At least four percent of the Salmonella typhimurium genome is required for fatal infection of mice

Typhoid fever

Typhoid fever is an invasive bacterial disease associated with bloodstream infection that causes a high burden of disease in Africa and Asia. Typhoid primarily affects individuals ranging from infants through to young adults. The causative organism, Salmonella enterica subsp. enterica serovar Typhi is transmitted via the faecal-oral route, crossing the intestinal epithelium and disseminating to systemic and intracellular sites, causing an undifferentiated febrile illness. Blood culture remains the practical reference standard for diagnosis of typhoid fever, where culture testing is available, but novel diagnostic modalities are an important priority under investigation. Since 2017, remarkable progress has been made in defining the global burden of both typhoid fever and antimicrobial resistance; in understanding disease pathogenesis and immunological protection through the use of controlled human infection; and in advancing effective vaccination programmes through strategic multipartner collaboration and targeted clinical trials in multiple high-incidence priority settings. This Primer thus offers a timely update of progress and perspective on future priorities for the global scientific community.

Novel method to recover Salmonella enterica cells for Tn-Seq approaches from lettuce leaves and agricultural environments using combination of sonication, filtration, and dialysis membrane

Salmonella enterica in agricultural environments has become an important concern, due to its potential transmission to humans and the associated public health risks. To identify genes contributing to Salmonella adaptation to such environments, transposon sequencing has been used in recent years. However, isolating Salmonella from atypical hosts, such as plant leaves, can pose technical challenges due to low bacterial content and the difficulty to separate an adequate number of bacteria from host tissues. In this study, we describe a modified methodology using a combination of sonication and filtration to recover S. enterica cells from lettuce leaves. We successfully recovered over a total of 3.5 × 106Salmonella cells in each biological replicate from two six-week old lettuce leaves, 7 days after infiltration with a Salmonella suspension of 5 × 107 colony forming units (CFU)/mL. Moreover, we have developed a dialysis membrane system as an alternative method for recovering bacteria from culture medium, mimicking a natural environment. Inoculating 107 CFU/mL of Salmonella into the media based on plant (lettuce and tomato) leaf and diluvial sand soil, a final concentration of 109.5 and 108.5 CFU/mL was obtained, respectively. One millilitre of the bacterial suspension after 24 h incubation at 28 °C using 60 rpm agitation was pelleted, corresponding to 109.5 and 108.5 cells from leaf- or soil-based media. The recovered bacterial population, from both lettuce leaves and environment-mimicking media, can adequately cover a presumptive library density of 106 mutants. In conclusion, this protocol provides an effective method to recover a Salmonella transposon sequencing library from in planta and in vitro systems. We expect this novel technique to foster the study of Salmonella in atypical hosts and environments, as well as other comparable scenarios.

Roles of the Tol/Pal System in Bacterial Pathogenesis and Its Application to Antibacterial Therapy

The Tol/Pal system (also written as “The Tol-Pal system”) is a set of protein complexes produced by most Gram-negative bacteria. It comprises the inner membrane-associated and the outer membrane-anchored subunits composed of the TolA, TolQ, and TolR proteins and the TolB and Pal proteins, respectively. Although the Tol/Pal system was first defined as bacterial proteins involved in colicin uptake of Escherichia coli, its global roles have been characterized in several studies as mentioned in this article. Pathogenesis of many Gram-negative pathogens is sustained by the Tol/Pal system. It is also essential for cell growth and fitness in some pathogens. Therefore, the Tol/Pal system is proposed as a potential target for antimicrobial chemotherapy. Although the tol/pal mutants are low in virulence, they still have the ability to stimulate the immune system. The Pal protein is highly immunogenic and induces both adaptive and innate immune responses. Therefore, the tol/pal mutant strains and Pal proteins also have potential vaccine properties. For these reasons, the Tol/Pal system represents a promising research target in the development of antibacterial therapeutic strategies for refractory infections caused by multi-drug-resistant (MDR), Gram-negative pathogens. In this paper, we summarize studies on the Tol/Pal system associated with bacterial pathogenesis and vaccine development.

Donnan Potential across the Outer Membrane of Gram-Negative Bacteria and Its Effect on the Permeability of Antibiotics

The cell envelope structure of Gram-negative bacteria is unique, composed of two lipid bilayer membranes and an aqueous periplasmic space sandwiched in between. The outer membrane constitutes an extra barrier to limit the exchange of molecules between the cells and the exterior environment. Donnan potential is a membrane potential across the outer membrane, resulted from the selective permeability of the membrane, which plays a pivotal role in the permeability of many antibiotics. In this review, we discussed factors that affect the intensity of the Donnan potential, including the osmotic strength and pH of the external media, the osmoregulated periplasmic glucans trapped in the periplasmic space, and the displacement of cell surface charges. The focus of our discussion is the impact of Donnan potential on the cellular permeability of selected antibiotics including fluoroquinolones, tetracyclines, β-lactams, and trimethoprim.

The multifarious roles of Tol-Pal in Gram-negative bacteria

In the 1960s several groups reported the isolation and preliminary genetic mapping of Escherichia coli strains tolerant towards the action of colicins. These pioneering studies kick-started two new fields in bacteriology; one centred on how bacteriocins like colicins exploit the Tol (or more commonly Tol-Pal) system to kill bacteria, the other on the physiological role of this cell envelope-spanning assembly. The following half century has seen significant advances in the first of these fields whereas the second has remained elusive, until recently. Here, we review work that begins to shed light on Tol-Pal function in Gram-negative bacteria. What emerges from these studies is that Tol-Pal is an energised system with fundamental, interlinked roles in cell division – coordinating the re-structuring of peptidoglycan at division sites and stabilising the connection between the outer membrane and underlying cell wall. This latter role is achieved by Tol-Pal exploiting the proton motive force to catalyse the accumulation of the outer membrane peptidoglycan associated lipoprotein Pal at division sites while simultaneously mobilising Pal molecules from around the cell. These studies begin to explain the diverse phenotypic outcomes of tol-pal mutations, point to other cell envelope roles Tol-Pal may have and raise many new questions.

Roles of the Tol-Pal system in the Type III secretion system and flagella-mediated virulence in enterohemorrhagic Escherichia coli

The Tol-Pal system is a protein complex that is highly conserved in many gram-negative bacteria. We show here that the Tol-Pal system is associated with the enteric pathogenesis of enterohemorrhagic E. coli (EHEC). Deletion of tolB, which is required for the Tol-Pal system decreased motility, secretion of the Type III secretion system proteins EspA/B, and the ability of bacteria to adhere to and to form attaching and effacing (A/E) lesions in host cells, but the expression level of LEE genes, including espA/B that encode Type III secretion system proteins were not affected. The Citrobacter rodentium, tolB mutant, that is traditionally used to estimate Type III secretion system associated virulence in mice did not cause lethality in mice while it induced anti-bacterial immunity. We also found that the pal mutant, which lacks activity of the Tol-Pal system, exhibited lower motility and EspA/B secretion than the wild-type parent. These combined results indicate that the Tol-Pal system contributes to the virulence of EHEC associated with the Type III secretion system and flagellar activity for infection at enteric sites. This finding provides evidence that the Tol-Pal system may be an effective target for the treatment of infectious diseases caused by pathogenic E. coli.

