Relationship between evolutionary rate and cellular location among the Inv/Spa invasion proteins of Salmonella enterica

Genomic Characterisation of Mushroom Pathogenic Pseudomonads and Their Interaction with Bacteriophages

Bacterial diseases of the edible white button mushroom Agaricus bisporus caused by Pseudomonas species cause a reduction in crop yield, resulting in considerable economic loss. We examined bacterial pathogens of mushrooms and bacteriophages that target them to understand the disease and opportunities for control. The Pseudomonastolaasii genome encoded a single type III protein secretion system (T3SS), but contained the largest number of non-ribosomal peptide synthase (NRPS) genes, multimodular enzymes that can play a role in pathogenicity, including a putative tolaasin-producing gene cluster, a toxin causing blotch disease symptom. However, Pseudomonasagarici encoded the lowest number of NRPS and three putative T3SS while non-pathogenic Pseudomonas sp. NS1 had intermediate numbers. Potential bacteriophage resistance mechanisms were identified in all three strains, but only P. agarici NCPPB 2472 was observed to have a single Type I-F CRISPR/Cas system predicted to be involved in phage resistance. Three novel bacteriophages, NV1, ϕNV3, and NV6, were isolated from environmental samples. Bacteriophage NV1 and ϕNV3 had a narrow host range for specific mushroom pathogens, whereas phage NV6 was able to infect both mushroom pathogens. ϕNV3 and NV6 genomes were almost identical and differentiated within their T7-like tail fiber protein, indicating this is likely the major host specificity determinant. Our findings provide the foundations for future comparative analyses to study mushroom disease and phage resistance.

A real-time PCR regimen for testing environmental samples for Salmonella enterica subsp. enterica serovars of concern to the poultry industry, with special focus on Salmonella Enteritidis

A real-time PCR (qPCR) regimen, using up to six genetic targets, was developed to rapidly detect Salmonella and in particular identify Salmonella Enteritidis. The test regimen was first evaluated using a reference culture collection of Salmonella to confirm the appropriateness of the selected targets, which included up to three genetic markers for discrimination of Salmonella Enteritidis from other Salmonella serovars commonly found in poultry facilities. The qPCR procedure was then compared with culture methods used to detect Salmonella using a collection of enrichment broths previously generated from 239 environmental samples collected from a large number of hatchery facilities across Canada over several years. The qPCR regimen facilitated specific detection of Salmonella Enteritidis, and on a sample basis, it showed excellent agreement with the culture methods. Moreover, in many cases, qPCR detected Salmonella earlier in the culture process than did the culture method. Application of this method will significantly shorten test times and allow more timely identification of infected poultry premises, thereby improving present programmes aimed at controlling Salmonella Enteritidis at the environmental source.

Functional relatedness in the Inv/Mxi-Spa type III secretion system family

Type III Secretion Systems (T3SSs) are structurally conserved nanomachines that span the inner and outer bacterial membranes, and via a protruding needle complex contact host cell membranes and deliver type III effector proteins. T3SS are phylogenetically divided into several families based on structural basal body components. Here we have studied the evolutionary and functional conservation of four T3SS proteins from the Inv/Mxi-Spa family: a cytosolic chaperone, two hydrophobic translocators that form a plasma membrane-integral pore, and the hydrophilic 'tip complex' translocator that connects the T3SS needle to the translocon pore. Salmonella enterica serovar Typhimurium (S. Typhimurium), a common cause of food-borne gastroenteritis, possesses two T3SSs, one belonging to the Inv/Mxi-Spa family. We used invasion-deficient S. Typhimurium mutants as surrogates for expression of translocator orthologs identified from an extensive phylogenetic analysis, and type III effector translocation and host cell invasion as a readout for complementation efficiency, and identified several Inv/Mxi-Spa orthologs that can functionally substitute for the S. Typhimurium chaperone and translocator proteins. Functional complementation correlates with amino acid sequence identity between orthologs, but varies considerably between the four proteins. This is the first in-depth survey of the functional interchangeability of Inv/Mxi-Spa T3SS proteins acting directly at the host-pathogen interface.

Protein Acetylation Is Involved in Salmonella enterica Serovar Typhimurium Virulence

Salmonella causes a range of diseases in different hosts, including enterocolitis and systemic infection. Lysine acetylation regulates many eukaryotic cellular processes, but its function in bacteria is largely unexplored. The acetyltransferase Pat and NAD(+)-dependent deacetylase CobB are involved in the reversible protein acetylation in Salmonella Typhimurium. Here, we used cell and animal models to evaluate the virulence of pat and cobB deletion mutants in S. Typhimurium and found that pat is critical for bacterial intestinal colonization and systemic infection. Next, to understand the underlying mechanism, genome-wide transcriptome was analyzed. RNA sequencing data showed that the expression of Salmonella pathogenicity island 1 (SPI-1) is partially dependent on pat In addition, we found that HilD, a key transcriptional regulator of SPI-1, is a substrate of Pat. The acetylation of HilD by Pat maintained HilD stability and was essential for the transcriptional activation of HilA. Taken together, these results suggest that a protein acetylation system regulates SPI-1 expression by controlling HilD in a posttranslational manner to mediate S. Typhimurium virulence.

The Pyromaniac Inside You: Salmonella Metabolism in the Host Gut

A metabolically diverse microbial community occupies all available nutrient-niches in the lumen of the mammalian intestine, making it difficult for pathogens to establish themselves in this highly competitive environment. Salmonella serovars sidestep the competition by using their virulence factors to coerce the host into creating a novel nutrient-niche. Inflammation-derived nutrients available in this new niche support a bloom of Salmonella serovars, thereby ensuring transmission of the pathogen to the next susceptible host by the fecal-oral route. Here we review the anaerobic food chain that characterizes resident gut-associated microbial communities along with the winning metabolic strategy Salmonella serovars use to edge out competing microbes in the inflamed intestine.

SYBR®Green qPCR Salmonella detection system allowing discrimination at the genus, species and subspecies levels

In this work, a three-level Salmonella detection system based on a combination of seven SYBR®Green qPCR was developed. This detection system discriminates Salmonella at the genus, species and subspecies levels using a single 96-well plate. The SYBR®Green qPCR assays target the invA, rpoD, iroB and safC genes, as well as the STM0296 locus, putatively coding for a cytoplasmic protein. This study includes the design of primer pairs, in silico and in situ selectivity, sensitivity, repeatability and reproducibility evaluations of the seven SYBR®Green qPCR assays. Each detection level displayed a selectivity of 100 %. This combinatory SYBR®Green qPCR system was also compared with three commercially available Salmonella qPCR detection kits. This comparison highlighted the importance of using a multi-gene detection system to be able to detect every target strain, even those with deletion or mutation of important genes.

