Disruption of the genes for ClpXP protease in Salmonella enterica serovar Typhimurium results in persistent infection in mice, and development of persistence requires endogenous gamma interferon and tumor necrosis factor alpha

A putative cytotoxic serine protease from Salmonella typhimurium UcB5 recovered from undercooked burger

A putative virulence exoprotease designated as UcB5 was successfully purified from the bacterium Salmonella typhimurium to the electrophoretic homogeneity with 13.2-fold and 17.1% recovery by hydrophobic, ion-exchange, and gel permeation chromatography using Phenyl-Sepharose 6FF, DEAE-Sepharose CL-6B, and Sephadex G-75, respectively. By applying SDS-PAGE, the molecular weight was confirmed at 35 kDa. The optimal temperature, pH, and isoelectric point were 35 °C, 8.0, 5.6 ± 0.2, respectively. UcB5 was found to have a broad substrate specificity against almost all the tested chromogenic substrates with maximal affinity against N-Succ-Ala-Ala-Pro-Phe-pNA achieving K_m of 0.16 mM, K_cat/K_m of 3.01 × 10⁵ S^-1 M^-1, and amidolytic activity of 28.9 µmol min^-1 L^-1. It was drastically inhibited by TLCK, PMSF, SBTI, and aprotinin while, DTT, β-mercaptoethanol, 2,2'-bipyridine, o-phenanthroline, EDTA, and EGTA had no effect, which suggested a serine protease-type. Also, it has shown a broad substrate specificity against a broad range of natural proteins including serum proteins. A cytotoxicity and electron microscopy study revealed that UcB5 could cause subcellular proteolysis that finally led to liver necrosis. For this, future research should focus on using a combination of external antiproteases and antimicrobial agents for the treatment of microbial diseases instead of using drugs alone.

Cellular functions of the ClpP protease impacting bacterial virulence

Proteostasis mechanisms significantly contribute to the sculpting of the proteomes of all living organisms. ClpXP is a central AAA+ chaperone-protease complex present in both prokaryotes and eukaryotes that facilitates the unfolding and subsequent degradation of target substrates. ClpX is a hexameric unfoldase ATPase, while ClpP is a tetradecameric serine protease. Substrates of ClpXP belong to many cellular pathways such as DNA damage response, metabolism, and transcriptional regulation. Crucially, disruption of this proteolytic complex in microbes has been shown to impact the virulence and infectivity of various human pathogenic bacteria. Loss of ClpXP impacts stress responses, biofilm formation, and virulence effector protein production, leading to decreased pathogenicity in cell and animal infection models. Here, we provide an overview of the multiple critical functions of ClpXP and its substrates that modulate bacterial virulence with examples from several important human pathogens.

Revisiting Persistent Salmonella Infection and the Carrier State: What Do We Know?

One characteristic of the few Salmonella enterica serovars that produce typhoid-like infections is that disease-free persistent infection can occur for months or years in a small number of individuals post-convalescence. The bacteria continue to be shed intermittently which is a key component of the epidemiology of these infections. Persistent chronic infection occurs despite high levels of circulating specific IgG. We have reviewed the information on the basis for persistence in S. Typhi, S. Dublin, S. Gallinarum, S. Pullorum, S. Abortusovis and also S. Typhimurium in mice as a model of persistence. Persistence appears to occur in macrophages in the spleen and liver with shedding either from the gall bladder and gut or the reproductive tract. The involvement of host genetic background in defining persistence is clear from studies with the mouse but less so with human and poultry infections. There is increasing evidence that the organisms (i) modulate the host response away from the typical Th1-type response normally associated with immune clearance of an acute infection to Th2-type or an anti-inflammatory response, and that (ii) the bacteria modulate transformation of macrophage from M1 to M2 type. The bacterial factors involved in this are not yet fully understood. There are early indications that it might be possible to remodulate the response back towards a Th1 response by using cytokine therapy.

Increased Production of Outer Membrane Vesicles by Salmonella Interferes with Complement-Mediated Innate Immune Attack

Bacterial outer membrane vesicles (OMVs) enriched with bioactive proteins, toxins, and virulence factors play a critical role in host-pathogen and microbial interactions. The two-component system PhoP-PhoQ (PhoPQ) of Salmonella enterica orchestrates the remodeling of outer membrane lipopolysaccharide (LPS) molecules and concomitantly upregulates OMV production. In this study, we document a novel use of nanoparticle tracking analysis to determine bacterial OMV size and number. Among the PhoPQ-activated genes tested, pagC expression had the most significant effect on the upregulation of OMV production. We provide the first evidence that PhoPQ-mediated upregulation of OMV production contributes to bacterial survival by interfering with complement activation. OMVs protected bacteria in a dose-dependent manner, and bacteria were highly susceptible to complement-mediated killing in their absence. OMVs from bacteria expressing PagC bound to complement component C3b in a dose-dependent manner and inactivated it by recruiting complement inhibitor Factor H. As we also found that Factor H binds to PagC, we propose that PagC interferes with complement-mediated killing of Salmonella in the following two steps: first by engaging Factor H, and second, through the production of PagC-enriched OMVs that divert and inactivate the complement away from the bacteria. Since PhoPQ activation occurs intracellularly, the resultant increase in PagC expression and OMV production is suggested to contribute to the local and systemic spread of Salmonella released from dying host cells that supports the infection of new cells. IMPORTANCE Bacterial outer membrane vesicles (OMVs) mediate critical bacterium-bacterium and host-microbial interactions that influence pathogenesis through multiple mechanisms, including the elicitation of inflammatory responses, delivery of virulence factors, and enhancement of biofilm formation. As such, there is a growing interest in understanding the underlying mechanisms of OMV production. Recent studies have revealed that OMV biogenesis is a finely tuned physiological process that requires structural organization and selective sorting of outer membrane components into the vesicles. In Salmonella, outer membrane remodeling and OMV production are tightly regulated by its PhoPQ system. In this study, we demonstrate that PhoPQ-regulated OMV production plays a significant role in defense against host innate immune attack. PhoPQ-activated PagC expression recruits the complement inhibitor Factor H and degrades the active C3 component of complement. Our results provide valuable insight into the combination of tools and environmental signals that Salmonella employs to evade complement-mediated lysis, thereby suggesting a strong evolutionary adaptation of this facultative intracellular pathogen to protect itself during its extracellular stage in the host.

Acyldepsipeptide activated ClpP1P2 macromolecule of Leptospira, an ideal Achilles' heel to hamper the cell survival and deregulate ClpP proteolytic activity

Antibiotic acyldepsipeptide (ADEP) targets the bacterial ClpP serine protease and can inhibit the growth of numerous bacterial species by activating/dysregulating the protease activity within the cell. The spirochete Leptospira interrogans harbors two ClpP isoforms (LepClpP1 and LepClpP2). Supplementation of ADEP in the Leptospira growth medium resulted in the inhibition of bacterial growth. The ADEP mediated activation of the LepClpP mixture was dependent on the time allowed for the self-assembly of LepClpP1 and LepClpP2. The dynamic light scattering of the LepClpP mixture in the presence of the ADEP indicated a conformational transformation of the LepClpP machinery. Serine 98, a catalytic triad residue of the LepClpP1 in the LepClpP1P2 heterocomplex, was critical for the ADEP mediated activation. The computational prototype of the LepClpP1P2 structure suggested that the hydrophobic pockets wherein the ADEPs or the physiological chaperone ClpX predominantly dock are exclusively present in the LepClpP2 heptamer. Using the ADEP as a tool, this investigation provides an insight into the molecular function of the LepClpP1P2 in a coalition with its ATPase chaperone LepClpX. The shreds of the evidence illustrated in this investigation verify that ADEP1 possesses the ability to control the LepClpP system in an unconventional approach than the other organisms.

