Plasmid-mediated virulence genes in non-typhoid Salmonella serovars

Infection biology of Salmonella enterica

Salmonella enterica is the leading cause of bacterial foodborne illness in the USA, with an estimated 95% of salmonellosis cases due to the consumption of contaminated food products. Salmonella can cause several different disease syndromes, with the most common being gastroenteritis, followed by bacteremia and typhoid fever. Among the over 2,600 currently identified serotypes/serovars, some are mostly host-restricted and host-adapted, while the majority of serotypes can infect a broader range of host species and are associated with causing both livestock and human disease. Salmonella serotypes and strains within serovars can vary considerably in the severity of disease that may result from infection, with some serovars that are more highly associated with invasive disease in humans, while others predominantly cause mild gastroenteritis. These observed clinical differences may be caused by the genetic make-up and diversity of the serovars. Salmonella virulence systems are very complex containing several virulence-associated genes with different functions that contribute to its pathogenicity. The different clinical syndromes are associated with unique groups of virulence genes, and strains often differ in the array of virulence traits they display. On the chromosome, virulence genes are often clustered in regions known as Salmonella pathogenicity islands (SPIs), which are scattered throughout different Salmonella genomes and encode factors essential for adhesion, invasion, survival, and replication within the host. Plasmids can also carry various genes that contribute to Salmonella pathogenicity. For example, strains from several serovars associated with significant human disease, including Choleraesuis, Dublin, Enteritidis, Newport, and Typhimurium, can carry virulence plasmids with genes contributing to attachment, immune system evasion, and other roles. The goal of this comprehensive review is to provide key information on the Salmonella virulence, including the contributions of genes encoded in SPIs and plasmids during Salmonella pathogenesis.

Molecular Detection of Virulence Factors in Salmonella serovars Isolated from Poultry and Human Samples

Salmonellosis is a common infectious disease in humans caused by Salmonella spp., which in recent years has shown an increase in its incidence, with products of avian origin being a common source of transmission. To present a successful infective cycle, there are molecular mechanisms such as virulence factors that provide characteristics that facilitate survival, colonization, and damage to the host. According to this, the study aims to characterize the virulence factors of Salmonella spp. strains isolated from broilers (n = 39) and humans (n = 10). The presence of 24 virulence genes was evaluated using end-point PCR. All the strains of Salmonella spp. isolated from broiler chickens revealed presence of 7/24 (29, 16%) virulence genes (lpfA, csgA, sitC, sipB, sopB, sopE, and sivH). Regarding the strains isolated from cases of gastroenteritis in humans, all strains contained (14/24, 58, 33%) virulence genes (lpfA, csgA, pagC, msgA, spiA, sitC, iroN, sipB, orgA, hilA, sopB, sifA, avrA, and sivH). In summary, the presence of virulence genes in different strains of Salmonella isolated from broilers and humans could be described as bacteria with potential pathogenicity due to the type and number of virulence genes detected. These findings are beneficial for the pathogenic monitoring of Salmonella in Colombia.

The Number and Type of Chaperone-Usher Fimbriae Reflect Phylogenetic Clade Rather than Host Range in Salmonella

Salmonella is one of the most successful foodborne pathogens worldwide, owing in part to its ability to colonize or infect a wide range of hosts. Salmonella serovars are known to encode a variety of different fimbriae (hairlike organelles that facilitate binding to surfaces); however, the distribution, number, and sequence diversity of fimbriae encoded across different lineages of Salmonella were unknown. We queried whole-genome sequence (WGS) data for 242 Salmonella enterica subsp. enterica (subspecies enterica) isolates from the top 217 serovars associated with isolation from humans and agricultural animals; this effort identified 2,894 chaperone-usher (CU)-type fimbrial usher sequences, representing the most conserved component of CU fimbriae. On average, isolates encoded 12 different CU fimbrial ushers (6 to 18 per genome), although the distribution varied significantly (P = 1.328E-08) by phylogenetic clade, with isolates in section Typhi having significantly fewer fimbrial ushers than isolates in clade A2 (medians = 10 and 12 ushers, respectively). Characterization of fimbriae in additional non-enterica subspecies genomes suggested that 8 fimbrial ushers were classified as being unique to subspecies enterica isolates, suggesting that the majority of fimbriae were most likely acquired prior to the divergence of subspecies enterica. Characterization of mobile elements suggested that plasmids represent an important vehicle facilitating the acquisition of a wide range of fimbrial ushers, particularly for the acquisition of fimbriae from other Gram-negative genera. Overall, our results suggest that differences in the number and type of fimbriae encoded most likely reflect differences in phylogenetic clade rather than differences in host range. IMPORTANCE Fimbriae of the CU assembly pathway represent important organelles that mediate Salmonella's interactions with host tissues and abiotic surfaces. Our analyses provide a comprehensive overview of the diversity of CU fimbriae in Salmonella spp., highlighting that the majority of CU fimbriae are distributed broadly across multiple subspecies and suggesting that acquisition most likely occurred prior to the divergence of subspecies enterica. Our data also suggest that plasmids represent the primary vehicles facilitating the horizontal transfer of diverse CU fimbriae in Salmonella. Finally, the observed high sequence similarity between some ushers suggests that different names may have been assigned to closely related fimbrial ushers that likely should be represented by a single designation. This highlights the need to establish standard criteria for fimbria classification and nomenclature, which will also facilitate future studies seeking to associate virulence factors with adaptation to or differences in the likelihood of causing disease in a given host.

Epidemiological Analysis of Salmonella enterica subsp. enterica Serovar Dublin in German Cattle Herds Using Whole-Genome Sequencing

Salmonella enterica subsp. enterica serovar Dublin is a cattle-adapted serovar that causes enteritis and systemic diseases in animals. In Germany, S. Dublin is not detected or is very rarely detected in some federal states but is endemic in certain regions. Information on detailed genetic characteristics of S. Dublin is not available. An understanding of the paths and spreading of S. Dublin within and between regions and over time is essential to establish effective control strategies. Whole-genome sequencing (WGS) and bioinformatic analysis were used to explore the genetic traits of S. Dublin and to determine their epidemiological context. Seventy-four S. Dublin strains collected in 2005 to 2018 from 10 federal states were studied. The phylogeny was analyzed using core-genome single-nucleotide polymorphisms (cgSNPs) and core-genome multilocus sequence typing. Genomic clusters at 100 cgSNPs, 40 cgSNPs, and 15 cgSNPs were selected for molecular epidemiology. WGS-based genoserotyping confirmed serotyping. Important specific virulence determinants were detected in all strains, but multidrug resistance in German S. Dublin organisms is uncommon. Use of different thresholds for cgSNP analysis enabled a broad view and also a detailed view of the occurrence of S. Dublin in Germany. Genomic clusters could be allocated nationwide, to a limited number of federal states, or to special regions only. Results indicate both persistence and spread of S. Dublin within and between federal states in short and longer time periods. However, to detect possible routes of infection or persistence of S. Dublin indicated by genomic analysis, information on the management of the cattle farms and contacts with corresponding farms are essential.

