Identification and characterization of OmpT-like proteases in uropathogenic Escherichia coli clinical isolates.

Bacterial colonization of the urogenital tract is limited by innate defenses, including the production of antimicrobial peptides (AMPs). Uropathogenic Escherichia coli (UPEC) resist AMP-killing to cause a range of urinary tract infections (UTIs) including asymptomatic bacteriuria, cystitis, pyelonephritis, and sepsis. UPEC strains have high genomic diversity and encode numerous virulence factors that differentiate them from non-UTI-causing strains, including ompT. As OmpT homologs cleave and inactivate AMPs, we hypothesized that UPEC strains from patients with symptomatic UTIs have high OmpT protease activity. Therefore, we measured OmpT activity in 58 clinical E. coli isolates. While heterogeneous OmpT activities were observed, OmpT activity was significantly greater in UPEC strains isolated from patients with symptomatic infections. Unexpectedly, UPEC strains exhibiting the greatest protease activities harbored an additional ompT-like gene called arlC (ompTp). The presence of two OmpT-like proteases in some UPEC isolates led us to compare the substrate specificities of OmpT-like proteases found in E. coli. While all three cleaved AMPs, cleavage efficiency varied on the basis of AMP size and secondary structure. Our findings suggest the presence of ArlC and OmpT in the same UPEC isolate may confer a fitness advantage by expanding the range of target substrates.

PmrC (EptA) and CptA Negatively Affect Outer Membrane Vesicle Production in Citrobacter rodentium.

Outer membrane vesicles (OMVs) are naturally produced by Gram-negative bacteria by a bulging of the outer membrane (OM) and subsequent release into the environment. By serving as vehicles for various cargos, including proteins, nucleic acids and small metabolites, OMVs are central to interbacterial interactions and both symbiotic and pathogenic host bacterial interactions. However, despite their importance, the mechanism of OMV formation remains unclear. Recent evidence indicates that covalent modifications of lipopolysaccharides (LPS) influence OMV biogenesis. Several enteric bacteria modify LPS with phosphoethanolamine (pEtN) using the iron-regulated PmrC (EptA) and CptA pEtN transferases. In wild-type Citrobacter rodentium, the presence of increasing subtoxic concentrations of iron was found to stimulate OMV production 4- to 9-fold above baseline. C. rodentium uses the two-component system PmrAB to sense and adapt to environmental iron. Compared to the wild type, the C. rodentium ΔpmrAB strain exhibited heightened OMV production at similar iron concentrations. PmrAB regulates transcription of pmrC (also known as eptA) and cptA OMV production in strains lacking either pmrC (eptA) or cptA was similarly increased in comparison to that of the wild type. Importantly, plasmid complementation of C. rodentium strains with either pmrC (eptA) or cptA resulted in a drastic inhibition of OMV production. Finally, we showed that ß-lactamase and CroP, two enzymes found in the C. rodentium periplasm and outer membrane (OM), respectively, are associated with OMVs. These data suggest a novel mechanism by which C. rodentium and possibly other Gram-negative bacteria can negatively affect OMV production through the PmrAB-regulated genes pmrC (eptA) and cptAIMPORTANCE Although OMVs secreted by Gram-negative bacteria fulfill multiple functions, the molecular mechanism of OMV biogenesis remains ill defined. Our group has previously shown that PmrC (also known as EptA) and CptA maintain OM integrity and provide resistance to iron toxicity and antibiotics in the murine pathogen Citrobacter rodentium In several enteric bacteria, these proteins modify the lipid A and core regions of lipopolysaccharide with phosphoethanolamine moieties. Here, we show that these proteins also repress OMV production in response to environmental iron in C. rodentium These data support the emerging understanding that lipopolysaccharide modifications are important regulators of OMV biogenesis in Gram-negative bacteria.

The Virulence Effect of CpxRA in Citrobacter rodentium Is Independent of the Auxiliary Proteins NlpE and CpxP.

