Nuclear factor kappa B-dependent persistence of Salmonella Typhi and Paratyphi in human macrophages

Salmonella serovars Typhi and Paratyphi cause a prolonged illness known as enteric fever, whereas other serovars cause acute gastroenteritis. Mechanisms responsible for the divergent clinical manifestations of nontyphoidal and enteric fever Salmonella infections have remained elusive. Here, we show that S. Typhi and S. Paratyphi A can persist within human macrophages, whereas S. Typhimurium rapidly induces apoptotic macrophage cell death that is dependent on Salmonella pathogenicity island 2 (SPI2). S. Typhi and S. Paratyphi A lack 12 specific SPI2 effectors with pro-apoptotic functions, including nine that target nuclear factor κB (NF-κB). Pharmacologic inhibition of NF-κB or heterologous expression of the SPI2 effectors GogA or GtgA restores apoptosis of S. Typhi-infected macrophages. In addition, the absence of the SPI2 effector SarA results in deficient signal transducer and activator of transcription 1 (STAT1) activation and interleukin 12 production, leading to impaired TH1 responses in macrophages and humanized mice. The absence of specific nontyphoidal SPI2 effectors may allow S. Typhi and S. Paratyphi A to cause chronic infections.

IMPORTANCE

Salmonella enterica is a common cause of gastrointestinal infections worldwide. The serovars Salmonella Typhi and Salmonella Paratyphi A cause a distinctive systemic illness called enteric fever, whose pathogenesis is incompletely understood. Here, we show that enteric fever Salmonella serovars lack 12 specific virulence factors possessed by nontyphoidal Salmonella serovars, which allow the enteric fever serovars to persist within human macrophages. We propose that this fundamental difference in the interaction of Salmonella with human macrophages is responsible for the chronicity of typhoid and paratyphoid fever, suggesting that targeting the nuclear factor κB (NF-κB) complex responsible for macrophage survival could facilitate the clearance of persistent bacterial infections.

Genomic reconstruction and directed interventions in a multidrug-resistant Shigellosis outbreak in Seattle, WA, USA: a genomic surveillance study

BACKGROUND

Shigella spp have been associated with community-wide outbreaks in urban settings. We analysed a sustained shigellosis outbreak in Seattle, WA, USA, to understand its origins and mechanisms of antimicrobial resistance, define ongoing transmission patterns, and optimise strategies for treatment and infection control.

METHODS

We did a retrospective study of all Shigella isolates identified from stool samples at the clinical laboratories at Harborview Medical Center and University of Washington Medical Center (Seattle, WA, USA) from May 1, 2017, to Feb 28, 2022. We characterised isolates by species identification, phenotypic susceptibility testing, and whole-genome sequencing. Demographic characteristics and clinical outcomes of the patients were retrospectively examined.

FINDINGS

171 cases of shigellosis were included. 78 (46%) patients were men who have sex with men (MSM), and 88 (52%) were people experiencing homelessness (PEH). Although 84 (51%) isolates were multidrug resistant, 100 (70%) of 143 patients with data on antimicrobial therapy received appropriate empirical therapy. Phylogenomic analysis identified sequential outbreaks of multiple distinct lineages of Shigella flexneri and Shigella sonnei. Discrete clonal lineages (ten in S flexneri and nine in S sonnei) and resistance traits were responsible for infection in different at-risk populations (ie, MSM, PEH), enabling development of effective guidelines for empirical treatment. The most prevalent lineage in Seattle was probably introduced to Washington State via international travel, with subsequent domestic transmission between at-risk groups.

INTERPRETATION

An outbreak in Seattle was driven by parallel emergence of multidrug-resistant strains involving international transmission networks and domestic transmission between at-risk populations. Genomic analysis elucidated not only outbreak origin, but directed optimal approaches to testing, treatment, and public health response. Rapid diagnostics combined with detailed knowledge of local epidemiology can enable high rates of appropriate empirical therapy even in multidrug-resistant infection.

FUNDING

None.

Analysis of Salmonella Typhi Pathogenesis in a Humanized Mouse Model

Efforts to understand molecular mechanisms of pathogenesis of the human-restricted pathogen Salmonella enterica serovar Typhi, the causative agent of typhoid fever, have been hampered by the lack of a tractable small animal model. This obstacle has been surmounted by a humanized mouse model in which genetically modified mice are engrafted with purified CD34+ stem cells from human umbilical cord blood, designated CD34+ Hu-NSG (formerly hu-SRC-SCID) mice. We have shown that these mice develop a lethal systemic infection with S. Typhi that is dependent on the presence of engrafted human hematopoietic cells. Immunological and pathological features of human typhoid are recapitulated in this model, which has been successfully employed for the identification of bacterial genetic determinants of S. Typhi virulence. Here we describe the methods used to infect CD34+ Hu-NSG mice with S. Typhi in humanized mice and to construct and analyze a transposon-directed insertion site sequencing S. Typhi library, and provide general considerations for the use of humanized mice for the study of a human-restricted pathogen.

Manganese import protects Salmonella enterica serovar Typhimurium against nitrosative stress

Nitric oxide (NO˙) is a radical molecule produced by mammalian phagocytic cells as part of the innate immune response to bacterial pathogens. It exerts its antimicrobial activity in part by impairing the function of metalloproteins, particularly those containing iron and zinc cofactors. The pathogenic Gram-negative bacterium Salmonella enterica serovar typhimurium undergoes dynamic changes in its cellular content of the four most common metal cofactors following exposure to NO˙ stress. Zinc, iron and magnesium all decrease in response to NO˙ while cellular manganese increases significantly. Manganese acquisition is driven primarily by increased expression of the mntH and sitABCD transporters following derepression of MntR and Fur. ZupT also contributes to manganese acquisition in response to nitrosative stress. S. Typhimurium mutants lacking manganese importers are more sensitive to NO˙, indicating that manganese is important for resistance to nitrosative stress.

Genome-wide Analysis of Salmonella enterica serovar Typhi in Humanized Mice Reveals Key Virulence Features

Salmonella enterica serovar Typhi causes typhoid fever only in humans. Murine infection with S. Typhimurium is used as a typhoid model, but its relevance to human typhoid is limited. Non-obese diabetic-scid IL2rγnull mice engrafted with human hematopoietic stem cells (hu-SRC-SCID) are susceptible to lethal S. Typhi infection. In this study, we use a high-density S. Typhi transposon library in hu-SRC-SCID mice to identify virulence loci using transposon-directed insertion site sequencing (TraDIS). Vi capsule, lipopolysaccharide (LPS), and aromatic amino acid biosynthesis were essential for virulence, along with the siderophore salmochelin. However, in contrast to the murine S. Typhimurium model, neither the PhoPQ two-component system nor the SPI-2 pathogenicity island was required for lethal S. Typhi infection, nor was the CdtB typhoid toxin. These observations highlight major differences in the pathogenesis of typhoid and non-typhoidal Salmonella infections and demonstrate the utility of humanized mice for understanding the pathogenesis of a human-specific pathogen.

