Complete genome sequence of Salmonella enterica subspecies arizonae str. RKS2983

Virulence Comparison of Salmonella enterica Subsp. enterica Isolates from Chicken and Whole Genome Analysis of the High Virulent Strain S. Enteritidis 211

Background: Salmonellaenterica is one of the common pathogens in both humans and animals that causes salmonellosis and threatens public health all over the world. Methods and Results: Here we determined the virulence phenotypes of nine Salmonellaenterica subsp. enterica (S. enterica) isolates in vitro and in vivo, including pathogenicity to chicken, cell infection, biofilm formation and virulence gene expressions. S. Enteritidis 211 (SE211) was highly pathogenic with notable virulence features among the nine isolates. The combination of multiple virulence genes contributed to the conferring of the high virulence in SE211. Importantly, many mobile genetic elements (MGEs) were found in the genome sequence of SE211, including a virulence plasmid, genomic islands, and prophage regions. The MGEs and CRISPR-Cas system might function synergistically for gene transfer and immune defense. In addition, the neighbor joining tree and the minimum spanning tree were constructed in this study. Conclusions: This study provided both the virulence phenotypes and genomic features, which might contribute to the understanding of bacterial virulence mechanisms in Salmonella enterica subsp. enterica. The first completed genomic sequence for the high virulent S. Enteritidis isolate SE211 and the comparative genomics and phylogenetic analyses provided a preliminary understanding of S. enterica genetics and laid the foundation for further study.

Salmonella secretion systems: Differential roles in pathogen-host interactions

The bacterial genus Salmonella includes a large group of food-borne pathogens that cause a variety of gastrointestinal or systemic diseases in hosts. Salmonella use several secretion devices to inject various effectors targeting eukaryotic hosts, or bacteria. In the past few years, considerable progress has been made towards understanding the structural features and molecular mechanisms of the secretion systems of Salmonella, particularly regarding their roles in host-pathogen interactions. In this review, we summarize the current advances about the main characteristics of the Salmonella secretion systems. Clarifying the roles of the secretion systems in the process of infecting various hosts will broaden our understanding of the importance of microbial interactions in maintaining human health and will provide information for developing novel therapeutic approaches.

The Salmonella enterica Serovar Typhi ltrR Gene Encodes Two Proteins Whose Transcriptional Expression Is Upregulated by Alkaline pH and Repressed at Their Promoters and Coding Regions by H-NS and Lrp

LtrR is a LysR-type regulator involved in the positive expression of ompR to promote ompC and ompF expression. This regulatory network is fundamental for the control of bacterial transformation and resistance to the bile salt sodium deoxycholate in Salmonella enterica serovar Typhi. In this work, the transcriptional regulation of ltrR was characterized, revealing that the use of alternative promoters results in two transcripts. The larger one, the ltrR2 mRNA, was repressed at promoter and coding regions by H-NS, whereas Lrp repressed its expression at the coding region. In the case of the second and shorter ltrR1 transcript, it was repressed only at the coding region by H-NS and Lrp. Remarkably, pH 7.5 is a positive signal involved in the transcriptional expression of both ltrR units. Translational fusions and Western blot experiments demonstrated that ltrR2 and ltrR1 mRNAs encode the LtrR2 and LtrR1 proteins. This study adds new data on the complex genetic and regulatory characteristics of one of the most predominant types of transcriptional factors in bacteria, the LysR-type transcriptional regulators.IMPORTANCE The LysR-type transcriptional regulators are present in viruses, archaea, bacteria, and eukaryotic cells. Furthermore, these proteins are the most abundant transcriptional factors in bacteria. Here, we demonstrate that two LysR-type proteins are generated from the ltrR gene. These proteins are genetically induced by pH and repressed at the promoter and coding regions by the global regulators H-NS and Lrp. Thus, novel basic aspects of the complex genetic regulation of the LysR-type transcriptional regulators are described.

Genetic diversity and evolutionary features of type VI secretion systems in Salmonella

AIM

Type VI secretion systems (T6SS) play key roles in bacterial pathogenesis, but their evolutionary features remain largely unclear. In this study, we conducted systematic comparisons among the documented T6SSs in Salmonella and determined their structural diversity, phylogenetic distribution and lineage-specific properties.

