Discovery of highly neutralizing human antibodies targeting Pseudomonas aeruginosa

Drug-resistant Pseudomonas aeruginosa (PA) poses an emerging threat to human health with urgent need for alternative therapeutic approaches. Here, we deciphered the B cell and antibody response to the virulence-associated type III secretion system (T3SS) in a cohort of patients chronically infected with PA. Single-cell analytics revealed a diverse B cell receptor repertoire directed against the T3SS needle-tip protein PcrV, enabling the production of monoclonal antibodies (mAbs) abrogating T3SS-mediated cytotoxicity. Mechanistic studies involving cryoelectron microscopy identified a surface-exposed C-terminal PcrV epitope as the target of highly neutralizing mAbs with broad activity against drug-resistant PA isolates. These anti-PcrV mAbs were as effective as treatment with conventional antibiotics in vivo. Our study reveals that chronically infected patients represent a source of neutralizing antibodies, which can be exploited as therapeutics against PA.

A VirB4 ATPase of the mobile accessory genome orchestrates core genome-encoded features of physiology, metabolism, and virulence of Pseudomonas aeruginosa TBCF10839

Pseudomonas aeruginosa TBCF10839 is a highly virulent strain that can persist and replicate in human neutrophils. Screening of a signature-tagged mutagenesis (STM) TBCF10839 transposon library in phagocytosis tests identified a mutant that carried the transposon in the VirB4 homolog 5PG21 of an integrative and conjugative element (ICE)-associated type IV secretion system of the pKLC102 subtype. 5P21 TBCF10839 insertion mutants were deficient in metabolic versatility, secretion, quorum sensing, and virulence. The mutants were efficiently killed in phagocytosis tests in vitro and were avirulent in an acute murine airway infection model in vivo. The inactivation of 5PG21 silenced the rhl, las, and pqs operons and the gene expression for the synthesis of hydrogen cyanide, the antimetabolite l-2-amino-4-methoxy-trans-3-butenoic acid, and the H2- and H3-type VI secretion systems and their associated effectors. The mutants were impaired in the utilization of carbon sources and stored compounds that are not funneled into intermediary metabolism. This showcase demonstrates that a single gene of the mobile accessory genome can become an essential element to operate the core genome-encoded features of metabolism and virulence.

Competitive survival of clonal serial Pseudomonas aeruginosa isolates from cystic fibrosis airways in human neutrophils

Chronic airway infections with Pseudomonas aeruginosa are the major co-morbidity in most people with cystic fibrosis (CF) sustained by neutrophils as the major drivers of lung inflammation, damage, and remodeling. Phagocytosis assays were performed with clonal consortia of longitudinal P. aeruginosa airway isolates collected from people with CF since the onset of lung colonization until patient's death or replacement by another clone. The extra- and intracellular abundance of individual strains was assessed by deep amplicon sequencing of strain-specific single nucleotide variants in the bacterial genome. The varied microevolution of the accessory genome of the P. aeruginosa clones during mild and severe courses of infection corresponded with a differential persistence of clonal progeny in the neutrophil phagosome. By simultaneously exposing the ancestor and its progeny to the same habitat, the study recapitulated the time lapse of the temporal change of the fitness of the clone to survive in neutrophils.

Competitive fitness of Pseudomonas aeruginosa isolates in human and murine precision-cut lung slices

Chronic respiratory infections with the gram-negative bacterium Pseudomonas aeruginosa are an important co-morbidity for the quality of life and prognosis of people with cystic fibrosis (CF). Such long-term colonization, sometimes lasting up to several decades, represents a unique opportunity to investigate pathogen adaptation processes to the host. Our studies aimed to resolve if and to what extent the bacterial adaptation to the CF airways influences the fitness of the pathogen to grow and to persist in the lungs. Marker-free competitive fitness experiments of serial P. aeruginosa isolates differentiated by strain-specific SNPs, were performed with murine and human precision cut lung slices (PCLS). Serial P. aeruginosa isolates were selected from six mild and six severe CF patient courses, respectively. MPCLS or hPCLS were inoculated with a mixture of equal numbers of the serial isolates of one course. The temporal change of the composition of the bacterial community during competitive growth was quantified by multi-marker amplicon sequencing. Both ex vivo models displayed a strong separation of fitness traits between mild and severe courses. Whereas the earlier isolates dominated the competition in the severe courses, intermediate and late isolates commonly won the competition in the mild courses. The status of the CF lung disease rather than the bacterial genotype drives the adaptation of P. aeruginosa during chronic CF lung infection. This implies that the disease status of the lung habitat governed the adaptation of P. aeruginosa more strongly than the underlying bacterial clone-type and its genetic repertoire.