The Evolution of SlyA/RovA Transcription Factors from Repressors to Countersilencers in Enterobacteriaceae

Gene duplication and subsequent evolutionary divergence have allowed conserved proteins to develop unique roles. The MarR family of transcription factors (TFs) has undergone extensive duplication and diversification in bacteria, where they act as environmentally responsive repressors of genes encoding efflux pumps that confer resistance to xenobiotics, including many antimicrobial agents. We have performed structural, functional, and genetic analyses of representative members of the SlyA/RovA lineage of MarR TFs, which retain some ancestral functions, including repression of their own expression and that of divergently transcribed multidrug efflux pumps, as well as allosteric inhibition by aromatic carboxylate compounds. However, SlyA and RovA have acquired the ability to countersilence horizontally acquired genes, which has greatly facilitated the evolution of Enterobacteriaceae by horizontal gene transfer. SlyA/RovA TFs in different species have independently evolved novel regulatory circuits to provide the enhanced levels of expression required for their new role. Moreover, in contrast to MarR, SlyA is not responsive to copper. These observations demonstrate the ability of TFs to acquire new functions as a result of evolutionary divergence of both cis-regulatory sequences and in trans interactions with modulatory ligands.IMPORTANCE Bacteria primarily evolve via horizontal gene transfer, acquiring new traits such as virulence and antibiotic resistance in single transfer events. However, newly acquired genes must be integrated into existing regulatory networks to allow appropriate expression in new hosts. This is accommodated in part by the opposing mechanisms of xenogeneic silencing and countersilencing. An understanding of these mechanisms is necessary to understand the relationship between gene regulation and bacterial evolution. Here we examine the functional evolution of an important lineage of countersilencers belonging to the ancient MarR family of classical transcriptional repressors. We show that although members of the SlyA lineage retain some ancestral features associated with the MarR family, their cis-regulatory sequences have evolved significantly to support their new function. Understanding the mechanistic requirements for countersilencing is critical to understanding the pathoadaptation of emerging pathogens and also has practical applications in synthetic biology.

The curli regulator CsgD mediates stationary phase counter-silencing of csgBA in Salmonella Typhimurium

Integration of horizontally acquired genes into transcriptional networks is essential for the regulated expression of virulence in bacterial pathogens. In Salmonella enterica, expression of such genes is repressed by the nucleoid-associated protein H-NS, which recognizes and binds to AT-rich DNA. H-NS-mediated silencing must be countered by other DNA-binding proteins to allow expression under appropriate conditions. Some genes that can be transcribed by RNA polymerase (RNAP) associated with the alternative sigma factor σ^S or the housekeeping sigma factor σ⁷⁰ in vitro appear to be preferentially transcribed by σ^S in the presence of H-NS, suggesting that σ^S may act as a counter-silencer. To determine whether σ^S directly counters H-NS-mediated silencing and whether co-regulation by H-NS accounts for the σ^S selectivity of certain promoters, we examined the csgBA operon, which is required for curli fimbriae expression and is known to be regulated by both H-NS and σ^S . Using genetics and in vitro biochemical analyses, we found that σ^S is not directly required for csgBA transcription, but rather up-regulates csgBA via an indirect upstream mechanism. Instead, the biofilm master regulator CsgD directly counter-silences the csgBA promoter by altering the DNA-protein complex structure to disrupt H-NS-mediated silencing in addition to directing the binding of RNAP.

Probiotic bacteria reduce salmonella typhimurium intestinal colonization by competing for iron

Host inflammation alters the availability of nutrients such as iron to limit microbial growth. However, Salmonella enterica serovar Typhimurium thrives in the inflamed gut by scavenging for iron with siderophores. By administering Escherichia coli strain Nissle 1917, which assimilates iron by similar mechanisms, we show that this nonpathogenic bacterium can outcompete and reduce S. Typhimurium colonization in mouse models of acute colitis and chronic persistent infection. This probiotic activity depends on E. coli Nissle iron acquisition, given that mutants deficient in iron uptake colonize the intestine but do not reduce S. Typhimurium colonization. Additionally, the ability of E. coli Nissle to overcome iron restriction by the host protein lipocalin 2, which counteracts some siderophores, is essential, given that S. Typhimurium is unaffected by E. coli Nissle in lipocalin 2-deficient mice. Thus, iron availability impacts S. Typhimurium growth, and E. coli Nissle reduces S. Typhimurium intestinal colonization by competing for this limiting nutrient.

Salmonella enterica causes more severe inflammatory disease in C57/BL6 Nramp1G169 mice than Sv129S6 mice

Salmonella enterica serovar Typhimurium (S. Typhimurium) causes systemic inflammatory disease in mice by colonizing cells of the mononuclear leukocyte lineage. Mouse strains resistant to S. Typhimurium, including Sv129S6, have an intact Nramp1 (Slc11a1) allele and survive acute infection, whereas C57/BL6 mice, homozygous for a mutant Nramp1 allele, Nramp1(G169D) , develop lethal infections. Restoration of Nramp1 (C57/BL6 Nramp1(G169) ) reestablishes resistance to S. Typhimurium; mice survive at least 3 to 4 weeks postinfection. Since many transgenic mouse strains are on a C57/BL6 genetic background, C57/BL6 Nramp1(G169) mice provide a model to examine host genetic determinants of resistance to infection. To further evaluate host immune response to S. Typhimurium, we performed comparative analyses of Sv129S6 and C57/BL6 Nramp1(G169) mice 3 weeks following oral S. Typhimurium infection. C57/BL6 Nramp1(G169) mice developed more severe inflammatory disease with splenic bacterial counts 1000-fold higher than Sv129S6 mice and relatively greater splenomegaly and blood neutrophil and monocyte counts. Infected C57/BL6 Nramp1(G169) mice developed higher proinflammatory serum cytokine and chemokine responses (interferon-γ, tumor necrosis factor-α, interleukin [IL]-1β, and IL-2 and monocyte chemotactic protein-1 and chemokine [C-X-C motif] ligand 1, respectively) and marked decreases in anti-inflammatory serum cytokine concentrations (IL-10, IL-4) compared with Sv129S6 mice postinfection. Splenic dendritic cells and macrophages in infected compared with control mice increased to a greater extent in C57/BL6 Nramp1(G169) mice than in Sv129S6 mice. Overall, data show that despite the Nramp1 gene present in both strains, C57/BL6 Nramp1(G169) mice develop more severe, Th1-skewed, acute inflammatory responses to S. Typhimurium infection compared with Sv129S6 mice. Both strains are suitable model systems for studying inflammation in the context of adaptive immunity.