MATERIALS & METHODS

We screened 295 Salmonella genomes for 13 T6SS core components by hidden Markov models and identified 363 T6SS clusters covering types i1, i2, i3 and i4a.

RESULTS

Type i3 and i4a T6SSs were restricted to Salmonella enterica subspecies enterica and Salmonella bongori, respectively. whereas type i2 T6SSs were conserved between S. enterica subspecies, arizonae and diarizonae. S. enterica subspecies salamae, indica and houtenae harbored only type i1 T6SSs, which had wide distribution and high sequence diversity.

CONCLUSION

The diverse Salmonella T6SSs have undergone purifying selection pressures during the bacterial evolution and may be involved in host adaptation.

Distribution of virulence factors, determinants of antibiotic resistance and molecular fingerprinting of Salmonella species isolated from cattle and beef samples: suggestive evidence of animal-to-meat contamination

In this study, three hundred presumptive Salmonella strains isolated from cattle faeces and raw beef samples were subjected to both preliminary and confirmatory tests specific for Salmonella. PCR assays revealed that 100%, 20% and 26.7% of the isolates were positive for 16S rRNA, fliC and fljB gene fragments, respectively. Large proportions (62.4 to 94.3%) of these isolates were multiple antibiotic resistant (MAR) strains that were resistant to three or more antibiotics belonging to different classes. MAR phenotypes Ab1, Ab2, Ab3, Ab7, Ab8, Ab9, Ab26 and Ab27 were dominant among the isolates. Cluster analysis of antibiotic inhibition zone diameter data revealed two major clusters (clusters 1 and 2), and each cluster contained two sub-clusters (1A, 1B, 2A and 2B). PCR data revealed that 27.1% and 30.7% of the isolates possessed the spvC and invA virulent genes, respectively. There was a significant correlation between the possession of MAR phenotypes and virulent gene determinants. Analysis of restriction fragment length polymorphism (RFLP) of 16S rRNA gene fragments using EcoRI and HaeIII showed that large proportions of isolates from beef and cattle faeces produced similar genetic fingerprints. From these results, it is suggested that Salmonella species in cattle are transmitted to beef and, therefore, the consumption of undercooked beef could pose severe health complications on consumers. These findings provide baseline data that could be of great epidemiological importance and, thus, the need to utilise more sensitive typing tools in determining the genetic relatedness of isolates from different sources.

Evolution of host adaptation in the Salmonella typhoid toxin

The evolution of virulence traits is central for the emergence or re-emergence of microbial pathogens and for their adaptation to a specific host 1-5 . Typhoid toxin is an essential virulence factor of the human-adapted bacterial pathogen Salmonella Typhi 6,7 , the cause of typhoid fever in humans 8-12 . Typhoid toxin has a unique A2B5 architecture with two covalently linked enzymatic 'A' subunits, PltA and CdtB, associated with a homopentameric 'B' subunit made up of PltB, which has binding specificity for the N-acetylneuraminic acid (Neu5Ac) sialoglycans 6,13 prominently present in humans 14 . Here, we examine the functional and structural relationship between typhoid toxin and ArtAB, an evolutionarily related AB5 toxin encoded by the broad-host Salmonella Typhimurium 15 . We find that ArtA and ArtB, homologues of PltA and PltB, can form a functional complex with the typhoid toxin CdtB subunit after substitution of a single amino acid in ArtA, while ArtB can form a functional complex with wild-type PltA and CdtB. We also found that, after addition of a single-terminal Cys residue, a CdtB homologue from cytolethal distending toxin can form a functional complex with ArtA and ArtB. In line with the broad host specificity of S. Typhimurium, we found that ArtB binds human glycans, terminated in N-acetylneuraminic acid, as well as glycans terminated in N-glycolylneuraminic acid (Neu5Gc), which are expressed in most other mammals 14 . The atomic structure of ArtB bound to its receptor shows the presence of an additional glycan-binding site, which broadens its binding specificity. Despite equivalent toxicity in vitro, we found that the ArtB/PltA/CdtB chimaeric toxin exhibits reduced lethality in an animal model, indicating that the host specialization of typhoid toxin has optimized its targeting mechanisms to the human host. This is a remarkable example of a toxin evolving to broaden its enzymatic activities and adapt to a specific host.