Bitter taste signaling in tracheal epithelial brush cells elicits innate immune responses to bacterial infection

Constant exposure of the airways to inhaled pathogens requires efficient early immune responses protecting against infections. How bacteria on the epithelial surface are detected and first-line protective mechanisms are initiated are not well understood. We have recently shown that tracheal brush cells (BCs) express functional taste receptors. Here we report that bitter taste signaling in murine BCs induces neurogenic inflammation. We demonstrate that BC signaling stimulates adjacent sensory nerve endings in the trachea to release the neuropeptides CGRP and substance P that mediate plasma extravasation, neutrophil recruitment, and diapedesis. Moreover, we show that bitter tasting quorum-sensing molecules from Pseudomonas aeruginosa activate tracheal BCs. BC signaling depends on the key taste transduction gene Trpm5, triggers secretion of immune mediators, among them the most abundant member of the complement system, and is needed to combat P. aeruginosa infections. Our data provide functional insight into first-line defense mechanisms against bacterial infections of the lung.

Rescue from Pseudomonas aeruginosa Airway Infection via Stem Cell Transplantation

Cystic fibrosis is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, which lead to impaired ion transport in epithelial cells. Although lung failure due to chronic infection is the major comorbidity in individuals with cystic fibrosis, the role of CFTR in non-epithelial cells has not been definitively resolved. Given the important role of host defense cells, we evaluated the Cftr deficiency in pulmonary immune cells by hematopoietic stem cell transplantation in cystic fibrosis mice. We transplanted healthy bone marrow stem cells and could reveal a stable chimerism of wild-type cells in peripheral blood. The outcome of stem cell transplantation and the impact of healthy immune cells were evaluated in acute Pseudomonas aeruginosa airway infection. In this study, mice transplanted with wild-type cells displayed better survival, lower lung bacterial numbers, and a milder disease course. This improved physiology of infected mice correlated with successful intrapulmonary engraftment of graft-derived alveolar macrophages, as seen by immunofluorescence microscopy and flow cytometry of graft-specific leucocyte surface marker CD45 and macrophage marker CD68. Given the beneficial effect of hematopoietic stem cell transplantation and stable engraftment of monocyte-derived CD68-positive macrophages, we conclude that replacement of mutant Cftr macrophages attenuates airway infection in cystic fibrosis mice.

Lung infection caused by Pseudomonas aeruginosa in a CD26/DPP4 deficient F344 rat model

Background

Pseudomonas aeruginosa (PA) is the most important opportunistic pathogen in causing nosocomial infections and, furthermore, poses a permanent threat for severe chronic infections in patients with cystic fibrosis or COPD. The transmembrane protein CD26 with dipeptidyl peptidase-4 (DPP4) activity shows an increased expression in inflamed tissue. We tested whether CD26/DPP4 deficiency leads to reduced inflammation and decreased structural damage when infected with PA.

Methods

CD26/DPP4⁺ and CD26/DPP4⁻ rats were instilled intratracheally with NaCl (controls) or with PA. Six hours later, bacterial distribution was detected with the in vivo imaging system 200 (IVIS). Lungs were then processed for molecular biology, light and electron microscopy and analyzed qualitatively, quantitatively and stereologically. Bacterial numbers were determined in homogenized lungs.

Results

Compared to saline treated controls, in both infected groups (1) the acinar airspace was significantly increased, (2) the volume density of the alveolar epithelium was significantly decreased, (3) the septal thickness was significantly reduced, (4) more than 40% of the alveolar epithelial surface was damaged, and up to 36% of the epithelial surface was covered with edema. In infected CD26⁻ rats, the increase in lung weight was significantly less pronounced, the portion of edematous alveolar airspace was significantly lower and the part of edema interspersed with PA was decreased significantly.

Conclusions

CD26/DPP4 deficiency resulted in reduced pulmonary edema under sublethal PA infection, implicating a role for CD26 in infection progression. The partly pronounced structural damage may mask further possible influences of CD26 on the inflammatory response.

The Pseudomonas aeruginosa ExoY phenotype of high-copy-number recombinants is not detectable in natural isolates

The nucleotidyl cyclase ExoY is an effector protein of the type III secretion system of Pseudomonas aeruginosa. We compared the cyclic nucleotide production and lung disease phenotypes caused by the ExoY-overexpressing strain PA103ΔexoUexoT::Tc pUCPexoY, its vector control strain PA103ΔexoUexoT::Tc pUCP18, its loss-of-function control PA103ΔexoUexoT::Tc pUCPexoY K81M and natural ExoY-positive and ExoY-negative isolates in a murine acute airway infection model. Only the P. aeruginosa carrier of the exoY-plasmid produced high levels of cUMP and caused the most severe course of infection. The pathology ascribed to ExoY from studies using the high-copy-number plasmid on mammalian cells in vitro and in vivo was not observed with natural P. aeruginosa isolates. This indicates that the role of ExoY during infection with real-life P. aeruginosa still needs to be resolved.