Citrobacter rodentium is a murine pathogen used to model the intestinal infection caused by Enteropathogenic and Enterohemorrhagic Escherichia coli (EPEC and EHEC), two diarrheal pathogens responsible for morbidity and mortality in developing and developed countries, respectively. During infection, these bacteria must sense and adapt to the gut environment of the host. In order to adapt to changing environmental cues and modulate expression of specific genes, bacteria can use two-component signal transduction systems (TCS). We have shown that the deletion of the Cpx TCS in C. rodentium leads to a marked attenuation in virulence in C3H/HeJ mice. In E. coli, the Cpx TCS is reportedly activated in response to signals from the outer-membrane lipoprotein NlpE. We therefore investigated the role of NlpE in C. rodentium virulence. We also assessed the role of the reported negative regulator of CpxRA, CpxP. We found that as opposed to the ΔcpxRA strain, neither the ΔnlpE, ΔcpxP nor the ΔnlpEΔcpxP strains were significantly attenuated, and had similar in vivo localization to wild-type C. rodentium. The in vitro adherence of the Cpx auxiliary protein mutants, ΔnlpE, ΔcpxP, ΔnlpEΔcpxP, was comparable to wild-type C. rodentium, whereas the ΔcpxRA strain showed significantly decreased adherence. To further elucidate the mechanisms behind the contrasting virulence phenotypes, we performed microarrays in order to define the regulon of the Cpx TCS. We detected 393 genes differentially regulated in the ΔcpxRA strain. The gene expression profile of the ΔnlpE strain is strikingly different than the profile of ΔcpxRA with regards to the genes activated by CpxRA. Further, there is no clear inverse correlation in the expression pattern of the ΔcpxP strain in comparison to ΔcpxRA. Taken together, these data suggest that in these conditions, CpxRA activates gene expression in a largely NlpE- and CpxP-independent manner. Compared to wildtype, 161 genes were downregulated in the ΔcpxRA strain, while being upregulated or unchanged in the Cpx auxiliary protein deletion strains. This group of genes, which we hypothesize may contribute to the loss of virulence of ΔcpxRA, includes T6SS components, ompF, the regulator for colanic acid synthesis, and several genes involved in maltose metabolism.

Sex-hormone-driven innate antibodies protect females and infants against EPEC infection.

Females have an overall advantage over males in resisting Gram-negative bacteremias, thus hinting at sexual dimorphism of immunity during infections. Here, through intravital microscopy, we observed a sex-biased difference in the capture of blood-borne bacteria by liver macrophages, a process that is critical for the clearance of systemic infections. Complement opsonization was indispensable for the capture of enteropathogenic Escherichia coli (EPEC) in male mice; however, a faster complement component 3-independent process involving abundant preexisting antibodies to EPEC was detected in female mice. These antibodies were elicited predominantly in female mice at puberty in response to estrogen regardless of microbiota-colonization conditions. Estrogen-driven antibodies were maternally transferrable to offspring and conferred protection during infancy. These antibodies were conserved in humans and recognized specialized oligosaccharides integrated into the bacterial lipopolysaccharide and capsule. Thus, an estrogen-driven, innate antibody-mediated immunological strategy conferred protection to females and their offspring.

Systematic Analysis of Two-Component Systems in Citrobacter rodentium Reveals Positive and Negative Roles in Virulence.

Citrobacter rodentium is a murine pathogen used to model intestinal infections caused by the human diarrheal pathogens enterohemorrhagic and enteropathogenic Escherichia coli During infection, bacteria use two-component systems (TCSs) to detect changing environmental cues within the host, allowing for rapid adaptation by altering the expression of specific genes. In this study, 26 TCSs were identified in C. rodentium, and quantitative PCR (qPCR) analysis showed that they are all expressed during murine infection. These TCSs were individually deleted, and the in vitro and in vivo effects were analyzed to determine the functional consequences. In vitro analyses only revealed minor differences, and surprisingly, type III secretion (T3S) was only affected in the ΔarcA strain. Murine infections identified 7 mutants with either attenuated or increased virulence. In agreement with the in vitro T3S assay, the ΔarcA strain was attenuated and defective in colonization and cell adherence. The ΔrcsB strain was among the most highly attenuated strains. The decrease in virulence of this strain may be associated with changes to the cell surface, as Congo red binding was altered, and qPCR revealed that expression of the wcaA gene, which has been implicated in colanic acid production in other bacteria, was drastically downregulated. The ΔuvrY strain exhibited increased virulence compared to the wild type, which was associated with a significant increase in bacterial burden within the mesenteric lymph nodes. The systematic analysis of virulence-associated TCSs and investigation of their functions during infection may open new avenues for drug development.

