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SummarySubversion of host cell actin microfilaments is the hallmark of enterohaemorrhagic (EHEC) and enteropathogenic (EPEC) Escherichia coli infections. Both pathogens translocate the trans -membrane receptor protein -translocated intimin receptor (Tir), which links the extracellular bacterium to the cell cytoskeleton. While both converge on neural Wiskott-Aldrich syndrome protein (N-WASP), Tir-mediated actin accretion by EPEC and EHEC differ in that Tir EPEC requires both tyrosine phosphorylation and the host adaptor protein Nck, whereas Tir EHEC is not phosphorylated and utilizes an unidentified linker. Here we report the identification of Tir-cytoskeleton coupling protein (TccP), a novel EHEC effector that displays an Ncklike coupling activity following translocation into host cells. A tccP mutant did not affect Tir translocation and focusing but failed to recruit a a a a -actinin, Arp3, N-WASP and actin to the site of bacterial adhesion.When expressed in EPEC, bacterial-derived TccP restored actin polymerization activity following infection of an Nck-deficient cell line. TccP has a similar biological activity on infected human intestinal explants ex vivo . Purified TccP activates N-WASP stimulating, in the presence of Arp2/3, actin polymerization in vitro . These results show that EHEC translocates both its own receptor (Tir) and an Nck-like protein (TccP) to facilitate actin polymerization.

Expression of the Salmonella enterica serovar Typhimurium pathogenicity island 2 (SPI-2) type III secretion system is controlled by the two-component regulatory system SsrA-SsrB. We used a transcriptomic approach to help define the SsrA-SsrB regulon. We identified a gene encoding an uncharacterized effector (SseL) whose translocation into host cells depends on the SPI-2 secretion system. SseL has similarities to cysteine proteases with deubiquitinating activity. A GST-SseL fusion protein specifically hydrolyzed mono-and polyubiquitin substrates in vitro with a preference for K63-linked ubiquitin chains. Ubiquitin-modified proteins accumulated in macrophages infected with Salmonella sseL mutant strains but to a lesser extent when infected with bacteria expressing active protein, demonstrating that SseL functions as a deubiquitinase in vivo. Salmonella sseL mutant strains did not show a replication defect or induce altered levels of cytokine production upon infection of macrophages but were defective for a delayed cytotoxic effect and were attenuated for virulence in mice.cytotoxicity ͉ SPI-2 ͉ deubiquitinating enzyme

The implementation of infection models that approximate human disease is essential for understanding pathogenesis at the molecular level and for testing new therapies before they are entered into clinical stages. Insects are increasingly being used as surrogate hosts because they share, with mammals, essential aspects of the innate immune response to infections. We examined whether the larva of the wax moth Galleria mellonella could be used as a host model to conceptually approximate Klebsiella pneumoniae-triggered pneumonia. We report that the G. mellonella model is capable of distinguishing between pathogenic and nonpathogenic Klebsiella strains. Moreover, K. pneumoniae infection of G. mellonella models some of the known features of Klebsiella-induced pneumonia, i.e., cell death associated with bacterial replication, avoidance of phagocytosis by phagocytes, and the attenuation of host defense responses, chiefly the production of antimicrobial factors. Similar to the case for the mouse pneumonia model, activation of innate responses improved G. mellonella survival against subsequent Klebsiella challenge. Virulence factors necessary in the mouse pneumonia model were also implicated in the Galleria model. We found that mutants lacking capsule polysaccharide, lipid A decorations, or the outer membrane proteins OmpA and OmpK36 were attenuated in Galleria. All mutants activated G. mellonella defensive responses. The Galleria model also allowed us to monitor Klebsiella gene expression. The expression levels of cps and the loci implicated in lipid A remodeling peaked during the first hours postinfection, in a PhoPQ-and PmrAB-governed process. Taken together, these results support the utility of G. mellonella as a surrogate host for assessing infections with K. pneumoniae. In 1890, Robert Koch formulated Koch's postulates as general guidelines for identifying disease-causing organisms. One century later, Stanley Falkow established the molecular version of Koch's postulates, this time to guide the identification of microbial genes encoding virulence factors. One of the key points of the molecular postulates is to test the virulence of a microorganism with an inactivated candidate virulence gene in an appropriate animal model. Therefore, the use of animal models to identify the virulence factors of human pathogens is indispensable. Currently, identification and characterization of novel virulence factors rely largely on assessing mutant bacteria for growth in the organs of infected mice. The dependence on mouse infection models limits large-scale analysis of virulence due to the large number of animals needed to obtain statistically significant results.To circumvent these issues, the search for alternative host models is ongoing. Ideally, these alternative models should be easy to maintain and infect, should be amenable to genetic manipulation, and should model aspects of vertebrate defenses upon infection, chiefly the immune response. The immune defense consists of two main parts, an innate and an adaptive response, wit...

