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

Experimental Models of Hospital-Acquired Infections After Traumatic Brain Injury: Challenges and Opportunities

Patients hospitalized after a moderate or severe traumatic brain injury (TBI) are at increased risk of nosocomial infections, including bacterial pneumonia and other upper respiratory tract infections. Infections represent a secondary immune challenge for vulnerable TBI patients that can lead to increased morbidity and poorer long-term prognosis. This review first describes the clinical significance of infections after TBI, delving into the known mechanisms by which a TBI can alter systemic immunological responses towards an immunosuppressive state, leading to promotion of increased vulnerability to infections. Pulmonary dysfunction resulting from respiratory tract infections is considered in the context of neurotrauma, including the bidirectional relationship between the brain and lungs. Turning to pre-clinical modeling, current laboratory approaches to study experimental TBI and lung infections are reviewed, to highlight findings from the limited key studies to date that have incorporated both insults. Then, practical decisions for the experimental design of animal studies of post-injury infections are discussed. Variables associated with the host animal, the infectious agent (e.g., species, strain, dose, and administration route), as well as the timing of the infection relative to the injury model are important considerations for model development. Together, the purpose of this review is to highlight the significant clinical need for increased pre-clinical research into the two-hit insult of a hospital-acquired infection after TBI to encourage further scientific enquiry in the field.

To bead or not to bead: A review of Pseudomonas aeruginosa lung infection models for cystic fibrosis

Cystic fibrosis (CF) lung disease is characterised by recurring bacterial infections resulting in inflammation, lung damage and ultimately respiratory failure. Pseudomonas aeruginosa is considered one of the most important lung pathogens in those with cystic fibrosis. While multiple cystic fibrosis animal models have been developed, many fail to mirror the cystic fibrosis lung disease of humans, including the colonisation by opportunistic environmental pathogens. Delivering bacteria to the lungs of animals in different forms is a way to model cystic fibrosis bacterial lung infections and disease. This review presents an overview of previous models, and factors to consider when generating a new P. aeruginosa lung infection model. The future development and application of lung infection models that more accurately reflect human cystic fibrosis lung disease has the potential to assist in understanding the pathophysiology of cystic fibrosis lung disease and for developing treatments.

Brd4 inhibition ameliorates Pyocyanin-mediated macrophage dysfunction via transcriptional repression of reactive oxygen and nitrogen free radical pathways

Macrophages play critical roles in the first-line immune defense against airway infections caused by Pseudomonas aeruginosa (PA). The redox-active phenazine-pyocyanin (PCN), as one of the most essential virulence factors, facilities PA-related infection via a wide spectrum of cellular oxidative damages. However, little is known for PCN cytotoxicity in macrophages. In this study, besides showing PCN-mediated reactive oxygen species (ROS) indeed involved in macrophage viability and function impairment, we at the first time demonstrated a novel role of reactive nitrogen species (RNS) pathway causing cellular damage in PCN-challenged macrophages. Using small molecule inhibitor JQ1 targeting Bromodomain and extra-terminal family proteins, we showed restrained iNOS-dependent nitric oxide (NO) production correlated with abolished Brd4 recruitment to the NOS2 (encoding inducible nitric oxide synthase-iNOS) promoter. Application of JQ1 diminished PCN-mediated peroxynitrite (ONOO-) that followed ROS and NO induction, restored macrophage survival and bacteria clearance as well as repressed local inflammation in PA/PCN-challenged mice lungs. Our results uncover a novel link between PCN-mediated macrophage dysfunction and reactive free radicals that rely on Brd4-dependent transcription modulation of multiple stress-response genes, suggesting Brd4 could be a promising therapeutic target in treating PA-related lung infection.