Non-contiguous finished genome sequence and description of Salmonella enterica subsp. houtenae str. RKS3027

Salmonella enterica subsp. houtenae serovar 16:z4, z32:-- str. RKS3027 was isolated from a human in Illinois, USA. S. enterica subsp. houtenae is a facultative aerobic rod-shaped Gram-negative bacterium. Here we describe the features of this organism, together with the draft genome sequence and annotation. The 4,404,136 bp long genome (97 contigs) contains 4,335 protein-coding gene and 28 RNA genes.

Mapping and regulation of genes within Salmonella pathogenicity island 12 that contribute to in vivo fitness of Salmonella enterica Serovar Typhimurium

Salmonella pathogenicity island 12 (SPI-12) of Salmonella enterica serovar Typhimurium is a 15-kb region that encompasses genes STM2230 to STM2245 and encodes a remnant phage known to contribute to bacterial virulence. In mouse infection experiments and replication assays in macrophages, we demonstrated a role for four genes in SPI-12 for bacterial survival in the host. STM2239, a potential Q antiterminator, showed a prominent contribution to bacterial fitness. Transcriptional reporter experiments, quantitative reverse transcription-PCR (RT-PCR), and immunoblotting demonstrated that the virulence regulator SsrB and STM2239 contribute to transcriptional activation of genes in SPI-12. SsrB was found to indirectly regulate this locus by transcriptional read-through from the sspH2 (STM2241) promoter. Chromatin immunoprecipitation showed that STM2239 copurified with the promoter regulating STM2237, suggesting that STM2239 may function as an antiterminator to activate adjacent genes. These results demonstrate that bacteriophage genes may be adapted by pathogenic bacteria to improve fitness in the host.

A plethora of virulence strategies hidden behind nuclear targeting of microbial effectors

Plant immune responses depend on the ability to couple rapid recognition of the invading microbe to an efficient response. During evolution, plant pathogens have acquired the ability to deliver effector molecules inside host cells in order to manipulate cellular and molecular processes and establish pathogenicity. Following translocation into plant cells, microbial effectors may be addressed to different subcellular compartments. Intriguingly, a significant number of effector proteins from different pathogenic microorganisms, including viruses, oomycetes, fungi, nematodes, and bacteria, is targeted to the nucleus of host cells. In agreement with this observation, increasing evidence highlights the crucial role played by nuclear dynamics, and nucleocytoplasmic protein trafficking during a great variety of analyzed plant-pathogen interactions. Once in the nucleus, effector proteins are able to manipulate host transcription or directly subvert essential host components to promote virulence. Along these lines, it has been suggested that some effectors may affect histone packing and, thereby, chromatin configuration. In addition, microbial effectors may either directly activate transcription or target host transcription factors to alter their regular molecular functions. Alternatively, nuclear translocation of effectors may affect subcellular localization of their cognate resistance proteins in a process that is essential for resistance protein-mediated plant immunity. Here, we review recent progress in our field on the identification of microbial effectors that are targeted to the nucleus of host plant cells. In addition, we discuss different virulence strategies deployed by microbes, which have been uncovered through examination of the mechanisms that guide nuclear localization of effector proteins.

Comparison of real-time PCR, reverse transcriptase real-time PCR, loop-mediated isothermal amplification, and the FDA conventional microbiological method for the detection of Salmonella spp. in produce

Contamination of foods, especially produce, with Salmonella spp. is a major concern for public health. Several methods are available for the detection of Salmonella in produce, but their relative efficiency for detecting Salmonella in commonly consumed vegetables, often associated with outbreaks of food poisoning, needs to be confirmed. In this study, the effectiveness of three molecular methods for detection of Salmonella in six produce matrices was evaluated and compared to the FDA microbiological detection method. Samples of cilantro (coriander leaves), lettuce, parsley, spinach, tomato, and jalapeno pepper were inoculated with Salmonella serovars at two different levels (10(5) and <10(1) CFU/25 g of produce). The inoculated produce was assayed by the FDA Salmonella culture method (Bacteriological Analytical Manual) and by three molecular methods: quantitative real-time PCR (qPCR), quantitative reverse transcriptase real-time PCR (RT-qPCR), and loop-mediated isothermal amplification (LAMP). Comparable results were obtained by these four methods, which all detected as little as 2 CFU of Salmonella cells/25 g of produce. All control samples (not inoculated) were negative by the four methods. RT-qPCR detects only live Salmonella cells, obviating the danger of false-positive results from nonviable cells. False negatives (inhibition of either qPCR or RT-qPCR) were avoided by the use of either a DNA or an RNA amplification internal control (IAC). Compared to the conventional culture method, the qPCR, RT-qPCR, and LAMP assays allowed faster and equally accurate detection of Salmonella spp. in six high-risk produce commodities.

The LysR-type regulator QseA regulates both characterized and putative virulence genes in enterohaemorrhagic Escherichia coli O157:H7

Enterohaemorrhagic Escherichia coli (EHEC) colonizes the large intestine, causing attaching and effacing (AE) lesions. Most of the genes involved in AE lesion formation are encoded within a chromosomal pathogenicity island termed the locus of enterocyte effacement (LEE). The LysR-type transcriptional factor QseA regulates the LEE by binding to the regulatory region of ler. We performed transcriptome analyses comparing wild-type (WT) EHEC and the qseA mutant to elucidate QseA's role in gene regulation. During both growth phases, several genes carried in O-islands were activated by QseA, whereas genes involved in cell metabolism were repressed. During late-logarithmic growth, QseA activated expression of the LEE genes as well as non-LEE-encoded effector proteins. We also performed electrophoretic mobility shift assays, competition experiments and DNase I footprints. The results demonstrated that QseA directly binds both the ler proximal and distal promoters, its own promoter, as well as promoters of genes encoded in EHEC-specific O-islands. Additionally, we mapped the transcriptional start site of qseA, leading to the identification of two promoter sequences. Taken together, these results indicate that QseA acts as a global regulator in EHEC, co-ordinating expression of virulence genes.

Type III secretion systems shape up as they ship out

Virulence associated protein type III secretion systems (T3SSs) are intricately structured organic nanosyringes that achieve the translocation of bacterial proteins from the prokaryotic cytoplasm across three membranes into the host cytosol. The substrates for these systems number in the hundreds, with remarkably diverse biological activities, modulating host cell biology for the benefit of the pathogen. Although there has been tremendous progress on the structure and function of the T3SS substrates, there has been comparatively little progress on the much more highly conserved secretion apparatus itself. This review summarizes recent advances in the field of structural microbiology that have begun to address this shortcoming, finally bringing to bear the power of structural biology to this central virulence system of Gram-negative bacterial pathogens.

