylation profiles of S. aureus clonal complexes 1, 8, 30, and ST93. The IMXXB strains enable direct, high-efficiency transformation and streamlined genetic manipulation of major S. aureus lineages.The genetic manipulation of clinical S. aureus isolates has been hampered due to the presence of restriction modification barriers that detect and subsequently degrade inappropriately methylated DNA. Current methods allow the introduction of plasmid DNA into a limited subset of S. aureus strains at high efficiency after passage of plasmid DNA through the restriction-negative, modification-proficient strain RN4220. Here, we have constructed and validated a suite of E. coli strains that mimic the adenine methylation profiles of different clonal complexes and show high-efficiency plasmid DNA transfer. The ability to bypass RN4220 will reduce the cost and time involved for plasmid transfer into S. aureus. The IMXXB series of E. coli strains should expedite the process of mutant construction in diverse genetic backgrounds and allow the application of new techniques to the genetic manipulation of S. aureus. Copyright © 2015 Monk et al.


July 19, 2019  |  

Genetic stabilization of the drug-resistant PMEN1 Pneumococcus lineage by its distinctive DpnIII restriction-modification system.

The human pathogen Streptococcus pneumoniae (pneumococcus) exhibits a high degree of genomic diversity and plasticity. Isolates with high genomic similarity are grouped into lineages that undergo homologous recombination at variable rates. PMEN1 is a pandemic, multidrug-resistant lineage. Heterologous gene exchange between PMEN1 and non-PMEN1 isolates is directional, with extensive gene transfer from PMEN1 strains and only modest transfer into PMEN1 strains. Restriction-modification (R-M) systems can restrict horizontal gene transfer, yet most pneumococcal strains code for either the DpnI or DpnII R-M system and neither limits homologous recombination. Our comparative genomic analysis revealed that PMEN1 isolates code for DpnIII, a third R-M system syntenic to the other Dpn systems. Characterization of DpnIII demonstrated that the endonuclease cleaves unmethylated double-stranded DNA at the tetramer sequence 5′ GATC 3′, and the cognate methylase is a C5 cytosine-specific DNA methylase. We show that DpnIII decreases the frequency of recombination under in vitro conditions, such that the number of transformants is lower for strains transformed with unmethylated DNA than in those transformed with cognately methylated DNA. Furthermore, we have identified two PMEN1 isolates where the DpnIII endonuclease is disrupted, and phylogenetic work by Croucher and colleagues suggests that these strains have accumulated genomic differences at a higher rate than other PMEN1 strains. We propose that the R-M locus is a major determinant of genetic acquisition; the resident R-M system governs the extent of genome plasticity.Pneumococcus is one of the most important community-acquired bacterial pathogens. Pneumococcal strains can develop resistance to antibiotics and to serotype vaccines by acquiring genes from other strains or species. Thus, genomic plasticity is associated with strain adaptability and pneumococcal success. PMEN1 is a widespread and multidrug-resistant highly pathogenic pneumococcal lineage, which has evolved over the past century and displays a relatively stable genome. In this study, we characterize DpnIII, a restriction-modification (R-M) system that limits recombination. DpnIII is encountered in the PMEN1 lineage, where it replaces other R-M systems that do not decrease plasticity. Our hypothesis is that this genomic region, where different pneumococcal lineages code for variable R-M systems, plays a role in the fine-tuning of the extent of genomic plasticity. It is possible that well-adapted lineages such as PMEN1 have a mechanism to increase genomic stability, rather than foster genomic plasticity. Copyright © 2015 Eutsey et al.


July 19, 2019  |  

TAL effectors and activation of predicted host targets distinguish Asian from African strains of the rice pathogen Xanthomonas oryzae pv. oryzicola while strict conservation suggests universal importance of five TAL effectors.

Xanthomonas oryzae pv. oryzicola (Xoc) causes the increasingly important disease bacterial leaf streak of rice (BLS) in part by type III delivery of repeat-rich transcription activator-like (TAL) effectors to upregulate host susceptibility genes. By pathogen whole genome, single molecule, real-time sequencing and host RNA sequencing, we compared TAL effector content and rice transcriptional responses across 10 geographically diverse Xoc strains. TAL effector content is surprisingly conserved overall, yet distinguishes Asian from African isolates. Five TAL effectors are conserved across all strains. In a prior laboratory assay in rice cv. Nipponbare, only two contributed to virulence in strain BLS256 but the strict conservation indicates all five may be important, in different rice genotypes or in the field. Concatenated and aligned, TAL effector content across strains largely reflects relationships based on housekeeping genes, suggesting predominantly vertical transmission. Rice transcriptional responses did not reflect these relationships, and on average, only 28% of genes upregulated and 22% of genes downregulated by a strain are up- and down- regulated (respectively) by all strains. However, when only known TAL effector targets were considered, the relationships resembled those of the TAL effectors. Toward identifying new targets, we used the TAL effector-DNA recognition code to predict effector binding elements in promoters of genes upregulated by each strain, but found that for every strain, all upregulated genes had at least one. Filtering with a classifier we developed previously decreases the number of predicted binding elements across the genome, suggesting that it may reduce false positives among upregulated genes. Applying this filter and eliminating genes for which upregulation did not strictly correlate with presence of the corresponding TAL effector, we generated testable numbers of candidate targets for four of the five strictly conserved TAL effectors.