Pathologic changes and immune responses against Coxiella burnetii in mice following infection via non-invasive intratracheal inoculation

Q fever is a worldwide zoonosis caused by Coxiella burnetii. Human Q fever is typically acquired through inhalation of contaminated aerosols, resulting in an initial pulmonary infection. In this study, BALB/c mice were infected with C. burnetii via an intratracheal (IT) route using a non-invasive aerosol pulmonary delivery device to directly place the living C. burnetii organisms into the lungs of the mice. The bacterial loads, pathological lesions, and antibody and cellular responses were analyzed and compared with those of mice infected via an intraperitoneal (IP) route. Compared with mice infected via an IP route, mice infected via an IT route exhibited a higher bacterial load and more severe pathological lesions in the heart and lungs at days 3 and 7 post-infection (pi). The levels of interferon-γ and IL-12p70 in the serum of mice infected via the IT route were significantly higher than those of mice infected via the IP route at day 3 pi. In conclusion, this murine model of acute C. burnetii infection via IT inoculation closely resembles the natural route of C. burnetii infection than that of IP injection. Thus, this newly developed model will be useful for investigating the pathogenesis and immunity of C. burnetii aerosol infection, as well as for the evaluation of therapeutic drugs and preventive vaccines of Q fever.

Pathogenesis of Respiratory Syncytial Virus Infection in BALB/c Mice Differs Between Intratracheal and Intranasal Inoculation

Human respiratory syncytial virus (RSV) is a major cause of severe lower respiratory tract disease requiring hospitalization in infants. There are no market-approved vaccines or antiviral agents available, but a growing number of vaccines and therapeutics are in (pre)clinical stages of development. Reliable animal models are crucial to evaluate new vaccine concepts, but in vivo RSV research is hampered by the lack of well-characterized animal models that faithfully mimic the pathogenesis of RSV infection in humans. Mice are frequently used in RSV infection and vaccination studies. However, differences in the use of mouse strains, RSV subtypes, and methodology often lead to divergent study outcomes. To our knowledge, a comparison between different RSV inoculation methods in mice has not been described in the literature, even though multiple methods are being used across different studies. In this study, we evaluated various pathological and immunological parameters in BALB/c mice after intratracheal or intranasal inoculation with RSV-A2. Our study reveals that intranasal inoculation induces robust pathology and inflammation, whereas this is not the case for intratracheal inoculation. As immunopathology is an important characteristic of RSV disease in infants, these data suggest that in mice intranasal inoculation is a more appropriate method to study RSV infection than intratracheal inoculation. These findings will contribute to the rational experimental design of future in vivo RSV experiments.

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.

A Mouse Model of Post-Stroke Pneumonia Induced by Intra-Tracheal Inoculation with Streptococcus pneumoniae

Background: Stroke-induced immunodeficiency increases the risk of infectious complications, which adversely affects neurological outcome. Among those, pneumonia affects as many as one third of stroke patients and is the main contributor to mortality in the post-acute phase of stroke. Experimental findings on post-stroke susceptibility to spontaneous pneumonia in mice are contradictory. Here, we established a mouse model inducing standardized bacterial pneumonia and characterized the impaired pulmonary cellular and humoral immune responses after experimental stroke. Methods: Bacterial pneumonia was induced by intra-tracheal inoculation with Streptococcus pneumoniae at different time points after transient middle cerebral artery occlusion (MCAO). Bacterial counts in lungs and blood, histological changes, and cytokine production in the lungs were assessed. Furthermore, we investigated the effect of pneumonia on stroke outcome. Results: Intra-tracheal inoculation resulted in reproducible pneumonia and bacteraemia, and demonstrated post-stroke susceptibility to streptococcal pneumonia developing with a delay of at least 24 h after MCAO. Higher bacterial counts in mice infected 3 days after stroke induction correlated with reduced neutrophil and macrophage infiltration in the lungs and lower levels of pro-inflammatory cytokines in the broncho-alveolar lavage compared to sham-operated animals. Pneumonia increased mortality without affecting brain-infiltrating leukocytes. Conclusions: In this standardized mouse model of post-stroke pneumonia, we describe attenuated leukocyte infiltration and cytokine production in response to bacterial infection in the lungs that has a profound effect on outcome.