Slc11a1 (Nramp1) impairs growth of Salmonella enterica serovar typhimurium in macrophages via stimulation of lipocalin-2 expression

The expression of the cation transporter Nramp1 (Slc11a1) in late phagolysosomes confers resistance to infection with several intracellular pathogens, such as Salmonella enterica, in mice. The antimicrobial actions of Nramp1 are attributable, in part, to modulation of macrophage immune function and cellular iron metabolism--the latter affecting the availability of the essential nutrient iron for intraphagosomal bacteria. Here, we provide novel evidence that Nramp1 functionality increases the expression of the peptide Lcn2, which exerts its antimicrobial activity by scavenging iron-loaded bacterial siderophores and mediating iron efflux from macrophages. With the use of macrophage cell lines expressing functional or nonfunctional Nramp1, we found significantly elevated Lcn2 mRNA and protein levels in Nramp1-expressing cells. These resulted from Nramp1-mediated alterations in the production of ROS, which stimulated NF-κ B activity and subsequently, Lcn2 transcription. We observed that increased Lcn2 levels in primary Nramp1-positive macrophages resulted in a significant suppression of S. enterica serovar typhimurium growth. Stimulation of Lcn2 expression is a novel mechanism by which Nramp1 confers resistance against infection with the intracellular bacterium S. typhimurium.

Evolution of Salmonella enterica virulence via point mutations in the fimbrial adhesin

Whereas the majority of pathogenic Salmonella serovars are capable of infecting many different animal species, typically producing a self-limited gastroenteritis, serovars with narrow host-specificity exhibit increased virulence and their infections frequently result in fatal systemic diseases. In our study, a genetic and functional analysis of the mannose-specific type 1 fimbrial adhesin FimH from a variety of serovars of Salmonella enterica revealed that specific mutant variants of FimH are common in host-adapted (systemically invasive) serovars. We have found that while the low-binding shear-dependent phenotype of the adhesin is preserved in broad host-range (usually systemically non-invasive) Salmonella, the majority of host-adapted serovars express FimH variants with one of two alternative phenotypes: a significantly increased binding to mannose (as in S. Typhi, S. Paratyphi C, S. Dublin and some isolates of S. Choleraesuis), or complete loss of the mannose-binding activity (as in S. Paratyphi B, S. Choleraesuis and S. Gallinarum). The functional diversification of FimH in host-adapted Salmonella results from recently acquired structural mutations. Many of the mutations are of a convergent nature indicative of strong positive selection. The high-binding phenotype of FimH that leads to increased bacterial adhesiveness to and invasiveness of epithelial cells and macrophages usually precedes acquisition of the non-binding phenotype. Collectively these observations suggest that activation or inactivation of mannose-specific adhesive properties in different systemically invasive serovars of Salmonella reflects their dynamic trajectories of adaptation to a life style in specific hosts. In conclusion, our study demonstrates that point mutations are the target of positive selection and, in addition to horizontal gene transfer and genome degradation events, can contribute to the differential pathoadaptive evolution of Salmonella.

Multiple targets of nitric oxide in the tricarboxylic acid cycle of Salmonella enterica serovar typhimurium

Host nitric oxide (NO⋅) production is important for controlling intracellular bacterial pathogens, including Salmonella enterica serovar Typhimurium, but the underlying mechanisms are incompletely understood. S. Typhmurium 14028s is prototrophic for all amino acids but cannot synthesize methionine (M) or lysine (K) during nitrosative stress. Here, we show that NO⋅-induced MK auxotrophy results from reduced succinyl-CoA availability as a consequence of NO⋅ targeting of lipoamide-dependent lipoamide dehydrogenase (LpdA) activity. LpdA is an essential component of the pyruvate and α-ketoglutarate dehydrogenase complexes. Additional effects of NO⋅ on gene regulation prevent compensatory pathways of succinyl-CoA production. Microarray analysis indicates that over 50% of the transcriptional response of S. Typhimurium to nitrosative stress is attributable to LpdA inhibition. Bacterial methionine transport is essential for virulence in NO⋅-producing mice, demonstrating that NO⋅-induced MK auxotrophy occurs in vivo. These observations underscore the importance of metabolic targets for antimicrobial actions of NO⋅.

Vaccination with attenuated Salmonella enterica Dublin expressing E coli O157:H7 outer membrane protein Intimin induces transient reduction of fecal shedding of E coli O157:H7 in cattle

Background

Escherichia coli serogroup O157:H7 has emerged as an important zoonotic bacterial pathogen, causing a range of symptoms from self-limiting bloody diarrhea to severe hemorrhagic colitis and hemolytic-uremic syndrome in humans. Beef and dairy cattle are considered the most important animal reservoirs for this pathogen. One of the important virulence characteristics of E. coli O157:H7 is the eaeA gene encoding the 97 kDa surface protein intimin. Intimin is required for attachment and effacement during the interaction of enterohemorrhagic E. coli with human and bovine neonatal enterocytes. The present study was undertaken to test the hypothesis that an adaptive mucosal immune response directed against intimin will reduce or prevent enteric colonization and fecal shedding of E. coli O157:H7 in cattle.

Results

Cattle were orally inoculated with either milk (control), milk with live attenuated Salmonella enterica serovar Dublin (vector), or milk with live attenuated recombinant S. Dublin expressing intimin (vaccinated) on days 0, 14 and 28. On day 98, all calves were challenged orally with E. coli O157:H7 to evaluate whether vaccination with the recombinant S. Dublin expressing intimin would reduce the level of E. coli O157:H7 fecal shedding.

During the first 28 days, vaccinated calves shed both the vector strain and the intimin-expressing S. Dublin strain at a similar level. The vector strain was shed for a significantly longer period as compared to the level of recombinant vaccine strain. Calves that received the intimin-expressed vaccine ceased shedding S. Dublin from day 28 to day 63. All calves were challenged with E. coli O157:H7 on day 98 to determine the effect on fecal shedding of E. coli O157:H7. The amount of E. coli O157:H7 in feces was measured for 30 days post-challenge. We observed a transient clearance of E. coli O157:H7 from the feces in the vaccinated calves. The magnitude of fecal E. coli O157:H7 shedding did not correlate with the presence of intimin-specific fecal IgA.