July 19, 2019  |  

Biosynthesis of the novel macrolide antibiotic anthracimycin.

We report the identification of the biosynthetic gene cluster for the unusual antibiotic anthracimycin (atc) from the marine derived producer strain Streptomyces sp. T676 isolated off St. John’s Island, Singapore. The 53?253 bps atc locus includes a trans-acyltransferase (trans-AT) polyketide synthase (PKS), and heterologous expression in Streptomyces coelicolor resulted in anthracimycin production. Analysis of the atc cluster revealed that anthracimycin is likely generated by four PKS gene products AtcC-AtcF without involvement of post-PKS tailoring enzymes, and a biosynthetic pathway is proposed. The availability of the atc cluster provides a basis for investigating the biosynthesis of anthracimycin and its subsequent bioengineering to provide novel analogues with improved pharmacological properties.


July 19, 2019  |  

Variable genetic architectures produce virtually identical molecules in bacterial symbionts of fungus-growing ants.

Small molecules produced by Actinobacteria have played a prominent role in both drug discovery and organic chemistry. As part of a larger study of the actinobacterial symbionts of fungus-growing ants, we discovered a small family of three previously unreported piperazic acid-containing cyclic depsipeptides, gerumycins A-C. The gerumycins are slightly smaller versions of dentigerumycin, a cyclic depsipeptide that selectively inhibits a common fungal pathogen, Escovopsis. We had previously identified this molecule from a Pseudonocardia associated with Apterostigma dentigerum, and now we report the molecule from an associate of the more highly derived ant Trachymyrmex cornetzi. The three previously unidentified compounds, gerumycins A-C, have essentially identical structures and were produced by two different symbiotic Pseudonocardia spp. from ants in the genus Apterostigma found in both Panama and Costa Rica. To understand the similarities and differences in the biosynthetic pathways that produced these closely related molecules, the genomes of the three producing Pseudonocardia were sequenced and the biosynthetic gene clusters identified. This analysis revealed that dramatically different biosynthetic architectures, including genomic islands, a plasmid, and the use of spatially separated genetic loci, can lead to molecules with virtually identical core structures. A plausible evolutionary model that unifies these disparate architectures is presented.


July 19, 2019  |  

Lifespan of restriction-modification systems critically affects avoidance of their recognition sites in host genomes.

Avoidance of palindromic recognition sites of Type II restriction-modification (R-M) systems was shown for many R-M systems in dozens of prokaryotic genomes. However the phenomenon has not been investigated systematically for all presently available genomes and annotated R-M systems. We have studied all known recognition sites in thousands of prokaryotic genomes and found factors that influence their avoidance.Only Type II R-M systems consisting of independently acting endonuclease and methyltransferase (called ‘orthodox’ here) cause avoidance of their sites, both palindromic and asymmetric, in corresponding prokaryotic genomes; the avoidance takes place for?~?50 % of 1774 studied cases. It is known that prokaryotes can acquire and lose R-M systems. Thus it is possible to talk about the lifespan of an R-M system in a genome. We have shown that the recognition site avoidance correlates with the lifespan of R-M systems. The sites of orthodox R-M systems that are encoded in host genomes for a long time are avoided more often (up to 100 % in certain cohorts) than the sites of recently acquired ones. We also found cases of site avoidance in absence of the corresponding R-M systems in the genome. An analysis of closely related bacteria shows that such avoidance can be a trace of lost R-M systems. Sites of Type I, II?/G, IIM, III, and IV R-M systems are not avoided in vast majority of cases.The avoidance of orthodox Type II R-M system recognition sites in prokaryotic genomes is a widespread phenomenon. Presence of an R-M system without an underrepresentation of its site may indicate that the R-M system was acquired recently. At the same time, a significant underrepresentation of a site may be a sign of presence of the corresponding R-M system in this organism or in its ancestors for a long time. The drastic difference between site avoidance for orthodox Type II R-M systems and R-M systems of other types can be explained by a higher rate of specificity changes or a less self-toxicity of the latter.


July 19, 2019  |  

The complete genome sequence of the murine pathobiont Helicobacter typhlonius.

