Publications (15) View all
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Article: Naturally occurring motility-defective mutants of Salmonella enterica serovar Enteritidis isolated preferentially from nonhuman rather than human sources.
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ABSTRACT: Salmonellosis represents a worldwide health problem because it is one of the major causes of food-borne disease. Although motility is postulated as an important Salmonella virulence attribute, there is little information about variation in motility in natural isolates. Here we report the identification of a point mutation (T551 → G) in motA, a gene essential for flagellar rotation, in several Salmonella enterica serovar Enteritidis field isolates. This mutation results in bacteria that can biosynthesize structurally normal but paralyzed flagella and are impaired in their capacity to invade human intestinal epithelial cells. Introduction of a wild-type copy of motA into one of these isolates restored both motility and cell invasiveness. The motA mutant triggered higher proinflammatory transcriptional responses than an aflagellate isolate in differentiated Caco-2 cells, suggesting that the paralyzed flagella are able to signal through pattern recognition receptors. A specific PCR was designed to screen for the T551 → G mutation in a collection of 266 S. Enteritidis field isolates from a nationwide epidemic, comprising 194 from humans and 72 from other sources. We found that 72 of the 266 (27%) isolates were nonmotile, including 24.7% (48/194) of human and 33.3% (24/72) of food isolates. Among nonmotile isolates, 15 carried the T551 → G mutation and, significantly, 13 were recovered from food, including 7 from eggs, but only 2 were from human sources. These results suggest that the presence of paralyzed flagella may impair the ability of S. Enteritidis to cause disease in the human host but does not prevent its ability to colonize chickens and infect eggs.Applied and environmental microbiology 09/2011; 77(21):7740-8. · 3.69 Impact Factor -
Article: Using human demographic history to infer natural selection reveals contrasting patterns on different families of immune genes.
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ABSTRACT: Detecting regions of the human genome that are, or have been, influenced by natural selection remains an important goal for geneticists. Many methods are used to infer selection, but there is a general reliance on an accurate understanding of how mutation and recombination events are distributed, and the well-known link between these processes and their evolutionary transience introduces uncertainty into inferences. Here, we present and apply two new, independent approaches; one based on single nucleotide polymorphisms (SNPs) that exploits geographical patterns in how humans lost variability as we colonized the world, the other based on the relationship between microsatellite repeat number and heterozygosity. We show that the two methods give concordant results. Of these, the SNP-based method is both widely applicable and detects selection over a well-defined time interval, the last 50 000 years. Analysis of all human genes by their Gene Ontology codes reveals how accelerated and decelerated loss of variability are both preferentially associated with immune genes. Applied to 168 immune genes used as the focus of a previous study, we show that members of the same gene family tend to yield similar indices of selection, even when located on different chromosomes. We hope our approach will provide a useful tool with which to infer where selection has acted to shape the human genome.Proceedings of the Royal Society B: Biological Sciences 11/2010; 278(1711):1587-94. · 5.41 Impact Factor -
Article: Differential phenotypic diversity among epidemic-spanning Salmonella enterica serovar enteritidis isolates from humans or animals.
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ABSTRACT: Nontyphoidal salmonellae are major causes of food-borne disease worldwide. In Uruguay, Salmonella enterica serovar Enteritidis was the most commonly isolated serovar throughout the last decade, with a marked epidemic period between 1995 and 2004. In a previous study, we conducted comparative genomics of 29 epidemic-spanning S. Enteritidis field isolates, and here we evaluated the pathogenic potential of the same set of isolates using several phenotypic assays. The sample included 15 isolates from human gastroenteritis, 5 from invasive disease, and 9 from nonhuman sources. Contrary to the genetic homogeneity previously observed, we found great phenotypic variability among these isolates. One-third of them were defective in at least one assay, namely, 10 isolates were defective in motility, 8 in invasion of Caco-2 cells, and 10 in survival in egg albumen. Twelve isolates were tested for invasiveness in 3-day-old chickens, and five of these were significantly less invasive than the reference strain. The two oldest preepidemic isolates were reduced in fitness in all assays, providing a plausible explanation for the previous negligible incidence of S. Enteritidis in Uruguay and supporting the view that the introduction or emergence of a more virulent strain was responsible for the marked rise of this serovar. Further, we found differences in fitness among the isolates which depended on the source of isolation. A total of 1 out of 14 isolates from human gastroenteritis, but 6 out of 13 isolates from other sources, was impaired in at least two assays, suggesting enhanced fitness among strains able to cause intestinal disease in humans.Applied and environmental microbiology 10/2010; 76(20):6812-20. · 3.69 Impact Factor -
Article: Genomic and phenotypic variation in epidemic-spanning Salmonella enterica serovar Enteritidis isolates.