Conclusion

Oral vaccination with live attenuated recombinant S. Dublin expressing intimin reduced enteric colonization and fecal shedding of E. coli O157:H7. However, the transient clearance of E. coli O157:H7 was not associated with an enhanced IgA-mediated mucosal immune response.

The Base Excision Repair system of Salmonella enterica serovar typhimurium counteracts DNA damage by host nitric oxide

Intracellular pathogens must withstand nitric oxide (NO.) generated by host phagocytes. Salmonella enterica serovar Typhimurium interferes with intracellular trafficking of inducible nitric oxide synthase (iNOS) and possesses multiple systems to detoxify NO.. Consequently, the level of NO. stress encountered by S. Typhimurium during infection in vivo has been unknown. The Base Excision Repair (BER) system recognizes and repairs damaged DNA bases including cytosine and guanine residues modified by reactive nitrogen species. Apurinic/apyrimidinic (AP) sites generated by BER glycosylases require subsequent processing by AP endonucleases. S. Typhimurium xth nfo mutants lacking AP endonuclease activity exhibit increased NO. sensitivity resulting from chromosomal fragmentation at unprocessed AP sites. BER mutant strains were thus used to probe the nature and extent of nitrosative damage sustained by intracellular bacteria during infection. Here we show that an xth nfo S. Typhimurium mutant is attenuated for virulence in C3H/HeN mice, and virulence can be completely restored by the iNOS inhibitor L-NIL. Inactivation of the ung or fpg glycosylase genes partially restores virulence to xth nfo mutant S. Typhimurium, demonstrating that NO. fluxes in vivo are sufficient to modify cytosine and guanine bases, respectively. Mutants lacking ung or fpg exhibit NO.-dependent hypermutability during infection, underscoring the importance of BER in protecting Salmonella from the genotoxic effects of host NO.. These observations demonstrate that host-derived NO. damages Salmonella DNA in vivo, and the BER system is required to maintain bacterial genomic integrity.

A nitric oxide-inducible lactate dehydrogenase enables Staphylococcus aureus to resist innate immunity

Staphylococcus aureus is one of the most successful human pathogens, colonizing 2 billion individuals worldwide and causing invasive infections even in immunocompetent hosts. S. aureus can evade multiple components of host innate immunity, including the antimicrobial radical nitric oxide (NO.) produced by activated phagocytes. We show that S. aureus is capable of metabolically adapting to nitrosative stress by expressing an NO.-inducible L-lactate dehydrogenase (ldh1, SACOL0222) divergently transcribed from the NO.-detoxifying flavohemoglobin (hmp). L-Lactate production allows S. aureus to maintain redox homeostasis during nitrosative stress and is essential for virulence. NO.-inducible lactate dehydrogenase activity and NO. resistance distinguish S. aureus from the closely related commensal species S. epidermidis and S. saprophyticus.

Regulatory and structural differences in the Cu,Zn-superoxide dismutases of Salmonella enterica and their significance for virulence

Many of the most virulent strains of Salmonella enterica produce two distinct Cu,Zn-superoxide dismutases (SodCI and SodCII). The bacteriophage-encoded SodCI enzyme makes the greater contribution to Salmonella virulence. We have performed a detailed comparison of the functional, structural, and regulatory properties of the Salmonella SodC enzymes. Here we demonstrate that SodCI and SodCII differ with regard to specific activity, protease resistance, metal affinity, and peroxidative activity, with dimeric SodCI exhibiting superior stability and activity. In particular, monomeric SodCII is unable to retain its catalytic copper ion in the absence of zinc. We have also found that SodCI and SodCII are differentially affected by oxygen, zinc availability, and the transcriptional regulator FNR. SodCII is strongly down-regulated under anaerobic conditions and dependent on the high affinity ZnuABC zinc transport system, whereas SodCI accumulation in vitro and within macrophages is FNR-dependent. We have confirmed earlier findings that SodCII accumulation in intracellular Salmonella is negligible, whereas SodCI is strongly up-regulated in macrophages. Our observations demonstrate that differences in expression, activity, and stability help to account for the unique contribution of the bacteriophage-encoded SodCI enzyme to Salmonella virulence.

The response regulator SsrB activates expression of diverse Salmonella pathogenicity island 2 promoters and counters silencing by the nucleoid-associated protein H-NS

The two-component system SsrA-SsrB activates expression of a type III secretion system required for replication in macrophages and systemic infection in mice. Here we characterize the SsrB-dependent regulation of genes within Salmonella pathogenicity island 2 (SPI-2). Primer extension and DNase I footprinting identified multiple SsrB-regulated promoters within SPI-2 located upstream of ssaB, sseA, ssaG and ssaM. We previously demonstrated that ssrA and ssrB transcription is uncoupled. Overexpression of SsrB in the absence of its cognate kinase, SsrA, is sufficient to activate SPI-2 transcription. Because SsrB requires phosphorylation to relieve inhibitory contacts that occlude its DNA-binding domain, additional components must phosphorylate SsrB. SPI-2 promoters examined in single copy were highly SsrB-dependent, activated during growth in macrophages and induced by acidic pH. The nucleoid structuring protein H-NS represses horizontally acquired genes; we confirmed that H-NS is a negative regulator of SPI-2 gene expression. In the absence of H-NS, the requirement for SsrB in activating SPI-2 genes is substantially reduced, suggesting a role for SsrB in countering H-NS silencing. SsrB activates transcription of multiple operons within SPI-2 by binding to degenerate DNA targets at diversely organized promoters. SsrB appears to possess dual activities to promote SPI-2 gene expression: activation of transcription and relief of H-NS-mediated repression.

Silencing of xenogeneic DNA by H-NS--facilitation of lateral gene transfer in bacteria by a defense system that recognizes foreign DNA

Lateral gene transfer has played a prominent role in bacterial evolution, but the mechanisms allowing bacteria to tolerate the acquisition of foreign DNA have been incompletely defined. Recent studies show that H-NS, an abundant nucleoid-associated protein in enteric bacteria and related species, can recognize and selectively silence the expression of foreign DNA with higher adenine and thymine content relative to the resident genome, a property that has made this molecule an almost universal regulator of virulence determinants in enteric bacteria. These and other recent findings challenge the ideas that curvature is the primary determinant recognized by H-NS and that activation of H-NS-silenced genes in response to environmental conditions occurs through a change in the structure of H-NS itself. Derepression of H-NS-silenced genes can occur at specific promoters by several mechanisms including competition with sequence-specific DNA-binding proteins, thereby enabling the regulated expression of foreign genes. The possibility that microorganisms maintain and exploit their characteristic genomic GC ratios for the purpose of self/non-self-discrimination is discussed.

