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PubMed2006

Modeling the regulation of the competence-evoking quorum sensing network in Streptococcus pneumoniae.

Competence for genetic transformation seems to play a fundamental role in the biology of Streptococcus pneumoniae and is believed to account for serotype switching, evolution of virulence factors, and rapid emergence of antibiotic resistance. The initiation of competence is regulated by the quorum sensing system referred as the ComABCDE pathway. Experimental studies reveal that competence is down-regulated a short time after its induction and several hypotheses about the mechanism(s) responsible for this shut-down have been presented. Possibly, a ComX-dependent gene product, such as a repressor or a phosphatase, is involved. To better understand the down-regulation of the competence-evoking system in S. pneumoniae, a mathematical model was set up. By analyzing the model, we suggest that shut-down of competence possibly occurs at the transcriptional level on the comCDE operon. As a result of introducing a putative comX-dependent repressor, which inhibits expression of comCDE and comX, in the mathematical model, competence is demonstrated to appear in waves. This is supported by experimental studies showing the appearance of successive competence cycles in pneumococcal batch cultures.

Antigens, Bacterial↗
PubMed2000

The effects of symbiotic crabs on the pumping activity and growth rates of Chaetopterus variopedatus.

This study investigates how the presence of symbiotic crabs (Pinnixa chaetopterana or Polyonyx gibbesi) in the tubes of the polychaete Chaetopterus variopedatus affects the worms' pumping activity and growth rates under laboratory and field conditions. In the field, worms whose tubes are inhabited by Pinnixa beat their fan segments significantly more frequently than do worms hosting Polyonyx, but other measures of pumping activity do not differ according to symbiont species. In the lab, worms tend to move water through their tubes at higher rates when crabs are present. In 7-month laboratory experiments, growth rates of worms hosting either species of crab did not differ from growth rates of worms without crab symbionts. Although worms hosting Polyonyx are, on average, significantly larger than worms hosting Pinnixa, this appears to be due to competition between the crab species for hosts and not due to differential effects on host growth. Unlike the crabs in this study, pea crab species inhabiting bivalves are known to have strong deleterious effects on host growth and reproduction, suggesting that the evolution of virulence in symbiotic interactions is dependent upon specific ecological context.

Journal Article↗
PubMed2003

A system for studying genetic changes in Candida albicans during infection.

Candida albicans is a diploid yeast with a dimorphic life history. It exists commensally in many healthy humans but becomes a potent pathogen in immunocompromised hosts. The underlying genetic mechanisms by which C. albicans switches from a commensal to a pathogenic form in the host are not well understood. To study the evolution of virulence in mammalian hosts, we used GAL1 as selectable marker system that allows for both positive and negative selection in selective media. We show that the deletion of one or both copies of GAL1 in the C. albicans genome does not change virulence in a systemic mouse model. We obtained estimates for the frequency of mitotic recombination at the GAL1 locus during systemic infection. Our observations suggest that genetic changes such as mitotic recombination and gene conversion occur at a high enough frequency to be important in the transition of C. albicans from a commensal to a pathogenic organism.

Animals↗
PubMed1995

Comparative effects of mites and lice on the reproductive success of rock doves (Columba livia).

We report experimental data comparing the effects of Mesostigmatid mites and Ischnoceran lice on the reproductive performance of a single group of captive rock doves (Columba livia). Several components of host reproductive success were compared for the two groups, including number of eggs laid, hatching success, nestling growth rates, fledging success, post-fledging body mass and survival. Adult body mass and survival were also compared. There was a dramatic difference in the effects of the mites and lice. The former drove host reproductive success to zero, mainly by agitating adults and causing them to incubate eggs less faithfully. Nestling growth rates and post-fledging survival were also significantly reduced by mites. Lice showed no effect on reproductive success whatsoever, even though the feather damage they cause is known to have energetic consequences (Booth, Clayton & Block, 1993). Neither parasite had a significant effect on adult birds. Although Ischnocera are found on most species of birds, our results for lice constitute the first experimental test of the impact of Ischnocera on avian reproductive success (preliminary report by Clayton & Tompkins, 1994). We discuss reasons for the different effects of mites and lice, including the relationship of horizontal (mites) and vertical (lice) transmission to the evolution of virulence.