Maintaining Integrity Under Stress: Envelope Stress Response Regulation of Pathogenesis in Gram-Negative Bacteria

The Gram-negative bacterial envelope is an essential interface between the intracellular and harsh extracellular environment. Envelope stress responses (ESRs) are crucial to the maintenance of this barrier and function to detect and respond to perturbations in the envelope, caused by environmental stresses. Pathogenic bacteria are exposed to an array of challenging and stressful conditions during their lifecycle and, in particular, during infection of a host. As such, maintenance of envelope homeostasis is essential to their ability to successfully cause infection. This review will discuss our current understanding of the σ^E- and Cpx-regulated ESRs, with a specific focus on their role in the virulence of a number of model pathogens.

Alpha-synuclein suppresses mitochondrial protease ClpP to trigger mitochondrial oxidative damage and neurotoxicity

Both α-Synuclein (αSyn) accumulation and mitochondrial dysfunction have been implicated in the pathology of Parkinson's disease (PD). Although studies suggest that αSyn and its missense mutant, A53T, preferentially accumulate in the mitochondria, the mechanisms by which αSyn and mitochondrial proteins regulate each other to trigger mitochondrial and neuronal toxicity are poorly understood. ATP-dependent Clp protease (ClpP), a mitochondrial matrix protease, plays an important role in regulating mitochondrial protein turnover and bioenergetics activity. Here, we show that the protein level of ClpP is selectively decreased in αSyn-expressing cell culture and neurons derived from iPS cells of PD patient carrying αSyn A53T mutant, and in dopaminergic (DA) neurons of αSyn A53T mice and PD patient postmortem brains. Deficiency in ClpP induces an overload of mitochondrial misfolded/unfolded proteins, suppresses mitochondrial respiratory activity, increases mitochondrial oxidative damage and causes cell death. Overexpression of ClpP reduces αSyn-induced mitochondrial oxidative stress through enhancing the level of Superoxide Dismutase-2 (SOD2), and suppresses the accumulation of αSyn S129 phosphorylation and promotes neuronal morphology in neurons derived from PD patient iPS cells carrying αSyn A53T mutant. Moreover, we find that αSyn WT and A53T mutant interact with ClpP and suppress its peptidase activity. The binding of αSyn to ClpP further promotes a distribution of ClpP from soluble to insoluble cellular fraction in vitro and in vivo, leading to reduced solubility of ClpP. Compensating for the loss of ClpP in the substantia nigra of αSyn A53T mice by viral expression of ClpP suppresses mitochondrial oxidative damage, and reduces αSyn pathology and behavioral deficits of mice. Our findings provide novel insights into the mechanism underlying αSyn-induced neuronal pathology, and they suggest that ClpP might be a useful therapeutic target for PD and other synucleinopathies.

ClpP Protease, a Promising Antimicrobial Target

The caseinolytic protease proteolytic subunit (ClpP) is a serine protease playing an important role in proteostasis of eukaryotic organelles and prokaryotic cells. Alteration of ClpP function has been proved to affect the virulence and infectivity of a number of pathogens. Increased bacterial resistance to antibiotics has become a global problem and new classes of antibiotics with novel mechanisms of action are needed. In this regard, ClpP has emerged as an attractive and potentially viable option to tackle pathogen fitness without suffering cross-resistance to established antibiotic classes and, when not an essential target, without causing an evolutionary selection pressure. This opens a greater window of opportunity for the host immune system to clear the infection by itself or by co-administration with commonly prescribed antibiotics. A comprehensive overview of the function, regulation and structure of ClpP across the different organisms is given. Discussion about mechanism of action of this protease in bacterial pathogenesis and human diseases are outlined, focusing on the compounds developed in order to target the activation or inhibition of ClpP.

The Role of ClpP Protease in Bacterial Pathogenesis and Human Diseases

In prokaryotic cells and eukaryotic organelles, the ClpP protease plays an important role in proteostasis. The disruption of the ClpP function has been shown to influence the infectivity and virulence of a number of bacterial pathogens. More recently, ClpP has been found to be involved in various forms of carcinomas and in Perrault syndrome, which is an inherited condition characterized by hearing loss in males and females and by ovarian abnormalities in females. Hence, targeting ClpP is a potentially viable, attractive option for the treatment of different ailments. Herein, the biochemical and cellular activities of ClpP are discussed along with the mechanisms by which ClpP affects bacterial pathogenesis and various human diseases. In addition, a comprehensive overview is given of the new classes of compounds in development that target ClpP. Many of these compounds are currently primarily aimed at treating bacterial infections. Some of these compounds inhibit ClpP activity, while others activate the protease and lead to its dysregulation. The ClpP activators are remarkable examples of small molecules that inhibit protein-protein interactions but also result in a gain of function.

ClpXP-Dependent RpoS Degradation Enables Full Activation of Type III Secretion System, Amylovoran Production, and Motility in Erwinia amylovora

Erwinia amylovora, the causal agent of fire blight disease of apple and pear, employs intracellular proteases, including Lon and ClpXP, for posttranslational regulation of various cellular proteins. It has been shown that Lon plays a critical role in E. amylovora virulence by directly targeting type III secretion system (T3SS) proteins and the Rcs phosphorelay system. In this study, we genetically examined the role of ClpXP and its potential interaction with Lon in E. amylovora. Mutation in clpXP diminished the expression of the T3SS, reduced exopolysaccharide amylovoran production and motility, and resulted in delayed disease progress. Western blot analyses showed highly accumulated RpoS proteins in the clpXP mutant. Moreover, mutation of rpoS in the clpXP mutant background rescued the expression of the T3SS and amylovoran production, suggesting that ClpXP-dependent RpoS degradation positively affects virulence traits. Interestingly, lack of both ClpXP and Lon resulted in significantly reduced virulence but increased expression of the T3SS and amylovoran production. However, this phenomenon was independent of RpoS accumulation, suggesting that ClpXP and Lon are indispensable for full virulence in E. amylovora.

The effect of mutation in the clpX gene on the synthesis of N-acyl-homoserine lactones and other properties of Burkholderia cenocepacia 370

In order to study the regulation of N-acyl-homoserine lactones synthesis (AHLs, the signal molecules of Quorum Sensing regulation) in Burkholderia cenocepacia strain 370 we obtained mutants with increased AHL production. One of the mutants, named BC-B6, was obtained by TnMod-RKm(r) plasposon mutagenesis. The plasposon insertion was located within the clpX gene encoding the ATPase subunit ClpX of the ClpXP protease. The mutation reduced bacterial virulence in mice intranasal infection. The results of proteomic analysis demonstrated that the expression of at least 19 proteins differed not less than 2-fold between the parental and mutant strains. 18 of the proteins were upregulated in the mutant, and one protein was downregulated. The proteins included those that involved in protein synthesis and modification, in energy production, in general metabolism, in transport and regulation. To check the effect of the clpX mutation on the AHL synthesis, a mutant with inactivated clpX gene (BC-clpX:Km(r)) was constructed by gene replacement method. This mutant also exhibited increased AHLs production. A swarming motility of both mutants was reduced compared to the original strain. Thus, the obtained results show that the clpX gene was involved in the regulation of AHL production and a number of cellular processes in B. cenocepacia 370.