FNR is a global regulator of virulence and anaerobic metabolism in Salmonella enterica serovar Typhimurium (ATCC 14028s)

Salmonella enterica serovar Typhimurium must successfully transition the broad fluctuations in oxygen concentrations encountered in the host. In Escherichia coli, FNR is one of the main regulatory proteins involved in O2 sensing. To assess the role of FNR in serovar Typhimurium, we constructed an isogenic fnr mutant in the virulent wild-type strain (ATCC 14028s) and compared their transcriptional profiles and pathogenicities in mice. Here, we report that, under anaerobic conditions, 311 genes (6.80% of the genome) are regulated directly or indirectly by FNR; of these, 87 genes (28%) are poorly characterized. Regulation by FNR in serovar Typhimurium is similar to, but distinct from, that in E. coli. Thus, genes/operons involved in aerobic metabolism, NO. detoxification, flagellar biosynthesis, motility, chemotaxis, and anaerobic carbon utilization are regulated by FNR in a fashion similar to that in E. coli. However, genes/operons existing in E. coli but regulated by FNR only in serovar Typhimurium include those coding for ethanolamine utilization, a universal stress protein, a ferritin-like protein, and a phosphotransacetylase. Interestingly, Salmonella-specific genes/operons regulated by FNR include numerous virulence genes within Salmonella pathogenicity island 1 (SPI-1), newly identified flagellar genes (mcpAC, cheV), and the virulence operon (srfABC). Furthermore, the role of FNR as a positive regulator of motility, flagellar biosynthesis, and pathogenesis was confirmed by showing that the mutant is nonmotile, lacks flagella, is attenuated in mice, and does not survive inside macrophages. The inability of the mutant to survive inside macrophages is likely due to its sensitivity to the reactive oxygen species generated by NADPH phagocyte oxidase.

Selective silencing of foreign DNA with low GC content by the H-NS protein in Salmonella

Horizontal gene transfer plays a major role in microbial evolution. However, newly acquired sequences can decrease fitness unless integrated into preexisting regulatory networks. We found that the histone-like nucleoid structuring protein (H-NS) selectively silences horizontally acquired genes by targeting sequences with GC content lower than the resident genome. Mutations in hns are lethal in Salmonella unless accompanied by compensatory mutations in other regulatory loci. Thus, H-NS provides a previously unrecognized mechanism of bacterial defense against foreign DNA, enabling the acquisition of DNA from exogenous sources while avoiding detrimental consequences from unregulated expression of newly acquired genes. Characteristic GC/AT ratios of bacterial genomes may facilitate discrimination between a cell's own DNA and foreign DNA.

Co-regulation of Salmonella enterica genes required for virulence and resistance to antimicrobial peptides by SlyA and PhoP/PhoQ

Analysis of the transcriptome of slyA mutant Salmonella enterica serovar Typhimurium revealed that many SlyA-dependent genes, including pagC, pagD, ugtL, mig-14, virK, phoN, pgtE, pipB2, sopD2, pagJ and pagK, are also controlled by the PhoP/PhoQ regulatory system. Many SlyA- and PhoP/PhoQ-co-regulated genes have functions associated with the bacterial envelope, and some have been directly implicated in virulence and resistance to antimicrobial peptides. Purified His-tagged SlyA binds to the pagC and mig-14 promoters in regions homologous to a previously proposed 'SlyA-box'. The pagC promoter lacks a consensus PhoP binding site and does not bind PhoP in vitro, suggesting that the effect of PhoP on pagC transcription is indirect. Stimulation of pagC expression by PhoP requires SlyA. Levels of SlyA protein and mRNA are not significantly changed under low-magnesium PhoP-inducing conditions in which pagC expression is profoundly elevated, however, indicating that the PhoP/PhoQ system does not activate pagC expression by altering SlyA protein concentration. Models are proposed in which PhoP may control SlyA activity via a soluble ligand or SlyA may function as an anti-repressor to allow PhoP activation. The absence of almost all SlyA-activated genes from the Escherichia coli K12 genome suggests that the functional linkage between the SlyA and PhoP/PhoQ regulatory systems arose as Salmonella evolved its distinctive pathogenic lifestyle.

Liver abscess caused by magA+ Klebsiella pneumoniae in North America

Taiwan has witnessed an emerging syndrome of liver abscess caused by Klebsiella pneumoniae carrying the magA gene required for exopolysaccharide web biosynthesis. We report a patient transferred from Alaska to Washington State with a magA(+) K. pneumoniae liver abscess and describe a simple approach for recognition of these hypervirulent strains.

Isocitrate lyase (AceA) is required for Salmonella persistence but not for acute lethal infection in mice

Isocitrate lyase is required for fatty acid utilization via the glyoxylate shunt. Although isocitrate lyase is essential for Salmonella persistence during chronic infection, it is dispensable for acute lethal infection in mice. Substrate availability in the phagosome appears to evolve over time, with increasing fatty acid dependence during chronic infection.

The Salmonella enterica serovar typhimurium divalent cation transport systems MntH and SitABCD are essential for virulence in an Nramp1G169 murine typhoid model

Nramp1 is a transporter that pumps divalent cations from the vacuoles of phagocytic cells and is associated with the innate resistance of mice to diverse intracellular pathogens. We demonstrate that sitA and mntH, genes encoding high-affinity metal ion uptake systems in Salmonella enterica serovar Typhimurium, are upregulated when Salmonella is internalized by Nramp1-expressing macrophages and play an essential role in systemic infection of congenic Nramp1-expressing mice.

The ferritin-like Dps protein is required for Salmonella enterica serovar Typhimurium oxidative stress resistance and virulence

Resistance to phagocyte-derived reactive oxygen species is essential for Salmonella enterica serovar Typhimurium pathogenesis. Salmonella can enhance its resistance to oxidants through the induction of specific genetic pathways controlled by SoxRS, OxyR, sigma(S), sigma(E), SlyA, and RecA. These regulons can be found in a wide variety of pathogenic and environmental bacteria, suggesting that evolutionarily conserved mechanisms defend against oxidative stress both endogenously generated by aerobic respiration and exogenously produced by host phagocytic cells. Dps, a ferritin-like protein found in many eubacterial and archaebacterial species, appears to protect cells from oxidative stress by sequestering iron and limiting Fenton-catalyzed oxyradical formation. In Escherichia coli and some other bacterial species, Dps has been shown to accumulate during stationary phase in a sigma(S)-dependent fashion, bind nonspecifically to DNA, and form a crystalline structure that compacts and protects chromatin from oxidative damage. In the present study, we provide evidence that Dps protects Salmonella from iron-dependent killing by hydrogen peroxide, promotes Salmonella survival in murine macrophages, and enhances Salmonella virulence. Reduced numbers of dps mutant bacteria in the livers and spleens of infected mice are consistent with a role of Dps in protecting Salmonella from oxidative stress encountered during infection.