The lipoprotein Pal stabilises the bacterial outer membrane during constriction by a mobilisation-and-capture mechanism

Coordination of outer membrane constriction with septation is critical to faithful division in Gram-negative bacteria and vital to the barrier function of the membrane. This coordination requires the recruitment of the peptidoglycan-binding outer-membrane lipoprotein Pal at division sites by the Tol system. Here, we show that Pal accumulation at Escherichia coli division sites is a consequence of three key functions of the Tol system. First, Tol mobilises Pal molecules in dividing cells, which otherwise diffuse very slowly due to their binding of the cell wall. Second, Tol actively captures mobilised Pal molecules and deposits them at the division septum. Third, the active capture mechanism is analogous to that used by the inner membrane protein TonB to dislodge the plug domains of outer membrane TonB-dependent nutrient transporters. We conclude that outer membrane constriction is coordinated with cell division by active mobilisation-and-capture of Pal at division septa by the Tol system.

Sensitivity of predatory bacteria to different surfactants and their application to check bacterial predation

We evaluated the toxicity of surfactants against different predatory bacteria. Tests with Bdellovibrio bacteriovorus HD100 and SDS, an anionic surfactant, showed the predator was very sensitive; 0.02% SDS completely killed the predatory population (7-log loss; < 10 PFU/ml remaining) both when free-swimming or within the bdelloplast, i.e., intraperiplasmic. Similar results were also observed with B. bacteriovorus 109J and Peredibacter starrii. In contrast, none of the prey (E. coli, Klebsiella pneumoniae, Acinetobacter baumannii, or Pseudomonas sp. DSM 50906) viabilities were negatively affected by SDS. Triton X-100, a nonionic surfactant, was slightly less toxic towards B. bacteriovorus HD100 (viability loss of only 4-log), while two cationic surfactants, i.e., benzalkonium chloride (BZC) and cetyltrimethylammonium bromide (CTAB), were toxic towards both the predator and prey. Based on the above findings, we tested the potential use of SDS as a means to control predation. Addition of 0.02% SDS immediately halted predation based upon the prey bioluminescence, which leveled off and remained steady. This was confirmed using the predator viabilities; no predators were found in any of the samples where SDS was added. Consequently, low concentrations of SDS can be used as a simple means to control B. bacteriovorus HD100 activities.

The Tol-Pal System of Uropathogenic Escherichia coli Is Responsible for Optimal Internalization Into and Aggregation Within Bladder Epithelial Cells, Colonization of the Urinary Tract of Mice, and Bacterial Motility

Urinary tracts infection (UTI) caused by uropathogenic Escherichia coli (UPEC) is a common infectious disease. With the shortage of new antimicrobial agents, the increase in UPEC resistance to commonly used drugs, such as fluoroquinolones and β-lactams including carbapenems is a critical issue. UPEC invades urinary tract cells, where it aggregates, and subsequently, forms biofilm-like multicellular colonies termed intracellular bacterial communities (IBCs). This process allows the bacteria to establish infections and so may be a good potential target for new drugs to treat infections. Here, we show that deletion of the tolB gene, encoding a protein of the Tol-Pal system that was originally characterized as a protein complex for colicin uptake and maintenance of the outer membrane, decreases the level of bacterial internalization into and aggregation within cultured bladder epithelial cells and also inhibits the colonization of mice urinary tracts. The tolB mutant also exhibited defective motility because of impaired flagellum syntheses. The fliC and motA mutants, which are non-motile strains, also exhibited lower levels of bacterial internalization and aggregation than their wild-type parent. Additional deletion of tolB in the fliC mutant did not further decrease these, suggesting that the attenuated virulence of the tolB mutant is a result of defective motility. The tolA, tolQ, tolR, and pal mutants that lack other members of the Tol-Pal system also exhibited lower levels of motility and aggregation within bladder epithelial cells compared to their wild-type parent. These combined results suggest another role of the Tol-Pal system, i.e., that it is responsible for optimal internalization, aggregation followed by IBC formation within urinary tract cells, and bacterial motility.

Transcriptomic Analysis of Vibrio parahaemolyticus Reveals Different Virulence Gene Expression in Response to Benzyl Isothiocyanate

Vibrio parahaemolyticus isolated from seafood is a pathogenic microorganism that leads to several acute diseases that are harmful to our health and is frequently transmitted by food. Therefore, there is an urgent need for the control and suppression of this pathogen. In this paper, transcriptional analysis was used to determine the effect of treatment with benzyl isothiocyanate (BITC) extracted from cruciferous vegetables on V. parahaemolyticus and to elucidate the molecular mechanisms underlying the response to BITC. Treatment with BITC resulted in 332 differentially expressed genes, among which 137 genes were downregulated, while 195 genes were upregulated. Moreover, six differentially expressed genes (DEGs) in RNA sequencing studies were further verified by quantitative real-time polymerase chain reaction (qRT-PCR). Genes found to regulate virulence encoded an l-threonine 3-dehydrogenase, a GGDEF family protein, the outer membrane protein OmpV, a flagellum-specific adenosine triphosphate synthase, TolQ protein and VirK protein. Hence, the results allow us to speculate that BITC may be an effective control strategy for inhibiting microorganisms growing in foods.

Beyond the antibody: B cells as a target for bacterial infection

It is well established that B cells play an important role during infections beyond antibody production. B cells produce cytokines and are APCs for T cells. Recently, it has become clear that several pathogenic bacterial genera, such as Salmonella, Brucella, Mycobacterium, Listeria, Francisella, Moraxella, and Helicobacter, have evolved mechanisms such as micropinocytosis induction, inflammasome down-regulation, inhibitory molecule expression, apoptosis induction, and anti-inflammatory cytokine secretion to manipulate B cell functions influencing immune responses. In this review, we summarize our current understanding of B cells as targets of bacterial infection and the mechanisms by which B cells become a niche for bacterial survival and replication away from extracellular immune responses such as complement and antibodies.