Self-assembled peptide-poloxamine nanoparticles enable in vitro and in vivo genome restoration for cystic fibrosis

Developing safe and efficient non-viral delivery systems remains a major challenge for in vivo applications of gene therapy, especially in cystic fibrosis. Unlike conventional cationic polymers or lipids, the emerging poloxamine-based copolymers display promising in vivo gene delivery capabilities. However, poloxamines are invalid for in vitro applications and their in vivo transfection efficiency is still low compared with viral vectors. Here, we show that peptides developed by modular design approaches can spontaneously form compact and monodisperse nanoparticles with poloxamines and nucleic acids via self-assembly. Both messenger RNA and plasmid DNA expression mediated by peptide-poloxamine nanoparticles are greatly boosted in vitro and in the lungs of cystic fibrosis mice with negligible toxicity. Peptide-poloxamine nanoparticles containing integrating vectors enable successful in vitro and in vivo long-term restoration of cystic fibrosis transmembrane conductance regulator deficiency with a safe integration profile. Our dataset provides a new framework for designing non-viral gene delivery systems qualified for in vivo genetic modifications.

Bioreactor-based mass production of human iPSC-derived macrophages enables immunotherapies against bacterial airway infections

The increasing number of severe infections with multi-drug-resistant pathogens worldwide highlights the need for alternative treatment options. Given the pivotal role of phagocytes and especially alveolar macrophages in pulmonary immunity, we introduce a new, cell-based treatment strategy to target bacterial airway infections. Here we show that the mass production of therapeutic phagocytes from induced pluripotent stem cells (iPSC) in industry-compatible, stirred-tank bioreactors is feasible. Bioreactor-derived iPSC-macrophages (iPSC-Mac) represent a highly pure population of CD45⁺CD11b⁺CD14⁺CD163⁺ cells, and share important phenotypic, functional and transcriptional hallmarks with professional phagocytes, however with a distinct transcriptome signature similar to primitive macrophages. Most importantly, bioreactor-derived iPSC-Mac rescue mice from Pseudomonas aeruginosa-mediated acute infections of the lower respiratory tract within 4-8 h post intra-pulmonary transplantation and reduce bacterial load. Generation of specific immune-cells from iPSC-sources in scalable stirred-tank bioreactors can extend the field of immunotherapy towards bacterial infections, and may allow for further innovative cell-based treatment strategies.

Pulmonary infections constitute a substantial health problem worldwide. Here the authors show that phagocytes similar to primitive macrophages can be generated from human induced pluripotent stem cells, by the use of industry-compatible, stirred-tank bioreactors, and applied as a cell-based therapy to treat acute bacterial infections in mice.

Key role of an ADP - ribose - dependent transcriptional regulator of NAD metabolism for fitness and virulence of Pseudomonas aeruginosa

NAD is an essential co-factor of redox reactions and metabolic conversions of NAD-dependent enzymes. NAD biosynthesis in the opportunistic pathogen Pseudomonas aeruginosa has yet not been experimentally explored. The in silico search for orthologs in the P. aeruginosa PAO1 genome identified the operon pncA - pncB1-nadE (PA4918-PA4920) to encode the nicotinamidase, nicotinate phosporibosyltransferase and Nad synthase of salvage pathway I. The functional role of the preceding genes PA4917 and PA4916 was resolved by the characterization of recombinant protein. PA4917 turned out to encode the nicotinate mononucleotide adenylyltransferase NadD2 and PA4916 was determined to encode the transcriptional repressor NrtR that binds to an intergenic sequence between nadD2 and pncA. Complex formation between the catalytically inactive Nudix protein NrtR and its DNA binding site was suppressed by the antirepressor ADP-ribose. NrtR plasposon mutagenesis abrogated virulence of P. aeruginosa TBCF10839 in a murine acute airway infection model and constrained its metabolite profile. When grown together with other isogenic plasposon mutants, the nrtR knock-out was most compromised in competitive fitness to persist in nutrient-rich medium in vitro or murine airways in vivo. This example demonstrates how tightly metabolism and virulence can be intertwined by key elements of metabolic control.