Inhibition of bacterial DNA replication by zinc mobilization during nitrosative stress

Phagocytic cells inhibit the growth of intracellular pathogens by producing nitric oxide (NO). NO causes cell filamentation, induction of the SOS response, and DNA replication arrest in the Gram-negative bacterium Salmonella enterica. NO also induces double-stranded chromosomal breaks in replication-arrested Salmonella lacking a functional RecBCD exonuclease. This DNA damage depends on actions of additional DNA repair proteins, the RecG helicase, and RuvC endonuclease. Introduction of a recG mutation restores both resistance to NO and the ability of an attenuated recBC mutant Salmonella strain to cause lethal infection in mice, demonstrating that bacterial DNA replication is inhibited during host-pathogen interactions. Inhibition of DNA replication during nitrosative stress is invariably accompanied by zinc mobilization, implicating DNA-binding zinc metalloproteins as critical targets of NO-related antimicrobial activity.

Characterization of the spv locus in Salmonella enterica serovar Arizona

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

The alternative sigma factor sigmaE controls antioxidant defences required for Salmonella virulence and stationary-phase survival

Bacteria must contend with conditions of nutrient limitation in all natural environments. Complex programmes of gene expression, controlled in part by the alternative sigma factors sigmaS (sigma38, RpoS) and sigmaH (sigma32, RpoH), allow a number of bacterial species to survive conditions of partial or complete starvation. We show here that the alternative sigma factor sigmaE (sigma24, RpoE) also facilitates the survival of Salmonella typhimurium under conditions of nutrient deprivation. Expression of the sigmaE regulon is strongly induced upon entry of Salmonella into stationary phase. A Salmonella mutant lacking sigmaE has reduced survival during stationary phase as well as increased susceptibility to oxidative stress. A Salmonella strain lacking both sigmaE and sigmaS is non-viable after just 24 h in stationary phase, but survival of these mutants is completely preserved under anaerobic stationary-phase conditions, suggesting that oxidative injury is one of the major mechanisms of reduced microbial viability during periods of nutrient deprivation. Moreover, the attenuated virulence of sigmaE-deficient Salmonella for mice can be largely restored by genetic abrogation of the host phagocyte respiratory burst, suggesting that the sigmaE regulon plays an important antioxidant role during Salmonella infection of mammalian hosts.

In vitro and in vivo assessment of Salmonella enterica serovar Typhimurium DT104 virulence

Multidrug-resistant Salmonella enterica serovar Typhimurium phage type DT104 has become a widespread cause of human and other animal infection worldwide. The severity of clinical illness in S. enterica serovar Typhimurium DT104 outbreaks has led to the suggestion that this strain possesses enhanced virulence. In the present study, in vitro and in vivo virulence-associated phenotypes of several clinical isolates of S. enterica serovar Typhimurium DT104 were examined and compared to S. enterica serovar Typhimurium ATCC 14028s. The ability of these DT104 isolates to survive within murine peritoneal macrophages, invade cultured epithelial cells, resist antimicrobial actions of reactive oxygen and nitrogen compounds, and cause lethal infection in mice were assessed. Our results failed to demonstrate that S. enterica serovar Typhimurium DT104 isolates are more virulent than S. enterica serovar Typhimurium ATCC 14028s.

Fis, a DNA nucleoid-associated protein, is involved in Salmonella typhimurium SPI-1 invasion gene expression

The ability of Salmonella enterica serovar Typhimurium to cause disease depends upon the co-ordinated expression of many genes located around the Salmonella chromosome. Specific pathogenicity loci, termed Salmonella pathogenicity islands, have been shown to be crucial for the invasion and survival of Salmonella within host cells. Salmonella pathogenicity island 1 (SPI-1) harbours the genes required for the stimulation of Salmonella uptake across the intestinal epithelia of the infected host. Regulation of SPI-1 genes is complex, as invasion gene expression responds to a number of different signals, presumably signals similar to those found within the environment of the intestinal tract. As a result of our continued studies of SPI-1 gene regulation, we have discovered that the nucleoid-binding protein Fis plays a pivotal role in the expression of HilA and InvF, two activators of SPI-1 genes. A S. typhimurium fis mutant demonstrates a two- to threefold reduction in hilA:Tn5lacZY and a 10-fold reduction in invF:Tn5lacZY expression, as well as a 50-fold decreased ability to invade HEp-2 tissue culture cells. This decreased expression of hilA and invF resulted in an altered secreted invasion protein profile in the fis mutant. Furthermore, the virulence of a S. typhimurium fis mutant is attenuated 100-fold when administered orally, but has wild-type virulence when administered intraperitoneally. Expression of hilA:Tn5lacZY and invF:Tn5lacZY in the fis mutant could be restored by introducing a plasmid containing the S. typhimurium fis gene or a plasmid containing hilD, a gene encoding an AraC-like regulator of Salmonella invasion genes.

The Salmonella virulence plasmid spv genes are required for cytopathology in human monocyte-derived macrophages

The pathogenesis of serious systemic Salmonella infections is characterized by survival and proliferation of bacteria inside macrophages. Infection of human monocyte-derived macrophages in vitro with S. typhimurium or S. dublin produces cytopathology characterized by detachment of cells that contain large numbers of proliferating bacteria. This cytopathology is dependent on the expression of the bacterial spv genes, a virulence locus previously shown to markedly enhance the ability of Salmonella to produce systemic disease. After 24 h of infection, macrophage cultures contain two populations of bacteria: (i) proliferating organisms present in a detached cell fraction; and (ii) a static bacterial population in macrophages remaining attached to the culture well. Mutations in either the essential transcriptional activator SpvR or the key SpvB protein markedly reduce the cytopathic effect of Salmonella infection. The spv-dependent cytopathology in macrophages exhibits characteristics of apoptosis, with release of nucleosomes into the cytoplasm, nuclear condensation and DNA fragmentation. The current findings suggest that the mechanism of the spv effect is through induction of increased cytopathology in host macrophages.

Differential regulation of enteric and systemic salmonellosis by slyA

Mutation of slyA, which reduces Salmonella typhimurium virulence in mice, caused only minor attenuation of S. typhimurium virulence in orally inoculated calves. This correlated with modest reductions in intestinal invasion and enteropathogenic responses in bovine ligated ileal loops. slyA appears to regulate virulence genes involved in systemic, but not enteric, salmonellosis.

