Mechanism Of Horizontal Transfer Research Articles

Mechanisms for horizontal transfer of methoprene from treated to untreated Tribolium castaneum (Herbst)

Experiments were performed to determine the relative impact of different mechanisms of methoprene horizontal transfer between Tribolium castaneum (Herbst) individuals. Larvae provided during later development with only dead methoprene-treated developmental stages (late-stage larvae, pupae, or adults) resulted in no survival to adulthood. This suggests that transfer through cuticle contact could be involved, but some cannibalization also occurred. The average level of cannibalization was not different between methoprene and control treatments, but the distribution of the amount of feeding did appear to be shifted to greater consumption by larvae exposed to methoprene-treated individuals compared to water-treated. The potential impact of cannibalism on horizontal transfer was further evaluated by scoring the level of feeding by untreated larvae on either water or methoprene-treated pupa and then comparing larval development and survival after different levels of cannibalization. In this experiment, the level of feeding on methoprene-treated individuals did not affect the percentage of normal surviving adults (insects which emerged as adults, exhibited no external morphological malformations, and were alive at the end of the observational period), suggesting that consumption of treated individuals was not contributing significantly to horizontal transfer. Short-term confinement of a methoprene or water treated adult with an untreated late-stage larva did not effect larval development and survival compared to the controls, indicating that brief contact between individuals was not sufficient for detectable negative effects. Transfer of methoprene from treated individuals to a flour substrate and then to an untreated individual through contact with and/or feeding on the flour was evaluated. Exposure of larvae to methoprene-contaminated flour during development resulted in a significantly lower number of normal surviving adults compared to controls. These results suggest that horizontal transfer of methoprene is probably due to prolonged contact with methoprene-treated individuals or methoprene-contaminated flour.

Streptococcus thermophilus의 자연적 형질전환 메커니즘과 Leuconostoc 속 세균에서 그 메커니즘의 추론

Natural genetic transformation is one of mechanism for horizontal transfer of genetic elements in bacteria which is an integral part of the physiology of bacteria. Streptococcus thermophilus utilize a novel quorum sensing system to control for development of natural competence which composes Rgg-type response regulator (ComR) and hydrophobic peptide pheromone (ComS). Matured ComS is released outside of cell during early exponential phase when growing in peptide free chemical defined medium (CDM). Matured ComS is transported via Ami (oligopeptide transport system) form outside to cytoplasm, and then bind to ComR. Activated ComR (matured ComS combining form of ComR) induces transcription of comX, which encodes an alternative sigma factor (σx) that regulates expression of late competence genes. Adding synthetic matured form of ComS to CDM, it can induce genetic transformation of S. thermophilus. The genus Leuconostoc has not been identified natural transformation mechanism even though presenting homologous genes related with late competence development. The study for identifying natural transformation mechanism of Leuconostoc can realize development of improved strains for kimchi industry.

Dynamics of the SetCD-Regulated Integration and Excision of Genomic Islands Mobilized by Integrating Conjugative Elements of the SXT/R391 Family

Mobilizable genomic islands (MGIs) are small genomic islands that are mobilizable by SXT/R391 integrating conjugative elements (ICEs) due to similar origins of transfer. Their site-specific integration and excision are catalyzed by the integrase that they encode, but their conjugative transfer entirely depends upon the conjugative machinery of SXT/R391 ICEs. In this study, we report the mechanisms that control the excision and integration processes of MGIs. We found that while the MGI-encoded integrase Int(MGI) is sufficient to promote MGI integration, efficient excision from the host chromosome requires the combined action of Int(MGI) and of a novel recombination directionality factor, RdfM. We determined that the genes encoding these proteins are activated by SetCD, the main transcriptional activators of SXT/R391 ICEs. Although they share the same regulators, we found that unlike rdfM, int(MGI) has a basal level of expression in the absence of SetCD. These findings explain how an MGI can integrate into the chromosome of a new host in the absence of a coresident ICE and shed new light on the cross talk that can occur between mobilizable and mobilizing elements that mobilize them, helping us to understand part of the rules that dictate horizontal transfer mechanisms.

Candida albicans.

Candida albicans.