IMPORTANCESalmonella enterica subsp. enterica serovar Dublin is a bovine host-adapted serovar that causes up to 50% of all registered outbreaks of salmonellosis in cattle in Germany. S. Dublin is not detected or is only rarely detected in some federal states but has been endemic in certain regions of the country for a long time. Information on genetic traits of the causative strains is essential to determine routes of infection. WGS and bioinformatic analysis should be used to explore the genetic characteristics of S. Dublin. Combining the genomic features of S. Dublin strains with information on the management of the cattle farms concerned should enable the detection of possible routes of infection or persistence of S. Dublin. This approach is regarded as a prerequisite to developing effective intervention strategies.

AT Homopolymer Strings in Salmonella enterica Subspecies I Contribute to Speciation and Serovar Diversity

Adenine and thymine homopolymer strings of at least 8 nucleotides (AT 8+mers) were characterized in Salmonella enterica subspecies I. The motif differed between other taxonomic classes but not between Salmonella enterica serovars. The motif in plasmids was possibly associated with serovar. Approximately 12.3% of the S. enterica motif loci had mutations. Mutability of AT 8+mers suggests that genomes undergo frequent repair to maintain optimal gene content, and that the motif facilitates self-recognition; in addition, serovar diversity is associated with plasmid content. A theory that genome regeneration accounts for both persistence of predominant Salmonella serovars and serovar diversity provides a new framework for investigating root causes of foodborne illness.

Genomic Characterization of a Nalidixic Acid-Resistant Salmonella Enteritidis Strain Causing Persistent Infections in Broiler Chickens

Virulent strains of Salmonella enterica subsp. enterica serovar Enteritidis (SE) harbored by poultry can cause disease in poultry flocks and potentially result in human foodborne illness. Two broiler flocks grown a year apart on the same premises experienced mortality throughout the growing period due to septicemic disease caused by SE. Gross lesions predominantly consisted of polyserositis followed by yolk sacculitis, arthritis, osteomyelitis, and spondylitis. Tissues with lesions were cultured yielding 59 SE isolates. These were genotyped by Rep-PCR followed by whole-genome sequencing (WGS) of 15 isolates which were clonal. The strain, SE_TAU19, was further characterized for antimicrobial susceptibility and virulence in a broiler embryo lethality assay. SE_TAU19 was resistant to nalidixic acid and sulfadimethoxine and was virulent to embryos with 100% mortality of all challenged broiler embryos within 3.5 days. Screening the SE_TAU19 whole-genome sequence revealed seven antimicrobial resistance (AMR) genes, 120 virulence genes, and two IncF plasmid replicons corresponding to a single, serovar-specific pSEV virulence plasmid. The pef, spv, and rck virulence genes localized to the plasmid sequence assembly. We report phenotypic and genomic features of a virulent SE strain from persistently infected broiler flocks and present a workflow for SE characterization from isolate collection to genome assembly and sequence analysis. Further SE surveillance and investigation of SE virulence in broiler chickens is warranted.

Copy Number Heterogeneity in the Virulence Plasmid of Salmonella enterica

Quantitative PCR analysis shows that the virulence plasmid of Salmonella enterica serovar Typhimurium (pSLT) is a low-copy-number plasmid, with 1–2 copies per chromosome. However, fluorescence microscopy observation of pSLT labeled with a lacO fluorescent tag reveals cell-to-cell differences in the number of foci, which ranges from 1 to 8. As each focus must correspond to ≥1 plasmid copy, the number of foci can be expected to indicate the minimal number of pSLT copies per cell. A correlation is found between the number of foci and the bacterial cell volume. In contrast, heterogeneity in the number of foci appears to be independent of the cell volume and may have stochastic origin. As a consequence of copy number heterogeneity, expression of a pSLT-bone reporter gene shows high levels of cell-to-cell variation, especially in actively dividing cultures. These observations support the notion that low-copy-number plasmids can be a source of gene expression noise in bacterial populations.

Characterization of non-typhoidal Salmonella from poultry in Punjab, India

Present study was done to determine the prevalence, virulence and antimicrobial spectrum of identified serotypes. A total of 693 samples, comprising 585 poultry fecal samples (420 from layer, 120 from broiler and 45 from backyard poultry), 54 each poultry feed and farm water were collected from 31 poultry farms of three districts of Punjab. A total of 6 Salmonella isolates were obtained from these samples. Out of 585 fecal samples, 6 (1.02%) were positive for Salmonella. Out of 31 farms, 2 (6.45%) farms were positive for Salmonella. Out of 6 Salmonella isolates, four isolates from one farm were serotype Salmonella IIIa, 35: z24: z23 and two Salmonella isolates from another farm were untypable and also carried spvC gene. Isolates showed resistance to gentamicin (2/6, 33.3%), co-trimoxazole (1/6, 16.7%). Antibiotics ampicillin, tetracycline, ciprofloxacin, chloramphenicol and enrofloxacin were effective against all the isolates. On PFGE analysis, four isolates from one farm were clustered in two clusters and two isolates from other farm were similar and clustered together. Based on the results we can say that drug resistant Salmonella is present in poultry flock in Punjab.