Antimicrobial Peptide Conformation as a Structural Determinant of Omptin Protease Specificity.

UNLABELLED: Bacterial proteases contribute to virulence by cleaving host or bacterial proteins to promote survival and dissemination. Omptins are a family of proteases embedded in the outer membrane of Gram-negative bacteria that cleave various substrates, including host antimicrobial peptides, with a preference for cleaving at dibasic motifs. OmpT, the enterohemorrhagic Escherichia coli (EHEC) omptin, cleaves and inactivates the human cathelicidin LL-37. Similarly, the omptin CroP, found in the murine pathogen Citrobacter rodentium, which is used as a surrogate model to study human-restricted EHEC, cleaves the murine cathelicidin-related antimicrobial peptide (CRAMP). Here, we compared the abilities of OmpT and CroP to cleave LL-37 and CRAMP. EHEC OmpT degraded LL-37 and CRAMP at similar rates. In contrast, C. rodentium CroP cleaved CRAMP more rapidly than LL-37. The different cleavage rates of LL-37 and CRAMP were independent of the bacterial background and substrate sequence specificity, as OmpT and CroP have the same preference for cleaving at dibasic sites. Importantly, LL-37 was α-helical and CRAMP was unstructured under our experimental conditions. By altering the α-helicity of LL-37 and CRAMP, we found that decreasing LL-37 α-helicity increased its rate of cleavage by CroP. Conversely, increasing CRAMP α-helicity decreased its cleavage rate. This structural basis for CroP substrate specificity highlights differences between the closely related omptins of C. rodentium and E. coli. In agreement with previous studies, this difference in CroP and OmpT substrate specificity suggests that omptins evolved in response to the substrates present in their host microenvironments. IMPORTANCE: Omptins are recognized as key virulence factors for various Gram-negative pathogens. Their localization to the outer membrane, their active site facing the extracellular environment, and their unique catalytic mechanism make them attractive targets for novel therapeutic strategies. Gaining insights into similarities and variations between the different omptin active sites and subsequent substrate specificities will be critical to develop inhibitors that can target multiple omptins. Here, we describe subtle differences between the substrate specificities of two closely related omptins, CroP and OmpT. This is the first reported example of substrate conformation acting as a structural determinant for omptin activity between OmpT-like proteases.

Inhibition of outer membrane proteases of the omptin family by aprotinin.

Bacterial proteases are important virulence factors that inactivate host defense proteins and contribute to tissue destruction and bacterial dissemination. Outer membrane proteases of the omptin family, exemplified by Escherichia coli OmpT, are found in some Gram-negative bacteria. Omptins cleave a variety of substrates at the host-pathogen interface, including plasminogen and antimicrobial peptides. Multiple omptin substrates relevant to infection have been identified; nonetheless, an effective omptin inhibitor remains to be found. Here, we purified native CroP, the OmpT ortholog in the murine pathogen Citrobacter rodentium. Purified CroP was found to readily cleave both a synthetic fluorescence resonance energy transfer substrate and the murine cathelicidin-related antimicrobial peptide. In contrast, CroP was found to poorly activate plasminogen into active plasmin. Although classical protease inhibitors were ineffective against CroP activity, we found that the serine protease inhibitor aprotinin displays inhibitory potency in the micromolar range. Aprotinin was shown to act as a competitive inhibitor of CroP activity and to interfere with the cleavage of the murine cathelicidin-related antimicrobial peptide. Importantly, aprotinin was able to inhibit not only CroP but also Yersinia pestis Pla and, to a lesser extent, E. coli OmpT. We propose a structural model of the aprotinin-omptin complex in which Lys15 of aprotinin forms salt bridges with conserved negatively charged residues of the omptin active site.