Animals↗
PubMed2001

Intra-stadial tick-borne Thogoto virus (Orthomyxoviridae) transmission: accelerated arbovirus transmission triggered by host death.

Haematophagous insect vectors of arthropod-borne viruses (arboviruses) feed repeatedly. Consequently, they can transmit arboviruses to more than one host during the same developmental stage (intra-stadial transmission). By contrast, ixodid ticks generally feed only once at each parasitic stage (larva, nymph, and adult) and hence they have only one opportunity for tickborne virus transmission per stadium (inter-stadial transmission). Under natural conditions, tick-infested hosts may die (from disease or other causes) before the ticks have completed their long period of engorgement. A laboratory model was used to investigate the consequences of premature host death on tick-borne virus transmission. We report intra-stadial transmission of Thogoto virus by the nymphal, male, and female ticks of Rhipicephalus appendiculatus. Tick-borne Thogoto virus infection caused viraemia and death of hamsters before the nymphal and adult ticks feeding on them had completed engorgement. The resulting partially fed ticks were allowed to continue engorgement on new, uninfected hosts (interrupted feeding). During feeding on the subsequent hosts, they transmitted the virus intra-stadially to susceptible hosts (hamsters), and to uninfected co-feeding ticks on non-susceptible hosts (guinea-pigs). Intra-stadial transmission, mediated by interrupted feeding, may help explain outbreaks of rapid and fatal tick-borne viral diseases, and the epidemiology as well as evolution of virulence, in a susceptible host population. Additionally, intra-stadial transmission provides an increased risk of tick-borne pathogen transmission to humans and domestic animals during slaughter and game hunting.

Animals↗
PubMed2004

Population genetic structure of the sugar beet cyst nematode Heterodera schachtii: a gonochoristic and amphimictic species with highly inbred but weakly differentiated populations.

The sugar beet cyst nematode Heterodera schachtii is a soil-dwelling phytoparasitic nematode that feeds on beet roots. It is an important pest in most sugar beet growing areas, and better knowledge of its genetic variability is an important step to preserve the durability of resistant sugar beet varieties. The population genetic structure of this species in northern France was studied using five microsatellite markers. A hierarchical sampling design was used to investigate spatial structuring at the scale of the region, the field and the plant. Multilocus genotypes were obtained for single individual second-stage larvae, using only one individual per cyst in order to avoid the analysis of closely allied individuals (larvae from the same cyst share at least the same mother). A consistent trend of heterozygote deficit at all loci was observed at all spatial scales. Heterozygote deficit at the level of individual plants argues against its generation through a Wahlund effect. Inbreeding could be due to very limited active dispersal of larvae in the soil, favouring mating between siblings, such as larvae emerging from the same cyst. Such behaviour could have important consequences for the evolution of virulence in increasing the production of homozygous virulent individuals. Moreover, an analysis of molecular variance (amova) reveals that only 1.6% of the genetic variability is observed among regions, 3.7% among fields of the same region and 94.6% within fields. The very low level of genetic differentiation among fields is also indicated by low values of FST (</= 0.105) even for fields 150 km apart. We suggest that long-distance gene flows occur due to passive transport of cysts by human activities, water or wind. As such mechanisms of gene flow would involve cysts and not larvae, the strong signals of inbreeding observed at the local scale should not be disrupted. To our knowledge, this study constitutes the first investigation of the genetic structure of a phytoparasitic nematode based on neutral codominant genetic markers scored in single individuals.

Analysis of Variance↗
PubMed2004

An interspersed refuge for Sitodiplosis mosellana (Diptera: Cecidomyiidae) and a biocontrol agent Macroglenes penetrans (Hymenoptera: Pteromalidae) to manage crop resistance in wheat.