Salmonella enterica serovar Typhimurium utilizes the ClpPX and Lon proteases for optimal fitness in the ceca of chickens

Salmonella enterica serovar Typhimurium (S. Typhimurium) is a leading cause of salmonellosis. Poultry and poultry products are implicated in transmission of Salmonella to humans. In 2013, an outbreak of S Typhimurium occurred that comprised 39 states within the United States and was associated with backyard flocks of chickens. Colonization of the avian host by S Typhimurium requires numerous genetic factors encoded within the bacterium. Of particular interest are genetic factors induced by alternative sigma factors within S Typhimurium since these genetic elements are important for adaptation to different environmental stresses. The heat shock response is a dedicated change in gene regulation within bacteria in response to several stresses, specifically growth at 42°C. Because chickens have a higher body temperature than other animals (42°C) the hypothesis was tested that components of the heat shock response are important for optimal fitness within the chicken. To this end, deletion of the heat shock proteases clpPX (BTNC0022) or lon (BTNC0021) was accomplished and the bacterial fitness in vivo was compared to the "wild-type" strain (NC1040) using a competition assay. One-day-old chicks were orally gavaged with an equal mixture of NC1040 and either BTNC0022 or BTNC0021. Quantification of viable bacteria over time by using plate counts indicated that deletion of either heat shock protease resulted in significantly reduced colonization of the chicken ceca compared to the wild-type strain. To satisfy the molecular Koch's postulates, clpPX and lon mutants were complemented in trans using a low-copy number plasmid for additional in vivo experiments. Complementation studies confirmed the importance of either heat shock protease to colonization of the chicken ceca. This report demonstrated that both ClpPX and Lon were important for optimal fitness within chickens. Moreover, these results suggested that components of the heat shock may be critical factors used by S. Typhimurium for colonization of poultry. The use of feed additives or other treatments that inactivate or inhibit ClpPX or Lon may reduce the bacterial burden of S. Typhimurium in poultry.

Specific Monoclonal Antibody Overcomes the Salmonella enterica Serovar Typhimurium's Adaptive Mechanisms of Intramacrophage Survival and Replication

Salmonella-specific antibodies play an important role in host immunity; however, the mechanisms of Salmonella clearance by pathogen-specific antibodies remain to be completely elucidated since previous studies on antibody-mediated protection have yielded inconsistent results. These inconsistencies are at least partially attributable to the use of polyclonal antibodies against Salmonella antigens. Here, we developed a new monoclonal antibody (mAb)-449 and identified its related immunogen that protected BALB/c mice from infection with Salmonella enterica serovar Typhimurium. In addition, these data indicate that the mAb-449 immunogen is likely a major protective antigen. Using in vitro infection studies, we also analyzed the mechanism by which mAb-449 conferred host protection. Notably, macrophages infected with mAb-449-treated S. Typhimurium showed enhanced pathogen uptake compared to counterparts infected with control IgG-treated bacteria. Moreover, these macrophages produced elevated levels of pro-inflammatory cytokine TNFα and nitric oxide, indicating that mAb-449 enhanced macrophage activation. Finally, the number of intracellular bacteria in mAb-449-activated macrophages decreased considerably, while the opposite was found in IgG-treated controls. Based on these findings, we suggest that, although S. Typhimurium has the potential to survive and replicate within macrophages, host production of a specific antibody can effectively mediate macrophage activation for clearance of intracellular bacteria.

Salmonella as an Inducer of Autoimmunity

A clear etiological link has been established between infection with several gram-negative enteric pathogens, including Salmonella spp., and the incidence of reactive arthritis (ReA), an autoimmune disease that largely affects the joints. ReA is sometimes referred to as Reiter's syndrome, particularly when accompanied by uveitis and urethritis. This review reviews the evidence etiologically linking Salmonella infection with autoimmune disease and addresses the roles that bacterial and host elements play in controlling disease outcome. ReA is an autoimmune disease that largely consists of painful joint inflammation but also can include inflammation of the eye, gastrointestinal tract, and skin. ReA is a member of a broad spectrum of chronic inflammatory disorders termed the seronegative spondyloarthropathies (SNSpAs) that includes ankylosing spondylitis (AS), psoriatic arthritis, and enteropathic arthritis. Salmonella species, as well as other enteric pathogens associated with postgastroenteritis ReA, are facultative intracellular gram-negative bacteria. Many studies have analyzed the association of the HLA class I molecule, HLA-B27, with SNSpAs. Whereas B27 has been shown to be present in 90 to 95% of cases of AS, the association of the B27 haplotype with other SNSpAs is more tenuous. The clear association between ReA and infection with Salmonella or other gram-negative enteric pathogens has led to the suggestion that the adaptive immune response to infection has an autoimmune component. In addition to various Salmonella species, other gram-negative enteric pathogens have been linked to the development of ReA. Given their close relationship to Salmonella, this review considers the involvement of Shigella species in ReA.

Pseudomonas fluorescens: identification of Fur-regulated proteins and evaluation of their contribution to pathogenesis

Pseudomonas fluorescens is a Gram-negative bacterium and a common pathogen to a wide range of farmed fish. In a previous study, we found that the ferric uptake regulator gene (fur) is essential to the infectivity of a pathogenic fish isolate of P. fluorescens (wild-type strain TSS). In the present work, we conducted comparative proteomic analysis to examine the global protein profiles of TSS and the P. fluorescens fur knockout mutant TFM. Twenty-eight differentially produced proteins were identified, which belong to different functional categories. Four of these proteins, viz. TssP (a type VI secretion protein), PspA (a serine protease), OprF (an outer membrane porin), and ClpP (the proteolytic subunit of an ATP-dependent Clp protease), were assessed for virulence participation in a model of turbot Scophthalmus maximus. The results showed that the oprF and clpP knockouts exhibited significantly reduced capacities in (1) resistance against the bactericidal effect of host serum, (2) dissemination into and colonization of host tissues, and (3) inducing host mortality. In contrast, mutation of tssP and pspA had no apparent effect on the pathogenicity of TSS. Purified recombinant OprF, when used as a subunit vaccine, induced production of specific serum antibodies in immunized fish and elicited significant protection against lethal TSS challenge. Antibody blocking of the OprF in TSS significantly impaired the ability of the bacteria to invade host tissues. Taken together, these results indicate for the first time that in pathogenic P. fluorescens, Fur regulates the expression of diverse proteins, some of which are required for optimal infection.

Mouse models for assessing the cross-protective efficacy of oral non-typhoidal Salmonella vaccine candidates harbouring in-frame deletions of the ATP-dependent protease lon and other genes

In BALB/c mouse models of Salmonella enterica serovar Typhimurium infection, a single oral immunization with a mutant strain with an insertion of the chloramphenicol resistance gene into the ATP-dependent protease clpP or lon gene decreased the number of salmonellae in each tissue sample 5 days after oral challenge with virulent S. Typhimurium at weeks 26 and 54 post-immunization. These data suggested that an oral immunization with the ClpP- or Lon-disrupted S. Typhimurium strain could provide long-term protection against oral challenge with virulent S. Typhimurium. Accordingly, recombinant oral non-typhoidal Salmonella (NTS) vaccines were constructed by incorporating mutants of both S. Typhimurium and S. enterica serovar Enteritidis harbouring stable in-frame markerless deletions of the clpP-lon-sulA (suppressor of lon), lon-sulA or lon-msbB (acyltransferase) genes. Amongst these orally administered vaccine candidates, those with the lon-sulA gene deletion mutants of S. Typhimurium and S. Enteritidis protected BALB/c and C57BL/6J mice against oral challenge with both virulent S. Typhimurium and virulent S. Enteritidis. Therefore, the in-frame markerless lon-sulA gene deletion mutant of S. Typhimurium or S. Enteritidis could be a promising cross-protective NTS live vaccine candidate for practical use in humans.