Horizontal transfer of methoprene in Tribolium castaneum

Aerosol applications of reduced risk insecticides such as pyrethrins, pyrethroids, and insect growth regulators are becoming more commonly used to manage stored-product insects in food facilities. However, these applications have a limited ability to penetrate into hidden refugia, where the majority of the pest population is located. Horizontal transfer of insecticides could occur as individuals directly treated or exposed to treated surfaces move into hidden refugia and encounter untreated individuals. In this series of studies, the potential for horizontal transfer of methoprene from treated Tribolium castaneum , the red flour beetle, to untreated individuals was evaluated. Adding larvae, pupae, or adults treated with methoprene to flour patches with untreated T. castaneum larvae, resulted in increased pupa and adult deformities and higher numbers of dead focal individuals, which suggests the potential for this mechanism. The transfer mechanism might be flour substrate contamination, transfer during contact of individuals, and/or cannibalism of individuals exposed to insecticides. Experiments focused on isolating the impact of contact and cannibalism on horizontal transfer did not detect a significant increase in mortality. Experiments focused on flour substrate contamination resulted in decreased adult emergence as well as lower survival, and higher rates of deformities. These findings suggest that substrate contamination is the more likely mechanism for horizontal transfer, and although horizontal transfer can occur, the impact of this process on populations needs further evaluation. Keywords: Red flour beetle, Tribolium castaneum , Methoprene, Horizontal transfer /* Style Definitions */ table.MsoNormalTable {mso-style-name:Normale Tabelle; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin-top:0cm; mso-para-margin-right:0cm; mso-para-margin-bottom:10.0pt; mso-para-margin-left:0cm; line-height:115%; mso-pagination:widow-orphan; font-size:11.0pt; font-family:Calibri,sans-serif; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:Times New Roman; mso-fareast-theme-font:minor-fareast; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin;} Normal 0 21 false false false DE X-NONE X-NONE MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable {mso-style-name:Normale Tabelle; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin-top:0cm; mso-para-margin-right:0cm; mso-para-margin-bottom:10.0pt; mso-para-margin-left:0cm; line-height:115%; mso-pagination:widow-orphan; font-size:11.0pt; font-family:Calibri,sans-serif; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:Times New Roman; mso-fareast-theme-font:minor-fareast; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin;}

Sequencing and transcriptional analysis of the Streptococcus thermophilus histamine biosynthesis gene cluster: factors that affect differential hdcA expression.

Histamine, a toxic compound that is formed by the decarboxylation of histidine through the action of microbial decarboxylases, can accumulate in fermented food products. From a total of 69 Streptococcus thermophilus strains screened, two strains, CHCC1524 and CHCC6483, showed the capacity to produce histamine. The hdc clusters of S. thermophilus CHCC1524 and CHCC6483 were sequenced, and the factors that affect histamine biosynthesis and histidine-decarboxylating gene (hdcA) expression were studied. The hdc cluster began with the hdcA gene, was followed by a transporter (hdcP), and ended with the hdcB gene, which is of unknown function. The three genes were orientated in the same direction. The genetic organization of the hdc cluster showed a unique organization among the lactic acid bacterial group and resembled those of Staphylococcus and Clostridium species, thus indicating possible acquisition through a horizontal transfer mechanism. Transcriptional analysis of the hdc cluster revealed the existence of a polycistronic mRNA covering the three genes. The histidine-decarboxylating gene (hdcA) of S. thermophilus demonstrated maximum expression during the stationary growth phase, with high expression levels correlated with high histamine levels. Limited expression was evident during the lag and exponential growth phases. Low-temperature (4 degrees C) incubation of milk inoculated with a histamine-producing strain showed lower levels of histamine than did inoculated milk kept at 42 degrees C. This reduction was attributed to a reduction in the activity of the HdcA enzyme itself rather than a reduction in gene expression or the presence of a lower cell number.