Lipocalin-2 resistance confers an advantage to Salmonella enterica serotype Typhimurium for growth and survival in the inflamed intestine

In response to enteric pathogens, the inflamed intestine produces antimicrobial proteins, a process mediated by the cytokines IL-17 and IL-22. Salmonella enterica serotype Typhimurium thrives in the inflamed intestinal environment, suggesting that the pathogen is resistant to antimicrobials it encounters in the intestinal lumen. However, the identity of these antimicrobials and corresponding bacterial resistance mechanisms remain unknown. Here, we report that enteric infection of rhesus macaques and mice with S. Typhimurium resulted in marked Il-17- and IL-22-dependent intestinal epithelial induction and luminal accumulation of lipocalin-2, an antimicrobial protein that prevents bacterial iron acquisition. Resistance to lipocalin-2, mediated by the iroBCDE iroN locus, conferred a competitive advantage to the bacterium in colonizing the inflamed intestine of wild-type but not of lipocalin-2-deficient mice. Thus, resistance to lipocalin-2 defines a specific adaptation of S. Typhimurium for growth in the inflamed intestine.

Pseudomonas syringae HrpP Is a type III secretion substrate specificity switch domain protein that is translocated into plant cells but functions atypically for a substrate-switching protein

Pseudomonas syringae delivers virulence effector proteins into plant cells via an Hrp1 type III secretion system (T3SS). P. syringae pv. tomato DC3000 HrpP has a C-terminal, putative T3SS substrate specificity switch domain, like Yersinia YscP. A DeltahrpP DC3000 mutant could not cause disease in tomato or elicit a hypersensitive response (HR) in tobacco, but the HR could be restored by expression of HrpP in trans. Though HrpP is a relatively divergent protein in the T3SS of different P. syringae pathovars, hrpP from P. syringae pv. syringae 61 and P. syringae pv. phaseolicola 1448A restored HR elicitation and pathogenicity to DC3000 DeltahrpP. HrpP was translocated into Nicotiana benthamiana cells via the DC3000 T3SS when expressed from its native promoter, but it was not secreted in culture. N- and C-terminal truncations of HrpP were tested for their ability to be translocated and to restore HR elicitation activity to the DeltahrpP mutant. No N-terminal truncation completely abolished translocation, implying that HrpP has an atypical T3SS translocation signal. Deleting more than 20 amino acids from the C terminus abolished the ability to restore HR elicitation. HrpP fused to green fluorescent protein was no longer translocated but could restore HR elicitation activity to the DeltahrpP mutant, suggesting that translocation is not essential for the function of HrpP. No T3SS substrates were detectably secreted by DC3000 DeltahrpP except the pilin subunit HrpA, which unexpectedly was secreted poorly. HrpP may function somewhat differently than YscP because the P. syringae T3SS pilus likely varies in length due to differing plant cell walls.

Recombinant SpaO and H1a as immunogens for protection of mice from lethal infection with Salmonella paratyphi A: implications for rational design of typhoid fever vaccines

The Vi capsular polysaccharide vaccine is one of two vaccines against typhoid recommended worldwide and is the vaccine generally used in China. However, in recent years a Salmonella paratyphi A strain that is naturally devoid of capsule has caused frequent outbreaks of typhoid fever in Southern China, leading to the need for identification of additional antigens that could be incorporated into new vaccines. SpaO acts as a major invasion factor of Salmonella enterica spp. and H1a is the unique flagellin subunit ofS. paratyphi A. In this study, the two prokaryotic recombinant antigens, rSpaO and rH1a, were expressed and their immunogenicity was demonstrated by the slide agglutination test and Western blot assays. Using PCR and sequencing analysis as well as ELISA, we find that the spaO and h1a genes are widely distributed in 196 S. paratyphi A isolates (97.5 and 100%, respectively), with high expression frequencies for the SpaO (98.0%) and H1a (100%) antigens. The two genes also show high sequence conservation (similarities from 99.31 to 99.88% for both genes). In sera from 172 paratyphoid A patients, anti-SpaO and anti-H1a IgGs were detectable by ELISA, in 94.8 and 98.8% of patients, respectively. Furthermore, 41.7-66.7% of mice immunized with rSpaO or rH1a alone were protected against subsequent infection, and the protection rate rose to 75.0-91.7% in mice co-immunized with the two antigens. As the spaO and h1a genes of S. paratyphi A are sequence conserved, extensively distributed and highly expressed, the rSpaO and rH1a immunogens should be considered in the development of novel vaccines to prevent S. paratyphi A-caused typhoid fever.

Rapid quantification of Salmonella typhimurium inoculated to meat products by real-time PCR

The objective of this study was to use a 5'-nuclease (TaqMan) real-time PCR method with primers and probe specific to the spaQ gene as a rapid approach to quantitatively determine Salmonella Typhimurium. The result showed that the correlation coefficient between real-time PCR estimates and bovine serum albumin (BSA) plate counts of S. Typhimurium was 0.99, independently of 10(5)-fold numbers of bystander Escherichia coli O157:H7 or total viable counts. The sensitivity of the real-time quantitative PCR assay was 10 CFU/mL for pure S. Typhimurium culture without enrichment. A known number of S. Typhimurium target cells were inoculated to dumpling fillings and chicken nuggets and DNA was extracted for real-time PCR analysis. The sensitivity was 60 CFU/g for S. Typhimurium inoculated to the food samples without any preceding procedure of enrichment. The duration of the entire experiment from DNA isolation and purification to PCR amplification was less than 12 h. This study demonstrated that real-time PCR is a rapid and reliable technique for quantifying S. Typhimurium possessing the spaQ gene in pure culture and in meat products.

Pathogenic adaptation of intracellular bacteria by rewiring a cis-regulatory input function

The acquisition of DNA by horizontal gene transfer enables bacteria to adapt to previously unexploited ecological niches. Although horizontal gene transfer and mutation of protein-coding sequences are well-recognized forms of pathogen evolution, the evolutionary significance of cis-regulatory mutations in creating phenotypic diversity through altered transcriptional outputs is not known. We show the significance of regulatory mutation for pathogen evolution by mapping and then rewiring a cis-regulatory module controlling a gene required for murine typhoid. Acquisition of a binding site for the Salmonella pathogenicity island-2 regulator, SsrB, enabled the srfN gene, ancestral to the Salmonella genus, to play a role in pathoadaptation of S. typhimurium to a host animal. We identified the evolved cis-regulatory module and quantified the fitness gain that this regulatory output accrues for the bacterium using competitive infections of host animals. Our findings highlight a mechanism of pathogen evolution involving regulatory mutation that is selected because of the fitness advantage the new regulatory output provides the incipient clones.