An interspersed refuge of susceptible plants in a resistant, spring-sown wheat crop was tested as a strategy to protect crop resistance against evolution of virulence by the wheat midge Sitodiplosis mosellana (Géhin), and also to conserve a biocontrol agent Macroglenes penetrans(Kirby). Eight replicated field experiments were conducted using seed mixtures of 0, 5, 10, 15 and 100% or 0, 5 and 100% susceptible wheat with an agronomically similar wheat expressing the antibiotic resistance gene Sm1. The frequencies of eggs, mature larvae and parasitized larvae in susceptible and resistant wheat spikes, and midge-affected seeds in the harvest, were recorded for each plot. In susceptible wheat, insect densities and seed damage were typical of those in commercial wheat. In resistant wheat, few larvae completed development, 2% or less compared with about 80% in susceptible wheat, when larvae were sampled at maturity. This resistant wheat also deterred midge oviposition, reducing egg densities by 65% compared with susceptible wheat. The wheat midge and its parasitoid oviposited throughout the plots, and parasitism was density independent. The densities of mature midge larvae and parasitoids were in proportion to the size of the refuge. A 5% susceptible refuge produced about 41 mature larvae for each mature larva from the resistant wheat, and provided effective control of damage. An interspersed refuge of susceptible plants in resistant wheat is a promising strategy for sustaining resistance conferred by Sm1 and biocontrol of the wheat midge.

Animals↗
PubMed2004

Sex ratios of Sitodiplosis mosellana (Diptera: Cecidomyiidae): implications for pest management in wheat (Poaceae).

Sex ratios of populations of the wheat midge Sitodiplosis mosellana Gehin, developing on wheat Triticum aestivum L., were determined at reproduction, adult emergence, and dispersal. The patterns of sex ratio through the life cycle of S. mosellana result from: (i) a genetic mechanism that causes all or nearly all of the progeny of individual females to be a single sex, with an overall sex ratio that is slightly biased at 54-57% females; (ii) a differential mortality during diapause that increases the sex ratio to 60-65% females; (iii) mating which occurs near the emergence site followed by female dispersal which causes the post-dispersal sex ratio to rise to nearly 100% females; and (iv) oviposition which spreads eggs among different plants and assures that the next generation has a local sex ratio close to the population average. These changes in sex ratio through the life cycle have implications for using crop resistance or pheromones to manage S. mosellana, because mating takes place quickly near emergence sites, and because mated females but not males disperse from emergence sites to oviposition sites. Crop refuges used to protect resistance genes against the evolution of virulence by S. mosellana must be interspersed to prevent assortative mating that would occur in separate blocks of resistant and susceptible plants. Monitoring or mating disruption using a pheromone would be ineffective when wheat is grown in rotation with a non-host crop.

Agriculture↗
PubMed1998

Stability of Mycobacterium tuberculosis DNA genotypes.

To assess genotype stability in Mycobacterium tuberculosis, DNA genotypes were compared in sequential isolates from 49 patients who had sputum cultures separated by at least 90 days that grew M. tuberculosis. By use of IS6110 and the polymorphic GC-rich sequence (PGRS) as markers, it was found that paired isolates from 14 (29%) of 49 patients showed changes in their DNA genotypes between isolates (12 in IS6110 genotypes and 2 in PGRS genotypes). Changed IS6110 genotypes were confined to strains with 8-14 bands and were not related to the bacterial drug susceptibility, the patients' human immunodeficiency virus serostatus, or adherence to therapy. Although this rate of change complicates the interpretation of molecular epidemiologic studies, it can be exploited to gain additional insight into disease transmission. Furthermore, IS6110-related mutations may be a major source of genetic plasticity in M. tuberculosis and provide insights into the organism's evolution and virulence.

Antitubercular Agents↗
PubMed1995

The population dynamics of vertically and horizontally transmitted parasites.

We analyse a model of the transmission dynamics of a parasite transmitted both vertically and horizontally. The basic reproductive ratio (R0) of the parasite is shown to be a sum of horizontal and vertical components. We derive expressions for the equilibrium prevalence of infection for a mixture of horizontal and vertical transmission; prevalence can reach 100% if transmission is sufficiently high. At the endemic equilibrium, if prevalence is high, most transmission will in general be vertical, but horizontal transmission rates must be high to reach and stably maintain such an equilibrium. Surprisingly, for such parasites the highest equilibrium rates of vertical transmission are observed when horizontal transmission is very effective. We discuss the implications for assessing the importance of horizontal v. vertical transmission from field data, and we suggest some implications for the evolution of virulence.

Animals↗
PubMed1995

A role for bacteriophages in the evolution and transfer of bacterial virulence determinants.