A Deadly Gift

Although many malignant gliomas express EGFRvIII, a constitutively active oncogenic mutant form of the epidermal growth factor receptor (EGFR), the mutant receptor may actually be present in only a subset of the tumor cells. Al-Nedawi et al . found that a human glioma cell line (U373 cells) stably transfected with and made to express EGFRvIII (U373vIII cells) produced more membrane-derived microvesicles than did the parent cell line. Increased formation of vesicular membrane protrusions was apparent in U373vIII cells by scanning electron microscopy, and the abundance of total protein in the microvesicle fraction of medium in which cells had grown was increased. U373vIII cell-derived microvesicles contained EGFRvIII, as well as the lipid raft marker flotillin-1. Unlike U373 cells, U373vIII cells form subcutaneous tumors when injected into immunodeficient mice; these EGFRvIII-immunoreactive tumors released EGFRvIII-containing microvesicles into the circulatory system. Experiments in which U373 cells were incubated with vesicles derived from U373vIII cells indicated that microvesicular EGFRvIII was transferred to the U373 cells, where it appeared on the cell surface. A day later, U373 cells that had been exposed to EGFRvIII-containing microvesicles showed increased phosphorylation of extracellular signal-regulated kinase 1 and 2 (ERK1/2). Pharmacological treatment of microvesicles to inhibit signaling by EGFR-family receptors attenuated this increase in ERK1/2 phosphorylation, as did treatment of the microvesicles with annexin V (to prevent their uptake). EGFRvIII-containing microvesicles also stimulated other events downstream of EGFRvIII: Akt phosphorylation, release of vascular endothelial growth factor, abundance of the antiapoptotic protein Bcl-x L , and the ability of U373 cells to grow in soft agar (indicative of malignant transformation). The authors thus conclude that membrane microvesicles provide a mechanism for horizontal transfer of the transformed phenotype. K. Al-Nedawi, B. Meehan, J. Micallef, V. Lhotak, L. May, A. Guha, J. Rak, Intercellular transfer of the oncogenic receptor EGFRvIII by microvesicles derived from tumour cells. Nat . Cell Biol . 10 , 619-624 (2008). [PubMed]

Are all horizontal gene transfers created equal? Prospects for mechanism-based studies of HGT patterns

Detecting patterns of horizontal gene transfer (HGT) in genomic sequences is an important problem, with implications for evolution, ecology, biotechnology and medicine. Extensive genetic, biochemical and genomic studies have provided a good understanding of sequence features that are associated with many (though not all) known mobile elements and mechanisms of gene transfer. This information, however, is not currently incorporated into automated methods for gene transfer detection in genomic data. In this review, we argue that automated annotation of sequence features associated with gene transfer mechanisms could be used both to build more sensitive, mechanism-specific compositional models for the detection of some types of HGT in genomic data, and to ask new questions about the classes of genes most frequently transferred by each mechanism. We then summarize the genes and sequence features associated with different mechanisms of horizontal transfer, emphasizing those that are most useful for distinguishing types of transfer when examining genomic data, and noting those classes of transfers that cannot be distinguished in genomic data using existing techniques. Finally, we describe software, databases and algorithms for identifying particular classes of mobile elements, and outline prospects for better detection of HGT based on specific mechanisms of transfer.

A global gene evolution analysis on Vibrionaceae family using phylogenetic profile

BackgroundVibrionaceae represent a significant portion of the cultivable heterotrophic sea bacteria; they strongly affect nutrient cycling and some species are devastating pathogens.In this work we propose an improved phylogenetic profile analysis on 14 Vibrionaceae genomes, to study the evolution of this family on the basis of gene content.The phylogenetic profile is based on the observation that genes involved in the same process (e.g. metabolic pathway or structural complex) tend to be concurrently present or absent within different genomes. This allows the prediction of hypothetical functions on the basis of a shared phylogenetic profiles. Moreover this approach is useful to identify putative laterally transferred elements on the basis of their presence on distantly phylogenetically related bacteria.ResultsVibrionaceae ORFs were aligned against all the available bacterial proteomes. Phylogenetic profile is defined as an array of distances, based on aminoacid substitution matrixes, from single genes to all their orthologues. Final phylogenetic profiles, derived from non-redundant list of all ORFs, was defined as the median of all the profiles belonging to the cluster. The resulting phylogenetic profiles matrix contains gene clusters on the rows and organisms on the columns.Cluster analysis identified groups of "core genes" with a widespread high similarity across all the organisms and several clusters that contain genes homologous only to a limited set of organisms. On each of these clusters, COG class enrichment has been calculated. The analysis reveals that clusters of core genes have the highest number of enriched classes, while the others are enriched just for few of them like DNA replication, recombination and repair.ConclusionWe found that mobile elements have heterogeneous profiles not only across the entire set of organisms, but also within Vibrionaceae; this confirms their great influence on bacteria evolution even inside the same family. Furthermore, several hypothetical proteins highly correlate with mobile elements profiles suggesting a possible horizontal transfer mechanism for the evolution of these genes. Finally, we suggested the putative role of some ORFs having an unknown function on the basis of their phylogenetic profile similarity to well characterized genes.