Rapid classification and identification of salmonellae at the species and subspecies levels by whole-cell matrix-assisted laser desorption ionization-time of flight mass spectrometry

Variations in the mass spectral profiles of multiple housekeeping proteins of 126 strains representing Salmonella enterica subsp. enterica (subspecies I), S. enterica subsp. salamae (subspecies II), S. enterica subsp. arizonae (subspecies IIIa), S. enterica subsp. diarizonae (subspecies IIIb), S. enterica subsp. houtenae (subspecies IV), and S. enterica subsp. indica (subspecies VI), and Salmonella bongori were analyzed to obtain a phylogenetic classification of salmonellae based on whole-cell matrix-assisted laser desorption ionization-time of flight mass spectrometric bacterial typing. Sinapinic acid produced highly informative spectra containing a large number of biomarkers and covering a wide molecular mass range (2,000 to 40,000 Da). Genus-, species-, and subspecies-identifying biomarker ions were assigned on the basis of available genome sequence data for Salmonella, and more than 200 biomarker peaks, which corresponded mainly to abundant and highly basic ribosomal or nucleic acid binding proteins, were selected. A detailed comparative analysis of the biomarker profiles of Salmonella strains revealed sequence variations corresponding to single or multiple amino acid changes in multiple housekeeping proteins. The resulting mass spectrometry-based bacterial classification was very comparable to the results of DNA sequence-based methods. A rapid protocol that allowed identification of Salmonella subspecies in minutes was established.

Protective role of Hemidesmus indicus R. Br. root extract against Salmonella typhimurium-induced cytotoxicity in Int 407 cell line

The present investigation deals with the effect of the chloroform fraction composed of sterols and fatty acids isolated from Hemidesmus indicus root extract (CHI) on Salmonella enterica serovar Typhimurium (S. typhimurium)-induced cytotoxicity in a human intestinal epithelial cell line (Int 407). The optimum dose was fixed as 100 microg/mL for CHI against S. typhimurium, which was quite safe for Int 407 cells as the CD(50) concentration (50% cell death) of CHI was determined to be 500 microg/mL in the Int 407 cell line. CHI-treated S. typhimurium were 10-fold less cytotoxic and 40% less adherent to host cells than wild-type. Treatment of CHI significantly abrogated the invasion ability to 10- to 15-fold in S. typhimurium. The cells infected with CHI-treated S. typhimurium had a comparable viability to uninfected cells in the epithelial cell detachment assay. Immunofluorescence showed the CHI-treated bacteria were unhealthy and shrunken rods in comparison with the wild-type bacteria; those were firmly attached and invaded to deceased and hypertrophoid Int 407 cells. Transmission electron micrographs of Int 407 cells infected with wild bacteria showed a coat of adherent and invaded bacteria completely occupying the cytoplasm with characteristic Salmonella-containing vacuoles (SCV). Both necrotic and apoptotic type of cell death were observed in cells infected with wild-type bacteria, whereas most of the cells infected with treated bacteria were normal in morphology and a few had invaded bacteria, but the typical proliferated SCV was not observed in cells infected with CHI-treated S. typhimurium. In summary, the sterols and fatty acids present in CHI may be capable of taming S. typhimurium by suppressing its cytotoxic activity in an intestinal epithelial cell line.

Salmonellae interplay with host cells

Salmonellae are important causes of enteric diseases in all vertebrates. Characterization of the molecular mechanisms that underpin the interactions of salmonellae with their animal hosts has advanced greatly over the past decade, mainly through the study of Salmonella enterica serovar Typhimurium in tissue culture and animal models of infection. Knowledge of these bacterial processes and host responses has painted a dynamic and complex picture of the interaction between salmonellae and animal cells. This Review focuses on the molecular mechanisms of these host-pathogen interactions, in terms of their context, significance and future perspectives.

Protein delivery into eukaryotic cells by type III secretion machines

Bacteria that have sustained long-standing close associations with eukaryotic hosts have evolved specific adaptations to survive and replicate in this environment. Perhaps one of the most remarkable of those adaptations is the type III secretion system (T3SS)--a bacterial organelle that has specifically evolved to deliver bacterial proteins into eukaryotic cells. Although originally identified in a handful of pathogenic bacteria, T3SSs are encoded by a large number of bacterial species that are symbiotic or pathogenic for humans, other animals including insects or nematodes, and plants. The study of these systems is leading to unique insights into not only organelle assembly and protein secretion but also mechanisms of symbiosis and pathogenesis.

Molecular Interactions between Bacterial Symbionts and Their Hosts

Symbiotic bacteria are important in animal hosts, but have been largely overlooked as they have proved difficult to culture in the laboratory. Approaches such as comparative genomics and real-time PCR have provided insights into the molecular mechanisms that underpin symbiont-host interactions. Studies on the heritable symbionts of insects have yielded valuable information about how bacteria infect host cells, avoid immune responses, and manipulate host physiology. Furthermore, some symbionts use many of the same mechanisms as pathogens to infect hosts and evade immune responses. Here we discuss what is currently known about the interactions between bacterial symbionts and their hosts.

Identification of core and variable components of the Salmonella enterica subspecies I genome by microarray

We have performed microarray hybridization studies on 40 clinical isolates from 12 common serovars within Salmonella enterica subspecies I to identify the conserved chromosomal gene pool. We were able to separate the core invariant portion of the genome by a novel mathematical approach using a decision tree based on genes ranked by increasing variance. All genes within the core component were confirmed using available sequence and microarray information for S. enterica subspecies I strains. The majority of genes within the core component had conserved homologues in Escherichia coli K-12 strain MG1655. However, many genes present in the conserved set which were absent or highly divergent in K-12 had close homologues in pathogenic bacteria such as Shigella flexneri and Pseudomonas aeruginosa. Genes within previously established virulence determinants such as SPI1 to SPI5 were conserved. In addition several genes within SPI6, all of SPI9, and three fimbrial operons (fim, bcf, and stb) were conserved within all S. enterica strains included in this study. Although many phage and insertion sequence elements were missing from the core component, approximately half the pseudogenes present in S. enterica serovar Typhi were conserved. Furthermore, approximately half the genes conserved in the core set encoded hypothetical proteins. Separation of the core and variant gene sets within S.enterica subspecies I has offered fundamental biological insight into the genetic basis of phenotypic similarity and diversity across S. enterica subspecies I and shown how the core genome of these pathogens differs from the closely related E. coli K-12 laboratory strain.