Comparative Genomic Analysis of 450 Strains of Salmonella enterica Isolated from Diseased Animals

Salmonella is a leading cause of bacterial infections in animals and humans. We sequenced a collection of 450 Salmonella strains from diseased animals to better understand the genetic makeup of their virulence and resistance features. The presence of Salmonella pathogenicity islands (SPIs) varied by serotype. S. Enteritidis carried the most SPIs (n = 15), while S. Mbandaka, S. Cerro, S. Meleagridis, and S. Havana carried the least (n = 10). S. Typhimurium, S. Choleraesuis, S. I 4,5,12:i:-, and S. Enteritidis each contained the spv operon on IncFII or IncFII-IncFIB hybrid plasmids. Two S. IIIa carried a spv operon with spvD deletion on the chromosome. Twelve plasmid types including 24 hybrid plasmids were identified. IncA/C was frequently associated with S. Newport (83%) and S. Agona (100%) from bovine, whereas IncFII (100%), IncFIB (100%), and IncQ1 (94%) were seen in S. Choleraesuis from swine. IncX (100%) was detected in all S. Kentucky from chicken. A total of 60 antimicrobial resistance genes (ARGs), four disinfectant resistances genes (DRGs) and 33 heavy metal resistance genes (HMRGs) were identified. The Salmonella strains from sick animals contained various SPIs, resistance genes and plasmid types based on the serotype and source of the isolates. Such complicated genomic structures shed light on the strain characteristics contributing to the severity of disease and treatment failures in Salmonella infections, including those causing illnesses in animals.

Assessing the heterogeneity of in silico plasmid predictions based on whole-genome-sequenced clinical isolates

High-throughput next-generation shotgun sequencing of pathogenic bacteria is growing in clinical relevance, especially for chromosomal DNA-based taxonomic identification and for antibiotic resistance prediction. Genetic exchange is facilitated for extrachromosomal DNA, e.g. plasmid-borne antibiotic resistance genes. Consequently, accurate identification of plasmids from whole-genome sequencing (WGS) data remains one of the major challenges for sequencing-based precision medicine in infectious diseases. Here, we assess the heterogeneity of four state-of-the-art tools (cBar, PlasmidFinder, plasmidSPAdes and Recycler) for the in silico prediction of plasmid-derived sequences from WGS data. Heterogeneity, sensitivity and precision were evaluated by reference-independent and reference-dependent benchmarking using 846 Gram-negative clinical isolates. Interestingly, the majority of predicted sequences were tool-specific, resulting in a pronounced heterogeneity across tools for the reference-independent assessment. In the reference-dependent assessment, sensitivity and precision values were found to substantially vary between tools and across taxa, with cBar exhibiting the highest median sensitivity (87.45%) but a low median precision (27.05%). Furthermore, integrating the individual tools into an ensemble approach showed increased sensitivity (95.55%) while reducing the precision (25.62%). CBar and plasmidSPAdes exhibited the strongest concordance with respect to identified antibiotic resistance factors. Moreover, false-positive plasmid predictions typically contained only few antibiotic resistance factors. In conclusion, while high degrees of heterogeneity and variation in sensitivity and precision were observed across the different tools and taxa, existing tools are valuable for investigating the plasmid-borne resistome. Nevertheless, additional studies on representative clinical data sets will be necessary to translate in silico plasmid prediction approaches from research to clinical application.

Comparative Genomic Analysis of Virulence, Antimicrobial Resistance, and Plasmid Profiles of Salmonella Dublin Isolated from Sick Cattle, Retail Beef, and Humans in the United States

Salmonella enterica serovar Dublin is a host-adapted serotype associated with typhoidal disease in cattle. While rare in humans, it usually causes severe illness, including bacteremia. In the United States, Salmonella Dublin has become one of the most multidrug-resistant (MDR) serotypes. To understand the genetic elements that are associated with virulence and resistance, we sequenced 61 isolates of Salmonella Dublin (49 from sick cattle and 12 from retail beef) using the Illumina MiSeq and closed 5 genomes using the PacBio sequencing platform. Genomic data of eight human isolates were also downloaded from NCBI (National Center for Biotechnology Information) for comparative analysis. Fifteen Salmonella pathogenicity islands (SPIs) and a spv operon (spvRABCD), which encodes important virulence factors, were identified in all 69 (100%) isolates. The 15 SPIs were located on the chromosome of the 5 closed genomes, with each of these isolates also carrying 1 or 2 plasmids with sizes between 36 and 329 kb. Multiple antimicrobial resistance genes (ARGs), including bla_CMY-2, bla_TEM-1B, aadA12, aph(3')-Ia, aph(3')-Ic, strA, strB, floR, sul1, sul2, and tet(A), along with spv operons were identified on these plasmids. Comprehensive antimicrobial resistance genotypes were determined, including 17 genes encoding resistance to 5 different classes of antimicrobials, and mutations in the housekeeping gene (gyrA) associated with resistance or decreased susceptibility to fluoroquinolones. Together these data revealed that this panel of Salmonella Dublin commonly carried 15 SPIs, MDR/virulence plasmids, and ARGs against several classes of antimicrobials. Such genomic elements may make important contributions to the severity of disease and treatment failures in Salmonella Dublin infections in both humans and cattle.

Characterization of Salmonella Typhimurium and its monophasic variant 1,4, [5],12:i:- isolated from different sources

In order to characterize the most commonly detected Salmonella serotypes, we tested 124 isolates of S. Typhimurium and 89 isolates of the monophasic variant of S. Typhimurium (S. 1,4, [5],12:i:-) for their antimicrobial susceptibility by means of the Kirby-Bauer disk-diffusion method, and for the detection of 19 genes (four Phage Markers (g13, Sieb, eat, g8), ten prophage-related virulence genes (gipA, gtgB, nanH, gogB, grvA, sopE, sspH1, sspH2, sodC1, gtgE), and five plasmid-borne virulence genes (spvC, pefA, mig5, rcK, srgA)) by means of PCR-based assays. A total of 213 strains were analyzed from, humans (n = 122), animals (n = 25), food (n = 46), and irrigation water (n = 20). S. Typhimurium isolates showed higher variability, in both their resistance profiles and molecular typing, than S. 1,4, [5],12:i:-. Strains from irrigation water displayed significantly higher susceptibility to antibiotics than those from the other sources. Resistance to ampicillin, streptomycin, sulfonamide, and tetracycline was the most commonly detected resistance profile (R-type), being in serovar S. 1,4, [5],12:i:-, frequently associated to resistance to other antimicrobials. Significant differences in genetic profiles in the two abovementioned Salmonella serotypes were found. None of the plasmid-borne virulence genes investigated were detected in S. 1,4, [5],12:i:- isolates, while those genes, characterized 37.9% of the S. Typhimurium strains. Differences in the prevalence of some molecular targets between the two Salmonella serotypes deserve further study. Importantly, the grvA gene was found exclusively in S. Typhimurium strains, whereas sopE, sodC, gtgB, and gipA were mainly detected, with a statistically significant difference, in S. 1,4, [5],12:i:- isolates.