The CpxRA two-component system is essential for Citrobacter rodentium virulence.

Citrobacter rodentium is a murine intestinal pathogen used as a model for the foodborne human pathogens enterohemorrhagic Escherichia coli and enteropathogenic E. coli. During infection, these pathogens use two-component signal transduction systems to detect and adapt to changing environmental conditions. In E. coli, the CpxRA two-component signal transduction system responds to envelope stress by modulating the expression of a myriad of genes. Quantitative real-time PCR showed that cpxRA was expressed in the colon of C57BL/6J mice infected with C. rodentium. To determine whether CpxRA plays a role during C. rodentium infection, a cpxRA deletion strain was generated and found to have a colonization defect during infection. This defect was independent of an altered growth rate or a defective type III secretion system, and single-copy chromosomal complementation of cpxRA restored virulence. The C. rodentium strains were then tested in C3H/HeJ mice, a lethal intestinal infection model. Mice infected with the ΔcpxRA strain survived infection, whereas mice infected with the wild-type or complemented strains succumbed to infection. Furthermore, we found that the cpxRA expression level was higher during early infection than at a later time point. Taken together, these data demonstrate that the CpxRA two-component signal transduction system is essential for the in vivo virulence of C. rodentium. In addition, these data suggest that fine-tuned cpxRA expression is important for infection. This is the first study that identifies a C. rodentium two-component transduction system required for pathogenesis. This study further indicates that CpxRA is an interesting target for therapeutics against enteric pathogens.

Enterohemorrhagic Escherichia coli OmpT regulates outer membrane vesicle biogenesis.

Enterohemorrhagic Escherichia coli (EHEC) infection from food or water often results in severe diarrheal disease and is a leading cause of death globally. Outer membrane vesicles (OMVs) secreted from E. coli induce lethality in mice. The omptin outer membrane protease OmpT from E. coli inactivates antimicrobial peptides and may enhance colonization of the uroepithelium, but its precise function remains unclear. Given OmpT is an outer membrane protease, we hypothesized it may have a role in OMV biogenesis. To further characterize the effect of OmpT on OMV production, a genetic approach using wild type, an ompT deletion mutant and an ompT overexpressing construct in EHEC were employed. ompT gene deletion markedly decreased OMV production and stainable lipid but increased vesicle diameter. Conversely, ompT overexpression profoundly increased OMV biogenesis but decreased stainable lipid, protein content, and vesicle diameter. Alterations in EHEC ompT gene expression have an impact on the biogenesis, composition, and size of OMVs. Changes in ompT gene expression may dynamically alter OMV formation, composition, and diameter in response to different host environments and contribute to cell-free intercellular communication to enhance bacterial growth and survival.

Host defense peptide resistance contributes to colonization and maximal intestinal pathology by Crohn's disease-associated adherent-invasive Escherichia coli.

Host defense peptides secreted by colonocytes and Paneth cells play a key role in innate host defenses in the gut. In Crohn's disease, the burden of tissue-associated Escherichia coli commonly increases at epithelial surfaces where host defense peptides concentrate, suggesting that this bacterial population might actively resist this mechanism of bacterial killing. Adherent-invasive E. coli (AIEC) is associated with Crohn's disease; however, the colonization determinants of AIEC in the inflamed gut are undefined. Here, we establish that host defense peptide resistance contributes to host colonization by Crohn's-associated AIEC. We identified a plasmid-encoded genomic island (called PI-6) in AIEC strain NRG857c that confers high-level resistance to α-helical cationic peptides and α- and ß-defensins. Deletion of PI-6 sensitized strain NRG857c to these host defense molecules, reduced its competitive fitness in a mouse model of infection, and attenuated its ability to induce cecal pathology. This phenotype is due to two genes in PI-6, arlA, which encodes a Mig-14 family protein implicated in defensin resistance, and arlC, an OmpT family outer membrane protease. Implicit in these findings are new bacterial targets whose inhibition might limit AIEC burden and disease in the gut.