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ABSTRACT: Salmonella enterica serovar Enteritidis (S. Enteritidis) has caused major epidemics of gastrointestinal infection in many different countries. In this study we investigate genome divergence and pathogenic potential in S. Enteritidis isolated before, during and after an epidemic in Uruguay. 266 S. Enteritidis isolates were genotyped using RAPD-PCR and a selection were subjected to PFGE analysis. From these, 29 isolates spanning different periods, genetic profiles and sources of isolation were assayed for their ability to infect human epithelial cells and subjected to comparative genomic hybridization using a Salmonella pan-array and the sequenced strain S. Enteritidis PT4 P125109 as reference. Six other isolates from distant countries were included as external comparators.Two hundred and thirty three chromosomal genes as well as the virulence plasmid were found as variable among S. Enteritidis isolates. Ten out of the 16 chromosomal regions that varied between different isolates correspond to phage-like regions. The 2 oldest pre-epidemic isolates lack phage SE20 and harbour other phage encoded genes that are absent in the sequenced strain. Besides variation in prophage, we found variation in genes involved in metabolism and bacterial fitness. Five epidemic strains lack the complete Salmonella virulence plasmid. Significantly, strains with indistinguishable genetic patterns still showed major differences in their ability to infect epithelial cells, indicating that the approach used was insufficient to detect the genetic basis of this differential behaviour. The recent epidemic of S. Enteritidis infection in Uruguay has been driven by the introduction of closely related strains of phage type 4 lineage. Our results confirm previous reports demonstrating a high degree of genetic homogeneity among S. Enteritidis isolates. However, 10 of the regions of variability described here are for the first time reported as being variable in S. Enteritidis. In particular, the oldest pre-epidemic isolates carry phage-associated genetic regions not previously reported in S. Enteritidis. Overall, our results support the view that phages play a crucial role in the generation of genetic diversity in S. Enteritidis and that phage SE20 may be a key marker for the emergence of particular isolates capable of causing epidemics.BMC Microbiology 11/2009; 9:237. · 3.04 Impact Factor -
Article: Molecular mechanisms involved in inflammasome activation.
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ABSTRACT: Germline-encoded pattern recognition receptors (PRRs) sense microbial or endogenous products released from damaged or dying cells and trigger innate immunity. In most cases, sensing of these signals is coupled to signal transduction pathways that lead to transcription of immune response genes that combat infection or lead to cell death. Members of the NOD-like receptor (NLR) family assemble into large multiprotein complexes, termed inflammasomes. Inflammasomes do not regulate transcription of immune response genes, but activate caspase-1, a proteolytic enzyme that cleaves and activates the secreted cytokines interleukin-1beta and interleukin-18. Inflammasomes also regulate pyroptosis, a caspase-1-dependent form of cell death that is highly inflammatory. Here, we review exciting recent developments on the role of inflammasome complexes in host defense and the discovery of a new DNA sensing inflammasome, and describe important progress made in our understanding of how inflammasomes are activated. Additionally, we highlight how dysregulation of inflammasomes contributes to human disease.Trends in cell biology 10/2009; 19(9):455-64. · 12.12 Impact Factor