Regulation of host hemoglobin binding by the Staphylococcus aureus Clp proteolytic system

Protein turnover is a key process for bacterial survival mediated by intracellular proteases. Proteolytic degradation reduces the levels of unfolded and misfolded peptides that accumulate in the cell during stress conditions. Three intracellular proteases, ClpP, HslV, and FtsH, have been identified in the Gram-positive bacterium Staphylococcus aureus, a pathogen responsible for significant morbidity and mortality worldwide. Consistent with their crucial role in protein turnover, ClpP, HslV, and FtsH affect a number of cellular processes, including metabolism, stress responses, and virulence. The ClpP protease is believed to be the principal degradation machinery in S. aureus. This study sought to identify the effect of the Clp protease on the iron-regulated surface determinant (Isd) system, which extracts heme-iron from host hemoglobin during infection and is critical to S. aureus pathogenesis. Inactivation of components of the Clp protease alters abundance of several Isd proteins, including the hemoglobin receptor IsdB. Furthermore, the observed changes in IsdB abundance are the result of transcriptional regulation, since transcription of isdB is decreased by clpP or clpX inactivation. In contrast, inactivation of clpC enhances isdB transcription and protein abundance. Loss of clpP or clpX impairs host hemoglobin binding and utilization and results in severe virulence defects in a systemic mouse model of infection. These findings suggest that the Clp proteolytic system is important for regulating nutrient iron acquisition in S. aureus. The Clp protease and Isd complex are widely conserved in bacteria; therefore, these data reveal a novel Clp-dependent regulation pathway that may be present in other bacterial pathogens.

Bacterial proteases and virulence

Bacterial pathogens rely on proteolysis for variety of purposes during the infection process. In the cytosol, the main proteolytic players are the conserved Clp and Lon proteases that directly contribute to virulence through the timely degradation of virulence regulators and indirectly by providing tolerance to adverse conditions such as those experienced in the host. In the membrane, HtrA performs similar functions whereas the extracellular proteases, in close contact with host components, pave the way for spreading infections by degrading host matrix components or interfering with host cell signalling to short-circuit host cell processes. Common to both intra- and extracellular proteases is the tight control of their proteolytic activities. In general, substrate recognition by the intracellular proteases is highly selective which is, in part, attributed to the chaperone activity associated with the proteases either encoded within the same polypeptide or on separate subunits. In contrast, substrate recognition by extracellular proteases is less selective and therefore these enzymes are generally expressed as zymogens to prevent premature proteolytic activity that would be detrimental to the cell. These extracellular proteases are activated in complex cascades involving auto-processing and proteolytic maturation. Thus, proteolysis has been adopted by bacterial pathogens at multiple levels to ensure the success of the pathogen in contact with the human host.

Salmonella's long-term relationship with its host

Host-adapted strains of Salmonella enterica cause systemic infections and have the ability to persist systemically for long periods of time and pose significant public-health problems. Multidrug-resistant S. enterica serovar Typhi (S. Typhi) and nontyphoidal Salmonella (NTS) are on the increase and are often associated with HIV infection. Chronically infected hosts are often asymptomatic and transmit disease to naïve hosts via fecal shedding of bacteria, thereby serving as a critical reservoir for disease. Salmonella utilizes multiple ways to evade and modulate host innate and adaptive immune responses in order to persist in the presence of a robust immune response. Survival in macrophages and modulation of immune cells migration allow Salmonella to evade various immune responses. The ability of Salmonella to persist depends on a balance between immune responses that lead to the clearance of the pathogen and avoidance of damage to host tissues.

Role of Streptococcus intermedius DnaK chaperone system in stress tolerance and pathogenicity

Streptococcus intermedius is a facultatively anaerobic, opportunistic pathogen that causes purulent infections and abscess formation. The DnaK chaperone system has been characterized in several pathogenic bacteria and seems to have important functions in stress resistance and pathogenicity. However, the role of DnaK in S. intermedius remains unclear. Therefore, we constructed a dnaK knockout mutant that exhibited slow growth, thermosensitivity, accumulation of GroEL in the cell, and reduced cytotoxicity to HepG2 cells. The level of secretion of a major pathogenic factor, intermedilysin, was not affected by dnaK mutation. We further examined the function and property of the S. intermedius DnaK chaperone system by using Escherichia coli ΔdnaK and ΔrpoH mutant strains. S. intermedius DnaK could not complement the thermosensitivity of E. coli ΔdnaK mutant. However, the intact S. intermedius DnaK chaperone system could complement the thermosensitivity and acid sensitivity of E. coli ΔdnaK mutant. The S. intermedius DnaK chaperone system could regulate the activity and stability of the heat shock transcription factor σ(32) in E. coli, although S. intermedius does not utilize σ(32) for heat shock transcription. The S. intermedius DnaK chaperone system was also able to efficiently eliminate the aggregated proteins from ΔrpoH mutant cells. Overall, our data showed that the S. intermedius DnaK chaperone system has important functions in quality control of cellular proteins but has less participation in the modulation of expression of pathogenic factors.

Enterobacterial common antigen mutants of Salmonella enterica serovar Typhimurium establish a persistent infection and provide protection against subsequent lethal challenge

Infection with Salmonella spp. is a significant source of disease globally. A substantial proportion of these infections are caused by Salmonella enterica serovar Typhimurium. Here, we characterize the role of the enterobacterial common antigen (ECA), a surface glycolipid ubiquitous among enteric bacteria, in S. Typhimurium pathogenesis. Construction of a defined mutation in the UDP-N-acetylglucosamine-1-phosphate transferase gene, wecA, in two clinically relevant strains of S. Typhimurium, TML and SL1344, resulted in strains that were unable to produce ECA. Loss of ECA did not affect the gross cell surface ultrastructure, production of lipopolysaccharide (LPS), flagella, or motility. However, the wecA mutant strains were attenuated in both oral and intraperitoneal mouse models of infection (P<0.001 for both routes of infection; log rank test), and virulence could be restored by complementation of the wecA gene in trans. Despite the avirulence of the ECA-deficient strains, the wecA mutant strains were able to persistently colonize systemic sites (spleen and liver) at moderate levels for up to 70 days postinfection. Moreover, immunization with the wecA mutant strains provided protection against a subsequent lethal oral or intraperitoneal challenge with wild-type S. Typhimurium. Thus, wecA mutant (ECA-negative) strains of Salmonella may be useful as live attenuated vaccine strains or as vehicles for heterologous antigen expression.