Organoid and Enteroid Modeling of Salmonella Infection

Salmonella are Gram-negative rod-shaped facultative anaerobic bacteria that are comprised of over 2,000 serovars. They cause gastroenteritis (salmonellosis) with headache, abdominal pain and diarrhea clinical symptoms. Salmonellosis brings a heavy burden for the public health in both developing and developed countries. Antibiotics are usually effective in treating the infected patients with severe gastroenteritis, although antibiotic resistance is on the rise. Understanding the molecular mechanisms of Salmonella infection is vital to combat the disease. In vitro immortalized 2-D cell lines, ex vivo tissues/organs and several animal models have been successfully utilized to study Salmonella infections. Although these infection models have contributed to uncovering the molecular virulence mechanisms, some intrinsic shortcomings have limited their wider applications. Notably, cell lines only contain a single cell type, which cannot reproduce some of the hallmarks of natural infections. While ex vivo tissues/organs alleviate some of these concerns, they are more difficult to maintain, in particular for long term experiments. In addition, non-human animal models are known to reflect only part of the human disease process. Enteroids and induced intestinal organoids are emerging as effective infection models due to their closeness in mimicking the infected tissues/organs. Induced intestinal organoids are derived from iPSCs and contain mesenchymal cells whereas enteroids are derive from intestinal stem cells and are comprised of epithelial cells only. Both enteroids and induced intestinal organoids mimic the villus and crypt domains comparable to the architectures of the in vivo intestine. We review here that enteroids and induced intestinal organoids are emerging as desired infection models to study bacterial-host interactions of Salmonella.

Prevalence and molecular characterization of Salmonella enterica isolates throughout an integrated broiler supply chain in China

A total of 1145 samples were collected from chicken breeder farms, hatcheries, broiler farms, a slaughterhouse and retail refrigerated chicken stores in an integrated broiler supply chain in Guangdong Province, China, in 2013. One-hundred and two Salmonella enterica strains were isolated and subjected to serotyping, antimicrobial susceptibility testing, virulence profile determination and molecular subtyping by pulsed field gel electrophoresis (PFGE). The contamination rates in samples from breeder farms, hatcheries, broiler farms, the slaughterhouse and retail stores were 1·46%, 4·31%, 7·00%, 62·86% and 54·67%, respectively. The isolated strains of S. enterica belonged to 10 serotypes; most of them were S. Weltevreden (46·08%, 47/102) and S. Agona (18·63%, 19/102). Isolates were frequently resistant to streptomycin (38·2%), tetracycline (36·3%), sulfisoxazole (35·3%) and gentamicin (34·3%); 31·4% of isolates were multidrug resistant. The isolates were screened for 10 virulence factors. The Salmonella pathogenicity island genes avrA, ssaQ, mgtC, siiD, and sopB and the fimbrial gene bcfC were present in 100% of the strains. PFGE genotyping of the 102 S. enterica isolates yielded 24 PFGE types at an 85% similarity threshold. The PFGE patterns show that the genotypes of S. enterica in the production chain are very diverse, but some strains have 100% similarity in different parts of the production chain, which indicates that some S. enterica persist throughout the broiler supply chain.

Virulence genes detection of Salmonella serovars isolated from pork and slaughterhouse environment in Ahmedabad, Gujarat

Aim:

The aim was to detect virulence gene associated with the Salmonella serovars isolated from pork and Slaughterhouse environment.

Materials and Methods:

Salmonella isolates (n=37) used in this study were isolated from 270 pork and slaughter house environmental samples collected from the Ahmedabad Municipal Corporation Slaughter House, Ahmedabad, Gujarat, India. Salmonella serovars were isolated and identified as per BAM USFDA method and serotyped at National Salmonella and Escherichia Centre, Central Research Institute, Kasauli (Himachal Pradesh, India). Polymerase chain reaction technique was used for detection of five genes, namely invA, spvR, spvC, fimA and stn among different serovars of Salmonella.

Results:

Out of a total of 270 samples, 37 (13.70%) Salmonella were isolated with two serovars, namely Enteritidis and Typhimurium. All Salmonella serovars produced 284 bp invA gene, 84 bp fimA and 260 bp amplicon for enterotoxin (stn) gene whereas 30 isolates possessed 310 bp spvR gene, but no isolate possessed spvC gene.

Conclusion:

Presence of invA, fimA and stn gene in all isolates shows that they are the specific targets for Salmonella identification and are capable of producing gastroenteric illness to humans, whereas 20 Typhimurium serovars and 10 Enteritidis serovars can able to produce systemic infection.

Plasmid-Mediated Antibiotic Resistance and Virulence in Gram-negatives: the Klebsiella pneumoniae Paradigm

Plasmids harbor genes coding for specific functions including virulence factors and antibiotic resistance that permit bacteria to survive the hostile environment found in the host and resist treatment. Together with other genetic elements such as integrons and transposons, and using a variety of mechanisms, plasmids participate in the dissemination of these traits resulting in the virtual elimination of barriers among different kinds of bacteria. In this article we review the current information about physiology and role in virulence and antibiotic resistance of plasmids from the gram-negative opportunistic pathogen Klebsiella pneumoniae. This bacterium has acquired multidrug resistance and is the causative agent of serious communityand hospital-acquired infections. It is also included in the recently defined ESKAPE group of bacteria that cause most of US hospital infections.

Complete sequence of multidrug resistance p9134 plasmid and its variants including natural recombinant with the virulence plasmid of Salmonella serovar Typhimurium

In this study we determined the complete nucleotide sequence of multidrug-resistance plasmid p9134, and its variants p9134dT and p9134dAT which spontaneously lost either tetracycline or both tetracycline and ampicillin resistance, respectively. The plasmids were 133,802 bp, 109,512 bp and 127,291 bp in size, respectively, and their basic backbone was similar to that of IncI plasmids. Genes coding for ampicillin (blaTEM), chloramphenicol (catA1), streptomycin (strA, strB), tetracycline (tetA(A)) and gentamicin (aac(3)-IV) resistance were confirmed in wild-type p9134. Moreover, a gene for hygromycine resistance (hph) and a putative gene for apramycin resistance were newly determined. In p9134dAT, a continuous sequence coding for ampicillin and tetracycline resistances was lost. Genetic rearrangements in p9134dT were more complex and 2 recombination events must have occurred. During the first one, the tetracycline resistance locus was replaced with rck, srgB, srgA, orf7 and pefI originating from Salmonella virulence plasmid pSLT. During the second one, ydjA, pifA and repC genes from p9134 were replaced with repA2, PSLT025 and PSLT026 genes from pSLT. Our findings indicate that recombination event between unrelated plasmids might be quite common and may lead to the generation and selection of plasmids both transferring antibiotic resistance and increasing virulence of their host.