Salmonella Tol-Pal Reduces Outer Membrane Glycerophospholipid Levels for Envelope Homeostasis and Survival during Bacteremia

Salmonellae regulate membrane lipids during infection, but the exact proteins and mechanisms that promote their survival during bacteremia remain largely unknown. Mutations in genes encoding the conserved Salmonella enterica serovar Typhimurium (S. Typhimurium) Tol-Pal apparatus caused the outer membrane (OM) sensor lipoprotein, RcsF, to become activated. The capsule activation phenotype for the mutants suggested that Tol-Pal might influence envelope lipid homeostasis. The mechanism involves reducing OM glycerophospholipid (GPL) levels, since the mutant salmonellae similarly accumulated phosphatidylglycerols (PGl) and phosphatidylethanolamines (PE) within the OM in comparison to the wild type. The data support the Escherichia coli model, whereby Tol-Pal directs retrograde GPL translocation across the periplasm. The S. Typhimurium mechanism involves contributions from YbgC, a cytoplasmic acyl coenzyme A (acyl-CoA) thioesterase, and CpoB, a periplasmic TolA-binding protein. The functional relationship between Tol-Pal and YbgC and CpoB was previously unresolved. The S. Typhimurium Tol-Pal proteins contribute similarly toward promoting OM-GPL homeostasis and Rcs signaling inactivity but differently toward promoting bacterial morphology, rifampin resistance, survival in macrophages, and survival in mice. For example, tolQ, tolR, tolA, and cpoB mutants were significantly more attenuated than ybgC, tolB, and pal mutants in a systemic mouse model of disease. Therefore, key roles exist for TolQ, TolR, TolA, and CpoB during murine bacteremia, which are independent of maintaining GPL homeostasis. The ability of TolQR to channel protons across the inner membrane (IM) is necessary for S. Typhimurium TolQRA function, since mutating conserved channel-facing residues rendered TolQ ineffective at rescuing deletion mutant phenotypes. Therefore, Tol-Pal promotes S. Typhimurium survival during bacteremia, in part, by reducing OM GPL concentrations, while TolQRA and CpoB enhance systemic virulence by additional mechanisms.

miR-143 inhibits intracellular salmonella growth by targeting ATP6V1A in macrophage cells in pig

Salmonella infects many vertebrate species, and animals such as pigs can be colonized with Salmonella and become established carriers. Analyzing the roles of microRNA in intracellular proliferation is important for understanding the process of Salmonella infection. The objective of this study is to verify the regulation effect of miR-143 on ATP6V1A and its functions in the intracellular growth of Salmonella. A new miR-143 binding site was discovered in the 3' UTR of ATP6V1A using a newly developed prediction tool. The binding site was confirmed by binding site deletion assay. Real-time PCR results indicated that ATP6V1A was predominantly expressed in bone-marrow-derived macrophages, and the expression of miR-143 in different tissues was negatively correlated with ATP6V1A. The Salmonella proliferation assay showed that the expression of miR-143 could inhibit intracellular Salmonella growth in macrophages by target ATP6V1A. The results strongly suggest that miR-143 plays important regulatory roles in the development of Salmonella infection in animals.

Involvement of tolQ and tolR genes in Edwardsiella ictaluri virulence

Edwardsiella ictaluri is a Gram-negative intracellular facultative pathogen causing enteric septicemia of channel catfish (ESC). The Tol system, consisting of four envelope proteins TolQ, TolR, TolA, and TolB, are required for colicin import and contributes to bacterial virulence in several pathogenic bacteria. However, the Tol system and its importance in E. ictaluri virulence have not been investigated. Here we present construction and evaluation of the E. ictaluri TolQ, TolR and TolQR mutants (EiΔtolQ, EiΔtolR, and EiΔtolQR). The Tol mutants were developed using in-frame gene deletion and their attenuation and vaccine efficacy were determined in catfish fingerlings. The EiΔtolQ, EiΔtolR, and EiΔtolQR mutants showed reduced virulence in catfish (28.93%, 19.70%, and 39.82% mortality, respectively) compared to wild type (46.91% mortality). Further, vaccination with these mutants protected catfish against subsequent wild-type infection. This study suggests that the Tol system contributes to E. ictaluri virulence in catfish.

Phosphate Limitation Induces Drastic Physiological Changes, Virulence-Related Gene Expression, and Secondary Metabolite Production in Pseudovibrio sp. Strain FO-BEG1

Phosphorus is a vital nutrient for living organisms and is obtained by bacteria primarily via phosphate uptake. However, phosphate is often scarcely accessible in nature, and there is evidence that in many areas of the ocean, its concentration limits bacterial growth. Surprisingly, the phosphate starvation response has been extensively investigated in different model organisms (e.g., Escherichia coli), but there is a dearth of studies on heterotrophic marine bacteria. In this work, we describe the response of Pseudovibrio sp. strain FO-BEG1, a metabolically versatile alphaproteobacterium and potential symbiont of marine sponges, to phosphate limitation. We compared the physiology, protein expression, and secondary metabolite production under phosphate-limited conditions to those under phosphate surplus conditions. We observed that phosphate limitation had a pleiotropic effect on the physiology of the strain, triggering cell elongation, the accumulation of polyhydroxyalkanoate, the degradation of polyphosphate, and the exchange of membrane lipids in favor of phosphorus-free lipids such as sulfoquinovosyl diacylglycerols. Many proteins involved in the uptake and degradation of phospho-organic compounds were upregulated, together with subunits of the ABC transport system for phosphate. Under conditions of phosphate limitation, FO-BEG1 secreted compounds into the medium that conferred an intense yellow coloration to the cultures. Among these compounds, we identified the potent antibiotic tropodithietic acid. Finally, toxin-like proteins and other proteins likely involved in the interaction with the eukaryotic host were also upregulated. Altogether, our data suggest that phosphate limitation leads to a pronounced reorganization of FO-BEG1 physiology, involving phosphorus, carbon, and sulfur metabolism; cell morphology; secondary metabolite production; and the expression of virulence-related genes.

The periplasmic protein TolB as a potential drug target in Pseudomonas aeruginosa

The Gram-negative bacterium Pseudomonas aeruginosa is one of the most dreaded pathogens in the hospital setting, and represents a prototype of multi-drug resistant "superbug" for which effective therapeutic options are very limited. The identification and characterization of new cellular functions that are essential for P. aeruginosa viability and/or virulence could drive the development of anti-Pseudomonas compounds with novel mechanisms of action. In this study we investigated whether TolB, the periplasmic component of the Tol-Pal trans-envelope protein complex of Gram-negative bacteria, represents a potential drug target in P. aeruginosa. By combining conditional mutagenesis with the analysis of specific pathogenicity-related phenotypes, we demonstrated that TolB is essential for P. aeruginosa growth, both in laboratory and clinical strains, and that TolB-depleted P. aeruginosa cells are strongly defective in cell-envelope integrity, resistance to human serum and several antibiotics, as well as in the ability to cause infection and persist in an insect model of P. aeruginosa infection. The essentiality of TolB for P. aeruginosa growth, resistance and pathogenicity highlights the potential of TolB as a novel molecular target for anti-P. aeruginosa drug discovery.

Defined single-gene and multi-gene deletion mutant collections in Salmonella enterica sv Typhimurium

We constructed two collections of targeted single gene deletion (SGD) mutants and two collections of targeted multi-gene deletion (MGD) mutants in Salmonella enterica sv Typhimurium 14028s. The SGD mutant collections contain (1), 3517 mutants in which a single gene is replaced by a cassette containing a kanamycin resistance (KanR) gene oriented in the sense direction (SGD-K), and (2), 3376 mutants with a chloramphenicol resistance gene (CamR) oriented in the antisense direction (SGD-C). A combined total of 3773 individual genes were deleted across these SGD collections. The MGD collections contain mutants bearing deletions of contiguous regions of three or more genes and include (3), 198 mutants spanning 2543 genes replaced by a KanR cassette (MGD-K), and (4), 251 mutants spanning 2799 genes replaced by a CamR cassette (MGD-C). Overall, 3476 genes were deleted in at least one MGD collection. The collections with different antibiotic markers permit construction of all viable combinations of mutants in the same background. Together, the libraries allow hierarchical screening of MGDs for different phenotypic followed by screening of SGDs within the target MGD regions. The mutants of these collections are stored at BEI Resources (www.beiresources.org) and publicly available.