Roles of Salmonella enterica serovar Typhimurium encoded Peptidase N during systemic infection of Ifnγ-/- mice

Pathogen encoded peptidases are known to be important during infection; however, their roles in modulating host responses in immunocompromised individuals are not well studied. The roles of S. typhimurium (WT) encoded Peptidase N (PepN), a major aminopeptidase and sole M1 family member, was studied in mice lacking Interferon-γ (IFNγ), a cytokine important for immunity. S. typhimurium lacking pepN (ΔpepN) displays enhanced colony forming units (CFU) compared to WT in peripheral organs during systemic infection in C57BL/6 mice. However, Ifnγ(-/-) mice show higher CFU compared to C57BL/6 mice, resulting in lower fold differences between WT and ΔpepN. Concomitantly, reintroduction of pepN in ΔpepN (ΔpepN/pepN) reduces CFU, demonstrating pepN-dependence. Interestingly, expression of a catalytically inactive PepN (ΔpepN/E298A) also lowers CFU, demonstrating that the decrease in CFU is independent of the catalytic activity of PepN. In addition, three distinct differences are observed between infection of C57BL/6 and Ifnγ(-/-) mice: First, serum amounts of TNFα and IL1β post infection are significantly lower in Ifnγ(-/-) mice. Second, histological analysis of C57BL/6 mice reveals that damage in spleen and liver upon infection with WT or ΔpepN is greater compared to ΔpepN/pepN or ΔpepN/E298A. On the other hand, Ifnγ(-/-) mice are highly susceptible to organ damage by all strains of S. typhimurium used in this study. Finally, greater survival of C57BL/6, but not Ifnγ(-/-) mice, is observed upon infection with ΔpepN/pepN or ΔpepN/E298A. Overall, the roles of the host encoded IFNγ during infection with S. typhimurium strains with varying degrees of virulence are highlighted.

Use of a Dictyostelium model for isolation of genetic loci associated with phagocytosis and virulence in Klebsiella pneumoniae

Phagocytosis resistance is an important virulence factor in Klebsiella pneumoniae. Dictyostelium has been used to study the interaction between phagocytes and bacteria because of its similarity to mammalian macrophages. In this study, we used a Dictyostelium model to investigate genes for resistance to phagocytosis in NTUH-K2044, a strain of K. pneumoniae causing pyogenic liver abscess that is highly resistant to phagocytosis. A total of 2,500 transposon mutants were screened by plaque assay, and 29 of them permitted phagocytosis by Dictyostelium. In the 29 mutants, six loci were identified; three were capsular synthesis genes. Of the other three, one was related to carnitine metabolism, one encoded a subunit of protease (clpX), and one encoded a lipopolysaccharide O-antigen transporter (wzm). Deletion and complementation of these genes showed that only ΔclpX and Δwzm mutants became susceptible to Dictyostelium phagocytosis, and their complementation restored the phagocytosis resistance phenotype. These two mutants were also susceptible to phagocytosis by human neutrophils and revealed attenuated virulence in a mouse model, implying that they play important roles in the pathogenesis of K. pneumoniae. Furthermore, we demonstrated that clpP, which exists in an operon with clpX, was also involved in resistance to phagocytosis. The transcriptional profile of ΔclpX was examined by microarray analysis and revealed a 3-fold lower level of expression of capsular synthesis genes. Therefore, we have identified genes involved in resistance to phagocytosis in K. pneumoniae using Dictyostelium, and this model is useful to explore genes associated with resistance to phagocytosis in heavily encapsulated bacteria.

Biogenesis of Salmonella enterica serovar typhimurium membrane vesicles provoked by induction of PagC

Gram-negative bacteria ubiquitously release membrane vesicles (MVs) into the extracellular milieu. Although MVs are the product of growing bacteria, not of cell lysis or death, the regulatory mechanisms underlying MV formation remained unknown. We have found that MV biogenesis is provoked by the induction of PagC, a Salmonella-specific protein whose expression is activated by conditions that mimic acidified macrophage phagosomes. PagC is a major constituent of Salmonella MVs, and increased expression accelerates vesiculation. Expression of PagC is regulated at the posttranscriptional and/or posttranslational level in a sigmaS (RpoS)-dependent manner. Serial quantitative analysis has demonstrated that MV formation can accelerate when the quantity of the MV constituents, OmpX and PagC, rises. Overproduction of PagC dramatically impacts the difference in the relative amount of vesiculation, but the corresponding overproduction of OmpX was less pronounced. Quantitative examination of the ratios of PagC and OmpX in the periplasm, outer membrane, and MVs demonstrates that PagC is preferentially enriched in MVs released from Salmonella cells. This suggests that specific protein sorting mechanisms operate when MVs are formed. The possible role(s) of PagC-MV in host cells is discussed.

Survival of Listeria monocytogenes in simulated gastrointestinal system and transcriptional profiling of stress- and adhesion-related genes

Food ingestion is the major route of exposure to the important human pathogen Listeria monocytogenes. An in vitro gastrointestinal model was used to (1) compare the survival rates of L. monocytogenes strains of serotypes 1/2a, 1/2c, and 4b; and (2) examine the transcription of stress- and adhesion-related genes after exposure to the conditions similar to those encountered in the mouth, stomach, and small intestine. None of the L. monocytogenes strains investigated could survive in the gastric juice at pH 2.5 or 3.0. Their survival increased at higher pH (3.5 and 4.0) in the gastric stress. Relative survival of L. monocytogenes serotypes 4b and 1/2a strains were higher than that of serotype 1/2c, suggesting that pathogenicity might be related to the viability in the gastrointestinal tract. The transcription levels of prfA and the general stress-related genes clpC, clpE, and clpP were upregulated after passing through the simulated gastrointestinal tract, whereas that of the adhesion-related gene ami was downregulated. Taken together, this study revealed that L. monocytogenes strains enhanced the expression of stress-related genes and decreased the transcription of adhesion-related gene in order to survive in the diverse microenvironments.

Virulence attenuation of Streptococcus pneumoniae clpP mutant by sensitivity to oxidative stress in macrophages via an NO-mediated pathway

ClpP protease is essential for virulence and survival under stress conditions in several pathogenic bacteria. The clpP mutation in a murine infection model has demonstrated both attenuation of virulence and a sensitivity to hydrogen peroxide. However, the underlying mechanisms for these changes have not been resolved. Because macrophages play a major role in immune response and activated macrophages can kill microbes via oxygen-dependant mechanisms, we investigated the effect of the clpP mutation on its sensitivity to macrophage-mediated oxygen-dependant mechanisms. The clpP mutant derived from D39 (serotype 2) exhibited a higher sensitivity to oxidative stresses such as reactive oxygen intermediates, reactive nitrogen intermediates, and H(2)O(2), but no sensitivity to osmotic stress (NaCl) and pH. Moreover, viability of the clpP mutant was significantly increased in murine macrophage cells by treatment with S-methylisothiourea sulfate, which inhibits inducible nitric oxide synthase (iNOS) activity and subsequently elicits lower level secretions of nitric oxide (NO). However, viability of wild type was unchanged. Taken together, these results indicate that ClpP is involved in the resistance to oxidative stresses after entrapment by macrophages and subsequently contributes to virulence via NO mediated pathway.

Virulence factor p60 of Listeria monocytogenes modulates innate immunity by inducing tumor necrosis factor alpha

We investigated the effect of p60, a virulence factor of Listeria monocytogenes, on host immune response in vitro and in vivo. Administration of p60 before a sublethal infection with L. monocytogenes enhanced innate host resistance in naïve mice. Mouse macrophage RAW264.7 cells produced tumor necrosis factor (TNF)-alpha in response to stimulation with recombinant p60. Toll-like receptor 4 may be involved in TNF-alpha production from RAW264.7 cells and enhanced host resistance induced by p60 administration. Our findings demonstrated that p60 modulates innate immune responses against L. monocytogenes infection.