Host specificity of septicemic Escherichia coli: human and avian pathogens

Extraintestinal pathogenic Escherichia coli (ExPEC) strains are the cause of a diverse spectrum of invasive human and animal infections, often leading to septicemia. ExPEC strains contain virulence factors that enable them to survive in the host blood and tissues. Most of these virulence factors are distributed in ExPEC strains in a host-independent fashion. Genomic analyses of these strains provide evidence for numerous recombinational events and horizontal gene transfer, as well as for a high diversity of virulence factors. In studies of human and avian septicemic strains of serotypes O2 and O78 it appears that there is a positive correlation between virulence, invasiveness and clonal origin. Yet, it is clear that clonal division in these strains, as well as distribution of virulence factors, is independent of the host and closely related clones reside in different hosts. Although the possibility exists that ExPEC strains do have a certain degree of host specificity, which is not obvious from genomic studies, it is clear that the similarity of virulence factors presents a significant zoonotic risk.

Molecular characterization of a new serovar of Salmonella bongori 13,22:z39:– isolated from a lizard

Three Salmonella strains isolated from a lizard (Gallotia simoni) in the "Isla del Hierro" (Canary Islands, Spain) were serotyped as Salmonella bongori serotype 13,22:z39:-, which has not been described in the Kauffmann-White scheme of Salmonella serovars. In order to shed light on the assignment of those strains to the S. bongori species, several genes were amplified and/or sequenced. The iroB gene has been reported to be present only in S. enterica, while the invA gene has been described as being a helpful tool in distinguishing Salmonella from other bacterial species. Both genes were amplified and, as expected, only invA could be amplified. The fliC gene, encoding the phase 1 flagellin fljB gene, encoding phase 2 flagellin, and the gapA gene, which is believed to present polymorphic alleles among different subspecies, were amplified and sequenced. The sequence obtained from fliC(z39) matched with the sequences fliC(z39) obtained from other serovars. The sequence obtained from gapA clustered into the S. bongori group when it was compared to others previously described. We conclude that these three isolates are members of the S. bongori species representing a new serovar that will be described in the next supplement to the Kauffmann-White scheme.

Evolution and Ecology of Salmonella

Over the past 120 to 160 million years, the genus Salmonella has evolved into a complex group of more than 2,300 genetically and phenotypically diverse serovars. Members of this genus are able to infect a wide diversity of vertebrate and invertebrate hosts; disease manifestations in humans range from gastroenteritis to typhoid fever. The evolution of the genus Salmonella and the divergence and radiation of particular lineages within this group have resulted from selection acting on new genetic variation generated by events such as the gain, loss, and/or rearrangement of genetic material. These types of genetic events have contributed to the speciation of Salmonella from its ancestral association with cold-blood animals to a pathogen of warm-blooded hosts. Moreover, adaptive radiation due to changes in gene content within S. enterica subspecies I has impacted host specificity and aided in the selection of host-restricted, host-adapted, and non-host-adapted serovars. In addition to the genetic diversity important for the wide phenotypic heterogeneity within the genus, a subset of core Salmonella-specific genes present in all Salmonella species and serovars has been identified that may contribute to the conserved aspects of the lifestyle of this microorganism, including the ability to survive in nutrient-poor nonhost environments such as soil and water. Whole-genome comparisons of isolates differing in host range and virulence will continue to elucidate the genetic mechanisms that have contributed to the evolution and diverse ecology of the genus Salmonella.

Salmonella enterica serotype Choleraesuis: epidemiology, pathogenesis, clinical disease, and treatment

Nontyphoid Salmonella strains are important causes of reportable food-borne infection. Among more than 2,000 serotypes, Salmonella enterica serotype Choleraesuis shows the highest predilection to cause systemic infections in humans. The most feared complication of serotype Cholearesuis bacteremia in adults is the development of mycotic aneurysm, which previously was almost uniformally fatal. The advances in diagnostic techniques, surgical care, and antimicrobial therapy have greatly improved the survival of these patients. However, the recent emergence of serotype Choleraesuis that is resistant to ampicillin, chloramphenicol, trimethoprim-sulfamethoxazole, and, notably, fluoroquinolone antibiotics has aroused concern about the use of these agents for the empirical treatment of systemic infection caused by this organism. In view of the serious implications of the situation, the chain of transmission and mechanism of resistance should be carefully studied to reduce the spread of infection and threat to human health. To date, there are no vaccines available to prevent serotype Choleraesuis infections in humans. The availability, in the near future, of the genome sequence of serotype Cholearesuis will facilitate the development of effective vaccines as well as the discovery of new targets for novel antimicrobial agents.

Molecular evolution of the dotA gene in Legionella pneumophila

The molecular evolution of dotA, which is related to the virulence of Legionella pneumophila, was investigated by comparing the sequences of 15 reference strains (serogroups 1 to 15). It was found that dotA has a complex mosaic structure. The whole dotA gene of Legionella pneumophila subsp. pneumophila serogroups 2, 6, and 12 has been transferred from Legionella pneumophila subsp. fraseri. A discrepancy was found between the trees inferred from the nucleotide and deduced amino acid sequences of dotA, which suggests that multiple hits, resulting in synonymous substitutions, have occurred. Gene phylogenies inferred from three different segments (the 5'-end region, the central, large periplasmic domain, and the 3'-end region) showed impressively dissimilar topologies. This was concordant with the sequence polymorphisms, indicating that each region has experienced an independent evolutionary history, and was evident even within the same domain of each strain. For example, the PP2 domain was found to have a heterogeneous structure, which led us hypothesize that the dotA gene of L. pneumophila may have originated from two or more different sources. Comparisons of synonymous and nonsynonymous substitutions demonstrated that the PP2 domain has been under strong selective pressure with respect to amino acid change. Split decomposition analysis also supported the intragenic recombination of dotA. Multiple recombinational exchange within the dotA gene, encoding an integral cytoplasmic membrane protein that is secreted, probably provided increased fitness in certain environmental niches, such as within a particular biofilm community or species of amoebae.