Immuno-compromised mice infected with Helicobacter typhlonius are used to model microbially inducted inflammatory bowel disease (IBD). The specific mechanism through which H. typhlonius induces and promotes IBD is not fully understood. Access to the genome sequence is essential to examine emergent properties of this organism, such as its pathogenicity. To this end, we present the complete genome sequence of H. typhlonius MIT 97-6810, obtained through single-molecule real-time sequencing.The genome was assembled into a single circularized contig measuring 1.92 Mbp with an average GC content of 38.8%. In total 2,117 protein-encoding genes and 43 RNA genes were identified. Numerous pathogenic features were found, including a putative pathogenicity island (PAIs) containing components of type IV secretion system, virulence-associated proteins and cag PAI protein. We compared the genome of H. typhlonius to those of the murine pathobiont H. hepaticus and human pathobiont H. pylori. H. typhlonius resembles H. hepaticus most with 1,594 (75.3%) of its genes being orthologous to genes in H. hepaticus. Determination of the global methylation state revealed eight distinct recognition motifs for adenine and cytosine methylation. H. typhlonius shares four of its recognition motifs with H. pylori.The complete genome sequence of H. typhlonius MIT 97-6810 enabled us to identify many pathogenic features suggesting that H. typhlonius can act as a pathogen. Follow-up studies are necessary to evaluate the true nature of its pathogenic capabilities. We found many methylated sites and a plethora of restriction-modification systems. The genome, together with the methylome, will provide an essential resource for future studies investigating gene regulation, host interaction and pathogenicity of H. typhlonius. In turn, this work can contribute to unraveling the role of Helicobacter in enteric disease.


July 19, 2019  |  

Nested Russian doll-like genetic mobility drives rapid dissemination of the Carbapenem resistance gene blaKPC

The recent widespread emergence of carbapenem resistance in Enterobacteriaceae is a major public health concern, as carbapenems are a therapy of last resort against this family of common bacterial pathogens. Resistance genes can mobilize via various mechanisms, including conjugation and transposition; however, the importance of this mobility in short-term evolution, such as within nosocomial outbreaks, is unknown. Using a combination of short- and long-read whole-genome sequencing of 281 blaKPC-positive Enterobacteriaceae isolates from a single hospital over 5 years, we demonstrate rapid dissemination of this carbapenem resistance gene to multiple species, strains, and plasmids. Mobility of blaKPC occurs at multiple nested genetic levels, with transmission of blaKPC strains between individuals, frequent transfer of blaKPC plasmids between strains/species, and frequent transposition of blaKPC transposon Tn4401 between plasmids. We also identify a common insertion site for Tn4401 within various Tn2-like elements, suggesting that homologous recombination between Tn2-like elements has enhanced the spread of Tn4401 between different plasmid vectors. Furthermore, while short-read sequencing has known limitations for plasmid assembly, various studies have attempted to overcome this by the use of reference-based methods. We also demonstrate that, as a consequence of the genetic mobility observed in this study, plasmid structures can be extremely dynamic, and therefore these reference-based methods, as well as traditional partial typing methods, can produce very misleading conclusions. Overall, our findings demonstrate that nonclonal resistance gene dissemination can be extremely rapid, presenting significant challenges for public health surveillance and achieving effective control of antibiotic resistance. Copyright © 2016 Sheppard et al.


July 19, 2019  |  

Winding paths to simplicity: genome evolution in facultative insect symbionts.

Symbiosis between organisms is an important driving force in evolution. Among the diverse relationships described, extensive progress has been made in insect-bacteria symbiosis, which improved our understanding of the genome evolution in host-associated bacteria. Particularly, investigations on several obligate mutualists have pushed the limits of what we know about the minimal genomes for sustaining cellular life. To bridge the gap between those obligate symbionts with extremely reduced genomes and their non-host-restricted ancestors, this review focuses on the recent progress in genome characterization of facultative insect symbionts. Notable cases representing various types and stages of host associations, including those from multiple genera in the family Enterobacteriaceae (class Gammaproteobacteria), Wolbachia (Alphaproteobacteria) and Spiroplasma (Mollicutes), are discussed. Although several general patterns of genome reduction associated with the adoption of symbiotic relationships could be identified, extensive variation was found among these facultative symbionts. These findings are incorporated into the established conceptual frameworks to develop a more detailed evolutionary model for the discussion of possible trajectories. In summary, transitions from facultative to obligate symbiosis do not appear to be a universal one-way street; switches between hosts and lifestyles (e.g. commensalism, parasitism or mutualism) occur frequently and could be facilitated by horizontal gene transfer. © FEMS 2016.


July 19, 2019  |  

Comprehensive genome analysis of carbapenemase-producing Enterobacter spp.: new insights into phylogeny, population structure and resistance mechanisms.