Helicobacter pylori mutants defective in the clpP ATP-dependant protease and the chaperone clpA display reduced macrophage and murine survival

The ATP-dependent caseinolytic proteases (Clp) are important in resistance against environmental stresses, antibiotic treatments and host immune defences for a number of pathogenic bacteria. ClpP is the proteolytic subunit, whilst ClpA acts both as a chaperone and as an ATPase driving the degradation of damaged or mis-made proteins. The gastric pathogen Helicobacter pylori infects approximately half of the world's population and can cause gastric or duodenal ulcers, gastric malignancies and mucosa-associated lymphoid tissue lymphomas. The conditions of its in vivo environment expose the organism to host immune cells and upon treatment, antibiotics, conditions likely to cause protein damage. We generated isogenic nonpolar mutants in strain SS1 of clpP and clpA and double mutants with both genes inactivated. Such mutants showed increased sensitivity to antibacterials causing protein damage and/or oxidative stress, in addition to a reduced survival in human macrophages. In the mouse infection model the double mutant SS1 clpAP lacked all ability to colonize the murine host. This suggests that the ability to recover from protein damage is of key importance in the pathogenesis of this organism.

Sigma32-mediated negative regulation of Salmonella pathogenicity island 1 expression

Salmonella pathogenicity island 1 (SPI1) enables infecting salmonellae to invade the intestinal epithelium and induce a proinflammatory response and macrophage cell death. SPI1 expression is controlled by a complex cascade with several transcriptional regulators within the island and global regulators outside it. Previously, we reported that DnaK-depleted salmonellae could neither invade epithelial cells nor secrete SPI1-encoded proteins, suggesting that DnaK is involved in the expression of SPI1. Here, we found that DnaK is involved in SPI1 expression through inhibition of sigma(32) protein, which directs the transcription of a group of genes in response to various global stresses. Overproduction of sigma(32) resulted in decreased levels of the SPI1-specific transcriptional regulators HilD and HilA. Further analysis demonstrated that the sigma(32)-mediated system negatively regulates HilD and HilA at the posttranslational and transcriptional levels, respectively. The executioner of this negative regulation was shown to be a sigma(32)-induced protein ATP-dependent Lon protease, which specifically degrades HilD. Since HilD can activate hilA transcription, is at the top of the hierarchical SPI1 regulatory loop, and has a dominant role, the posttranslational control of HilD by Lon is critically important for precise expression of SPI1. Consequently, we suggest that SPI1 expression is controlled by the feedback regulatory loop in which sigma(32) induces Lon to control turnover of HilD, and DnaK, which inhibits sigma(32) function, leading to the modulation of lon expression. This regulation in response to a specific combination of environmental signals would ensure that SPI1 expression is restricted to a few specific locations in the host.

Coordinated regulation of expression of Salmonella pathogenicity island 1 and flagellar type III secretion systems by ATP-dependent ClpXP protease

Salmonella enterica serovar Typhimurium delivers a variety of proteins via the Salmonella pathogenicity island 1 (SPI1)-encoded type III secretion system into host cells, where they elicit several physiological changes, including bacterial invasion, macrophage apoptosis, and enteropathogenesis. Once Salmonella has established a systemic infection, excess macrophage apoptosis would be detrimental to the pathogen, as it utilizes macrophages as vectors for systemic dissemination throughout the host. Therefore, SPI1 expression must be restricted to one or a few specific locations in the host. In the present study, we have demonstrated that the expression of this complex of genes is repressed by the ATP-dependent ClpXP protease, which therefore suppresses macrophage apoptosis. Depletion of ClpXP caused significant increases in the amounts of two SPI1-encoded transcriptional regulators, HilC and HilD, leading to the stimulation of hilA induction and therefore activation of SPI1 expression. Our evidence shows that ClpXP regulates cellular levels of HilC and HilD via the control of flagellar gene expression. Subsequent experiments demonstrated that the flagellum-related gene product FliZ controls HilD posttranscriptionally, and this in turn activates HilC. These findings suggest that the ClpXP protease coregulates SPI1-related virulence phenotypes and motility. ClpXP is a member of the stress protein family induced in bacteria exposed to hostile environments such as macrophages.

Peptide-assisted degradation of the Salmonella MgtC virulence factor

MgtC is a virulence factor common to several intracellular pathogens that is required for intramacrophage survival and growth in magnesium-depleted medium. In Salmonella enterica, MgtC is coexpressed with the MgtB magnesium transporter and transcription of the mgtCB operon is induced by magnesium deprivation. Despite the high level of mgtCB transcriptional induction in magnesium-depleted medium, the MgtC protein is hardly detected in a wild-type Salmonella strain. Here, we show that downregulation of MgtC expression is dependent on a hydrophobic peptide, MgtR, which is encoded by the mgtCB operon. Our results suggest that MgtR promotes MgtC degradation by the FtsH protease, providing a negative regulatory feedback. Bacterial two-hybrid assays demonstrate that MgtR interacts with the inner-membrane MgtC protein. We identified mutant derivatives of MgtR and MgtC that prevent both regulation and interaction between the two partners. In macrophages, overexpression of the MgtR peptide led to a decrease of the replication rate of Salmonella. This study highlights the role of peptides in bacterial regulatory mechanisms and provides a natural antagonist of the MgtC virulence factor.

Peptidase N encoded by Salmonella enterica serovar Typhimurium modulates systemic infection in mice

The cytosolic protein degradation pathway, involving ATP-dependent proteases and ATP-independent peptidases, is important for modulating several cellular responses. The involvement of pathogen-encoded ATP-dependent proteases is well established during infection. However, the roles of ATP-independent peptidases in this process are not well studied. The functional role of Peptidase N (PepN), an ATP-independent enzyme belonging to the M1 family, during systemic infection of mice by Salmonella enterica serovar Typhimurium (Salmonella typhimurium) was investigated. In a systemic model of infection, the number of CFU of S. typhimurium containing a targeted deletion in peptidase N (DeltapepN), compared with wild type, was significantly higher in the lymph node and spleen. In addition, S. typhimurium replicated in the thymus and greatly reduced the number of immature CD4(+)CD8(+) thymocytes in a dose- and time-dependent manner. Strains lacking or overexpressing pepN were used to show that the reduction in the number of thymocytes, but not lymph node cells, depends on a critical number of CFU. These findings establish a role for PepN in reducing the in vivo CFU of S. typhimurium during systemic infection. The implications of these results, in the context of the roles of proteases and peptidases, during host-pathogen interactions are discussed.