Salmonella pathogenicity and host adaptation in chicken-associated serovars

Enteric pathogens such as Salmonella enterica cause significant morbidity and mortality. S. enterica serovars are a diverse group of pathogens that have evolved to survive in a wide range of environments and across multiple hosts. S. enterica serovars such as S. Typhi, S. Dublin, and S. Gallinarum have a restricted host range, in which they are typically associated with one or a few host species, while S. Enteritidis and S. Typhimurium have broad host ranges. This review examines how S. enterica has evolved through adaptation to different host environments, especially as related to the chicken host, and continues to be an important human pathogen. Several factors impact host range, and these include the acquisition of genes via horizontal gene transfer with plasmids, transposons, and phages, which can potentially expand host range, and the loss of genes or their function, which would reduce the range of hosts that the organism can infect. S. Gallinarum, with a limited host range, has a large number of pseudogenes in its genome compared to broader-host-range serovars. S. enterica serovars such as S. Kentucky and S. Heidelberg also often have plasmids that may help them colonize poultry more efficiently. The ability to colonize different hosts also involves interactions with the host's immune system and commensal organisms that are present. Thus, the factors that impact the ability of Salmonella to colonize a particular host species, such as chickens, are complex and multifactorial, involving the host, the pathogen, and extrinsic pressures. It is the interplay of these factors which leads to the differences in host ranges that we observe today.

Novel tools to analyze the function of Salmonella effectors show that SvpB ectopic expression induces cell cycle arrest in tumor cells

In order to further characterize its role in pathogenesis and to establish whether its overproduction can lead to eukaryotic tumor cell death, Salmonella strains able to express its virulence factor SpvB (an ADP-ribosyl transferase enzyme) in a salicylate-inducible way have been constructed and analyzed in different eukaryotic tumor cell lines. To do so, the bacterial strains bearing the expression system have been constructed in a ∆purD background, which allows control of bacterial proliferation inside the eukaryotic cell. In the absence of bacterial proliferation, salicylate-induced SpvB production resulted in activation of caspases 3 and 7 and apoptotic cell death. The results clearly indicated that controlled SpvB production leads to F-actin depolimerization and either G1/S or G2/M phase arrest in all cell lines tested, thus shedding light on the function of SpvB in Salmonella pathogenesis. In the first place, the combined control of protein production by salicylate regulated vectors and bacterial growth by adenine concentration offers the possibility to study the role of Salmonella effectors during eukaryotic cells infection. In the second place, the salicylate-controlled expression of SpvB by the bacterium provides a way to evaluate the potential of other homologous or heterologous proteins as antitumor agents, and, eventually to construct novel potential tools for cancer therapy, given that Salmonella preferentially proliferates in tumors.

An outbreak of Salmonella serotype Typhimurium infections with an unusually long incubation period

A 1998 investigation of an outbreak of Salmonella serotype Typhimurium infections among children tasting unpasteurized milk during tours of a dairy farm demonstrated a distribution of unusually long incubation periods (median, 8 days; interquartile range [IQR], 6-14 days). Bacterial isolates were highly acid tolerant and contained genes associated with protection against destructive phagocytic reactive oxygen intermediates. We hypothesize that exposure to low-dose oral inoculum of a pathogen with these properties could have contributed to cases of non-typhoidal salmonellosis with the longest incubation period reported to the Centers for Disease Control and Prevention (CDC).

The Role of the spv Genes in Salmonella Pathogenesis

Salmonella strains cause three main types of diseases in people: gastroenteritis, enteric (typhoid) fever, and non-typhoid extra-intestinal disease with bacteremia. Genetic analysis indicates that each clinical syndrome requires distinct sets of virulence genes, and Salmonella isolates differ in their constellation of virulence traits. The spv locus is strongly associated with strains that cause non-typhoid bacteremia, but are not present in typhoid strains. The spv region contains three genes required for the virulence phenotype in mice: the positive transcriptional regulator spvR and two structural genes spvB and spvC. SpvB and SpvC are translocated into the host cell by the Salmonella pathogenicity island-2 type-three secretion system. SpvB prevents actin polymerization by ADP-ribosylation of actin monomers, while SpvC has phosphothreonine lyase activity and has been shown to inhibit MAP kinase signaling. The exact mechanisms by which SpvB and SpvC act in concert to enhance virulence are still unclear. SpvB exhibits a cytotoxic effect on host cells and is required for delayed cell death by apoptosis following intracellular infection. Strains isolated from systemic infections of immune compromised patients, particularly HIV patients, usually carry the spv locus, strongly suggesting that CD4 T cells are required to control disease due to Salmonella that are spv positive. This association is not seen with typhoid fever, indicating that the pathogenesis and immunology of typhoid have fundamental differences from the syndrome of non-typhoid bacteremia.

Salmonella Cerro isolated over the past twenty years from various sources in the US represent a single predominant pulsed-field gel electrophoresis type

Salmonella Cerro prevalence in US dairy cattle has increased significantly during the past decade. Comparison of 237 Salmonella isolates collected from various human and animal sources between 1986 and 2009 using pulsed-field gel electrophoresis, antimicrobial resistance typing, and spvA screening, showed very limited genetic diversity, indicating clonality of this serotype. Improved subtyping methods are clearly needed to analyze the potential emergence of this serotype. Our results thus emphasize the critical importance of population-based pathogen surveillance for the detection and characterization of potentially emerging pathogens, and caution to critically evaluate the adequacy of diagnostic tests for a given study population and diagnostic application.