cCMP and cUMP occur in vivo

Mammalian cells contain the cyclic pyrimidine nucleotides cCMP and cUMP. It is unknown whether these tentative new second messenger molecules occur in vivo. We used high performance liquid chromatography quadrupole tandem mass spectrometry to quantitate nucleoside 3',5'-cyclic monophosphates. cCMP was detected in all organs studied, most notably pancreas, spleen and the female reproductive system. cUMP was not detected in organs, probably due to the intrinsically low sensitivity of mass spectrometry to detect this molecule and organ matrix effects. Intratracheal infection of mice with recombinant Pseudomonas aeruginosa harboring the nucleotidyl cyclase toxin ExoY massively increased cUMP in lung. The identity of cCMP and cUMP in organs was confirmed by high performance liquid chromatography quadrupole time of flight mass spectrometry. cUMP also appeared in serum, urine and faeces following infection. Taken together, this report unequivocally shows for the first time that cCMP and cUMP occur in vivo.

The histamine H4 -receptor (H4 R) regulates eosinophilic inflammation in ovalbumin-induced experimental allergic asthma in mice

Via the histamine H4 -receptor (H4 R), histamine promotes the pathogenesis of experimental allergic asthma in mice. Application of H4 R antagonists during sensitization as well as during provocation reduces the severity of the disease. However, the specific cell types functionally expressing H4 R in experimental allergic asthma have not been well characterized in vivo. In this study, we identified the cell type(s) responsible for H4 R activity in experimental asthma and related physiological mechanisms. Using H4 R-deficient mice, we studied the role of H4 R in the sensitization and effector phase. DCs lacking H4 R expression during the in vitro sensitization reaction resulted in effector T cells unable to induce an entire eosinophilic inflammation in the lung upon adoptive transfer in vivo. Recipient mice lacking H4 R expression, which were adoptively transferred with H4 R(+/+) T cells polarized in the presence of H4 R(+/+) DCs, showed reduced signs of inflammation and ameliorated lung function. Here, we provide in vivo evidence that in experimental asthma in mice the H4 R specifically regulates activation of DCs during sensitization, while in the effector phase the H4 R is active in cells involved in the activation of eosinophils, and possibly other cells. A putative therapy targeting the H4 R may be an option for asthma patients developing IL-5-dependent eosinophilia.

Impaired TLR4 and HIF expression in cystic fibrosis bronchial epithelial cells downregulates hemeoxygenase-1 and alters iron homeostasis in vitro

Hemeoxygenase-1 (HO-1), an inducible heat shock protein, is upregulated in response to multiple cellular insults via oxidative stress, lipopolysaccharides (LPS), and hypoxia. In this study, we investigated in vitro the role of Toll-like receptor 4 (TLR4), hypoxia-inducible factor 1α (HIF-1α), and iron on HO-1 expression in cystic fibrosis (CF). Immunohistochemical analysis of TLR4, HO-1, ferritin, and HIF-1α were performed on lung sections of CFTR−/− and wild-type mice. CFBE41o- and 16HBE14o- cell lines were employed for in vitro analysis via immunoblotting, immunofluorescence, real-time PCR, luciferase reporter gene analysis, and iron quantification. We observed a reduced TLR4, HIF-1α, HO-1, and ferritin in CFBE41o- cell line and CF mice. Knockdown studies using TLR4-siRNA in 16HBE14o- revealed significant decrease of HO-1, confirming the role of TLR4 in HO-1 downregulation. Inhibition of HO-1 using tin protoporphyrin in 16HBE14o- cells resulted in increased iron levels, suggesting a probable role of HO-1 in iron accumulation. Additionally, sequestration of excess iron using iron chelators resulted in increased hypoxia response element response in CFBE41o- and 16HBE14o-, implicating a role of iron in HIF-1α stabilization and HO-1. To conclude, our in vitro results demonstrate that multiple regulatory factors, such as impaired TLR4 surface expression, increased intracellular iron, and decreased HIF-1α, downregulate HO-1 expression in CFBE41o- cells.

Assessing Pseudomonas virulence using mammalian models: acute infection model

The acute murine lung infection model monitors Pseudomonas aeruginosa airway infections by multiple continuous and endpoint parameters. After intratracheal or intranasal infection it characterizes the course of infection via head-out spirometry, rectal temperature, weight loss, a body condition score based on nine physiological parameters, lung bacterial numbers, organ dissemination of bacteria, and a semiquantitative assessment of lung inflammation and further analysis. The generated data allows a robust classification of virulence of mutant or wild-type P. aeruginosa strains and/or of the susceptibility of wild-type or engineered mouse strains to infection. If standardized, the model is applicable to the preclinical assessment of antipseudomonal prevention and intervention strategies.