A virulence-associated region in the genome of Dichelobacter nodosus has been shown to contain an integrase gene which is highly related to the integrases of Shigella flexneri phage Sf6 and coliphages P4 and phi R73, together with open reading frames (vapB, C and D) related to genes borne on plasmids in Neisseria gonorrhoeae, Escherichia coli, Actinobacillus actinomycetemcomitans and Treponema denticola. Similar to P4 and phi R73, the vap region is bracketed by putative bacteriophage att sites and is adjacent to a tRNA gene, which suggests that the vap region has been derived by the integration of a bacteriophage, or a plasmid carrying a bacteriophage-related integrase gene. Many similarities in genes and genes clusters encoding virulence determinants have been found in distantly related bacteria. These genes are often located on plasmids in one organism but on the chromosome in others, implying that transmission of the genes has been followed by integration. Thus, the events which have generated the vap regions of D. nodosus may represent a common mechanism for transfer of virulence determinants. A number of genes involved in the virulence of bacterial pathogens are found on integrated bacteriophages, and we suggest that others will prove to be associated with tRNA genes and/or integrase genes derived from bacteriophages. The use of tRNA genes as integration sites for many bacteriophages and plasmids may favour intergeneric transmission, as tRNA genes are highly conserved.

Attachment Sites, Microbiological↗
PubMed2001

Population biology of multihost pathogens.

The majority of pathogens, including many of medical and veterinary importance, can infect more than one species of host. Population biology has yet to explain why perceived evolutionary advantages of pathogen specialization are, in practice, outweighed by those of generalization. Factors that predispose pathogens to generalism include high levels of genetic diversity and abundant opportunities for cross-species transmission, and the taxonomic distributions of generalists and specialists appear to reflect these factors. Generalism also has consequences for the evolution of virulence and for pathogen epidemiology, making both much less predictable. The evolutionary advantages and disadvantages of generalism are so finely balanced that even closely related pathogens can have very different host range sizes.

Animals↗
PubMed2006

Bacteriophage migration via nematode vectors: host-parasite-consumer interactions in laboratory microcosms.

Pathogens vectored by nematodes pose serious agricultural, economic, and health threats; however, little is known of the ecological and evolutionary aspects of pathogen transmission by nematodes. Here we describe a novel model system with two trophic levels, bacteriophages and nematodes, each of which competes for bacteria. We demonstrate for the first time that nematodes are capable of transmitting phages between spatially distinct patches of bacteria. This model system has considerable advantages, including the ease of maintenance and manipulation at the laboratory bench, the ability to observe many generations in short periods, and the capacity to freeze evolved strains for later comparison to their ancestors. More generally, experimental studies of complex multispecies interactions, host-pathogen coevolution, disease dynamics, and the evolution of virulence may benefit from this model system because current models (e.g., chickens, mosquitoes, and malaria parasites) are costly to maintain, are difficult to manipulate, and require considerable space. Our initial explorations centered on independently assessing the impacts of nematode, bacterium, and phage population densities on virus migration between host patches. Our results indicated that virus transmission increases with worm density and host bacterial abundance; however, transmission decreases with initial phage abundance, perhaps because viruses eliminate available hosts before migration can occur. We discuss the microbial growth dynamics that underlie these results, suggest mechanistic explanations for nematode transmission of phages, and propose intriguing possibilities for future research.

Animals↗
PubMed1995

Adaptive mutation and cocolonization during Helicobacter pylori infection of gnotobiotic piglets.

Clinical isolates of Helicobacter pylori, the gastric pathogen implicated in gastritis, peptic ulcers, and gastric cancer in humans, are diverse in traits likely to be important for colonization and disease. Here we report studies using a gnotobiotic piglet-H. pylori infection model to test for host-specific adaptation and to detect cocolonization by different strains. First, an H. pylori strain that initially had grown only weakly in piglets was adapted to them by spontaneous mutation and selection during 12 serial passages; this resulted in an increase in yield from about 10(3) to > 10(7) bacteria per g of mucosa. Second, piglets were fed mixtures of two different well-adapted strains and the presence of one or both strains was monitored by restriction analysis of a PCR-amplified flagellar (flaA) gene segment. The restriction fragment patterns from pools of bacteria indicated that both strains had colonized most piglets and that both strains were present at more than half of the individual biopsy sites, although often at unequal ratios. This suggests a microcolonial mode of growth with limited migration of bacteria between neighboring sites in the gastric mucosa. We propose that the gnotobiotic piglet-H. pylori infection model will be useful for testing how spontaneous mutation, selection, and DNA transfer between strains during mixed infection may each contribute to adaptation to specific hosts and the evolution of virulence of this important pathogen.