ExpressedSalmonellaantigens within macrophages enhance the proliferation of CD4+and CD8+T lymphocytes by means of bystander dendritic cells

ATP-dependent Lon protease-deficient Salmonella enterica serovar Typhimurium (strain CS2022) appeared to invade successfully the mesenteric lymph nodes (MLN) and Peyer's patches (PP) of BALB/c mice and appeared to be easily eradicated by the host after oral immunization. As detected by flow cytometry, the population of major histocompatibility complex class I (MHC-I)-expressing macrophages and dendritic cells (DCs) was increased in the PP of mice immunized with CS2022 on day 6 after immunization. Thereafter, the population of splenic surface CD69(+) T lymphocytes prepared from mice immunized with CS2022 6 weeks prior to measurement increased as a result of the administration of the extracellular vesicles of RAW264.7 macrophage-like cells derived by Salmonella challenge. In addition, the proliferation of CD8(+) and even of CD4(+)T cells isolated from mouse spleens immunized with CS2022 was enhanced after cocultivation with naive DCs in the presence of the extracellular vesicles. These findings indicate that the extracellular vesicles prepared from the Salmonella-challenged macrophages carried salmonellae antigens to bystander DCs, thereby stimulating T-cell responses. Therefore, as antigen presentation after phagocytosis should be a central process in the T-cell activation that occurs in response to Salmonella infection, an oral immunization with CS2022 sufficiently induces T cell-mediated immunity in mice.

An oralSalmonellavaccine promotes the down-regulation of cell surface Toll-like receptor 4 (TLR4) and TLR2 expression in mice

A single oral immunization with the Lon-protease-deficient Salmonella enterica serovar Typhimurium (strain CS2022) induced protective immunity in mice against a subcutaneous challenge with virulent Listeria monocytogenes as well as virulent Salmonella serovar Typhimurium. The populations of cell surface Toll-like receptor 4 (TLR4) and TLR2 on peritoneal macrophages decreased at week 6 after immunization. This population decrease was not reversed after a challenge with either Salmonella or Listeria. These results suggest that oral immunization with CS2022 induced immune tolerance correlated with the down-regulation of cell surface TLR expression. This down-regulation may in part account for the development of cross-protection against a Listeria challenge by immunization with CS2022.

Susceptibility of germ-free pigs to challenge with protease mutants of Salmonella enterica serovar Typhimurium

Salmonella protease mutants, clpP and especially htrA, are candidate live oral vaccines in humans. A functional and mature immune system is, however, required to cope with them in mice. Here, we test the cytokine response of highly susceptible germ-free pigs to infection with Salmonella Typhimurium clpP and htrA mutants. Cytokine levels (IL-4, IL-10, IL-18 and IFN-gamma) were measured by ELISA in plasma and washes from the terminal small bowel 24h after oral challenge. Unlike the infection with the wild type strain, no IFN-gamma response and low IL-18 intestinal levels were found in pigs infected with the protease mutants. Despite this and regardless of partially reduced ability of htrA and clpP mutants to invade and multiply in a 3D4 porcine macrophage-like cell line, both the mutants were as virulent as was the wild type LT2 strain and caused fatal septicaemia in germ-free pigs. IFN-gamma and IL-18 response therefore did not correlate with the virulence of Salmonella Typhimurium. Our results indicate that htrA and clpP attenuations should be used with caution in populations in which an increased number of immunocompromised individuals can be expected.

[Bacterial strategies for escaping the bactericidal mechanisms by macrophage]

Phagocytosis with macrophages provides a specialized mechanism for regulated ingestion and intracellular destruction of bacteria. Bacteria are first engulfed by endocytosis into a phagosome. The fusion of phagosomes and lysosomes releases toxic products that kill most bacteria and degrade them into fragments. Debris from dead bacteria is then released by exocytosis. However, some bacteria that survive within host phagocytes have evolved strategies to escape the bactericidal mechanisms associated with phagocytosis: i) antiphagocytosis (Yersinia), ii) escaping from the phagosome into cytoplasm (Listeria), and iii) remodeling their phagosome by inhibiting the maturation of phagosomes (Salmonella, Mycobacterium, Legionella). In this review, I first summarize various strategies by bacteria to avoid phagocytosis by emphasizing the steps that have been subverted by bacteria. Then, I highlight the mechanisms for surviving phagocytosis by Salmonella, with a focus on the induction of macrophage-apoptosis and modulation of membrane traffic in host cells.

Reactive arthritis

Reactive arthritis (ReA) has been recognized as a clinical disease entity for nearly 100 years. The prevalence is estimated to be 30-40/100,000 adults. The HLA-B27-associated form is part of the spondyloarthritis concept. According to the current hypothesis the arthritis follows a primary extra-articular infection and is characterized by the presence of bacterial antigen and/or of viable but non-culturable bacteria persisting within the joint. Pathogenesis involves the modification of host cells by pathogen-associated molecular patterns (PAMPs, e.g. lipopolysaccharide), bacterial effector proteins, the adaptive immune system, and the genetic background. Up to 30% of patients develop chronic symptoms, and therapeutic options for these patients are still limited. Data for recommendations to apply conventional disease-modifying anti-rheumatic drugs (DMARDs) are rare; however, sulfasalazine seems to be effective, and first reports on agents that block tumour necrosis factor (TNF) are promising. Combination therapy of several antibiotics might open the window to curing the disease; however, controlled clinical studies are needed.

A homolog of Bacillus subtilis trigger factor in Listeria monocytogenes is involved in stress tolerance and bacterial virulence

Molecular chaperones play an essential role in the folding of nascent chain polypeptides, as well as in the refolding and degradation of misfolded or aggregated proteins. They also assist in protein translocation and participate in stress functions. We identified a gene, designated tig, encoding a protein homologous to trigger factor (TF), a cytosolic ribosome-associated chaperone, in the genome of Listeria monocytogenes. We constructed a chromosomal Delta tig deletion and evaluated the impact of the mutation on bacterial growth in broth under various stress conditions and on pathogenesis. The Delta tig deletion did not affect cell viability but impaired survival in the presence of heat and ethanol stresses. We also identified the ffh gene, encoding a protein homologous to the SRP54 eukaryotic component of the signal recognition particle. However, a Delta ffh deletion was not tolerated, suggesting that Ffh is essential, as it is in Bacillus subtilis and Escherichia coli. Thus, although dispensable for growth, TF is involved in the stress response of L. monocytogenes. The Delta tig mutant showed no or very modest intracellular survival defects in eukaryotic cells. However, in vivo it showed a reduced capacity to persist in the spleens and livers of infected mice, revealing that TF has a role in the pathogenicity of L. monocytogenes.

In vitro and in vivo stability of plasmids in attenuated Salmonella enterica serovar typhimurium used as a carrier of DNA vaccine is associated with its replication origin

The ability of live attenuated Salmonella enterica serovar typhimurium (S. typhimurium) as a carrier of DNA vaccine was evaluated using model plasmid encoding beta-galactosidase (beta-Gal) and BALB/c mice. We constructed pBRCMVbeta, beta-Gal expression apparatus having a replication origin from low copy pBR322. Comparison of the plasmid stability showed that pBRCMVbeta remained stable in Salmonella even after oral administration, while pUC-based pCMVbeta tended to be lost quickly. However, titers for beta-Gal specific IgG in sera did not significantly increase in mice orally administered S. typhimurium harboring pBRCMVbeta. These data suggest that the stability of plasmid in S. typhimurium is associated with its replication origin. Further studies are required to scientifically establish this methodology.

Self-compartmentalized bacterial proteases and pathogenesis

Protein degradation is required for homeostasis of all living organisms. Self-compartmentalized ATP-dependent proteases are required for virulence of several pathogenic bacteria. Among the proteases implicated are ClpP and Lon, as well as the more recently identified bacterial proteasome. It is generally assumed that when a pathogen invades a host, microbial proteins become irreversibly damaged and need to be degraded. However, recent data suggest that proteolysis is also essential for virulence gene regulation. In this review, we will discuss what is known about the relationship between ATP-dependent proteolysis and pathogenesis. In addition, we will propose other potential roles these chambered proteases may have in bacterial virulence. Importantly, these proteases show promise as targets for antimicrobial therapy.