A new role of the complement system: C3 provides protection in a mouse model of lung infection with intracellular Chlamydia psittaci

The complement system modulates the intensity of innate and specific immunity. While it protects against infections by extracellular bacteria its role in infection with obligate intracellular bacteria, such as the avian and human pathogen Chlamydia (C.) psittaci, is still unknown. In the present study, knockout mice lacking C3 and thus all main complement effector functions were intranasally infected with C. psittaci strain DC15. Clinical parameters, lung histology, and cytokine levels were determined. A subset of infections was additionally performed with mice lacking C5 or C5a receptors. Complement activation occurred before symptoms of pneumonia appeared. Mice lacking C3 were ∼100 times more susceptible to the intracellular bacteria compared to wild-type mice, with all C3^−/− mice succumbing to infection after day 9. At a low infective dose, C3^−/− mice became severely ill after an even longer delay, the kinetics suggesting a so far unknown link of complement to the adaptive, protective immune response against chlamydiae. The lethal phenotype of C3^−/− mice is not based on differences in the anti-chlamydial IgG response (which is slightly delayed) as demonstrated by serum transfer experiments. In addition, during the first week of infection, the absence of C3 was associated with partial protection characterized by reduced weight loss, better clinical score and lower bacterial burden, which might be explained by a different mechanism. Lack of complement functions downstream of C5 had little effect. This study demonstrates for the first time a strong and complex influence of complement effector functions, downstream of C3 and upstream of C5, on the outcome of an infection with intracellular bacteria, such as C. psittaci.

Friend or foe: genetic and functional characterization of plant endophytic Pseudomonas aeruginosa

Endophytic Pseudomonas aeruginosa strain BP35 was originally isolated from black pepper grown in the rain forest in Kerala, India. Strain PaBP35 was shown to provide significant protection to black pepper against infections by Phytophthora capsici and Radopholus similis. For registration and implementation in disease management programmes, several traits of PaBP35 were investigated including its endophytic behaviour, biocontrol activity, phylogeny and toxicity to mammals. The results showed that PaBP35 efficiently colonized black pepper shoots and displayed a typical spatiotemporal pattern in its endophytic movement with concomitant suppression of Phytophthora rot. Confocal laser scanning microscopy revealed high populations of PaBP35::gfp2 inside tomato plantlets, supporting its endophytic behaviour in other plant species. Polyphasic approaches to genotype PaBP35, including BOX-PCR, recN sequence analysis, multilocus sequence typing and comparative genome hybridization analysis, revealed its uniqueness among P. aeruginosa strains representing clinical habitats. However, like other P. aeruginosa strains, PaBP35 exhibited resistance to antibiotics, grew at 25-41°C and produced rhamnolipids and phenazines. PaBP35 displayed strong type II secretion effectors-mediated cytotoxicity on mammalian A549 cells. Coupled with pathogenicity in a murine airway infection model, we conclude that this plant endophytic strain is as virulent as clinical P. aeruginosa strains. Safety issues related to the selection of plant endophytic bacteria for crop protection are discussed.

Acute intratracheal Pseudomonas aeruginosa infection in cystic fibrosis mice is age-independent

Background

Since the discovery of the human CFTR gene in 1989 various mouse models for cystic fibrosis (CF) have been generated and used as a very suitable and popular tool to approach research on this life-threatening disease. Age related changes regarding the course of disease and susceptibility towards pulmonary infections have been discussed in numerous studies.

Methods

Here, we investigated Cftr^{TgH(neoim)Hgu} and Cftr^tm1Unc-Tg(FABPCFTR)1Jaw/J CF mice and their non-CF littermates during an acute lung infection with Pseudomonas aeruginosa for age dependent effects of their lung function and immune response.

Mice younger than three or older than six months were intratracheally infected with P. aeruginosa TBCF10839. The infection was monitored by lung function of the animals using non-invasive head-out spirometry and the time course of physiological parameters over 192 hours. Quantitative bacteriology and lung histopathology of a subgroup of animals were used as endpoint parameters.

Results

Age-dependent changes in lung function and characteristic features for CF like a shallower, faster breathing pattern were observed in both CF mouse models in uninfected state. In contrast infected CF mice did not significantly differ from their non-CF littermates in susceptibility and severity of lung infection in both mouse models and age groups. The transgenic Cftr^tm1Unc-Tg(FABPCFTR)1Jaw/J and their non-CF littermates showed a milder course of infection than the Cftr^{TgH(neoim)Hgu} CF and their congenic C57Bl/6J non-CF mice suggesting that the genetic background was more important for outcome than Cftr dysfunction.