Both group 4 capsule and lipopolysaccharide O-antigen contribute to enteropathogenic Escherichia coli resistance to human α-defensin 5.

Enteropathogenic and enterohemorrhagic Escherichia coli (EPEC and EHEC) are food-borne pathogens that colonize the small intestine and colon, respectively. To cause disease, these pathogens must overcome the action of different host antimicrobial peptides (AMPs) secreted into these distinct niches. We have shown previously that EHEC expresses high levels of the OmpT protease to inactivate the human cathelicidin LL-37, an AMP present in the colon. In this study, we investigate the mechanisms used by EPEC to resist human α-defensin 5 (HD-5), the most abundant AMP in the small intestine. Quantitative PCR was used to measure transcript levels of various EPEC surface structures. High transcript levels of gfcA, a gene required for group 4 capsule (G4C) production, were observed in EPEC, but not in EHEC. The unencapsulated EPEC ∆gfcA and EHEC wild-type strains were more susceptible to HD-5 than EPEC wild-type. Since the G4C is composed of the same sugar repeats as the lipopolysaccharide O-antigen, an -antigen ligase (waaL) deletion mutant was generated in EPEC to assess its role in HD-5 resistance. The ∆waaL EPEC strain was more susceptible to HD-5 than both the wild-type and ∆gfcA strains. The ∆gfcA∆waaL EPEC strain was not significantly more susceptible to HD-5 than the ∆waaL strain, suggesting that the absence of -antigen influences G4C formation. To determine whether the G4C and -antigen interact with HD-5, total polysaccharide was purified from wild-type EPEC and added to the ∆gfcA∆waaL strain in the presence of HD-5. The addition of exogenous polysaccharide protected the susceptible strain against HD-5 killing in a dose-dependent manner, suggesting that HD-5 binds to the polysaccharides present on the surface of EPEC. Altogether, these findings indicate that EPEC relies on both the G4C and the -antigen to resist the bactericidal activity of HD-5.

Role of uropathogenic Escherichia coli OmpT in the resistance against human cathelicidin LL-37.

Uropathogenic Escherichia coli (UPEC) strains are among the most prevalent causative agents of urinary tract infections. To establish infection, UPEC must overcome the bactericidal action of host antimicrobial peptides. Previously, the enterohaemorrhagic E. coli outer membrane protease, OmpT, was shown to degrade and inactivate the human antimicrobial peptide LL-37. This study aims to investigate the involvement of UPEC OmpT in LL-37 degradation. An ompT deletion mutant was generated in the prototypical UPEC strain CFT073. Western blot analysis showed that the OmpT protein level is moderate in CFT073. In agreement, OmpT was shown to partially cleave LL-37. However, no difference in the minimum inhibitory concentration of LL-37 was observed between CFT073 and the ompT mutant. Plasmid complementation of ompT, which led to increased OmpT levels, resulted in complete cleavage of LL-37 and a fourfold increase in the minimum inhibitory concentration. The analysis of other UPEC isolates showed similar OmpT activity levels as CFT073. Although UPEC OmpT can cleave LL-37, we conclude that the low level of OmpT limits its contribution to LL-37 resistance. Collectively, these data suggest that UPEC OmpT is likely accompanied by other LL-37 resistance mechanisms.

Enterohemorrhagic and enteropathogenic Escherichia coli evolved different strategies to resist antimicrobial peptides.

Enterohemorrhagic and enteropathogenic Escherichia coli (EHEC and EPEC) are enteric human pathogens that colonize the large and small intestines, respectively. To establish infection EHEC and EPEC must overcome innate host defenses, such as antimicrobial peptides (AMPs) produced by the intestinal epithelium. Gram-negative pathogens have evolved different mechanisms to resist AMPs, including outer-membrane proteases that degrade AMPs. We showed that the protease OmpT degrades the human AMP LL-37 more rapidly in EHEC than in EPEC. Promoter-swap experiments showed that this is due to differences in the promoters of the two genes, leading to greater ompT expression and subsequently greater levels of OmpT in EHEC. Here, we propose that the different ompT expression in EHEC and EPEC reflects the varying levels of LL-37 throughout the human intestinal tract. These data suggest that EHEC and EPEC adapted to their specific niches by developing distinct AMP-specific resistance mechanisms.