Somatic stem cell niche tropism in Wolbachia

Wolbachia are intracellular bacteria found in the reproductive tissue of all major groups of arthropods. They are transmitted vertically from the female hosts to their offspring, in a pattern analogous to mitochondria inheritance. But Wolbachia phylogeny does not parallel that of the host, indicating that horizontal infectious transmission must also occur. Insect parasitoids are considered the most likely vectors, but the mechanism for horizontal transfer is largely unknown. Here we show that newly introduced Wolbachia cross several tissues and infect the germline of the adult Drosophila melanogaster female. Through investigation of bacterial migration patterns during the course of infection, we found that Wolbachia reach the germline through the somatic stem cell niche in the D. melanogaster germarium. In addition, our data suggest that Wolbachia are highly abundant in the somatic stem cell niche of long-term infected hosts, implying that this location may also contribute to efficient vertical transmission. This is, to our knowledge, the first report of an intracellular parasite displaying tropism for a stem cell niche.

Recent horizontal intron transfer to a chloroplast genome

Evidence is presented for the recent, horizontal transfer of a self-splicing, homing group II intron from a cyanobacteria to the chloroplast genome of Euglena myxocylindracea. The psbA gene of E.myxocylindracea was found to contain a single 2566 nt group II intron with a gene in domain 4 for a 575 amino acid maturase. The predicted secondary structure and tertiary interactions of the group II intron, as well as the derived maturase primary sequence, most closely resemble the homing intron of the cyanobacterium Calothrix and the rnl introns of Porphyra purpurea mitochondria, while being only distantly related to all other Euglena plastid introns and maturases. All main functional domains of the intron-encoded proteins of known homing introns are conserved, including reverse transcriptase domains 1-7, the zinc finger domain and domain X. The close relationship with cyanobacterial introns was confirmed by phylogenetic analysis. Both the full-length psbA intron and a Delta-maturase variant self-splice in vitro in two independent assays. The psbA intron is the first example of a self-splicing chloroplast group II intron from any organism. These results support the conclusion that the psbA intron is the result of a recent horizontal transfer into the E.myxocylindracea chloroplast genome from a cyanobacterial donor and should prompt a reconsideration of horizontal transfer mechanisms to account for the origin of other chloroplast genetic elements.

Examination of diverse toxin-coregulated pilus-positive Vibrio cholerae strains fails to demonstrate evidence for Vibrio pathogenicity island phage.

The major virulence factors of toxigenic Vibrio cholerae are cholera toxin, which is encoded by a lysogenic filamentous bacteriophage (CTXPhi), and toxin-coregulated pilus (TCP), an essential colonization factor that is also the receptor for CTXPhi. The genes involved in the biosynthesis of TCP reside in a pathogenicity island, which has been reported to correspond to the genome of another filamentous phage (designated VPIPhi) and to encode functions necessary for the production of infectious VPIPhi particles. We examined 46 V. cholerae strains having diverse origins and carrying different genetic variants of the TCP island for the production of the VPIPhi and CTXPhi in different culture conditions, including induction of prophages with mitomycin C and UV irradiation. Although 9 of 10 V. cholerae O139 strains and 12 of 15 toxigenic El Tor strains tested produced extracellular CTXPhi, none of the 46 TCP-positive strains produced detectable VPIPhi in repeated assays, which detected as few as 10 particles of a control CTX phage per ml. These results contradict the previous report regarding VPIPhi-mediated horizontal transfer of the TCP genes and suggest that the TCP island is unable to support the production of phage particles. Further studies are necessary to understand the mechanism of horizontal transfer of the TCP island.

VO1, a temperate bacteriophage of the type 19A multiresistant epidemic 8249 strain of Streptococcus pneumoniae: analysis of variability of lytic and putative C5 methyltransferase genes.