Conclusions

Previous investigations of the same mouse lines have shown a higher airway susceptibility of older CF mice to intranasally applied P. aeruginosa. The different outcome of intranasal and intratracheal instillation of bacteria implies that infected CF epithelium is impaired during the initial colonization of upper airways, but not in the subsequent response of host defense.

Lung function and inflammation during murine Pseudomonas aeruginosa airway infection

BACKGROUND

Following any acute irritation lung function declines rapidly. Reasons for pulmonary deterioration in humans had been attributed to the action of either interleukin-6 or interleukin-8 in the lungs.

OBJECTIVES

The present study investigates the association between immune response and decline in lung function in a murine bacterial lung infection model.

METHODS

Upon intratracheal inoculation of C57BL/6J mice with a sublethal dose of Pseudomonas aeruginosa lung function, cytokine, chemokine and cytometry in bronchoalveolar lavage fluid, bacterial counts and lung histology was assessed at 2, 4, 6, 8, 10, 12, 18, 24, 48, 72, 96 and 120 h post inoculation.

RESULTS

Lung function measured by non-invasive head-out spirometry decreased most strongly between 6 and 10 h post inoculation and required up to 72 h to recover for selected parameters. CFU counts in the lungs peaked at 4h post inoculation with subsequent decline until at 24-48 h post inoculation background levels were reached. Cytokine and chemokine responses could be separated into an early pro-inflammatory phase (2-8h post inoculation; mainly tumor-necrosis factor α (TNFα) and interleukin-1α driven) and a late anti-inflammatory resolution phase (starting at 24h post inoculation; mainly interleukin-10 and interleukin-4 driven). Interleukin-6 levels correlated with the deterioration of lung function. Lung histology showed maximal changes in terms of inflammation and edema between 24 and 48 h post inoculation.

CONCLUSIONS

In summary, elevated interleukin-6, high local neutrophil counts and lung edema were found to be the most characteristic signs of the transient period of deterioration of lung function.

Beneficial effects of TLR-2/6 ligation in pulmonary bacterial infection and immunization with Pseudomonas aeruginosa

Pseudomonas aeruginosa (P. aeruginosa) is the major pathogen in nosocomial and life-threatening infections of immunocompromised or critically ill patients. The macrophage-activating lipopeptide-2 (MALP-2) activates the immune system via Toll-like receptors (TLR) 2 and 6 and leads to an accumulation of immune cells in lungs of young adult (8-10 week old) rats after intratracheal application. This is characterized by a high increase of granulocyte numbers in the BAL 24 h after MALP-2 treatment. It was hypothesized that MALP-2 may have a positive effect on the clinical course of an experimental infection. Therefore, rats were treated with MALP-2 at different time points following an infection with P. aeruginosa. The effect of MALP-2 in combination with immunization with inactivated P. aeruginosa was also investigated. Rats (n = 10) were infected intratracheally (i.t.) with 1 x 10(8) CFU P. aeruginosa on day 0. They were treated on day -3, -1, 0 and +1 with 2.5 microg MALP-2 or the vehicle i.t. In additional experiments, rats were immunized on day -21 and -14 with 1 x 10(8) CFU of inactivated P. aeruginosa bacteria and 2.5 microg MALP-2 or vehicle with 1 x 10(8) CFU of inactivated bacteria and isopropanol. The clinical score, rectal temperature and weight of the rats were checked in both treatment and immunization experiments twice a day. On day 2 they were sacrificed, CFU were determined in the left lung, the right lung being used for histology. In the group treated with MALP-2 1 day prior to infection significant effects were seen: The rectal temperature was about 2 degrees C higher in comparison to the controls at 6 h and also 1 day after infection. Both the symptoms of the infection and the weight loss were significantly reduced. In addition, the CFU and the inflammation in the lung tissue were significantly lower. These effects were not observed after treatment on day -3, 0 or +1. The MALP-2 enhanced immunization only resulted in a tendency to clinical improvement. In conclusion, local immunostimulation at the appropriate time can enhance the host defense against bacteria in the lung.

Caenorhabditis elegans semi-automated liquid screen reveals a specialized role for the chemotaxis gene cheB2 in Pseudomonas aeruginosa virulence