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.

Imaging regional lung function: a critical tool for developing inhaled antimicrobial therapies

Alterations in regional lung function due to respiratory infection have a significant effect on the deposition of inhaled treatments. This has consequences for treatment effectiveness and hence recovery of lung function. In order to advance our understanding of respiratory infection and inhaled treatment delivery, we must develop imaging techniques that can provide regional functional measurements of the lung. In this review, we explore the role of functional imaging for the assessment of respiratory infection and development of inhaled treatments. We describe established and emerging functional lung imaging methods. The effect of infection on lung function is described, and the link between regional disease, function, and inhaled treatments is discussed. The potential for lung function imaging to provide unique insights into the functional consequences of infection, and its treatment, is also discussed.

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.

Use of a safe, reproducible, and rapid aerosol delivery method to study infection by Burkholderia pseudomallei and Burkholderia mallei in mice

Burkholderia pseudomallei, the etiologic agent of melioidosis, is a saprophytic bacterium readily isolated from wet soils of countries bordering the equator. Burkholderia mallei is a host-adapted clone of B. pseudomallei that does not persist outside of its equine reservoir and causes the zoonosis glanders, which is endemic in Asia, Africa, the Middle East and South America. Infection by these organisms typically occurs via percutaneous inoculation or inhalation of aerosols, and the most common manifestation is severe pneumonia leading to fatal bacteremia. Glanders and melioidosis are difficult to diagnose and require prolonged antibiotic therapy with low success rates. There are no vaccines available to protect against either Burkholderia species, and there is concern regarding their use as biological warfare agents given that B. mallei has previously been utilized in this manner. Hence, experiments were performed to establish a mouse model of aerosol infection to study the organisms and develop countermeasures. Using a hand-held aerosolizer, BALB/c mice were inoculated intratracheally with strains B. pseudomallei 1026b and B. mallei ATCC23344 and growth of the agents in the lungs, as well as dissemination to the spleen, were examined. Mice infected with 10², 10³ and 10⁴ organisms were unable to control growth of B. mallei in the lungs and bacteria rapidly disseminated to the spleen. Though similar results were observed in mice inoculated with 10³ and 10⁴B. pseudomallei cells, animals infected with 10² organisms controlled bacterial replication in the lungs, dissemination to the spleen, and the extent of bacteremia. Analysis of sera from mice surviving acute infection revealed that animals produced antibodies against antigens known to be targets of the immune response in humans. Taken together, these data show that small volume aerosol inoculation of mice results in acute disease, dose-dependent chronic infection, and immune responses that correlate with those seen in human infections.

A non-invasive intratracheal inoculation method for the study of pulmonary melioidosis

Pulmonary melioidosis, a disease manifestation caused by the bacterium Burkholderia pseudomallei, has been studied using aerosols or intranasal (IN) inoculation in small animal models. Both have inherent disadvantages which may not accurately model primary pulmonary melioidosis in humans. Intratracheal inoculation (IT) by direct visualization of the tracheal opening offers an alternative technique for infection that overcomes the disadvantages of aerosol and IN challenge. In this study, we describe a method which requires relatively inexpensive equipment, little training, and is compliant with the operational constraints of a BSL3 laboratory. Results obtained using trypan blue demonstrated that an inoculum can be accurately delivered into the lungs of mice within a biosafety cabinet (BSC). Whole body imaging and histopathology confirmed that mice inoculated intratracheally with B. pseudomallei develop the primary focus of infection in the lungs, and not the nasal passages which can lead to invasion of the central nervous system and potential neurologic complications. Further, based on colony counts and bioluminescent imaging, dissemination to secondary organs occurred as expected. Taken together, this intratracheal method of inoculation fulfills four goals: (1) to accurately deliver B. pseudomallei into the lungs of the animal model, (2) to avoid potentially confounding complications due to primary infections at sites other than the lung, (3) to maintain normal organ dissemination, and (4) to be BSL3 compliant.