Regulation of the Escherichia coli sheA gene and characterization of its encoded hemolytic activity

Escherichia coli K-12 carries the cryptic hemolysin gene sheA which is under the control of positive and negative transcriptional regulators. The objectives of the present study were to further analyze the regulation of the sheA gene in E. coli, to compare the sheA genes from E. coli K-12 and a pathogenic E. coli strain, and to characterize the SheA hemolytic activity. Northern blot analysis demonstrated that the transcriptional regulator SlyA activates the E. coli K-12 sheA gene. The main transcriptional start site of the sheA gene was 56 nucleotides upstream from the start codon as determined by primer extension analysis. The sheA genes from E. coli K-12 and a pathogenic E. coli strain were identical. SheA hemolytic activity was cell associated and Ca2+ independent.

Periplasmic superoxide dismutase protects Salmonella from products of phagocyte NADPH-oxidase and nitric oxide synthase

Superoxide dismutase (SOD) catalyzes the conversion of superoxide radical to hydrogen peroxide. Periplasmic localization of bacterial Cu,Zn-SOD has suggested a role of this enzyme in defense against extracellular phagocyte-derived reactive oxygen species. Sequence analysis of regions flanking the Salmonella typhimurium sodC gene encoding Cu,Zn-SOD demonstrates significant homology to lambda phage proteins, reflecting possible bacteriophage-mediated horizontal gene transfer of this determinant among pathogenic bacteria. Salmonella deficient in Cu,Zn-SOD has reduced survival in macrophages and attenuated virulence in mice, which can be restored by abrogation of either the phagocyte respiratory burst or inducible nitric oxide synthase. Moreover, a sodC mutant is extremely susceptible to the combination of superoxide and nitric oxide. These observations suggest that SOD protects periplasmic or inner membrane targets by diverting superoxide and limiting peroxynitrite formation, and they demonstrate the ability of the respiratory burst and nitric oxide synthase to synergistically kill microbial pathogens in vivo.

SlyA, a transcriptional regulator of Salmonella typhimurium, is required for resistance to oxidative stress and is expressed in the intracellular environment of macrophages

Appropriate regulation of genes enables Salmonella typhimurium to adapt to the intracellular environment of the host. The Salmonella slyA gene is in a family of transcriptional regulators that may play an important role in this adaptation. We have previously shown that slyA mutant Salmonella strains are profoundly attenuated for virulence and do not survive in macrophages. In this study, we demonstrate that the expression of multiple Salmonella proteins is regulated by SlyA during stationary phase and during infection of macrophages. Both of these conditions also induced the expression of a slyA::lacZ transcriptional fusion. Expression of the slyA::lacZ transcriptional fusion increased 15-fold in stationary phase and was not dependent on the stationary-phase sigma factor, RpoS. slyA mutant Salmonella strains were sensitive to oxidative products of the respiratory burst, including hydrogen peroxide and the products of the redox cycling compound paraquat, but not to nitric oxide donors. These results suggest that the SlyA regulon is activated during infection of the host and is required for resistance to toxic oxidative products of the reticuloendothelial system.

The spv genes on the Salmonella dublin virulence plasmid are required for severe enteritis and systemic infection in the natural host

The pathogenic role of the spv (Salmonella plasmid virulence) genes of Salmonella dublin was determined in the natural, bovine host. Since the lack of overt signs of enteritis or enterocolitis due to Salmonella infections in mice has limited the development of a convenient experimental system to study enteric disease, we used calves to study the contribution of the spv genes to S. dublin-induced salmonellosis. Since the SpvR transcriptional regulator is required for expression of the spvABCD operon, we constructed an spvR knockout mutation in a calf-virulent strain of S. dublin. Calves were infected with the wild-type strain, an spvR mutant, and an spvR mutant containing a complementing plasmid. Calves that were infected with the wild type or the complemented spvR mutant rapidly developed severe diarrhea and became moribund. Calves that were infected with the spvR mutant showed little or no clinical signs of systemic salmonellosis and developed only mild diarrhea. The survival and growth of the wild-type strain and the spvR mutant were determined by using blood-derived bovine monocytes. Wild-type S. dublin survived and grew inside cells, while the spvR mutant did not proliferate. These results suggest that the spv genes of S. dublin promote enhanced intracellular proliferation in intestinal tissues and at extraintestinal sites in the natural host.

Dynamics of growth and death within a Salmonella typhimurium population during infection of macrophages

Survival of Salmonella typhimurium within macrophages is associated with virulence. Most data on the fate of Salmonella during infection of macrophages are derived from viable counts of intracellular bacteria. These counts are a result of a combination of bacterial death and growth within the intracellular population but may not reflect the true levels of either macrophage killing of Salmonella or bacterial growth inside cells. In this study, two independent methods have been used to obtain a more accurate measurement of absolute levels of both death and growth of Salmonella inside macrophages. A purine auxotroph (purD) was used to measure Salmonella death in the absence of bacterial growth and then bacterial growth was measured by supplementing the purD cultures with adenosine. Numbers of dead and live Salmonella were also quantitated using the BacLight staining system, which distinguishes dead from live bacteria. Both methods demonstrate that killing of Salmonella by macrophages is considerably greater than detected using traditional cell counts and that bacterial inactivation occurs throughout the infection period. Salmonella was inactivated at a similar rate in both J774 macrophages (most permissive macrophages) and peritoneal exuadate macrophages (least permissive macrophages), suggesting that the major difference between these cells is the ability to limit bacterial growth. These studies also demonstrate that growth of Salmonella within murine macrophages occurs simultaneously with significant amounts of bacterial death. Identifying the factors responsible for shifting the interaction between macrophages and bacteria toward conditions that favor bacterial growth will be critical to understanding Salmonella virulence.

Expression of Salmonella typhimurium rpoS and rpoS-dependent genes in the intracellular environment of eukaryotic cells

Adaptation to the intracellular environment of host cells is crucial for the pathogenesis of Salmonella infections. The alternative sigma factor RpoS is a global regulator of gene expression during starvation and stress conditions and is required for virulence in Salmonella spp. We have used lacZ reporter fusions to rpoS and rpoS-dependent genes to study rpoS regulation after entry of Salmonella typhimurium into macrophages and epithelial cells. The results demonstrate that expression of an rpoS::lacZ translational fusion increases rapidly in S. typhimurium after phagocytosis. Activity of RpoS also increases after bacterial entry into both macrophages and epithelial cells, as demonstrated by the induction of the rpoS-regulated genes katE and spvB. A control rpoS-independent promoter for neomycin resistance does not show significant induction after cell entry. These results demonstrate that the regulatory system mediated by RpoS in S. typhimurium is activated by the intracellular environment of eukaryotic cells.