A temperate bacteriophage (VO1) has been isolated from the Streptococcus pneumoniae type 19F multiresistant epidemic 8249 strain (South African strain). Structural analysis of the specific integration site, protein composition, restriction patterns, and molecular dissection of the lytic system of this phage revealed high sequence similarity with MM1, a temperate phage from the Spain23F-1 strain of pneumococcus, another multiresistant epidemic clone. The different pneumococcal strains sequenced so far exhibit an identical and single attB located in the same site of the genome. Remarkably, the LytA amidase coded by VO1 showed clear differences with that of the host bacterium in contrast with the situation previously documented for bacterial- and phage-coded amidases of pneumococcus. In addition, a new gene (orfmet) putatively coding for a C5 methyltransferase has been identified. A noticeable variability affecting the presence (or absence) of this supernumerary gene(s) in the same region of the genomes of three otherwise highly similar phages (i.e., VO1, MM1, and HB-3) suggests frequent recombinational events leading to introduce variability in this genome region. The peculiarities of genes like lytA and orfmet in VO1 provide interesting insights on mechanisms of horizontal transfer and lysogenic state co-evolution.

Transformation of malaria parasites by the spontaneous uptake and expression of DNA from human erythrocytes.

The uptake and expression of extracellular DNA has been established as a mechanism for horizontal transfer of genes between bacterial species. Such transfer can support acquisition of advantageous elements, including determinants that affect the interactions between infectious organisms and their hosts. Here we show that erythrocyte-stage Plasmodium falciparum malaria parasites spontaneously take up DNA from the host cell cytoplasm into their nuclei. We have exploited this finding to produce levels of reporter expression in P.falciparum that are substantially improved over those obtained by electroporation protocols currently used to transfect malaria parasites. Parasites were transformed to a drug-resistant state when placed into cell culture with erythrocytes containing a plasmid encoding the human dihydrofolate reductase sequence. The findings reported here suggest that the malaria genome may be continually exposed to exogenous DNA from residual nuclear material in host erythrocytes.

Transgenic organisms: should we use them or not?: Transgenic Organisms: Biological and Social Implications. Edited by J. Tomiuk, K. Wöhrmann and A. Sentker. Birkhauser, Basel. 1996. Price sFr. 88,-/DM 108. ISBN 3-7643-5262-0.

The production and use of transgenic organisms is controversial in western societies, and has been actively debated by various groups and in various media ever since the potential risks involved in producing transgenic organisms were first brought to general attention by a group of scientists in the early 1970s. Today it is mainly the concern of the general (consumer) public that slows down the speed of widespread introduction of transgenes in society. Meanwhile, the technical development in the scientific community and in the R & D groups of large companies proceeds rapidly, as witnessed by the regular hot news items that sweep through the public media. As usual with developments that on the one hand promise interesting economic gains for some and on the other hand have or seem to have a large impact on our society or on nature, risk assessment studies and certainly the ethical discussion tend to lag behind. One of the prerequisites for proper risk assessments as well as for a successful ethical discussion is the availability of good information. This book is meant as a contribution to the field by providing an overview of the state of the art with respect to biological and social phenomena related to the use of transgenic organisms. The book results from a meeting of scientists in 1995 in Germany and is one in a series, as is explained in one of the chapters written by the editors, where the history and progress in the study of risk assessment is discussed. A remarkable feature of the book that struck me when glancing over the contents pages is that all chapters have either a German (co)author or an author working in a German-speaking country, but this appears not to affect the general value of the book. It only shows that the topic is relatively hot in that part of Europe. Most chapters deal with the biological side. Fundamental molecular–genetic aspects are addressed in chapters on gene-silencing as a mechanism behind the often encountered instability of transgene expression, and on transposable elements as possible mechanisms of horizontal gene transmission. Mechanisms of horizontal transfer are considered further in separate chapters on the situation in bacteria (among themselves) and insects (via baculo viruses). Oddly placed between the chapters on bacteria and insects is a chapter on transgenic crop plants, which unfortunately is little more than a student course in theoretical population genetics. A similar limitation holds for a chapter on effects of transgenes on coevolutionary processes: the general set-up brings little news for the average JEB reader. Informative with respect to risk assessment aspects are the chapters on biological containment of bacteria and on possibilities for monitoring their presence in the soil, as well as a chapter on plants that uses the Weed Beet example to illustrate both the strength and the weakness of current biosafety research. Most chapters end with pointing out factors that need to be taken into account in future research. In the last part of the book attention shifts towards aspects of human health, economics and public opinion. Classical and modern plant breeding methods are compared from the point of view of the breeder, with the conclusion that they are complementary. Introduction of genetically modified food is discussed from a historical and procedural perspective. The state of the art in the HUman GenOme project is described and discussed with respect to its usefulness to human health care. Finally, the role of the media in shaping public opinion and ethical aspects is discussed. Although the chapter on ethical aspects calls for making explicit one’s assumptions and premises when discussing what to allow and what not, the book as a whole gave me no stimulus to develop my own ethical point of view. The atmosphere in the book is either scientifically neutral or implicitly positive towards the new technology. The significance of the book is not so much to inspire one to develop an individual point of view on the matter (although to my mind this is very important to do), but to provide a factual basis of knowledge. Most chapters give a good description of their respective fields, and together they cover many relevant aspects of the introduction of transgenic organisms in society. As such, the book is informative for someone who wants an introduction into the field of facts surrounding transgenic organisms.