Ex vivo transcriptional profiling reveals a common set of genes important for the adaptation of Pseudomonas aeruginosa to chronically infected host sites

The opportunistic bacterium Pseudomonas aeruginosa is a major nosocomial pathogen causing both devastating acute and chronic persistent infections. During the course of an infection, P.  aeruginosa rapidly adapts to the specific conditions within the host. In the present study, we aimed at the identification of genes that are highly expressed during biofilm infections such as in chronically infected lungs of patients with cystic fibrosis (CF), burn wounds and subcutaneous mouse tumours. We found a common subset of differentially regulated genes in all three in vivo habitats and evaluated whether their inactivation impacts on the bacterial capability to form biofilms in vitro and to establish biofilm-associated infections in a murine model. Additive effects on biofilm formation and host colonization were discovered by the combined inactivation of several highly expressed genes. However, even combined inactivation was not sufficient to abolish the establishment of an infection completely. These findings can be interpreted as evidence that either redundant traits encode functions that are essential for in vivo survival and chronic biofilm infections and/or bacterial adaptation is considerably achieved independently of transcription levels. Supplemental screens, will have to be applied in order to identify the minimal set of key genes essential for the establishment of chronic infectious diseases.

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.

Bacterial distribution in lung parenchyma early after pulmonary infection with Pseudomonas aeruginosa

Nosocomial infections often cause lethal pneumogenic sepsis. Information on early bacteria-host interaction in the lung is limited. In the present study, mice were sacrificed 60 min and 4 h after Pseudomonas aeruginosa (PA) infection to investigate lung morphology by using electron microscopy and light microscopy. After 1 h, bacteria were found in the alveoli partly in contact with surfactant. Alveolar macrophages were seen with up to 10 intracellular bacteria close to protrusions of alveolar epithelial type I cells and the gas/blood barrier. A rare but surprising finding was bacteria and even replicating bacteria in alveolar epithelial type II cells (AEII). No bacteria were seen in capillaries. Neither engulfment of bacteria by neutrophils nor structural damage of the pulmonary barrier was visible. After 4 h, many neutrophils were found within the capillaries, but also in the alveolar space. Thus, we hypothesize that, in early stages of infection, the uptake of PA even by single AEII can influence the course of the disease.

Genome diversity of Pseudomonas aeruginosa PAO1 laboratory strains

Pseudomonas aeruginosa PAO1 is the most commonly used strain for research on this ubiquitous and metabolically versatile opportunistic pathogen. Strain PAO1, a derivative of the original Australian PAO isolate, has been distributed worldwide to laboratories and strain collections. Over decades discordant phenotypes of PAO1 sublines have emerged. Taking the existing PAO1-UW genome sequence (named after the University of Washington, which led the sequencing project) as a blueprint, the genome sequences of reference strains MPAO1 and PAO1-DSM (stored at the German Collection for Microorganisms and Cell Cultures [DSMZ]) were resolved by physical mapping and deep short read sequencing-by-synthesis. MPAO1 has been the source of near-saturation libraries of transposon insertion mutants, and PAO1-DSM is identical in its SpeI-DpnI restriction map with the original isolate. The major genomic differences of MPAO1 and PAO1-DSM in comparison to PAO1-UW are the lack of a large inversion, a duplication of a mobile 12-kb prophage region carrying a distinct integrase and protein phosphatases or kinases, deletions of 3 to 1,006 bp in size, and at least 39 single-nucleotide substitutions, 17 of which affect protein sequences. The PAO1 sublines differed in their ability to cope with nutrient limitation and their virulence in an acute murine airway infection model. Subline PAO1-DSM outnumbered the two other sublines in late stationary growth phase. In conclusion, P. aeruginosa PAO1 shows an ongoing microevolution of genotype and phenotype that jeopardizes the reproducibility of research. High-throughput genome resequencing will resolve more cases and could become a proper quality control for strain collections.

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.