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.

Diverse secreted effectors are required for Salmonella persistence in a mouse infection model

Salmonella enterica serovar Typhimurium causes typhoid-like disease in mice and is a model of typhoid fever in humans. One of the hallmarks of typhoid is persistence, the ability of the bacteria to survive in the host weeks after infection. Virulence factors called effectors facilitate this process by direct transfer to the cytoplasm of infected cells thereby subverting cellular processes. Secretion of effectors to the cell cytoplasm takes place through multiple routes, including two separate type III secretion (T3SS) apparati as well as outer membrane vesicles. The two T3SS are encoded on separate pathogenicity islands, SPI-1 and -2, with SPI-1 more strongly associated with the intestinal phase of infection, and SPI-2 with the systemic phase. Both T3SS are required for persistence, but the effectors required have not been systematically evaluated. In this study, mutations in 48 described effectors were tested for persistence. We replaced each effector with a specific DNA barcode sequence by allelic exchange and co-infected with a wild-type reference to calculate the ratio of wild-type parent to mutant at different times after infection. The competitive index (CI) was determined by quantitative PCR in which primers that correspond to the barcode were used for amplification. Mutations in all but seven effectors reduced persistence demonstrating that most effectors were required. One exception was CigR, a recently discovered effector that is widely conserved throughout enteric bacteria. Deletion of cigR increased lethality, suggesting that it may be an anti-virulence factor. The fact that almost all Salmonella effectors are required for persistence argues against redundant functions. This is different from effector repertoires in other intracellular pathogens such as Legionella.

Proteomics of protein secretion by Aggregatibacter actinomycetemcomitans

The extracellular proteome (secretome) of periodontitis-associated bacteria may constitute a major link between periodontitis and systemic diseases. To obtain an overview of the virulence potential of Aggregatibacter actinomycetemcomitans, an oral and systemic human pathogen implicated in aggressive periodontitis, we used a combined LC-MS/MS and bioinformatics approach to characterize the secretome and protein secretion pathways of the rough-colony serotype a strain D7S. LC-MS/MS revealed 179 proteins secreted during biofilm growth. Further to confirming the release of established virulence factors (e.g. cytolethal distending toxin [CDT], and leukotoxin [LtxA]), we identified additional putative virulence determinants in the secretome. These included DegQ, fHbp, LppC, Macrophage infectivity protein (MIP), NlpB, Pcp, PotD, TolB, and TolC. This finding indicates that the number of extracellular virulence-related proteins is much larger than previously demonstrated, which was also supported by in silico analysis of the strain D7S genome. Moreover, our LC-MS/MS and in silico data revealed that at least Type I, II, and V secretion are actively used to excrete proteins directly into the extracellular space, or via two-step pathways involving the Sec/Tat systems for transport across the inner membrane, and outer membrane factors, secretins and auto-transporters, respectively for delivery across the outer membrane. Taken together, our results provide a molecular basis for further elucidating the role of A. actinomycetemcomitans in periodontal and systemic diseases.

Prc contributes to Escherichia coli evasion of classical complement-mediated serum killing

Escherichia coli is a common Gram-negative organism that causes bacteremia. Prc, a bacterial periplasmic protease, and its homologues are known to be involved in the pathogenesis of Gram-negative bacterial infections. The present study examined the role of Prc in E. coli bacteremia and characterized the ability of the prc mutant of the pathogenic E. coli strain RS218 to cause bacteremia and survive in human serum. The prc mutant of RS218 exhibited a decreased ability to cause a high level of bacteremia and was more sensitive to serum killing than strain RS218. This sensitivity was due to the mutant's decreased ability to avoid the activation of the antibody-dependent and -independent classical complement cascades as well as its decreased resistance to killing mediated by the membrane attack complex, the end product of complement system activation. The demonstration of Prc in the evasion of classical complement-mediated serum killing of pathogenic E. coli makes this factor a potential target for the development of therapeutic and preventive measures against E. coli bacteremia.

Role of anionic charges of osmoregulated periplasmic glucans of Salmonella enterica serovar Typhimurium SL1344 in mice virulence

opgB gene of Salmonella enterica serovar Typhimurium was identified earlier in a genome-wide screen for mice virulence (Valentine et al. in Infect Immun 66:3378-3383, 1998). Although mutation in opgB resulted in avirulent Salmonella strain, how this gene contributes to pathogenesis remains unclear. Based on DNA homology, opgB is predicted to be responsible for adding phosphoglycerate residues to osmoregulated periplasmic glucans (OPGs) giving them anionic characteristics. In Escherichia coli, yet another gene, opgC, is also reported to contribute to anionic characteristics of OPGs by adding succinic acid residues. We constructed opgB, opgC, and opgBC double mutants of S. enterica serovar Typhimurium strain SL1344. As predicted opgBC mutant synthesized neutral OPGs that were devoid of any anionic substituents. However, opgB, opgC, and opgBC mutations had no significant impact on mice virulence as well as on competitive organ colonization. In low osmotic conditions, opgB, opgC, and opgBC mutants exhibited delay in growth initiation in the presence of sodium deoxycholate. Anionic substituents of OPGs from Salmonella although appear to be needed to overcome resistance of deoxycholate in hypoosmotic growth media, no evidence was found for their role in mice virulence.

Burkholderia cenocepacia differential gene expression during host-pathogen interactions and adaptation to the host environment

Members of the Burkholderia cepacia complex (Bcc) are important in medical, biotechnological, and agricultural disciplines. These bacteria naturally occur in soil and water environments and have adapted to survive in association with plants and animals including humans. All Bcc species are opportunistic pathogens including Burkholderia cenocepacia that causes infections in cystic fibrosis and chronic granulomatous disease patients. The adaptation of B. cenocepacia to the host environment was assessed in a rat chronic respiratory infection model and compared to that of high cell-density in vitro grown cultures using transcriptomics. The distribution of genes differentially expressed on chromosomes 1, 2, and 3 was relatively proportional to the size of each genomic element, whereas the proportion of plasmid-encoded genes differentially expressed was much higher relative to its size and most genes were induced in vivo. The majority of genes encoding known virulence factors, components of types II and III secretion systems and chromosome 2-encoded type IV secretion system were similarly expressed between in vitro and in vivo environments. Lower expression in vivo was detected for genes encoding N-acyl-homoserine lactone synthase CepI, orphan LuxR homolog CepR2, zinc metalloproteases ZmpA and ZmpB, LysR-type transcriptional regulator ShvR, nematocidal protein AidA, and genes associated with flagellar motility, Flp type pilus formation, and type VI secretion. Plasmid-encoded type IV secretion genes were markedly induced in vivo. Additional genes induced in vivo included genes predicted to be involved in osmotic stress adaptation or intracellular survival, metal ion, and nutrient transport, as well as those encoding outer membrane proteins. Genes identified in this study are potentially important for virulence during host–pathogen interactions and may be associated with survival and adaptation to the host environment during chronic lung infections.