DNA repair is more important than catalase for Salmonella virulence in mice

Pathogenic microorganisms possess antioxidant defense mechanisms for protection from reactive oxygen metabolites such as hydrogen peroxide (H2O2), which are generated during the respiratory burst of phagocytic cells. These defense mechanisms include enzymes such as catalase, which detoxify reactive oxygen species, and DNA repair systems which repair damage resulting from oxidative stress. To determine the relative importance of these two potentially protective defense mechanisms against oxidative stress encountered by Salmonella during infection of the host, a Salmonella typhimurium double mutant unable to produce either the HPI or HPII catalase was constructed, and compared with an isogenic recA mutant deficient in DNA repair. The recA mutant was hypersusceptible to H2O2 at low cell densities in vitro, while the catalase mutant was more susceptible to high H2O2 concentrations at high cell densities. The catalase mutant was found to be resistant to macrophages and retained full murine virulence, in contrast to the recA mutant which previously was shown to be macrophage-sensitive and attenuated in mice. These observations suggest that Salmonella is subjected to low concentrations of H2O2 while at relatively low cell density during infection, conditions requiring an intact DNA repair system but not functional catalase activity.

Cloning and sequencing of the gene encoding the RpoS (KatF) sigma factor from Salmonella typhimurium 14028s

The gene encoding the alternative sigma factor RpoS in Salmonella typhimurium was cloned by its ability to complement acid susceptibility in rpoS mutant Escherichia coli. Sequence determination and comparison with rpoS from E. coli demonstrates a high degree of conservation, although significant differences are found within the extragenic regulatory regions.

RpoS is necessary for both the positive and negative regulation of starvation survival genes during phosphate, carbon, and nitrogen starvation in Salmonella typhimurium

The starvation stress response of Salmonella typhimurium encompasses the genetic and physiologic changes that occur when this bacterium is starved for an essential nutrient such as phosphate (P), carbon (C), or nitrogen (N). The responses to the limitation of each of these nutrients involve both unique and overlapping sets of proteins important for starvation survival and virulence. The role of the alternative sigma factor RpoS in the regulation of the starvation survival loci, stiA, stiB, and stiC, has been characterized. RpoS (sigma S) was found to be required for the P, C, and N starvation induction of stiA and stiC. In contrast, RpoS was found to be required for the negative regulation of stiB during P and C starvation-induced stationary phase but not during logarithmic phase. This role was independent of the relA gene (previously found to be needed for stiB induction). The role of RpoS alone and in combination with one or more sti mutations in the starvation survival of the organism was also investigated. The results clearly demonstrate that RpoS is an integral component of the complex interconnected regulatory systems involved in S. typhimurium's response to nutrient deprivation. However, differential responses of various sti genes indicate that additional signals and regulatory proteins are also involved.

A cytolysin encoded by Salmonella is required for survival within macrophages

A Salmonella gene encoding a cytolysin has been identified by screening for hemolysis on blood agar. DNA sequence analyses together with genetic mapping in Salmonella suggest that it is unrelated to other toxins or hemolysins. The gene (slyA) is present in every strain of Salmonella examined, in Shigella, and in enteroinvasive Escherichia coli but not in other Enterobacteriaceae. SlyA (salmolysin) purified from a derivative of the original clone has hemolytic and cytolytic activity and has a molecular weight predicted by the DNA sequence. The median lethal dose and infection kinetics in mice suggest that the toxin is required for virulence and facilitates Salmonella survival within mouse peritoneal macrophages.

The alternative sigma factor katF (rpoS) regulates Salmonella virulence

Nutrient limitation is a critical signal in Salmonella virulence gene regulation. The katF (rpoS) gene mediates the expression of the Salmonella spv plasmid virulence genes during bacterial starvation. A katF Salmonella mutant has increased susceptibility to nutrient deprivation, oxidative stress, acid stress, and DNA damage, conditions which are relevant to the intraphagosomal environment of host macrophages. Moreover, the katF mutant has significantly reduced virulence in mice. katF encodes an alternative sigma factor of RNA polymerase which coordinately regulates Salmonella virulence.

Cloning and expression of a Serpula (Treponema) hyodysenteriae hemolysin gene

Serpula (Treponema) hyodysenteriae, the etiologic agent of swine dysentery, produces a hemolysin which is thought to be an important factor in the pathogenesis of the disease. We report the cloning, sequencing, and expression of a hemolysin gene (tly) from S. hyodysenteriae B204. A pUC19 gene bank of strain B204 was constructed in the Escherichia coli K-12 strain DH5 alpha, and hemolytic recombinants were identified by plating the library on blood agar plates. From the hemolytic recombinants, a 1.5-kb DNA fragment could be isolated that contained information necessary for the production of a hemolysin/cytotoxin in E. coli. Nucleotide sequence determination of this 1.5-kb fragment showed that it contained an open reading frame capable of encoding a 26.9-kDa protein. The recombinant hemolysin was easily released from E. coli by osmotic shock. As with the native hemolysin, the recombinant hemolysin is EDTA insensitive, thermolabile, and cytotoxic for several eukaryotic cell lines. Southern blot hybridization showed that the cloned S. hyodysenteriae hemolysin gene tly is present in all pathogenic strains of S. hyodysenteriae tested and absent in the nonpathogenic, weakly hemolytic spirochete S. innocens.

Transformation of Corynebacterium pseudotuberculosis by electroporation

Corynebacterium pseudotuberculosis was transformed by electroporation, using pNG2, an erythromycin-resistance plasmid from C diphtheriae. Corynebacterium pseudotuberculosis cultivated in brain-heart infusion broth was washed 3 times with water, and resuspended to a final concentration of about 5 x 10(13) colony-forming units/ml. An electroporator constructed in our laboratory incorporated an electrode with 0.8-mm interelectrode gap, using disposable spectrophotometer cuvettes as containers for electroporation. The pNG2 was prepared in Escherichia coli and 4 to 16 micrograms of pNG2 DNA was mixed with 400-microliters amounts of cell suspension in prechilled cuvettes. After incubation on ice for 5 to 10 minutes, the mixture was electroporated at field strengths of up to 18 kV/cm, mixed with 1.5 ml of brain-heart infusion broth, and incubated at 37 C for 2 hours with agitation. Aliquots were then plated on brain-heart infusion blood agar with 15 micrograms of erythromycin/ml. Corynebacterium pseudotuberculosis was transformed at a maximal efficiency of approximately 4 x 10(4) transformants/micrograms of pNG2 DNA. Most total transformants and most transformants per microgram of pNG2 were generated at a field strength of 18 kV/cm. When the concentration of pNG2 DNA was varied, the average total number of transformants increased through a concentration of 30 micrograms/ml, but the efficiency of transformation was highest at the lowest DNA concentration. Transformants contained unmodified pNG2.