Virulence plasmid interchange between strains ATCC 14028, LT2, and SL1344 of Salmonella enterica serovar Typhimurium

Strains ATCC 14028 and SL1344 of Salmonella enterica serovar Typhimurium are more virulent than LT2 in the BALB/c mouse model. Virulence plasmid swapping between strains ATCC 14208, LT2, and SL1344 does not alter their competitive indexes during mouse infection, indicating that the three plasmids are functionally equivalent, and that their contribution to virulence is independent from the host background. Strains ATCC 14028 and LT2 are more efficient than SL1344 as conjugal donors of the virulence plasmid. Virulence plasmid swapping indicates that reduced ability of conjugal transfer is a property of the SL1344 plasmid, not of the host strain. An A→V amino acid substitution in the TraG protein appears to be the major cause that reduces conjugal transfer in the virulence plasmid of SL1344. Additional sequence differences in the tra operon are found between the SL1344 plasmid and the ATCC 14028 and LT2 plasmids. Divergence in the tra operon may reflect the occurrence of genetic drift either after laboratory domestication or in the environment. The latter might provide evidence that possession of conjugal transfer functions is a neutral trait in Salmonella populations, a view consistent with the abundance of Salmonella isolates whose virulence plasmids are non-conjugative.

Salmonella antibiotic-mutant strains reduce fecal shedding and organ invasion in broiler chicks

We investigated the exposure to antibiotics in the production of antibiotic-mutant strains of Salmonella. Ten isolates of poultry origin were assayed for antibiotic susceptibilities. One strain of Salmonella Enteritidis, one of Salmonella Heidelberg, and one of Salmonella Typhimurium were selected to induce antimicrobial resistance. Each strain was exposed to high concentrations of streptomycin, rifampicin, and nalidixic acid, respectively. Parent and antibiotic-mutant strains were assayed for antibiotic susceptibilities using a commercial microdilution test and the disk susceptibility test. The strains were assessed for virulence genes and evaluated for fecal shedding, cecal colonization, organ invasion, and mean Salmonella counts after inoculation in 1-day-old chicks. The study revealed that exposure to high concentrations of streptomycin produced the antibiotic-mutant strain SE/LABOR/USP/08 and the exposure to rifampicin produced the antibiotic-mutant SH/LABOR/USP/08. These strains showed significantly reduced fecal shedding (P≤0.05) and organ invasion, persisting less than the parental strains and showing no clinical signs in inoculated chicks. High concentrations of nalidixic acid produced the antibiotic-mutant strain ST/LABOR/USP/08, which did not show any differences compared with the parent strain. Likewise, SE/LABOR/USP/08 did not show the expression of plasmid-encoded fimbriae (pefA) and plasmid virulence protein (spvC), suggesting that after exposure to streptomycin, the parent isolate lost the original gene expression, reducing fecal shedding and organ invasion in inoculated chicks.

Occurrence of spvA virulence gene and clinical significance for multidrug-resistant Salmonella strains

Nontyphoidal Salmonella strains are important reservoirs of antimicrobial resistance. An important issue that has not been investigated is whether the multiresistant Salmonella strains are more virulent than their susceptible counterparts. Salmonella isolates collected from clinical human (n=888) and porcine (n=2,120) cases at the same time period and geographic location were investigated. Antimicrobial susceptibility, PCR analysis for the spvA virulence gene, and pulsed-field gel electrophoresis (PFGE) genotyping were done. Carriage of spvA was associated with multidrug-resistant (MDR) type ACSSuT strains (odds ratio, 7.1; P<0.05), a type often implicated in bacteremic human cases. PFGE revealed that clinical isolates from pigs were more clonally related to those of human origin than the nonclinical porcine isolates. The findings suggest that MDR strains that also carry specific virulence factors are more likely to be of clinical significance.

Complete nucleotide sequence of pSCV50, the virulence plasmid of Salmonella enterica serovar Choleraesuis SC-B67

We carried out comparative analysis on the sequences of two 50-kb virulence plasmids of Salmonella enterica serovar Choleraesuis strains SC-B67 (pSCV50) and RF-1 (pKDSC50). The two plasmids share over 99% sequence similarity. Ninety-two nucleotide variations at 42 sites were detected between the two plasmids; pSCV50 contains 24 nucleotide substitutions, 6 deletions, and 62 insertions, compared to pKDSC50. Two regions in pSCV50 appeared to be more susceptible to changes: one is the non-virulence-associated transfer region (27.5-33.0 K) and the other a function-unknown region (9.0-10.5 K). We re-annotated pSCV50 using more advanced tools and the up-to-date databases and corrected the inaccurate annotation in pKDSC50. The results indicate that virulence-related genes on the 50-kb plasmid are under negative selection, suggesting that they play important roles in the expression of virulence during the process of infection, while other genes in this plasmid tend to evolve neutrally.

Systematic targeted mutagenesis of Brucella melitensis 16M reveals a major role for GntR regulators in the control of virulence

In order to identify transcriptional regulators involved in virulence gene control in Brucella melitensis, we generated a collection of 88 mutants in the AraC, ArsR, Crp, DeoR, GntR, IclR, LysR, MerR, RpiR, and TetR families of regulators. This collection was named LiMuR (library of mutants for regulators). We developed a method to test several mutants simultaneously in one animal in order to identify those unable to survive. This method, called the plasmid-tagged mutagenesis method, was used to test the residual virulence of mutants after 1 week in a mouse model of infection. Ten attenuated mutants, of which six and three belong to the GntR and LysR families, respectively, were identified and individually confirmed to replicate at lower rates in mice. Among these 10 mutants, only gntR10 and arsR6 are attenuated in cellular models. The LiMuR also allows simple screenings to identify regulators of a particular gene or operon. As a first example, we analyzed the expression of the virB operon in the LiMuR mutants. We carried out Western blottings of whole-cell extracts to analyze the production of VirB proteins using polyclonal antisera against VirB proteins. Four mutants produced small amounts of VirB proteins, and one mutant overexpressed VirB proteins compared to the wild-type strain. In these five mutants, reporter analysis using the virB promoter fused to lacZ showed that three mutants control virB at the transcriptional level. The LiMuR is a resource that will provide straightforward identification of regulators involved in the control of genes of interest.

Characterization of the spv locus in Salmonella enterica serovar Arizona

Salmonella enterica serovar Arizona (S. enterica subspecies IIIa) is a common Salmonella isolate from reptiles and can cause serious systemic disease in humans. The spv virulence locus, found on large plasmids in Salmonella subspecies I serovars associated with severe infections, was confirmed to be located on the chromosome of serovar Arizona. Sequence analysis revealed that the serovar Arizona spv locus contains homologues of spvRABC but lacks the spvD gene and contains a frameshift in spvA, resulting in a different C terminus. The SpvR protein functions as a transcriptional activator for the spvA promoter, and SpvB and SpvC are highly conserved. The analysis supports the proposal that the chromosomal spv sequence more closely corresponds to the ancestral locus acquired during evolution of S. enterica, with plasmid acquisition of spv genes in the subspecies I strains involving addition of spvD and polymorphisms in spvA.