Structure and immunological characterization of the capsular polysaccharide of a pyrogenic liver abscess caused by Klebsiella pneumoniae: activation of macrophages through Toll-like receptor 4

The active components of a primary pyrogenic liver abscess (PLA) Klebsiella pneumoniae in stimulating cytokine expression in macrophages are still unclear. The capsular polysaccharide (CPS) of PLA K. pneumoniae is important in determining clinical manifestations, and we have shown that it consists of repeating units of the trisaccharide (→3)-β-D-Glc-(1→4)-[2,3-(S)-pyruvate]-β-D-GlcA-(1→4)-α-L-Fuc-(1→) and has the unusual feature of extensive pyruvation of glucuronic acid and acetylation of C(2)-OH or C(3)-OH of fucose. We demonstrated that PLA K. pneumoniae CPS induces secretion of tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) by macrophages through Toll-like receptor 4 (TLR4) and that this effect was lost when pyruvation and O-acetylation were chemically destroyed. Furthermore, expression of TNF-α and IL-6 in PLA K. pneumoniae CPS-stimulated macrophages was shown to be regulated by the TLR4/ROS/PKC-δ/NF-κB, TLR4/PI3-kinase/AKT/NF-κB, and TLR4/MAPK signaling pathways.

Analysis of the ArcA regulon in anaerobically grown Salmonella enterica sv. Typhimurium

Background

Salmonella enterica serovar Typhimurium (S. Typhimurium) is a Gram-negative pathogen that must successfully adapt to the broad fluctuations in the concentration of dissolved dioxygen encountered in the host. In Escherichia coli, ArcA (Aerobic Respiratory Control) helps the cells to sense and respond to the presence of dioxygen. The global role of ArcA in E. coli is well characterized; however, little is known about its role in anaerobically grown S. Typhimurium.

Results

We compared the transcriptional profiles of the virulent wild-type (WT) strain (ATCC 14028s) and its isogenic arcA mutant grown under anaerobic conditions. We found that ArcA directly or indirectly regulates 392 genes (8.5% of the genome); of these, 138 genes are poorly characterized. Regulation by ArcA in S. Typhimurium is similar, but distinct from that in E. coli. Thus, genes/operons involved in core metabolic pathways (e.g., succinyl-CoA, fatty acid degradation, cytochrome oxidase complexes, flagellar biosynthesis, motility, and chemotaxis) were regulated similarly in the two organisms. However, genes/operons present in both organisms, but regulated differently by ArcA in S. Typhimurium included those coding for ethanolamine utilization, lactate transport and metabolism, and succinate dehydrogenases. Salmonella-specific genes/operons regulated by ArcA included those required for propanediol utilization, flagellar genes (mcpAC, cheV), Gifsy-1 prophage genes, and three SPI-3 genes (mgtBC, slsA, STM3784). In agreement with our microarray data, the arcA mutant was non-motile, lacked flagella, and was as virulent in mice as the WT. Additionally, we identified a set of 120 genes whose regulation was shared with the anaerobic redox regulator, Fnr.

Conclusion(s)

We have identified the ArcA regulon in anaerobically grown S. Typhimurium. Our results demonstrated that in S. Typhimurium, ArcA serves as a transcriptional regulator coordinating cellular metabolism, flagella biosynthesis, and motility. Furthermore, ArcA and Fnr share in the regulation of 120 S. Typhimurium genes.

Identification of genes that are dispensable for animal infection by Salmonella typhimurium

In the current study, we generated a pool of Salmonella typhimurium mutants using the Tn10d-cam mini-transposon. This pool of mutants was administered to BALB/c mice through the oral route, and bacteria were recovered from the spleen 3 days post-infection. After three rounds of serial passage, we observed enrichment of two insertion mutants, a yddG insertion and an amyA insertion. These two genes have been implicated in growth on plant products (amyA) and survival in the presence of paraquat (yddG), both of which are natural environments for Salmonella. Thus, while in vivo expression technology has identified S. typhimurium genes that are absolutely necessary for animal infection, other genes involved in vegetative growth also appear to play role in the establishment of pathogenesis.

Expression-based identification of genetic determinants of the bacterial symbiosis 'Chlorochromatium aggregatum'

The phototrophic consortium 'Chlorochromatium aggregatum' is a highly structured association of green sulfur bacterial epibionts surrounding a central, motile bacterium and is the most specific symbiosis currently known between two phylogenetically distinct bacterial species. Genes and gene products potentially involved in the symbiotic interaction were identified on the genomic, transcriptomic and proteomic level. As compared with the 11 available genomes of free-living relatives, only 186 open reading frames were found to be unique to the epibiont genome. 2-D differential gel electrophoresis (2-D DIGE) of the soluble proteomes recovered 1612 protein spots of which 54 were detected exclusively in consortia but not in pure epibiont cultures. Using mass spectrometry analyses, the 13 most intense of the 54 spots could be attributed to the epibiont. Analyses of the membrane proteins of consortia, of consortia treated with cross-linkers and of pure cultures indicated that a branched chain amino acid ABC-transporter binding protein is only expressed in the symbiotic state of the epibiont. Furthermore, analyses of chlorosomes revealed that an uncharacterized 11 kDa epibiont protein is only expressed during symbiosis. This protein may be involved in the intracellular sorting of chlorosomes. Application of a novel prokaryotic cDNA suppression subtractive hybridization technique led to identification of 14 differentially regulated genes, and comparison of the transcriptomes of symbiotic and free-living epibionts indicated that 328 genes were differentially transcribed. The three approaches were mostly complementary and thereby yielded a first inventory of 352 genes that are likely to be involved in the bacterial interaction in 'C. aggregatum'. Notably, most of the regulated genes encoded components of central metabolic pathways whereas only very few (7.5%) of the unique 'symbiosis genes' turned out to be regulated under the experimental conditions tested. This pronounced regulation of central metabolic pathways may serve to fine-tune the symbiotic interaction in 'C. aggregatum' in response to environmental conditions.

