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1.  Horizontally Acquired Glycosyltransferase Operons Drive Salmonellae Lipopolysaccharide Diversity 
PLoS Genetics  2013;9(6):e1003568.
The immunodominant lipopolysaccharide is a key antigenic factor for Gram-negative pathogens such as salmonellae where it plays key roles in host adaptation, virulence, immune evasion, and persistence. Variation in the lipopolysaccharide is also the major differentiating factor that is used to classify Salmonella into over 2600 serovars as part of the Kaufmann-White scheme. While lipopolysaccharide diversity is generally associated with sequence variation in the lipopolysaccharide biosynthesis operon, extraneous genetic factors such as those encoded by the glucosyltransferase (gtr) operons provide further structural heterogeneity by adding additional sugars onto the O-antigen component of the lipopolysaccharide. Here we identify and examine the O-antigen modifying glucosyltransferase genes from the genomes of Salmonella enterica and Salmonella bongori serovars. We show that Salmonella generally carries between 1 and 4 gtr operons that we have classified into 10 families on the basis of gtrC sequence with apparent O-antigen modification detected for five of these families. The gtr operons localize to bacteriophage-associated genomic regions and exhibit a dynamic evolutionary history driven by recombination and gene shuffling events leading to new gene combinations. Furthermore, evidence of Dam- and OxyR-dependent phase variation of gtr gene expression was identified within eight gtr families. Thus, as O-antigen modification generates significant intra- and inter-strain phenotypic diversity, gtr-mediated modification is fundamental in assessing Salmonella strain variability. This will inform appropriate vaccine and diagnostic approaches, in addition to contributing to our understanding of host-pathogen interactions.
Author Summary
Bacterial pathogens frequently evolve mechanisms to vary the composition of their surface structures. The consequence is enhanced long-term survival by facilitating persistence and evasion of the host immune system. Salmonella sp., cause severe infections in a range of mammalian hosts and guard themselves with a protective coat, termed the O-antigen. Through genome sequence analyses we found that Salmonella have acquired an unprecedented repertoire of genetic sequences for modifying their O-antigen coat. There is strong evidence that these genetic factors have a dynamic evolutionary history and are spread through the bacterial population by bacteriophage. In addition to this genetic repertoire, we determined that Salmonella can and often do employ stochastic mechanisms for expression of these genetic factors. This means that O-antigen coat diversity can be generated within a Salmonella population that otherwise has a common genome. Our data significantly enhance our appreciation of the genetic and regulatory characteristics underpinning Salmonella O-antigen diversity. The role attributed to bacteriophage in generating this diversity highlights that Salmonella are acquiring an extensive repertoire of O-antigen modifying traits that may enhance the pathogen's ability to persist and cause disease in mammalian hosts. Such genetic traits may make useful markers for defining new epidemiological and diagnostic tools.
doi:10.1371/journal.pgen.1003568
PMCID: PMC3688519  PMID: 23818865
2.  YhdJ, a Nonessential CcrM-Like DNA Methyltransferase of Escherichia coli and Salmonella enterica▿  
Journal of Bacteriology  2007;189(11):4325-4327.
The Caulobacter crescentus DNA adenine methyltransferase CcrM and its homologs in the α-Proteobacteria are essential for viability. CcrM is 34% identical to the yhdJ gene products of Escherichia coli and Salmonella enterica. This study provides evidence that the E. coli yhdJ gene encodes a DNA adenine methyltransferase. In contrast to an earlier report, however, we show that yhdJ is not an essential gene in either E. coli or S. enterica.
doi:10.1128/JB.01854-06
PMCID: PMC1913422  PMID: 17400740

Results 1-2 (2)