Resistance to antimicrobial peptides in Gram-negative bacteria.

Antimicrobial peptides (AMPs) are present in virtually all organisms and are an ancient and critical component of innate immunity. In mammals, AMPs are present in phagocytic cells, on body surfaces such as skin and mucosa, and in secretions and body fluids such as sweat, saliva, urine, and breast milk, consistent with their role as part of the first line of defense against a wide range of pathogenic microorganisms including bacteria, viruses, and fungi. AMPs are microbicidal and have also been shown to act as immunomodulators with chemoattractant and signaling activities. During the co-evolution of hosts and bacterial pathogens, bacteria have developed the ability to sense and initiate an adaptive response to AMPs to resist their bactericidal activity. Here, we review the various mechanisms used by Gram-negative bacteria to sense and resist AMP-mediated killing. These mechanisms play an important role in bacterial resistance to host-derived AMPs that are encountered during the course of infection. Bacterial resistance to AMPs should also be taken into consideration in the development and use of AMPs as anti-infective agents, for which there is currently a great deal of academic and commercial interest.

OmpT outer membrane proteases of enterohemorrhagic and enteropathogenic Escherichia coli contribute differently to the degradation of human LL-37.

Enterohemorrhagic Escherichia coli (EHEC) and enteropathogenic E. coli (EPEC) are food-borne pathogens that cause serious diarrheal diseases. To colonize the human intestine, these pathogens must overcome innate immune defenses such as antimicrobial peptides (AMPs). Bacterial pathogens have evolved various mechanisms to resist killing by AMPs, including proteolytic degradation of AMPs. To examine the ability of the EHEC and EPEC OmpT outer membrane (OM) proteases to degrade α-helical AMPs, ompT deletion mutants were generated. Determination of MICs of various AMPs revealed that both mutant strains are more susceptible than their wild-type counterparts to α-helical AMPs, although to different extents. Time course assays monitoring the degradation of LL-37 and C18G showed that EHEC cells degraded both AMPs faster than EPEC cells in an OmpT-dependent manner. Mass spectrometry analyses of proteolytic fragments showed that EHEC OmpT cleaves LL-37 at dibasic sites. The superior protection provided by EHEC OmpT compared to EPEC OmpT against α-helical AMPs was due to higher expression of the ompT gene and, in turn, higher levels of the OmpT protein in EHEC. Fusion of the EPEC ompT promoter to the EHEC ompT open reading frame resulted in decreased OmpT expression, indicating that transcriptional regulation of ompT is different in EHEC and EPEC. We hypothesize that the different contributions of EHEC and EPEC OmpT to the degradation and inactivation of LL-37 may be due to their adaptation to their respective niches within the host, the colon and small intestine, respectively, where the environmental cues and abundance of AMPs are different.

Absence of PmrAB-mediated phosphoethanolamine modifications of Citrobacter rodentium lipopolysaccharide affects outer membrane integrity.

The PmrAB two-component system of enterobacteria regulates a number of genes whose protein products modify lipopolysaccharide (LPS). The LPS is modified during transport to the bacterial outer membrane (OM). A subset of PmrAB-mediated LPS modifications consists of the addition of phosphoethanolamine (pEtN) to lipid A by PmrC and to the core by CptA. In Salmonella enterica, pEtN modifications have been associated with resistance to polymyxin B and to excess iron. To investigate putative functions of pEtN modifications in Citrobacter rodentium, ΔpmrAB, ΔpmrC, ΔcptA, and ΔpmrC ΔcptA deletion mutants were constructed. Compared to the wild type, most mutant strains were found to be more susceptible to antibiotics that must diffuse across the LPS layer of the OM. All mutant strains also showed increased influx rates of ethidium dye across their OM, suggesting that PmrAB-regulated pEtN modifications affect OM permeability. This was confirmed by increased partitioning of the fluorescent dye 1-N-phenylnaphthylamine (NPN) into the OM phospholipid layer of the mutant strains. In addition, substantial release of periplasmic ß-lactamase was observed for the ΔpmrAB and ΔpmrC ΔcptA strains, indicating a loss of OM integrity. This study attributes a new role for PmrAB-mediated pEtN LPS modifications in the maintenance of C. rodentium OM integrity.