Influence of RpoS, cAMP-receptor protein, and ppGpp on expression of the opgGH operon and osmoregulated periplasmic glucan content of Salmonella enterica serovar Typhimurium

A transcriptional fusion (opgG1::MudJ) to the opgGH operon of Salmonella enterica serovar Typhimurium (S. Typhimurium) LT2, isolated by resistance to mecillinam, was used to study the influence of global regulators RpoS, ppGpp, and cAMP/cAMP-receptor protein (CRP) on expression of the opgGH operon and osmoregulated periplasmic glucan (OPG) content. Neither high growth medium osmolarity nor absence of ppGpp or CRP had important effects on opgG1::MudJ expression in exponential cultures. However, under the same conditions, OPG content was strongly decreased by high osmolarity or cAMP/CRP defectiveness, and reduced to a half by lack of ppGpp. In stationary cultures, high osmolarity as well as CRP loss caused significant descents in opgG1::MudJ expression that were compensated by inactivation of RpoS sigma factor. No effect of RpoS inactivation on OPG content was observed. It is concluded that opgGH expression in S. Typhimurium is only slightly affected by high osmolarity, but is inversely modulated by RpoS level. On the other hand, osmolarity and the cAMP/CRP global regulatory system appear to control OPG content, either directly or indirectly, mainly at the post-transcriptional level.

ppGpp-mediated stationary phase induction of the genes encoded by horizontally acquired pathogenicity islands and cob/pdu locus in Salmonella enterica serovar Typhimurium

Salmonella enterica is highly diverse in terms of genome structure, which is at least partly due to the horizontal transfer of genetic elements from various sources. In this study, we examined the expression profiles of such genes in Salmonella Pathogenicity Islands (SPIs) and the cob/pdu locus, horizontally acquired large DNA segments, during growth under standard growth conditions. Transcripts from exponentially growing and early stationary phase Salmonellae were compared using various methods including cDNA microarray analysis. Nearly all genes encoded by SPIs and the cob/pdu locus were induced at the onset of the stationary phase in a stringent molecule ppGpp-dependent but stationary phase sigma, sigma38-independent manner. Although, it has been suggested that ppGpp acts in concert with DksA, we found the stationary phase induction of those SPI genes was not DksA dependent. It is suggested that ppGpp stimulates the expression of these stress-inducible genes encoded by horizontally acquired DNA, by itself or in concert with DksA.

Short-term signatures of evolutionary change in the Salmonella enterica serovar typhimurium 14028 genome

Salmonella enterica serovar Typhimurium is a Gram-negative pathogen that causes gastroenteritis in humans and a typhoid-like disease in mice and is often used as a model for the disease promoted by the human-adapted S. enterica serovar Typhi. Despite its health importance, the only S. Typhimurium strain for which the complete genomic sequence has been determined is the avirulent LT2 strain, which is extensively used in genetic and physiologic studies. Here, we report the complete genomic sequence of the S. Typhimurium strain 14028s, as well as those of its progenitor and two additional derivatives. Comparison of these S. Typhimurium genomes revealed differences in the patterns of sequence evolution and the complete inventory of genetic alterations incurred in virulent and avirulent strains, as well as the sequence changes accumulated during laboratory passage of pathogenic organisms.

The crystal structure of the TolB box of colicin A in complex with TolB reveals important differences in the recruitment of the common TolB translocation portal used by group A colicins

Interaction of the TolB box of Group A colicins with the TolB protein in the periplasm of Escherichia coli cells promotes transport of the cytotoxic domain of the colicin across the cell envelope. The crystal structure of a complex between a 107-residue peptide (TA_1–107) of the translocation domain of colicin A (ColA) and TolB identified the TolB box as a 12-residue peptide that folded into a distorted hairpin within a central canyon of the β-propeller domain of TolB. Comparison of this structure with that of the colicin E9 (ColE9) TolB box–TolB complex, together with site-directed mutagenesis of the ColA TolB box residues, revealed important differences in the interaction of the two TolB boxes with an overlapping binding site on TolB. Substitution of the TolB box residues of ColA with those of ColE9 conferred the ability to competitively recruit TolB from Pal but reduced the biological activity of the mutant ColA. This datum explains (i) the difference in binding affinities of ColA and ColE9 with TolB, and (ii) the inability of ColA, unlike ColE9, to competitively recruit TolB from Pal, allowing an understanding of how these two colicins interact in a different way with a common translocation portal in E. coli cells.

Analysis of pools of targeted Salmonella deletion mutants identifies novel genes affecting fitness during competitive infection in mice

Pools of mutants of minimal complexity but maximal coverage of genes of interest facilitate screening for genes under selection in a particular environment. We constructed individual deletion mutants in 1,023 Salmonella enterica serovar Typhimurium genes, including almost all genes found in Salmonella but not in related genera. All mutations were confirmed simultaneously using a novel amplification strategy to produce labeled RNA from a T7 RNA polymerase promoter, introduced during the construction of each mutant, followed by hybridization of this labeled RNA to a Typhimurium genome tiling array. To demonstrate the ability to identify fitness phenotypes using our pool of mutants, the pool was subjected to selection by intraperitoneal injection into BALB/c mice and subsequent recovery from spleens. Changes in the representation of each mutant were monitored using T7 transcripts hybridized to a novel inexpensive minimal microarray. Among the top 120 statistically significant spleen colonization phenotypes, more than 40 were mutations in genes with no previously known role in this model. Fifteen phenotypes were tested using individual mutants in competitive assays of intraperitoneal infection in mice and eleven were confirmed, including the first two examples of attenuation for sRNA mutants in Salmonella. We refer to the method as Array-based analysis of cistrons under selection (ABACUS).

Coordinated regulation of virulence during systemic infection of Salmonella enterica serovar Typhimurium

To cause a systemic infection, Salmonella must respond to many environmental cues during mouse infection and express specific subsets of genes in a temporal and spatial manner, but the regulatory pathways are poorly established. To unravel how micro-environmental signals are processed and integrated into coordinated action, we constructed in-frame non-polar deletions of 83 regulators inferred to play a role in Salmonella enteriditis Typhimurium (STM) virulence and tested them in three virulence assays (intraperitoneal [i.p.], and intragastric [i.g.] infection in BALB/c mice, and persistence in 129X1/SvJ mice). Overall, 35 regulators were identified whose absence attenuated virulence in at least one assay, and of those, 14 regulators were required for systemic mouse infection, the most stringent virulence assay. As a first step towards understanding the interplay between a pathogen and its host from a systems biology standpoint, we focused on these 14 genes. Transcriptional profiles were obtained for deletions of each of these 14 regulators grown under four different environmental conditions. These results, as well as publicly available transcriptional profiles, were analyzed using both network inference and cluster analysis algorithms. The analysis predicts a regulatory network in which all 14 regulators control the same set of genes necessary for Salmonella to cause systemic infection. We tested the regulatory model by expressing a subset of the regulators in trans and monitoring transcription of 7 known virulence factors located within Salmonella pathogenicity island 2 (SPI-2). These experiments validated the regulatory model and showed that the response regulator SsrB and the MarR type regulator, SlyA, are the terminal regulators in a cascade that integrates multiple signals. Furthermore, experiments to demonstrate epistatic relationships showed that SsrB can replace SlyA and, in some cases, SlyA can replace SsrB for expression of SPI-2 encoded virulence factors.