Pseudomonas aeruginosa is an opportunistic human pathogen that causes infections in a variety of animal and plant hosts. Caenorhabditis elegans is a simple model with which one can identify bacterial virulence genes. Previous studies with C. elegans have shown that depending on the growth medium, P. aeruginosa provokes different pathologies: slow or fast killing, lethal paralysis and red death. In this study, we developed a high-throughput semi-automated liquid-based assay such that an entire genome can readily be scanned for virulence genes in a short time period. We screened a 2,200-member STM mutant library generated in a cystic fibrosis airway P. aeruginosa isolate, TBCF10839. Twelve mutants were isolated each showing at least 70% attenuation in C. elegans killing. The selected mutants had insertions in regulatory genes, such as a histidine kinase sensor of two-component systems and a member of the AraC family, or in genes involved in adherence or chemotaxis. One mutant had an insertion in a cheB gene homologue, encoding a methylesterase involved in chemotaxis (CheB2). The cheB2 mutant was tested in a murine lung infection model and found to have a highly attenuated virulence. The cheB2 gene is part of the chemotactic gene cluster II, which was shown to be required for an optimal mobility in vitro. In P. aeruginosa, the main player in chemotaxis and mobility is the chemotactic gene cluster I, including cheB1. We show that, in contrast to the cheB2 mutant, a cheB1 mutant is not attenuated for virulence in C. elegans whereas in vitro motility and chemotaxis are severely impaired. We conclude that the virulence defect of the cheB2 mutant is not linked with a global motility defect but that instead the cheB2 gene is involved in a specific chemotactic response, which takes place during infection and is required for P. aeruginosa pathogenicity.

The increase in hospital acquired and multi-drug resistant bacterial infections calls for an urgent development of new antimicrobials. As such, the identification and characterization of novel molecular targets involved in bacterial virulence has become a common goal for researchers. The use of non-mammalian hosts, such as the nematode Caenorhabditis elegans, is useful to accelerate this process. In our study, we developed a high-throughput screening method, which further facilitates the use of C. elegans, and allows the rapid screening of a large collection of bacterial mutants at the genomic scale. We have used Pseudomonas aeruginosa, a potent opportunistic pathogen, to perform this study. The screening of more than 2,000 mutant strains allowed the characterization of a mutant affected in the cheB2 gene. Importantly, this mutant was shown to be impaired in a mouse model of infection, supporting that our new screen is a good model to identify virulence genes relevant for infection in mammals. The cheB2 gene encodes a component of a chemotaxis pathway, which is likely involved in the perception of stimuli during the infection process, and allows an appropriate adaptive response for a successful infection. Our method could be applied to other bacterial pathogens and will help researchers discover candidate genes leading to the design of novel antimicrobials.

Multiple roles of Pseudomonas aeruginosa TBCF10839 PilY1 in motility, transport and infection

Polymorphonuclear neutrophils are the most important mammalian host defence cells against infections with Pseudomonas aeruginosa. Screening of a signature tagged mutagenesis library of the non-piliated P. aeruginosa strain TBCF10839 uncovered that transposon inactivation of its pilY1 gene rendered the bacterium more resistant against killing by neutrophils than the wild type and any other of the more than 3000 tested mutants. Inactivation of pilY1 led to the loss of twitching motility in twitching-proficient wild-type PA14 and PAO1 strains, predisposed to autolysis and impaired the secretion of quinolones and pyocyanin, but on the other hand promoted growth in stationary phase and bacterial survival in murine airway infection models. The PilY1 population consisted of a major full-length and a minor shorter PilY1* isoform. PilY1* was detectable in small extracellular quinolone-positive aggregates, but not in the pilus. P. aeruginosa PilY1 is not an adhesin on the pilus tip, but assists in pilus biogenesis, twitching motility, secretion of secondary metabolites and in the control of cell density in the bacterial population.

Fitness of isogenic colony morphology variants of Pseudomonas aeruginosa in murine airway infection

Chronic lung infections with Pseudomonas aeruginosa are associated with the diversification of the persisting clone into niche specialists and morphotypes, a phenomenon called 'dissociative behaviour'. To explore the potential of P. aeruginosa to change its morphotype by single step loss-of-function mutagenesis, a signature-tagged mini-Tn5 plasposon library of the cystic fibrosis airway isolate TBCF10839 was screened for colony morphology variants under nine different conditions in vitro. Transposon insertion into 1% of the genome changed colony morphology into eight discernable morphotypes. Half of the 55 targets encode features of primary or secondary metabolism whereby quinolone production was frequently affected. In the other half the transposon had inserted into genes of the functional categories transport, regulation or motility/chemotaxis. To mimic dissociative behaviour of isogenic strains in lungs, pools of 25 colony morphology variants were tested for competitive fitness in an acute murine airway infection model. Six of the 55 mutants either grew better or worse in vivo than in vitro, respectively. Metabolic proficiency of the colony morphology variant was a key determinant for survival in murine airways. The most common morphotype of self-destructive autolysis did unexpectedly not impair fitness. Transposon insertions into homologous genes of strain PAO1 did not reproduce the TBCF10839 mutant morphotypes for 16 of 19 examined loci pointing to an important role of the genetic background on colony morphology. Depending on the chosen P. aeruginosa strain, functional genome scans will explore other areas of the evolutionary landscape. Based on our discordant findings of mutant phenotypes in P. aeruginosa strains PAO1, PA14 and TBCF10839, we conclude that the current focus on few reference strains may miss modes of niche adaptation and dissociative behaviour that are relevant for the microevolution of complex traits in the wild.