In vivo, fliC expression by Salmonella enterica serovar Typhimurium is heterogeneous, regulated by ClpX, and anatomically restricted

FliC is a natural antigen recognized by the innate and adaptive immune systems during Salmonella infection in mice and humans; however, the regulatory mechanisms governing its expression in vivo are incompletely understood. Here, we use flow cytometry to quantify fliC gene expression in single bacteria. In vitro, fliC transcription was not uniformly positive; a viable fliC-negative subpopulation was also identified. Intracellular Salmonella repressed transcription of fliC and its positive regulator fliA, but constitutively transcribed the master regulator flhD; fliC repression required ClpXP protease, known to degrade FlhD. In orally infected mice, fliC transcription was anatomically restricted: Salmonella transcribed fliC in the Peyer's Patches (PP) but not in the mesenteric lymph nodes and spleen. The intracellularly transcribed pagC promoter was upregulated by Salmonella in all tissues, defining the infected PP as a unique environment that initiates expression of intracellularly induced genes and yet permits transcription of fliC. Because a single bacterium can escape the GI tract to colonize deeper tissues, heterogeneous gene expression may have important implications for Salmonella pathogenesis: FliC-positive bacteria in the PP could stimulate inflammation and facilitate the priming of FliC-specific immune responses, while FliC-negative bacteria escape host detection in the gut and spread to systemic sites of replication.

Evaluation of the Lon-deficient Salmonella strain as an oral vaccine candidate

We evaluated the efficacy of CS2022 (the Lon protease-deficient mutant strain of Salmonella enterica serovar Typhimurium) as a candidate live oral vaccine strain against subsequent oral challenge with a virulent strain administered to BALB/c and C57BL/6 mice. CS2022 persistently resided in the spleen, mesenteric lymph nodes, Peyer's patches, and cecum of both strains of mice after a single oral inoculation with 1 x 10(8) colony-forming units. Finally, CS2022 almost disappeared from each tissue sample by week 12 in BALB/c mice, whereas CS2022 still resided in each tissue type at week 12 after inoculation of C57BL/6 mice. A significant increase in the serovar Typhimurium lipopolysaccharide-specific secretory immunoglobulin A (s-IgA), as measured for one of the mucosal immune responses, was detected in bile and intestinal samples of both strains of immunized mice at week 4 after immunization. In addition, the expression of gamma interferon mRNA in the spleens of both strains of immunized mice, especially those of C57BL/6 mice, was significantly increased at week 4 after immunization and was boosted during the following 5 days after the challenge was administered to the mice. Furthermore, peritoneal macrophages isolated from immunized mice at week 4 after immunization exhibited an increase in intracellular killing activity against both virulent and avirulent Salmonella. The present results suggested that salmonellae-specific s-IgA on the mucosal surfaces induced by immunization with CS2022 generally prevented mice from succumbing to an oral challenge with a virulent strain. Simultaneously, CS2022 promoted the protective immunity associated with macrophages in both strains of mice.

Intracellular gene expression profile of Listeria monocytogenes

Listeria monocytogenes is a gram-positive, food-borne microorganism responsible for invasive infections with a high overall mortality. L. monocytogenes is among the very few microorganisms that can induce uptake into the host cell and subsequently enter the host cell cytosol by breaching the vacuolar membrane. We infected the murine macrophage cell line P388D1 with L. monocytogenes strain EGD-e and examined the gene expression profile of L. monocytogenes inside the vacuolar and cytosolic environments of the host cell by using whole-genome microarray and mutant analyses. We found that approximately 17% of the total genome was mobilized to enable adaptation for intracellular growth. Intracellularly expressed genes showed responses typical of glucose limitation within bacteria, with a decrease in the amount of mRNA encoding enzymes in the central metabolism and a temporal induction of genes involved in alternative-carbon-source utilization pathways and their regulation. Adaptive intracellular gene expression involved genes that are associated with virulence, the general stress response, cell division, and changes in cell wall structure and included many genes with unknown functions. A total of 41 genes were species specific, being absent from the genome of the nonpathogenic Listeria innocua CLIP 11262 strain. We also detected 25 genes that were strain specific, i.e., absent from the genome of the previously sequenced L. monocytogenes F2365 serotype 4b strain, suggesting heterogeneity in the gene pool required for intracellular survival of L. monocytogenes in host cells. Overall, our study provides crucial insights into the strategy of intracellular survival and measures taken by L. monocytogenes to escape the host cell responses.

Salmonella stress management and its relevance to behaviour during intestinal colonisation and infection

The enteric pathogen Salmonella enterica is exposed to a number of stressful environments during its life cycle within and outside its various hosts. During intestinal colonisation Salmonella is successively exposed to acid pH in the stomach, to the detergent-like activity of bile, to decreasing oxygen supply, to the presence of multiple metabolites produced by the normal gut microflora and finally it is exposed to cationic antimicrobial peptides present on the surface of epithelial cells. There are four major regulators controlling relevant stress responses in Salmonella, namely RpoS, PhoPQ, Fur and OmpR/EnvZ. Except for Fur, inactivation of genes encoding the other stress regulators results in attenuated virulence and such mutants can therefore be considered as vaccine candidates. In contrast, a decrease in oxygen supply monitored by Fnr and ArcAB, or oxidative stress controlled by OxyR and SoxRS is not regarded as a stress associated with host colonisation since inactivation of either of these systems does not result in reductions in colonisation. The role of quorum-sensing through luxS and sdiA is also considered as a regulator of virulence and colonisation.

Infection of the reproductive tract and eggs with Salmonella enterica serovar pullorum in the chicken is associated with suppression of cellular immunity at sexual maturity

Salmonella enterica serovar Pullorum causes persistent infections in laying hens. Splenic macrophages are the main site of persistence. At sexual maturity, numbers of bacteria increase and spread to the reproductive tract, which may result in vertical transmission to eggs or chicks. In this study we demonstrate that both male and female chickens may develop a carrier state following infection but that the increases in bacterial numbers and spread to the reproductive tract are phenomena restricted to hens, indicating that such changes are likely to be related to the onset of egg laying. The immunological responses during the carrier state and through the onset of laying in hens were determined. These indicate that chickens produce both humoral and T-cell responses to infection, but at the onset of laying both the T-cell response to Salmonella and nonspecific responses to mitogenic stimulation fall sharply in both infected and noninfected birds. The fall in T-cell responsiveness coincided with the increase in numbers of Salmonella serovar Pullorum and its spread to the reproductive tract. Three weeks after the onset of egg laying, T-cell responsiveness began to increase and bacterial numbers declined. Specific antibody levels changed little at the onset of laying but increased following the rise in bacterial numbers in a manner reminiscent of a secondary antibody response to rechallenge. These findings indicate that a nonspecific suppression of cellular responses occurs at the onset of laying and plays a major role the ability of Salmonella serovar Pullorum to infect the reproductive tract, leading to transmission to eggs. The loss of T-cell activity at the point of laying also has implications for Salmonella enterica serovar Enteritidis infection and transmission to eggs, along with its control by vaccination offering a "window of opportunity" in which infection may occur.