Deletion of tolA in Salmonella Typhimurium generates an attenuated strain with vaccine potential

The Gram-negative Tol-Pal system of envelope proteins plays a key role in maintaining outer membrane integrity and contributes to the virulence of several pathogens. We have investigated the role of one of these proteins, TolA, in the biology of Salmonella enterica serovar Typhimurium. Deletion of tolA rendered strain SL1344 more susceptible to killing by bile and human serum. In addition the mutant had impaired membrane integrity and displayed alterations in LPS production. The tolA mutant was highly attenuated in mouse infections via the oral and intravenous routes. Importantly, each phenotype displayed by the mutant was complemented by provision of tolA in trans. The tolA gene therefore contributes to virulence, membrane integrity, LPS production and bile and serum resistance in S. enterica serovar Typhimurium SL1344. Finally, immunization with the tolA mutant provided significant protection against subsequent challenge with wild-type SL1344. The Tol-Pal system is therefore a potential target in the development of novel attenuated live vaccines against Salmonella and other Gram-negative pathogens.

The case for hypervirulence through gene deletion in Mycobacterium tuberculosis

Deletion of genes in a pathogen is commonly associated with a reduction in its ability to cause disease. However, some rare cases have been described in the literature whereby deletion of a gene results in an increase in virulence. Recently, there have been several reports of hypervirulence resulting from gene deletion in Mycobacterium tuberculosis. Here, we explore this phenomenon in the context of the interaction between the pathogen and the host response.

Pathogen proliferation governs the magnitude but compromises the function of CD8 T cells

CD8+ T cell memory is critical for protection against many intracellular pathogens. However, it is not clear how pathogen virulence influences the development and function of CD8+ T cells. Salmonella typhimurium (ST) is an intracellular bacterium that causes rapid fatality in susceptible mice and chronic infection in resistant strains. We have constructed recombinant mutants of ST, expressing the same immunodominant Ag OVA, but defective in various key virulence genes. We show that the magnitude of CD8+ T cell response correlates directly to the intracellular proliferation of ST. Wild-type ST displayed efficient intracellular proliferation and induced increased numbers of OVA-specific CD8+ T cells upon infection in mice. In contrast, mutants with defective Salmonella pathogenicity island II genes displayed poor intracellular proliferation and induced reduced numbers of OVA-specific CD8+ T cells. However, when functionality of the CD8+ T cell response was measured, mutants of ST induced a more functional response compared with the wild-type ST. Infection with wild-type ST, in contrast to mutants defective in pathogenicity island II genes, induced the generation of mainly effector-memory CD8+ T cells that expressed little IL-2, failed to mediate efficient cytotoxicity, and proliferated poorly in response to Ag challenge in vivo. Taken together, these results indicate that pathogens that proliferate rapidly and chronically in vivo may evoke functionally inferior memory CD8+ T cells which may promote the survival of the pathogen.

Pregnancy impairs the innate immune resistance to Salmonella typhimurium leading to rapid fatal infection

Typhoid fever and gastroenteritis caused by Salmonella enterica species are increasing globally. Pregnancy poses a high risk, but it is unclear how maternal immunity to infection is altered. In mice, susceptible strains die of S. enterica serovar typhimurium (ST) infection within 7 days whereas resistant mice (129 x 1/SvJ) develop a chronic infection. We found that virulent ST infection during pregnancy, in normally resistant 129 x 1/SvJ mice, evoked approximately 100% fetal loss and surprisingly >60% host fatality, with a median survival of 6 days. Splenic bacterial load was 1000-fold higher in pregnant mice. This correlated to a diminished splenic recruitment/expansion of innate immune cells: dendritic cells, neutrophils, and NK cells. In particular, the splenic expansion and activation of NK cells postinfection seen in nonpregnant mice was lacking in pregnancy. Most notably, pregnant-infected mice had decreased production of serum IL-12 and increased IL-6 levels. Moreover, uteroplacental tissue of pregnant-infected mice exhibited an approximately 40-fold increase in IL-6 mRNA expression relative to noninfected placenta, whereas IL-12p40 was not increased. In vivo blocking of IL-6 significantly reduced the splenic bacterial burden in pregnant mice yet failed to prevent fetal loss. Fetal demise correlated to the rapidity of infection; by 14 h, ST expanded to >10(5) in the placenta and had reached the fetus. Therefore, the preferential placental expansion of ST plausibly altered the inflammatory response toward IL-6 and away from IL-12, reducing the recruitment/activation of splenic innate immune cells. Thus, highly virulent pathogens may use placental invasion to alter systemic host resistance to infection.

Identification of novel genes in genomic islands that contribute to Salmonella typhimurium replication in macrophages

Salmonella enterica serovar Typhimurium (S. typhimurium) survives and proliferates within macrophage cells. A mutant library of strain ATCC 14028 based on gene disruption by homologous recombination was screened in order to identify genes that are required for wild-type-like intracellular replication. Randomly generated chromosomal fragments from the genome of S. typhimurium were cloned into a temperature-sensitive vector, and approximately 8000 individual mutant clones were obtained by insertional-duplication mutagenesis (IDM) upon selection at non-permissive temperature. Large-scale screening for replication defects in mouse macrophages, but not during growth in rich or minimal medium, revealed a set of attenuated mutants that were further characterized by PCR amplification and sequencing of the mutagenic fragments. Following analysis of a Salmonella genome map with the annotated positions of vector insertions, an accumulation of 33 attenuating insertions within genes of ten non-collinear regions was found. Insertions in virK, gipA and five SPI-2 genes as well as seven non-polar deletions validated the screen. No invasion deficiencies of the mutants were observed. The cob-cbi-pdu cluster containing the genes for cobalamin synthesis and 1,2-propanediol degradation was shown to be required for Salmonella replication within macrophages. These data gave rise to a model of eukaryotic glycoconjugates and phospholipids as alternative carbon, nitrogen and energy sources for intracellularly replicating bacteria. The contribution of as yet unknown components of SPI-6 and the Gifsy-1 and Gifsy-2 prophage islands to intracellular replication is reported, as well as the fivefold reduced intracellular growth rate of a mutant with a deletion of STM1677, which probably encodes a LysR-like transcriptional regulator. The intracellular replication rate of three double mutants, each lacking two gene products of the cob-cbi-pdu cluster or the Gifsy-1 prophage, was shown to be lower than that of the respective single mutants, suggesting that additive effects of subtle intracellular advantages contribute to Salmonella fitness in vivo.

Oligonucleotide microarray for molecular characterization and genotyping of Salmonella spp. strains

OBJECTIVES

To characterize and subtype multidrug-resistant Salmonella isolates by determining the virulence factors, prophage sequences and antimicrobial resistance genes using a novel Salmonella-specific oligonucleotide microarray.

METHODS

Preliminary screening of 24 Salmonella clinical isolates was carried out by using susceptibility testing, plasmid profiling and class 1 integron PCR. Subsequently, oligonucleotide microarray was involved in genotypic characterization and localization of monitored genetic markers. The presence of antimicrobial resistance genes was also detected and confirmed by PCR and subsequent sequencing. The potential spread of emerging bla(SHV-2) was investigated by bacterial conjugation.