Spontaneous resolution of internal jugular vein thrombosis in a Salmonella neck abscess patient

This article describes a rare case of Salmonella neck abscess complicated by internal jugular vein thrombosis in a 51-year-old patient with previously undiagnosed diabetes. The patient was discharged without any complications after a combination of medical and surgical treatment. Also discussed here are the clinical manifestations, imaging findings, and spontaneous resolution of the internal jugular vein thrombosis. Being immunocompromised is a critical predisposing factor for Salmonella neck abscess. Patient recovery is largely determined by proper incision, drainage of pus and adequate intravenous antibiotics according to bacterial sensitivity tests. Detection of an internal jugular vein thrombosis does not signify a poor prognosis. Spontaneous resolution of thrombosis is encountered after treating the infection.

The virulence plasmid of Salmonella typhimurium is self-transmissible

Most isolates of Salmonella enterica serovar Typhimurium contain a 90-kb virulence plasmid. This plasmid is reported to be mobilizable but nonconjugative. However, we have determined that the virulence plasmid of strains LT2, 14028, and SR-11 is indeed self-transmissible. The plasmid of strain SL1344 is not. Optimal conjugation frequency requires filter matings on M9 minimal glucose plates with a recipient strain lacking the virulence plasmid. These conditions result in a frequency of 2.9 x 10(-4) transconjugants/donor. Matings on Luria-Bertani plates, liquid matings, or matings with a recipient strain carrying the virulence plasmid reduce the efficiency by up to 400-fold. Homologs of the F plasmid conjugation genes are physically located on the virulence plasmid and are required for the conjugative phenotype.

Mutational characterization of promoter regions recognized by the Salmonella dublin virulence plasmid regulatory protein SpvR

The virulence plasmid-encoded spv regulon is essential for virulence of Salmonella dublin in mice. The spvR gene product belongs to the LysR family of transcriptional regulator proteins. SpvR induces the expression of the spvABCD operon and positively regulates its own expression. DNase I protection analysis with purified SpvR fusion proteins identified SpvR binding sites within the spvA and spvR promoters (P. Grob and D. G. Guiney, J. Bacteriol. 178:1813-1820, 1996). We have used PCR mutagenesis, combined with functional selection for reduced SpvR affinity, to define the DNA elements essential for SpvR binding. For the spvR promoter fragment, a screen for reduced expression was also applied. Sequence analysis of the resulting mutant fragments reveals that the base pair changes are clustered in distinct regions. Determination of the apparent dissociation constants of SpvR for the mutant promoters showed that the spvA LysR-type motif and the upstream palindromic sequences of both promoters play an important role in SpvR recognition.

Expression profile and subcellular location of the plasmid-encoded virulence (Spv) proteins in wild-type Salmonella dublin

The plasmid-encoded virulence genes (spvABCD) in nontyphoid Salmonella strains mediate lethal infections in a variety of animals. Previous studies have shown that these genes are transcriptionally regulated by stationary-phase growth. We studied the expression profile and the subcellular locations of the SpvABCD proteins in wild-type S. dublin by using polyclonal antibodies against SpvA, SpvB, SpvC, and SpvD. The cellular levels of the individual proteins were determined during growth by quantitative immunoblotting. As expected, SpvA, SpvB, SpvC, and SpvD were not detectable before the late logarithmic growth phase and appeared in the sequence SpvA, SpvB, SpvC, and SpvD. In contrast to the transcriptional regulation, however, SpvA and SpvB reached their maximal expression shortly after induction and declined during further growth whereas SpvC and SpvD expression remained high throughout the stationary phase, indicating that the Spv proteins are individually regulated at a posttranscriptional level. To localize SpvABCD within the bacteria, the cells were fractionated into the periplasmic, cytoplasmic, inner membrane, and outer membrane components. The cell fractions and the culture supernatant were analyzed by immunoblotting. SpvA was present in the outer membrane, SpvB was present in the cytoplasm and the inner membrane, and SpvC was present in the cytoplasm. SpvD was secreted into the supernatant; however, a substantial portion of this protein was also detected in the cytoplasm and membranes. The molecular weights of SpvD in the supernatant and in the cytoplasm appeared to be equal, suggesting that SpvD is not cleaved upon secretion.

Common themes in microbial pathogenicity revisited

Bacterial pathogens employ a number of genetic strategies to cause infection and, occasionally, disease in their hosts. Many of these virulence factors and their regulatory elements can be divided into a smaller number of groups based on the conservation of similar mechanisms. These common themes are found throughout bacterial virulence factors. For example, there are only a few general types of toxins, despite a large number of host targets. Similarly, there are only a few conserved ways to build the bacterial pilus and nonpilus adhesins used by pathogens to adhere to host substrates. Bacterial entry into host cells (invasion) is a complex mechanism. However, several common invasion themes exist in diverse microorganisms. Similarly, once inside a host cell, pathogens have a limited number of ways to ensure their survival, whether remaining within a host vacuole or by escaping into the cytoplasm. Avoidance of the host immune defenses is key to the success of a pathogen. Several common themes again are employed, including antigenic variation, camouflage by binding host molecules, and enzymatic degradation of host immune components. Most virulence factors are found on the bacterial surface or secreted into their immediate environment, yet virulence factors operate through a relatively small number of microbial secretion systems. The expression of bacterial pathogenicity is dependent upon complex regulatory circuits. However, pathogens use only a small number of biochemical families to express distinct functional factors at the appropriate time that causes infection. Finally, virulence factors maintained on mobile genetic elements and pathogenicity islands ensure that new strains of pathogens evolve constantly. Comprehension of these common themes in microbial pathogenicity is critical to the understanding and study of bacterial virulence mechanisms and to the development of new "anti-virulence" agents, which are so desperately needed to replace antibiotics.

Further search for virulence factors encoded by Salmonella serovar-specific plasmids

The so called Salmonella virulence plasmids which are specifically prevalent among some of the S. enterica serovars were shown to contribute only marginally to the virulence make-up of salmonella, which is in contrast to Shigella and Yersinia spp. The experiments reported in this paper failed to find encoded plasmid factors which contribute to serum resistance, surface antigens, immunoinsufficiency or to up-regulation of chromosomally encoded factors such as toxins, surface antigens etc. Taking into consideration the rare prevalence of these plasmids among S. enterica but their common occurrence among a few of its serovars, their virulence implication remains an enigma.