Functional genomics of Pseudomonas aeruginosa to identify habitat-specific determinants of pathogenicity

Half of all genes in the Pseudomonas aeruginosa genome have either no homology to any previously reported sequence or are homologues of previously reported genes of unknown function. The signature-tagged mutagenesis (STM) screening method allows to explore the role of these hypothetical and unknown proteins for the colonization and persistence of P. aeruginosa in eukaryotic hosts. A plasposon STM library was constructed in the virulent clinical P. aeruginosa isolate TBCF10839 that can persist in polymorphonuclear leukocytes (PMNs). The STM library was screened for plasposon mutants that were attenuated in the killing of the nematode Caenorhabditis elegans, deficient in quorum sensing and production of type II secretion effector proteins or had become more susceptible to killing by PMNs in phagocytosis assays. The three screens revealed in total 69 attenuated mutants. Fifteen mutants that carried the transposon in potential novel virulence determinants of yet unknown function were selected for further analysis. The mutants were characterized in their transcriptome and proteome and their cytotoxicity in vitro and their virulence in worm and mouse infection models in vivo. Previous studies had revealed a remarkable degree of conservation in the virulence mechanisms used by P. aeruginosa to infect hosts of divergent evolutionary origins. Testing of our novel targets did not reveal such a strict conservation. The functional characterization revealed that the fifteen proteins play highly diverse roles in the cell and become habitat-specific virulence factors upon exposure to specific hosts and/or upon exposure to specific stress conditions or host defense mechanisms.

Murine pulmonary infection with Listeria monocytogenes: differential susceptibility of BALB/c, C57BL/6 and DBA/2 mice

Murine listeriosis is a paradigm to understand host pathogen interactions. Airway infections with Listeria monocytogenes, although representing a serious problem in early onset neonatal listeriosis, has not been investigated in detail in animal models so far. Here, the susceptibility of BALB/c, DBA/2 and C57BL/6 mice towards an intratracheal (i.t.) infection with virulent L. monocytogenes EGDe and the attenuated variant L. monocytogenes EGD hlyW491A(pERL3-CMVGFP) is reported. The course of infection was characterized by determination of bacterial numbers in the organs and assessment of the health condition of the mice. The distribution and cellular localization of Listeria in the airways was assessed by immunocytochemistry and confocal and electron microscopy. The differential susceptibility of inbred mouse strains to airway infections with L. monocytogenes could be assigned to the major virulence factor listeriolysin O. Resistant C57BL/6 mice were not affected by the two listerial strains. In contrast, BALB/c and DBA/2 mice showed differential susceptibility towards L. monocytogenes EGDe and attenuated bacteria, with all the mice being killed by the wild-type bacteria but rarely by the variant that secretes a listeriolysin of only 10% activity of that of the wild-type toxin. Thus, listeriolysin is a decisive factor for differential susceptibility against Listeria. After i.t. application, bacteria were predominantly localized in the peribronchiolar space and invaded alveolar macrophages but rarely lung epithelial cells. Dissemination from the lung into the deep organs started almost immediately after application, although a pulmonary bacterial reservoir remained during the first 4 days.

Pulmonary microbial infection in mice: comparison of different application methods and correlation of bacterial numbers and histopathology

Many investigations have been performed in characterising experimental bacterial infections in the lung of mice using several pathogens. Robust experimental pulmonary infection models require a reproducible method of application with defined numbers of pathogens to the respiratory tract without contaminating extrapulmonary tissues. At the same time trauma due to the experimental procedure should be kept to a minimum. So far several routes of administration have been used but a systematic comparison of these methods is still missing. Here we provide a comprehensive evaluation of view controlled i.t. instillation, tracheotomy, intranasal application, blind instillation and aerosol infection. An infection dose of up to 5 x 10(4) bacteria (L. monocytogenes) was applied to a group of ten mice by each technique and the animals were killed after 1 h or 24h. The number of viable bacteria was estimated by plating homogenates of the lungs and spleens. In addition, pathological effects on lung tissue were examined by histology 24h after infection. The highest reproducibility was achieved after applying Listeria directly in the trachea under view or by tracheotomy. However, mice were severely affected in their vitality after tracheotomy. Thus, for topical application of bacterial suspension into the lung the view controlled i.t. instillation is most appropriate.