Comparison of two major forms of the Shigella virulence plasmid pINV: positive selection is a major force driving the divergence

All Shigella and enteroinvasive Escherichia coli (EIEC) strains carry a 230-kb virulence plasmid (pINV) which is essential for their invasiveness. There are two sequence forms, pINV A and pINV B, of the plasmid (R. Lan, B. Lumb, D. Ryan, and P. R. Reeves, Infect. Immun. 69:6303-6309, 2001), and the recently sequenced pINV plasmid from Shigella flexneri serotype 5 is a pINV B form. In this study we sequenced the majority of the coding region of the pINV A form from S. flexneri serotype 6 other than insertion sequence or related sequences and compared it with the pINV B form. More than half of the genes sequenced appear to be under positive selection based on their low ratio of synonymous to nonsynonymous substitutions. This high proportion of selected differences indicates that the two pINV forms have functional differences, and comparative studies of pathogenicity in different Shigella-EIEC strains could be informative. There are also genes absent in the S. flexneri serotype 6 plasmid, including the sepA gene encoding serine protease, the major secreted protein of S. flexneri serotype 2a, and the stbAB genes, which encode one of the two partition systems found in S. flexneri serotype 5. The incompatibility of the two pINV forms appears to be due to either small differences in the mvpAT postsegregational killing system or the presence of an unknown system in pINVA.

Differences in gene content among Salmonella enterica serovar typhi isolates

We used a nonredundant microarray of the Salmonella enterica serovar Typhimurium LT2 and Typhi CT18 genomes to assess the genomic content of a diverse set of isolates of serovar Typhi. Comparative genomic hybridization revealed 13 regions of absent or divergent gene content in the eight Typhi strains examined compared to Typhi CT18. In particular, two Typhi CT18 prophage regions, STY1048 to STY1077 and STY2038 to STY2077, as well as a five-gene islet (STY3188 to STY3193) were absent or divergent in all other Typhi strains examined. Seven Typhi strains lacked most or all of the IS1 elements present in strain CT18, and three Typhi strains lacked a P4-like phage (STY4821 to STY4834). One strain was devoid of a 149-gene region (STY4521 to STY4680), which encodes numerous phage genes and the Vi antigen biosynthesis and export gene cluster, a type IV pilus, and numerous phage genes. In Typhi strain 26T25, an amplification of an entire inter-ribosomal region encompassing 31 genes has occurred. Furthermore, a 257-gene region (STY1360 to STY1639) showed an aberrant replication pattern in three Typhi isolates. Overall, these differences in gene content indicate that even within a highly clonal bacterial population the genomic reservoir is unstable.

Bacterial type III secretion systems are ancient and evolved by multiple horizontal-transfer events

Type III secretion systems (TTSS) are unique bacterial mechanisms that mediate elaborate interactions with their hosts. The fact that several of the TTSS proteins are closely related to flagellar export proteins has led to the suggestion that TTSS had evolved from flagella. Here we reconstruct the evolutionary history of four conserved type III secretion proteins and their phylogenetic relationships with flagellar paralogs. Our analysis indicates that the TTSS and the flagellar export mechanism share a common ancestor, but have evolved independently from one another. The suggestion that TTSS genes have evolved from genes encoding flagellar proteins is effectively refuted. A comparison of the species tree, as deduced from 16S rDNA sequences, to the protein phylogenetic trees has led to the identification of several major lateral transfer events involving clusters of TTSS genes. It is hypothesized that horizontal gene transfer has occurred much earlier and more frequently than previously inferred for TTSS genes and is, consequently, a major force shaping the evolution of species that harbor type III secretion systems.

Molecular characterization of the prototrophic Salmonella mutants defective for intraepithelial replication

Three MudJ prototrophs demonstrated that intracellular replication is a Salmonella virulence trait (K. Y. Leung and B. B. Finlay, Proc. Natl. Acad. Sci. USA, 88:11470-11474, 1991). mutS and mutH are disrupted in mutants 3-11 and 12-23, and ssaQ is disrupted in mutant 17-21. Further analysis revealed that loss of Salmonella pathogenicity island 2 function underlies the intracellular replication defect of 3-11 and 17-21.

A pseudomonas syringae pv. tomato DC3000 Hrp (Type III secretion) deletion mutant expressing the Hrp system of bean pathogen P. syringae pv. syringae 61 retains normal host specificity for tomato

The plant pathogenic species Pseudomonas syringae is divided into numerous pathovars based on host specificity. For example, P. syringae pv. tomato DC3000 is pathogenic on tomato and Arabidopsis, whereas P. syringae pv. syringae 61 is pathogenic on bean. The ability of P. syringae strains to elicit the hypersensitive response (HR) in non-hosts or be pathogenic (or parasitic) in hosts is dependent on the Hrp (type III secretion) system and effector proteins this system is thought to inject into plant cells. To test the role of the Hrp system in determining host range, the hrp/hrc gene cluster (hrpK through hrpR) was deleted from DC3000 and complemented in trans with the orthologous cluster from strain 61. Mutant CUCPB5114 expressing the bean pathogen Hrp system on plasmid pCPP2071 retained the ability of wild-type DC3000 to elicit the HR in bean, to grow and cause bacterial speck in tomato, and to elicit a cultivar-specific (gene-for-gene) HR in tomato plants carrying the Pto resistance gene. However, the symptoms produced in compatible tomato plants involved markedly reduced chlorosis, and CUCPB5114(pCPP2071) did not grow or produce symptoms in Arabidopsis Col-0 although it was weakly virulent in NahG Arabidopsis. A hypersensitive-like collapse was produced by CUCPB5114(pCPP2071) in Arabidopsis Col-0 at 1 x 10(7) CFU/ml, but only if the bacteria also expressed AvrB, which is recognized by the RPM1 resistance gene in Col-0 and confers incompatibility. These observations support the concept that the P. syringae effector proteins, rather than secretion system components, are the primary determinants of host range at both the species and cultivar levels of host specificity.

Phylogenetic analysis of Salmonella, Shigella, and Escherichia coli strains on the basis of the gyrB gene sequence

Phylogenetic analysis of about 200 strains of Salmonella, Shigella, and Escherichia coli was carried out using the nucleotide sequence of the gene for DNA gyrase B (gyrB), which was determined by directly sequencing PCR fragments. The results establish a new phylogenetic tree for the classification of Salmonella, Shigella, and Escherichia coli in which Salmonella forms a cluster separate from but closely related to Shigella and E. coli. In comparison with 16S rRNA analysis, the gyrB sequences indicated a greater evolutionary divergence for the bacteria. Thus, in screening for the presence of bacteria, the gyrB gene might be a useful tool for differentiating between closely related species of bacteria such as Shigella spp. and E. coli. At present, 16S rRNA sequence analysis is an accurate and rapid method for identifying most unknown bacteria to the genus level because the highly conserved 16S rRNA region is easy to amplify; however, analysis of the more variable gyrB sequence region can identify unknown bacteria to the species level. In summary, we have shown that gyrB sequence analysis is a useful alternative to 16S rRNA analysis for constructing the phylogenetic relationships of bacteria, in particular for the classification of closely related bacterial species.