Plasmid virulence gene expression induced by short-chain fatty acids in Salmonella dublin: identification of rpoS-dependent and rpo-S-independent mechanisms

The Salmonella plasmid virulence spvABCD genes are growth phase regulated and require RpoS for maximal expression in stationary phase. We identified a growth phase-independent expression of spv which is mediated by short-chain fatty acids. During this fatty acid-mediated expression of spv, RpoS is required for induction only during exponential phase. In stationary phase, an rpoS-independent mechanism is responsible for expression of spv.

In vitro binding of the Salmonella dublin virulence plasmid regulatory protein SpvR to the promoter regions of spvA and spvR

The spv regulon of Salmonella dublin is essential for virulence in mice. SpvR, a LysR-type regulator, induces the expression of the spvABCD operon and its own expression in the stationary phase of bacterial growth and in macrophages. We constructed fusion proteins to the maltose-binding protein (MBP) and a His tag peptide (His) to overcome the insolubility and to facilitate purification of SpvR. We demonstrated that both fusion proteins, MBP-SpvR and His-SpvR, were able to induce spvA expression in vivo. MBP-SpvR was produced as soluble protein, whereas His-SpvR was only marginally present in the soluble cell fraction. Affinity chromatography resulted in at least 95% pure MBP-SpvR protein and in an enrichment of His-SpvR. Gel mobility shift assay revealed that the SpvR fusion proteins were able to bind to 125-and 147-bp DNA fragments of the spvA and spvR promoter regions, respectively. DNase I footprint experiments showed that the fusion proteins protected DNA regions of 54 and 50 bp within the spvA and spvR promoter regions, respectively.

Role of Escherichia coli rpoS and associated genes in defense against oxidative damage

The first phenotype described for mutations in the Escherichia coli rpoS gene was hypersensitivity to near-ultraviolet radiation and to its oxidative photoproduct, hydrogen peroxide. Initially named nur, this gene is now known to code for a sigma factor, and has acquired new names such as katF and rpoS. The role of its protein product (sigma-38) is to regulate a battery of genes as cells enter and rest in stationary phase. Some of the gene products are involved in protection against oxidants (e.g., catalases) and repair of oxidative damage (e.g., exonuclease III). Sigma-38 may also modulate transcription of certain growth phase genes, including hydroperoxidase I and glutathione reductase. Sigma-38 activity is regulated at transcriptional, translational, and protein stabilization levels. This review describes the complex mechanisms whereby sigma-38 controls various genes, the interaction of sigma-38 with other regulators, and a possible role of sigma-38 in bacterial virulence.

SlyA, a regulatory protein from Salmonella typhimurium, induces a haemolytic and pore-forming protein in Escherichia coli

A chromosomal fragment from Salmonella typhimurium, when cloned in Escherichia coli, generates a haemolytic phenotype. This fragment carries two genes, termed slyA and slyB. The expression of slyA is sufficient for the haemolytic phenotype. The haemolytic activity of E. coli carrying multiple copies of slyA is found mainly in the cytoplasm, with some in the periplasm of cells grown to stationary phase, but overexpression of SlyB, a 15 kDa lipoprotein probably located in the outer membrane, may lead to enhanced, albeit unspecific, release of the haemolytic activity into the medium. Polyclonal antibodies raised against a purified SlyA-HlyA fusion protein identified the overexpressed monomeric 17 kDa SlyA protein mainly in the cytoplasm of E. coli grown to stationary phase, although smaller amounts were also found in the periplasm and even in the culture supernatant. However, the anti-SlyA antibodies reacted with the SlyA protein in a periplasmic fraction that did not contain the haemolytic activity. Conversely, the periplasmic fraction exhibiting haemolytic activity did not contain the 17 kDa SlyA protein. Furthermore, S. typhimurium transformed with multiple copies of the slyA gene did not show a haemolytic phenotype when grown in rich culture media, although the SlyA protein was expressed in amounts similar to those in the recombinant E. coli strain. These results indicate that SlyA is not itself a cytolysin but rather induces in E. coli (but not in S. typhimurium) the synthesis of an uncharacterised, haemolytically active protein which forms pores with a diameter of about 2.6 nm in an artificial lipid bilayer. The SlyA protein thus seems to represent a regulation factor in Salmonella, as is also suggested by the similarity of the SlyA protein to some other bacterial regulatory proteins. slyA- and slyB-related genes were also obtained by PCR from E. coli, Shigella sp. and Citrobacter diversus but not from several other gram-negative bacteria tested.

Central regulatory role for the RpoS sigma factor in expression of Salmonella dublin plasmid virulence genes

The plasmid virulence genes spvABCD of Salmonella spp. are regulated by SpvR and the stationary-phase sigma factor RpoS. The transcription of spv genes is induced during the post-exponential phase of bacterial growth in vitro. We sought to investigate the relationship between growth phase and RpoS in spv regulation. rpoS insertion mutations were constructed in S. dublin Lane and plasmid-cured LD842 strains, and the mutants were found to be attenuated for virulence and deficient in spv gene expression. We utilized the plasmid pBAD::rpoS to express rpoS independent of the growth phase under the control of the arabinose-inducible araBAD promoter. SpvA expression was induced within 2 h after the addition of 0.1% arabinose, even when bacteria were actively growing. This suggested that the level of RpoS, instead of the growth phase itself, controls induction of the spv genes. However, RpoS did not activate transcription of spvA in the absence of SpvR protein. Using a constitutive tet promoter to express spvR, we found that the spvA gene can be partially expressed in the rpoS mutant, suggesting that RpoS is required for SpvR synthesis. We confirmed that spvR is poorly expressed in the absence of RpoS. With an intact rpoS gene, spvR expression is not dependent on an intact spvR gene but is enhanced by spvR supplied in trans. We propose a model for Salmonella spv gene regulation in which both RpoS and SpvR are required for maximal expression at the spvR and spvA promoters.

Growth-phase regulation of plasmid virulence genes in Salmonella

Virulence genes in the genus Salmonella are regulated by growth phase and by environmental signals, which allows a sequential program of expression during infection. Conditions that promote the expression of loci required in systemic infection, including the plasmid-encoded spv genes, are the opposite of the factors that induce genes involved in the invasion of epithelial cells in the gastrointestinal tract.