An outer membrane protease of the omptin family prevents activation of the Citrobacter rodentium PhoPQ two-component system by antimicrobial peptides.

The PhoPQ two-component system of the intracellular pathogen Salmonella enterica senses and controls resistance to alpha-helical antimicrobial peptides (AMPs) by regulating covalent modifications of lipid A. A homologue of the phoPQ operon was found in the genome of the murine enteric extracellular pathogen, Citrobacter rodentium. Here we report that C. rodentium PhoPQ was apparently unable to mediate activation of target genes in the presence of alpha-helical AMPs. However, these AMPs activated C. rodentium PhoPQ expressed in a S. entericaDeltaphoPQ mutant. Analysis of the outer membrane (OM) fractions of the C. rodentium wild-type and DeltaphoPQ strains led to the identification of an omptin family protease (CroP) that was absent in DeltaphoPQ. Deletion of croP in C. rodentium resulted in higher susceptibility to alpha-helical AMPs, indicating a direct role of CroP in AMP resistance. CroP greatly contributed to the protection of the OM from AMP damage by actively degrading alpha-helical AMPs before they reach the periplasmic space. Accordingly, transcriptional activation of PhoP-regulated genes by alpha-helical AMPs was restored in the DeltacroP mutant. This study shows that resistance to alpha-helical AMPs by the extracellular pathogen C. rodentium relies primarily on the CroP OM protease.

The prpZ gene cluster encoding eukaryotic-type Ser/Thr protein kinases and phosphatases is repressed by oxidative stress and involved in Salmonella enterica serovar Typhi survival in human macrophages.

The prpZ gene cluster consists of three ORFs coding for proteins with homology to eukaryotic-type Ser/Thr protein phosphatases 2C (prpZ) and Ser/Thr protein kinases (prkY and prkX). This cluster is present in the sequenced genomes of Salmonella enterica serovar Typhi (S. Typhi) strains Ty2 and CT18. This study investigated the genetic organization of this gene cluster, its regulation and its putative involvement in virulence. The three genes are transcribed as a polycistronic mRNA as demonstrated by reverse transcriptase (RT)-PCR. Analysis of a prpZ::lacZ transcriptional fusion showed that the prpZ locus is expressed throughout the growth phase. LacZ activity and real-time RT-PCR showed that transcription of the mRNA is negatively regulated upon exposure of cells to HOCl and, to a lesser extent, hydrogen peroxide. A deletion mutant of the prpZ gene cluster showed a significantly lower level of survival than the parental strain Ty2 in human macrophages at 48 h postinfection. Together these data suggest that prpZ, prkY and prkX are virulence genes that may be part of a signaling pathway controlling long-term survival of S. Typhi in host cells.

Activation of the bacterial sensor kinase PhoQ by acidic pH.

The Salmonellae PhoQ sensor kinase senses the mammalian phagosome environment to activate a transcriptional program essential for virulence. The PhoQ periplasmic domain binds divalent cations, forming bridges with inner membrane phospholipids to maintain PhoQ repression. PhoQ also binds and is activated by cationic antimicrobial peptides. In this work, PhoQ is directly activated by exposure of the sensor domain to pH 5.5. NMR spectroscopy indicates that at acidic pH, the PhoQ periplasmic domain adopts a conformation different from that in the presence of divalent cations or antimicrobial peptides. The conformation is partially simulated by mutation of histidine 157, which is part of an interaction network that distinguishes the repressed conformation. The effects of antimicrobial peptides and pH on PhoQ activity are additive. We propose a model of activation by antimicrobial peptides via disruption of the cation bridges and/or by acidification of the periplasm through destabilization of the interaction network.