Knowledge regarding the genomic structure of Enterobacter spp., the second most prevalent carbapenemase-producing Enterobacteriaceae, remains limited. Here we sequenced 97 clinical Enterobacter species isolates that were both carbapenem susceptible and resistant from various geographic regions to decipher the molecular origins of carbapenem resistance and to understand the changing phylogeny of these emerging and drug-resistant pathogens. Of the carbapenem-resistant isolates, 30 possessed blaKPC-2, 40 had blaKPC-3, 2 had blaKPC-4, and 2 had blaNDM-1 Twenty-three isolates were carbapenem susceptible. Six genomes were sequenced to completion, and their sizes ranged from 4.6 to 5.1 Mbp. Phylogenomic analysis placed 96 of these genomes, 351 additional Enterobacter genomes downloaded from NCBI GenBank, and six newly sequenced type strains into 19 phylogenomic groups-18 groups (A to R) in the Enterobacter cloacae complex and Enterobacter aerogenes Diverse mechanisms underlying the molecular evolutionary trajectory of these drug-resistant Enterobacter spp. were revealed, including the acquisition of an antibiotic resistance plasmid, followed by clonal spread, horizontal transfer of blaKPC-harboring plasmids between different phylogenomic groups, and repeated transposition of the blaKPC gene among different plasmid backbones. Group A, which comprises multilocus sequence type 171 (ST171), was the most commonly identified (23% of isolates). Genomic analysis showed that ST171 isolates evolved from a common ancestor and formed two different major clusters; each acquiring unique blaKPC-harboring plasmids, followed by clonal expansion. The data presented here represent the first comprehensive study of phylogenomic interrogation and the relationship between antibiotic resistance and plasmid discrimination among carbapenem-resistant Enterobacter spp., demonstrating the genetic diversity and complexity of the molecular mechanisms driving antibiotic resistance in this genus.Enterobacter spp., especially carbapenemase-producing Enterobacter spp., have emerged as a clinically significant cause of nosocomial infections. However, only limited information is available on the distribution of carbapenem resistance across this genus. Augmenting this problem is an erroneous identification of Enterobacter strains because of ambiguous typing methods and imprecise taxonomy. In this study, we used a whole-genome-based comparative phylogenetic approach to (i) revisit and redefine the genus Enterobacter and (ii) unravel the emergence and evolution of the Klebsiella pneumoniae carbapenemase-harboring Enterobacter spp. Using genomic analysis of 447 sequenced strains, we developed an improved understanding of the species designations within this complex genus and identified the diverse mechanisms driving the molecular evolution of carbapenem resistance. The findings in this study provide a solid genomic framework that will serve as an important resource in the future development of molecular diagnostics and in supporting drug discovery programs. Copyright © 2016 Chavda et al.


July 19, 2019  |  

The composite 259-kb plasmid of Martelella mediterranea DSM 17316(T)-a natural replicon with functional RepABC modules from Rhodobacteraceae and Rhizobiaceae.

A multipartite genome organization with a chromosome and many extrachromosomal replicons (ECRs) is characteristic for Alphaproteobacteria. The best investigated ECRs of terrestrial rhizobia are the symbiotic plasmids for legume root nodulation and the tumor-inducing (Ti) plasmid of Agrobacterium tumefaciens. RepABC plasmids represent the most abundant alphaproteobacterial replicon type. The currently known homologous replication modules of rhizobia and Rhodobacteraceae are phylogenetically distinct. In this study, we surveyed type-strain genomes from the One Thousand Microbial Genomes (KMG-I) project and identified a roseobacter-specific RepABC-type operon in the draft genome of the marine rhizobium Martelella mediterranea DSM 17316(T). PacBio genome sequencing demonstrated the presence of three circular ECRs with sizes of 593, 259, and 170-kb. The rhodobacteral RepABC module is located together with a rhizobial equivalent on the intermediate sized plasmid pMM259, which likely originated in the fusion of a pre-existing rhizobial ECR with a conjugated roseobacter plasmid. Further evidence for horizontal gene transfer (HGT) is given by the presence of a roseobacter-specific type IV secretion system on the 259-kb plasmid and the rhodobacteracean origin of 62% of the genes on this plasmid. Functionality tests documented that the genuine rhizobial RepABC module from the Martelella 259-kb plasmid is only maintained in A. tumefaciens C58 (Rhizobiaceae) but not in Phaeobacter inhibens DSM 17395 (Rhodobacteraceae). Unexpectedly, the roseobacter-like replication system is functional and stably maintained in both host strains, thus providing evidence for a broader host range than previously proposed. In conclusion, pMM259 is the first example of a natural plasmid that likely mediates genetic exchange between roseobacters and rhizobia.


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