eThe opportunistic pathogen Pseudomonas aeruginosa is capable of establishing severe and persistent infections in various eukaryotic hosts. It encodes a wide array of virulence factors and employs several strategies to evade immune detection. In the present study, we screened the Harvard Medical School transposon mutant library of P. aeruginosa PA14 for bacterial factors that modulate interleukin-8 responses in A549 human airway epithelial cells. We found that in addition to the previously identified alkaline protease AprA, the elastase LasB is capable of degrading exogenous flagellin under calcium-replete conditions and prevents flagellin-mediated immune recognition. Our results indicate that the production of two proteases with anti-flagellin activity provides a failsafe mechanism for P. aeruginosa to ensure the maintenance of protease-dependent immune-modulating functions.

fThe recent finding that high numbers of strict anaerobes are present in the respiratory tract of cystic fibrosis (CF) patients has drawn attention to the pathogenic contribution of the CF microbiome to airway disease. In this study, we investigated the specific interactions of the most dominant bacterial CF pathogen, Pseudomonas aeruginosa, with the anaerobic bacterium Veillonella parvula, which has been recovered at comparable cell numbers from the respiratory tract of CF patients. In addition to growth competition experiments, transcriptional profiling, and analyses of biofilm formation by in vitro studies, we used our recently established in vivo murine tumor model to investigate mutual influences of the two pathogens during a biofilm-associated infection process. We found that P. aeruginosa and V. parvula colonized distinct niches within the tumor. Interestingly, significantly higher cell numbers of P. aeruginosa could be recovered from the tumor tissue when mice were coinfected with both bacterial species than when mice were monoinfected with P. aeruginosa. Concordantly, the results of in vivo transcriptional profiling implied that the presence of V. parvula supports P. aeruginosa growth at the site of infection in the host, and the higher P. aeruginosa load correlated with clinical deterioration of the host. Although many challenges must be overcome to dissect the specific interactions of coinfecting bacteria during an infection process, our findings exemplarily demonstrate that the complex interrelations between coinfecting microorganisms and the immune responses determine clinical outcome to a much greater extent than previously anticipated.

Pseudomonas aeruginosa is an important opportunistic pathogen that can cause acute respiratory infections in immunocompetent patients or chronic infections in immunocompromised individuals and in patients with cystic fibrosis. When acquiring the chronic infection state, bacteria are encapsulated within biofilm structures enabling them to withstand diverse environmental assaults, including immune reactions and antimicrobial therapy. Understanding the molecular interactions within the bacteria, as well as with the host or other bacteria, is essential for developing innovative treatment strategies. Such knowledge might be accumulated in vitro. However, it is ultimately necessary to confirm these findings in vivo. In the present Review, we describe state‐of‐the‐art in vivo models that allow studying P. aeruginosa infections in molecular detail. The portrayed mammalian models exclusively focus on respiratory infections. The data obtained by alternative animal models which lack lung tissue, often provide molecular insights that are easily transferable to mammals. Importantly, these surrogate in vivo systems reveal complex molecular interactions of P. aeruginosa with the host. Herein, we also provide a critical assessment of the advantages and disadvantages of such models.