Plasmids responsible for horizontal transfer of naphthalene catabolism genes between bacteria at a coal tar-contaminated site are homologous to pDTG1 from pseudomonas putida NCIB 9816-4

The presence of a highly conserved nahAc allele among phylogenetically diverse bacteria carrying naphthalene-catabolic plasmids provided evidence for in situ horizontal gene transfer at a coal tar-contaminated site (J. B. Herrick, K. G. Stuart-Keil, W. C. Ghiorse, and E. L. Madsen, Appl. Environ. Microbiol. 63:2330-2337, 1997). The objective of the present study was to identify and characterize the different-sized naphthalene-catabolic plasmids in order to determine the probable mechanism of horizontal transfer of the nahAc gene in situ. Filter matings between naphthalene-degrading bacterial isolates and their cured progeny revealed that the naphthalene-catabolic plasmids were self-transmissible. Limited interstrain transfer was also found. Analysis of the restriction fragment length polymorphism (RFLP) patterns indicated that catabolic plasmids from 12 site-derived isolates were closely related to each other and to the naphthalene-catabolic plasmid (pDTG1) of Pseudomonas putida NCIB 9816-4, which was isolated decades ago in Bangor, Wales. The similarity among all site-derived naphthalene-catabolic plasmids and pDTG1 was confirmed by using the entire pDTG1 plasmid as a probe in Southern hybridizations. Two distinct but similar naphthalene-catabolic plasmids were retrieved directly from the microbial community indigenous to the contaminated site in a filter mating by using a cured, rifampin-resistant site-derived isolate as the recipient. RFLP patterns and Southern hybridization showed that both of these newly retrieved plasmids, like the isolate-derived plasmids, were closely related to pDTG1. These data indicate that a pDTG1-like plasmid is the mobile genetic element responsible for transferring naphthalene-catabolic genes among bacteria in situ. The pervasiveness and persistence of this naphthalene-catabolic plasmid suggest that it may have played a role in the adaptation of this microbial community to the coal tar contamination at our study site.

Large genome rearrangements discovered by the detailed analysis of 21 Pseudomonas aeruginosa clone C isolates found in environment and disease habitats

In order to determine primary genetic events which occur during the diversification of a Pseudomonas aeruginosa clone in natural habitats, comparative genome analysis of 21 isolates of a predominant clone, called clone C, derived mainly from patients with cystic fibrosis (CF) and the aquatic environment, was carried out. Physical chromosome maps were constructed for the restriction enzymes SpeI, PacI, SwaI and I-CeuI by one and two-dimensional pulsed-field gel electrophoresis and by comparison with the existing strain C map. The positioning of 26 genes generated the genetic maps. Chromosome size varied between 6345 and 6606 kilobase-pairs (kb). A plasmid of 95 kb was detected in the strains of non-CF origin and, in addition, was found to be integrated into the chromosome of all strains but one CF isolate. Four subgroups of clone C strains were discriminated by the acquisition and loss of large blocks of DNA that could cover more than 10% of the chromosome size. The exchange of DNA blocks which ranged in size from 1 kb to 214 kb occurred preferentially around the terminus of replication region which is poor in biosynthetic genes. Genetic material which was additionally introduced into strain C in comparison with strain PAO seems to be a target of mutational processes in clone C strains. Within and among subgroups CF isolates frequently exhibited large inversions affecting the whole chromosomal structure. We concluded that the exchange of DNA blocks by mechanisms of horizontal transfer and large chromosomal inversions are major factors leading to the divergence of a clone in the species P. aeruginosa.