RESULTS

All Salmonella strains revealed resistance to two or more (up to nine) antibiotics. Nineteen of them carried class 1 integrons including dfrA1, dfrA12, aadA1, aadA2, bla(PSE-1) and bla(TEM-1) gene cassettes, respectively. Twenty-three out of 24 Salmonella isolates possessed one or more plasmids. Oligonucleotide microarray characterization and typing revealed the conserved character of Salmonella pathogenicity island virulence factors among three Salmonella enterica serovars, significant variability in prophage sequences and many different antimicrobial resistance gene patterns. Differential labelling of genomic and plasmid DNA, respectively, and hybridization to the microarray made it possible to localize important resistance determinants. Microarray results were successfully confirmed and verified by using PCR. The emerging bla(SHV-2) gene from Salmonella Kentucky SK10944 conferring resistance to ceftriaxone and cefotaxime was transferred via bacterial conjugation to Escherichia coli K-12 3110.

CONCLUSIONS

Salmonella isolates were quickly and thoroughly characterized by a novel oligonucleotide microarray, which could become a useful tool for detection of virulence and resistance genes and monitoring of their dissemination among salmonellae and closely related bacteria.

Oral delivery of tumor-targeting Salmonella for cancer therapy in murine tumor models

Tumor-targeting bacteria have been investigated intensively in recent years as anticancer agents. To ensure the reliability of infection, bacteria have conventionally been injected intravenously or intraperitoneally into animals or humans. However, systemic infection of bacteria is rather inconvenient and carries the risk of obvious toxicity. Here we tested whether Salmonella typhimurium VNP20009, a tumor-targeting strain, could be administrated orally for tumor therapy. Tumor-targeting potential, antitumor effects, as well as toxicity of orally administrated VNP20009 were investigated in this study. Oral delivery of VNP20009 not only exhibited high tumor-targeting potential, but also led to a significant anticancer effect by delaying tumor growth and prolonging survival in murine tumor models. As part of combination therapy, orally administrated bacteria notably improved the antitumor effect of cyclophosphamide. In vitro and in vivo studies showed that VNP20009 significantly induced tumor cell apoptosis. No obvious toxicity was observed during the treatments with oral inoculation of VNP20009. Comparative analysis of toxicity in tumor-bearing and tumor-free mice further revealed that orally administrated Salmonella had high safety compared to conventional systemic infection of bacteria. The findings indicated that oral administration of tumor-targeting bacteria is effective and safe. This approach provides a novel avenue in the application of bacteria as a potential antitumor agent.

Analysis of whole cell lysate from the intercellular bacterium Coxiella burnetii using two gel-based protein separation techniques

Coxiella burnetii, the causative agent of Q fever, is an obligate intracellular gamma-proteobacterium, which replicates within large phagolysosome-like compartments formed in the host cell. The global protein profile of intracellular C. burnetii strain Nine Mile phase II was analyzed by two gel-based approaches coupled to MALDI-TOF MS. Colloidal Coomassie brilliant blue-stained 2-DE gels at the pH range 3-10 resolved over 600 protein spots and 125 spots in doubled-SDS-PAGE gels. Mass spectra obtained for each trypsin-digested protein-spot were compared to the C. burnetii genome database, and a total number of 185 different C. burnetii proteins were identified by both techniques. 2-DE in combination with MALDI-TOF MS, as a high-throughput method, allowed the identification of 172 proteins. On the other hand, the application of doubled-SDS-PAGE allowed the identification of 38 proteins, with some of them being very alkaline and membrane proteins not identified in the 2-DE approach. Most identified proteins were predicted to be involved in metabolism and biosynthesis. Several identified proteins are speculated to have a distinct and vital role in the pathogenesis and survival of C. burnetii within the harsh phagolysosomal environment.

The MgtC virulence factor of Salmonella enterica serovar Typhimurium activates Na(+),K(+)-ATPase

The mgtC gene of Salmonella enterica serovar Typhimurium encodes a membrane protein of unknown function that is important for full virulence in the mouse. Since mgtC is part of an operon with mgtB which encodes a Mg(2+)-transporting P-type ATPase, MgtC was hypothesized to function in ion transport, possibly in Mg(2+) transport. Consequently, MgtC was expressed in Xenopus laevis oocytes, and its effect on ion transport was evaluated using ion selective electrodes. Oocytes expressing MgtC did not exhibit altered currents or membrane potentials in response to changes in extracellular H(+), Mg(2+), or Ca(2+), thus ruling out a previously postulated function as a Mg(2+)/H(+) antiporter. However, addition of extracellular K(+) markedly hyperpolarized membrane potential instead of the expected depolarization. Addition of ouabain to block the oocyte Na(+),K(+)-ATPase completely prevented hyperpolarization and restored the normal K(+)-induced depolarization response. These results suggested that the Na(+),K(+)-ATPase was constitutively activated in the presence of MgtC resulting in a membrane potential largely dependent on Na(+),K(+)-ATPase. Consistent with the involvement of Na(+),K(+)-ATPase, oocytes expressing MgtC exhibited an increased rate of (86)Rb(+) uptake and had increased intracellular free [K(+)] and decreased free [Na(+)] and ATP. The free concentrations of Mg(2+) and Ca(2+) and cytosolic pH were unchanged, although the total intracellular Ca(2+) content was slightly elevated. These results suggest that the serovar Typhimurium MgtC protein may be involved in regulating membrane potential but does not directly transport Mg(2+) or another ion.

The MgtC virulence factor of Salmonella enterica serovar Typhimurium activates Na(+),K(+)-ATPase

The mgtC gene of Salmonella enterica serovar Typhimurium encodes a membrane protein of unknown function that is important for full virulence in the mouse. Since mgtC is part of an operon with mgtB which encodes a Mg(2+)-transporting P-type ATPase, MgtC was hypothesized to function in ion transport, possibly in Mg(2+) transport. Consequently, MgtC was expressed in Xenopus laevis oocytes, and its effect on ion transport was evaluated using ion selective electrodes. Oocytes expressing MgtC did not exhibit altered currents or membrane potentials in response to changes in extracellular H(+), Mg(2+), or Ca(2+), thus ruling out a previously postulated function as a Mg(2+)/H(+) antiporter. However, addition of extracellular K(+) markedly hyperpolarized membrane potential instead of the expected depolarization. Addition of ouabain to block the oocyte Na(+),K(+)-ATPase completely prevented hyperpolarization and restored the normal K(+)-induced depolarization response. These results suggested that the Na(+),K(+)-ATPase was constitutively activated in the presence of MgtC resulting in a membrane potential largely dependent on Na(+),K(+)-ATPase. Consistent with the involvement of Na(+),K(+)-ATPase, oocytes expressing MgtC exhibited an increased rate of (86)Rb(+) uptake and had increased intracellular free [K(+)] and decreased free [Na(+)] and ATP. The free concentrations of Mg(2+) and Ca(2+) and cytosolic pH were unchanged, although the total intracellular Ca(2+) content was slightly elevated. These results suggest that the serovar Typhimurium MgtC protein may be involved in regulating membrane potential but does not directly transport Mg(2+) or another ion.