Adaptation, Biological↗
PubMed2003

In vivo lysogenic conversion of Tox(-) Streptococcus pyogenes to Tox(+) with Lysogenic Streptococci or free phage.

Temperate bacteriophage can transfer toxin-encoding genes between bacteria, often resulting in acquired pathogenicity. However, little is known regarding the effects of the eukaryotic host on the phage-pathogen interaction. Using Streptococcus pyogenes as a model, we demonstrate, both in vitro and in vivo, that the eukaryote mediates the efficient induction of toxin-encoding temperate phage and the resultant conversion of Tox(-) flora to Tox(+). Furthermore, we show that both phage induction and subsequent conversion need not happen in the same mammalian host, as host-to-host phage transmission can result in toxigenic conversion within the secondary host. Ultimately, our findings demonstrate that the eukaryotic host serves as an essential component in the phage-mediated evolution of virulence within the microbial population.

Animals↗
PubMed2000

Metronidazole activation is mutagenic and causes DNA fragmentation in Helicobacter pylori and in Escherichia coli containing a cloned H. pylori RdxA(+) (Nitroreductase) gene.

Much of the normal high sensitivity of wild-type Helicobacter pylori to metronidazole (Mtz) depends on rdxA (HP0954), a gene encoding a novel nitroreductase that catalyzes the conversion of Mtz from a harmless prodrug to a bactericidal agent. Here we report that levels of Mtz that partially inhibit growth stimulate forward mutation to rifampin resistance in rdxA(+) (Mtz(s)) and also in rdxA (Mtz(r)) H. pylori strains, and that expression of rdxA in Escherichia coli results in equivalent Mtz-induced mutation. A reversion test using defined lac tester strains of E. coli carrying rdxA(+) indicated that CG-to-GC transversions and AT-to-GC transitions are induced more frequently than other base substitutions. Alkaline gel electrophoretic tests showed that Mtz concentrations near or higher than the MIC for growth also caused DNA breakage in H. pylori and in E. coli carrying rdxA(+), suggesting that this damage may account for most of the bactericidal action of Mtz. Coculture of Mtz(s) H. pylori with E. coli (highly resistant to Mtz) in the presence of Mtz did not stimulate forward mutation in E. coli, indicating that the mutagenic and bactericidal products of Mtz metabolism do not diffuse significantly to neighboring (bystander) cells. Our results suggest that the widespread use of Mtz against other pathogens in people chronically infected with H. pylori may stimulate mutation and recombination in H. pylori, thereby speeding host-specific adaptation, the evolution of virulence, and the emergence of resistance against Mtz and other clinically useful antimicrobials.

Amino Acid Substitution↗
PubMed2026

Unveiling a missing component of the atypical type IV secretion system required for natural transformation of Helicobacter pylori.

Exchange of genetic information by natural transformation shapes bacterial evolution. In Helicobacter pylori it is thought to drive its unusually high recombination rate, which has a crucial role in the evolution of virulence and the propagation of antibiotics resistance genes. While in most cases uptake of the incoming DNA into the periplasm is mediated by type IV pili, in H. pylori this initial step of natural transformation requires ComB, a unique competence-specific type IV secretion system (T4SS). The mechanisms by which ComB mediates DNA uptake are still poorly understood, since T4SS are usually involved in an opposite process of DNA export. Here, we identify a gene (hp1421) that is absolutely required for uptake of the transforming DNA into the periplasm, although distant from the comB operons. We show that hp1421 codes for a hexameric ATPase from the VirB11 family. HP1421 is present in the cytoplasm and interacts with ComB4, another ATPase of the T4SS inner membrane subcomplex. The structural modelling and functional analysis of HP1421 and its interaction with ComB4 indicate that HP1421 is a missing component of the ComB inner-membrane subcomplex that we propose to name ComB11. Phylogenetic analyses show that comB11 is a H. pylori core gene and suggest that the competence-dedicated ComB T4SS was a recent acquisition within Helicobacteraceae. Hence, co-option of the T4SS for DNA transformation requires nearly all the proteins that were previously essential for DNA conjugation.

Helicobacter pylori↗