Detection of recombination among Salmonella enterica strains using the incongruence length difference test

Particular serovars of Salmonella enterica have emerged as significant foodborne pathogens in humans. At the chromosomal level, discrete regions in the Salmonella genome have been identified that are known to play important roles in the maintenance, survival, and virulence of S. enterica within the host. Interestingly, several of these loci appear to have been acquired by horizontal transfer of DNA among and between bacterial species. The profound importance of recombination in pathogen emergence is just now being realized, perhaps explaining the sudden interest in developing novel and facile ways for detecting putative horizontal transfer events in bacteria. The incongruence length difference (ILD) test offers one such means. ILD uses phylogeny to trace sequences that may have been acquired promiscuously by exchange of DNA during chromosome evolution. We show here that the ILD test readily detects recombinations that have taken place in several housekeeping genes in Salmonella as well as genes composing the type 1 pilin complex (14 min) and the inv-spa invasion gene complex (63 min). Moreover, the ILD test indicated that the mutS gene (64 min), whose product helps protect the bacterial genome from invasion by foreign DNA, appears to have undergone intragenic recombination within S. enterica subspecies I. ILD findings were supported using additional tests known to be independent of the ILD approach (e.g., split decomposition analysis and compatibility of sites). Taken together, these data affirm the application of the ILD test as one approach for identifying recombined sequences in the Salmonella chromosome. Furthermore, horizontally acquired sequences within mutS support a model whereby evolutionarily important recombinants of S. enterica are rescued from strains carrying defective mutS alleles via horizontal transfer.

Evolutionary genomics of Salmonella: gene acquisitions revealed by microarray analysis

The presence of homologues of Salmonella enterica sv. Typhimurium LT2 genes was assessed in 22 other Salmonella including members of all seven subspecies and Salmonella bongori. Genomes were hybridized to a microarray of over 97% of the 4,596 annotated ORFs in the LT2 genome. A phylogenetic tree based on homologue content, relative to LT2, was largely concordant with previous studies using sequence information from several loci. Based on the topology of this tree, homologues of genes in LT2 acquired by various clades were predicted including 513 homologues acquired by the ancestor of all Salmonella, 111 acquired by S. enterica, 105 by diphasic Salmonella, and 216 by subspecies 1, most of which are of unknown function. Because this subspecies is responsible for almost all Salmonella infections of mammals and birds, these genes will be of particular interest for further mechanistic studies. Overall, a high level of gene gain, loss, or rapid divergence was predicted along all lineages. For example, at least 425 close homologues of LT2 genes may have been laterally transferred into Salmonella and then between Salmonella lineages.

Evolutionary and functional analyses of variants of the toxin-coregulated pilus protein TcpA from toxigenic Vibrio cholerae non-O1/non-O139 serogroup isolates

The toxin-coregulated pilus (TCP) is a critical determinant of the pathogenicity of Vibrio cholerae. This bundle-forming pilus is an essential intestinal colonization factor and also serves as a receptor for CTXphi, the filamentous phage that encodes cholera toxin (CT). TCP is a polymer of repeating subunits of the major pilin protein TcpA and tcpA is found within the Vibrio pathogenicity island (VPI). In this study genetic variation at the tcpA locus in toxigenic isolates of V. cholerae was investigated and three novel TcpA sequences from V. cholerae strains V46, V52 and V54, belonging to serogroups O141, O37 and O8, respectively, were identified. These novel tcpA alleles grouped into three distinct clonal lineages. The polymorphisms in TcpA were predominantly located in the carboxyl region of TcpA in surface-exposed regions of TCP fibres. Comparison of the genetic diversity among V. cholerae isolates at the tcpA locus with that of aldA, another locus within the VPI, and mdh, a chromosomal locus, revealed that tcpA sequences are far more diverse than these other loci. Most likely, this diversity is a reflection of diversifying selection in adaptation to the host immune response or to CTXphi susceptibility. An assessment of the functional properties of the variant tcpA sequences in the non-O1 V. cholerae strains was carried out by analysing whether these strains could be infected by CTXphi and colonize the suckling mouse. Similar to El Tor strains of V. cholerae O1, in vitro CTXphi infection of these strains required the exogenous expression of toxT, suggesting that in these strains ToxT regulates TCP expression and that these TcpA variants can serve as CTXphi receptors. All the V. cholerae non-O1 serogroup isolates tested were capable of colonizing the suckling mouse small intestine, suggesting that the different TcpA variants could function as colonization factors.

Characterization of effector proteins translocated via the SPI1 type III secretion system of Salmonella typhimurium

Salmonella spp. employ a conserved type III secretion system encoded within the pathogenicity island 1 (SPI1; centisome 63) to translocate effector proteins into the host cytosol. The translocated effector proteins trigger diverse responses including bacterial internalization. In a mutation analysis we have defined the set of effector proteins mediating tissue culture cell invasion. This set includes sopE2 (centisome 40-42), sopB (SPI5, centisome 20) and in the case of S. typhimurium SL1344 also the phage-encoded effector sopE (SopEphi, centisome 59-60). A triple mutant SL1344 derivative deficient of SopE, SopE2 and SopB was more than 100-fold attenuated in tissue culture cell invasion. Phylogenetic analyses indicate that the last common ancestor of all contemporary Salmonella lineages already harbored all genes necessary for host cell invasion, namely the SPI1 type III secretion system, sopE2 and sopB. SopE, which is 70% identical to sopE2 is only present in some Salmonella strains and emerged later well after the divergence of the contemporary Salmonella lineages. Interestingly, S. typhimurium strains that harbor sopE are associated with epidemics, arguing that sopE is one of the factors determining the "fitness" of a strain. We found that SopE can specifically activate a different set of host cellular RhoGTPases than SopE2. This allows the bacteria to fine tune host cellular responses very precisely and may offer an explanation for the improved epidemic fitness of sopE-positive S. typhimurium strains.

Molecular evolution of the intimin gene in O111 clones of pathogenic Escherichia coli

Intimin is an important virulence factor in two groups of enteric pathogens: enteropathogenic Escherichia coli (EPEC), which is a major cause of infant diarrhea in the developing world, and enterohemorrhagic E. coli (EHEC), which has caused large food-borne outbreaks of hemorrhagic colitis in the United States and other developed countries. Intimin is encoded on a 35-kb pathogenicity island called the locus of enterocyte effacement (LEE). At least five antigenic types have been described for the highly variable gene, and each type is generally characteristic of particular evolutionary lineages. We determined the nucleotide sequences of intimin and other LEE genes in two O111 clones that have not been amenable to typing. The sequences from both O111:H8 and O111:H9 differed from the Int-beta that is typical of other clones in the same evolutionary lineage. The sequence from the O111:H8 strains was a mosaic of divergent segments that alternately clustered with Int-alpha, Int-beta, or Int-gamma. The sequence from the O111:H9 clone consistently showed a close relationship with that from E2348/69, a distantly related strain that expresses Int-alpha. The results suggest that there have been multiple acquisitions of the LEE in the EHEC 2/EPEC 2 clonal lineage, with a recent turnover in either O111:H8 or its close relatives. Amino acid substitutions that alter residue charge occurred more frequently than would be expected under random substitution in the extracellular domains of intimin, suggesting that diversifying selection has promoted divergence in this region of the protein. An N-terminal domain that presumably functions in the periplasm may also be under positive selection.