Horizontal gene transfer

This review explores examples of horizontal genetic transfer in eukaryotes and prokaryotes. The best understood of these involves various conserved families of transposable elements, but examples of non-transposable-element-based movement of genes or gene clusters have also been identified in prokaryotic genomes. A unifying theme is the structural and DNA-sequence homology of transposable elements from widely unrelated genomes, suggesting evolutionarily conserved mechanisms for horizontal transfer. This is reinforced by the fundamental similarity in the enzymatic mechanisms of retro viral integration (by integrases) and of transposition (by transposases). The review deals with various types of horizontal transfer, the mechanisms available for such transfer, potential barriers, and the evolutionary significance of horizontal genetic transfer.

Facteurs de virulence des anaerobies

Summary Whether spore-forming or non-spore forming, some anaerobes are important pathogens to consider by reason of their presence in the environment and in the human natural cavities. We will discuss new data in clostridia which are characterized by their ubiquity, their ability to survive as spores under adverse conditions and their capacity to produce potent toxins. New toxins have been recently described, i.e. an hemolysin in C. septicm and a larvicidal toxin, of potential interest as an anti-vectorial agent, in C. bifermentans . A C. butyricum strain and a C. barati strain producing botulinal toxins of type E and F, respectively have been isolated in infant botulism; also a C. botulinum strain producing two toxins, A and B has been isolated in a botulism outbreak. Taken together, these observations raise the question of the genetic determinism underlying the occurrence of toxins amongst previously non toxinogenic clostridial species. Infant botulism is an old disease, but a recently described clinical entity. The precise mechanisms underlying this toxi-infection are actually unknown, and probable involve a temporary immunity deficiency in some newbornsk. In C. difficile , several points have been partially solved. There is now a consensus concerning the existence of two toxins of high molecular weight, A and B that cross react against the present available antisera and thus should share common epitopes. Cloning studies suggest that gene(s) coding for both toxins should be very close, if not identical. Both toxins apparently have a similar mode of action (cytotoxic activity), the toxin A having in addition an enterotoxin action. The receptors for each toxin should be different, and should also differ in man and animal. Several questions are presently still unsolved, particularly; (i) the question in C. difficile of additional virulence determinants; (ii) the lack of virulence of C. difficile in newborns: the lack of specific toxin(s) receptors in the newborn has been suggested, but has yet to be confirmed; ( iii ) the role of antibiotic administration in the onset of the disease: if, in some cases, the administered antibiotic may directly select a C. difficile strain resistant to this antibiotic, in other cases, the antibiotic would exert an indirect action by inhibiting bacterial flora involved in resistance to colonization by C. difficile . The role of Clostridia , and particularly of C. butyricum in neonatal enterocolitis (NNE) is presently a matter of controversy. Epidemiological and experimental evidence have raised the question of the potential role of C. butyricum in NNE, but this species was also found in control infants and no specific toxicity determinant of C. butyricum has been shown in experiments with mice or culture cells, except a thermostable factor of low molecular weight identified as butyric acid. An experimental axenic chicken/quail model was then developed. Whether it is possible to extrapolate the chicken/quail model to human is a mute point. Virulence factors of non-spore forming anaerobes are discussed in most of the communications of the present symposium, and they are dealing with adherence, tissu damaging products, and host-protective properties. We wish just to emphasize the recent characterization of enterotoxigenic Bacteroides fragilis strains causing severe diarrhea in man and livestock, and experimentally capable of producing an enteric disease in infant rabbits. Transferable antibiotic resistance in anaerobes, either in Clostridium or in Bacteroides , are now well documented and such resistances may be considered as virulence cofactors. The genetic determinants of the antibiotic resistances are chromosomally-determined, plasmid-borne or localized on transposons. A relatively great variety of mechanisms of horizontal transfer of these genetic determinants have been identified in those anaerobes: self-mobilizing plasmids in C. perfringens and Bacteroides , conjugal Tc R and Tc R Cc R elements, conjugative transposon-like elements, and a conjugal mobilizing transposon in Bacteroides . In other groups, i.e. C. difficile and C. innocuum chromosomally-determined antibiotic resistance are transferable through a mechanism presently uncharacterized. Recent epidemiological data suggest the spreading of antibiotic resistance in other groups, i.e. the anaerobic cocci.