The Salmonella pathogenicity island-1 type III secretion system

Salmonella pathogenicity island 1 (SPI1) encodes a type III secretion system that is required for virulence during the intestinal phase of infection. The expression of SPI1 genes is controlled by many global regulatory pathways that affect the expression/activity of transcriptional regulators encoded on SPI1.

Genetic and structural organization of the aminophenol catabolic operon and its implication for evolutionary process

The aminophenol (AP) catabolic operon in Pseudomonas putida HS12 mineralizing nitrobenzene was found to contain all the enzymes responsible for the conversion of AP to pyruvate and acetyl coenzyme A via extradiol meta cleavage of 2-aminophenol. The sequence and functional analyses of the corresponding genes of the operon revealed that the AP catabolic operon consists of one regulatory gene, nbzR, and the following nine structural genes, nbzJCaCbDGFEIH, which encode catabolic enzymes. The NbzR protein, which is divergently transcribed with respect to the structural genes, possesses a leucine zipper motif and a MarR homologous domain. It was also found that NbzR functions as a repressor for the AP catabolic operon through binding to the promoter region of the gene cluster in its dimeric form. A comparative study of the AP catabolic operon with other meta cleavage operons led us to suggest that the regulatory unit (nbzR) was derived from the MarR family and that the structural unit (nbzJCaCbDGFEIH) has evolved from the ancestral meta cleavage gene cluster. It is also proposed that these two functional units assembled through a modular type gene transfer and then have evolved divergently to acquire specialized substrate specificities (NbzCaCb and NbzD) and catalytic function (NbzE), resulting in the creation of the AP catabolic operon. The evolutionary process of the AP operon suggests how bacteria have efficiently acquired genetic diversity and expanded their metabolic capabilities by modular type gene transfer.

Salmonella host cell invasion emerged by acquisition of a mosaic of separate genetic elements, including Salmonella pathogenicity island 1 (SPI1), SPI5, and sopE2

Salmonella spp. possess a conserved type III secretion system encoded within the pathogenicity island 1 (SPI1; centisome 63), which mediates translocation of effector proteins into the host cell cytosol to trigger responses such as bacterial internalization. Several translocated effector proteins are encoded in other regions of the Salmonella chromosome. It remains unclear how this complex chromosomal arrangement of genes for the type III apparatus and the effector proteins emerged and how the different effector proteins cooperate to mediate virulence. By Southern blotting, PCR, and phylogenetic analyses of highly diverse Salmonella spp., we show here that effector protein genes located in the core of SPI1 are present in all Salmonella lineages. Surprisingly, the same holds true for several effector protein genes located in distant regions of the Salmonella chromosome, namely, sopB (SPI5, centisome 20), sopD (centisome 64), and sopE2 (centisomes 40 to 42). Our data demonstrate that sopB, sopD, and sopE2, along with SPI1, were already present in the last common ancestor of all contemporary Salmonella spp. Analysis of Salmonella mutants revealed that host cell invasion is mediated by SopB, SopE2, and, in the case of Salmonella enterica serovar Typhimurium SL1344, by SopE: a sopB sopE sopE2-deficient triple mutant was incapable of inducing membrane ruffling and was >100-fold attenuated in host cell invasion. We conclude that host cell invasion emerged early during evolution by acquisition of a mosaic of genetic elements (SPI1 itself, SPI5 [sopB], and sopE2) and that the last common ancestor of all contemporary Salmonella spp. was probably already invasive.

Spa33, a cell surface-associated subunit of the Mxi-Spa type III secretory pathway of Shigella flexneri, regulates Ipa protein traffic

The Mxi-Spa type III secretion system of Shigella flexneri directs the host cell contact-induced secretion of a set of invasins, referred to as Ipas. In this study, we examined the role of Spa33 in Ipa secretion. A spa33-null mutant was both noninvasive and unable to translocate the Ipas from inner membrane to outer membrane (OM) positions of the Mxi-Spa transmembrane channel. Spa33 was found to be a Mxi-Spa substrate that is translocated to the bacterial cell surface upon the induction of Ipa secretion. This mobility may serve to drive Ipa translocation within Mxi-Spa toward OM positions. Consistent with a second distinct role in regulating Ipa traffic, the overexpression of Spa33 also blocked Ipa secretion and resulted in Ipa accumulation at the OM. Co-overexpression of Spa33 and another OM-associated element, Spa32, did not disrupt Ipa secretion, suggesting an interaction between the two proteins and an effect on the mechanism which serves to regulate Ipa release from the OM. These findings indicate that Spa33 is a mobile element within Mxi-Spa, which is required to control Ipa translocation into and out of OM positions of the secretory structure.

The insect endosymbiont Sodalis glossinidius utilizes a type III secretion system for cell invasion

Sodalis glossinidius is a maternally transmitted secondary endosymbiont residing intracellularly in tissues of the tsetse flies, Glossina spp. In this study, we have used Tn5 mutagenesis and a negative selection procedure to derive a S. glossinidius mutant that is incapable of invading insect cells in vitro and is aposymbiotic when microinjected into tsetse. This mutant strain harbors Tn5 integrated into a chromosomal gene sharing high sequence identity with a type III secretion system invasion gene (invC) previously identified in Salmonella enterica. With the use of degenerate PCR, we have amplified a further six Sodalis inv/spa genes sharing high sequence identity with type III secretion system genes encoded by Salmonella pathogenicity island 1. Phylogenetic reconstructions based on the inv/spa genes of Sodalis and other members of the family Enterobacteriaceae have consistently identified a well-supported clade containing Sodalis and the enteric pathogens Shigella and Salmonella. These results suggest that Sodalis may have evolved from an ancestor with a parasitic intracellular lifestyle, possibly a latter-day entomopathogen. These observations lend credence to a hypothesis suggesting that vertically transmitted mutualistic endosymbionts evolve from horizontally transmitted parasites through a parasitism-mutualism continuum.