Nontypable Haemophilus influenzae (NTHi) is a Gram-negative, non-capsulated human bacterial pathogen, a major cause of a repertoire of respiratory infections, and intimately associated with persistent lung bacterial colonization in patients suffering from chronic obstructive pulmonary disease (COPD). Despite its medical relevance, relatively little is known about its mechanisms of pathogenicity. In this study, we found that NTHi invades the airway epithelium by a distinct mechanism, requiring microtubule assembly, lipid rafts integrity, and activation of phosphatidylinositol 3-kinase (PI3K) signalling. We found that the majority of intracellular bacteria are located inside an acidic subcellular compartment, in a metabolically active and non-proliferative state. This NTHi-containing vacuole (NTHi-CV) is endowed with late endosome features, co-localizing with LysoTracker, lamp-1, lamp-2, CD63 and Rab7. The NTHi-CV does not acquire Golgi-or autophagy-related markers. These observations were extended to immortalized and primary human airway epithelial cells. By using NTHi clinical isolates expressing different amounts of phosphocholine (PCho), a major modification of NTHi lipooligosaccharide, on their surfaces, and an isogenic lic1BC mutant strain lacking PCho, we showed that PCho is not responsible for NTHi intracellular location. In sum, this study indicates that NTHi can survive inside airway epithelial cells. INTRODUCTIONHaemophilus influenzae is a Gram-negative human pathogen that colonizes asymptomatically the nasopharynx of healthy individuals. Disease occurs when the bacterium reaches privileged anatomical sites under various predisposing conditions, including, among others, age, viral infections or a constant exposure to pollutants, leading to otitis media, sinusitis, meningitis, septicaemia or respiratory infections (Foxwell et al., 1998;Rao et al., 1999). Nontypable H. influenzae (NTHi) is the most frequently isolated bacterial pathogen in the lungs of patients suffering from chronic obstructive pulmonary disease (COPD), and furthermore, NTHi accounts for the majority of bacterial COPD exacerbation episodes (Sethi et al., 2002;Sethi & Murphy, 2001.One of the NTHi virulence factors that has been most extensively analysed is its lipooligosaccharide (LOS). It consists of lipid A, an inner core consisting of a Abbreviations: COPD, chronic obstructive pulmonary disease; EEA1, early endosome antigen 1; FISH, fluorescent in situ hybridization; LOS, lipooligosaccharide; MbCD, methyl-b-cyclodextrin; NHBE cells, normal human bronchial epithelial cells; NTHi, nontypable Haemophilus influenzae; NTHi-CV, NTHi-containing vacuole; PCho, phosphocholine; PFA, paraformaldehyde; PI3K, phosphatidylinositol 3-kinase; SCV, Salmonella-containing vacuole; TEM, transmission electron microscopy. phosphorylated 2-keto-3-deoxyoctulosonic acid (KDO) linked to three heptoses and an outer core consisting of a heteropolymer of glucose and galactose (Schweda et al., 2007). Similar to other bacterial pathogens adapted to mucosal surfaces,...

This is the peer reviewed version of the following article: Cano, V., March, C., Insua, J. L., Aguilo, N., Llobot, E., MorataD., Regueiro, V., Brenan, G. P., Millan-Lou, M. I., Martin, C., Garmendia, J., Bengoechea, J. A. (2015),Klebsiella pneumoniae survives within macrophages by avoiding delivery to lysosomes. Cellular Microbiology, VOL: PAGE NO's. which has been published in final form at http://onlinelibrary.wiley.com/doi/10.1111/cmi.12466/abstract. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving. General rightsCopyright for the publications made accessible via the Queen's University Belfast Research Portal is retained by the author(s) and / or other copyright owners and it is a condition of accessing these publications that users recognise and abide by the legal requirements associated with these rights.Take down policy The Research Portal is Queen's institutional repository that provides access to Queen's research output. Every effort has been made to ensure that content in the Research Portal does not infringe any person's rights, or applicable UK laws. If you discover content in the Research Portal that you believe breaches copyright or violates any law, please contact openaccess@qub.ac.uk.

The outcome of an infection depends on host recognition of the pathogen, hence leading to the activation of signaling pathways controlling defense responses. A long-held belief is that the modification of the lipid A moiety of the lipopolysaccharide could help Gram-negative pathogens to evade innate immunity. However, direct evidence that this happens in vivo is lacking. Here we report the lipid A expressed in the tissues of infected mice by the human pathogen Klebsiella pneumoniae. Our findings demonstrate that Klebsiella remodels its lipid A in a tissue-dependent manner. Lipid A species found in the lungs are consistent with a 2-hydroxyacyl-modified lipid A dependent on the PhoPQ-regulated oxygenase LpxO. The in vivo lipid A pattern is lost in minimally passaged bacteria isolated from the tissues. LpxO-dependent modification reduces the activation of inflammatory responses and mediates resistance to antimicrobial peptides. An lpxO mutant is attenuated in vivo thereby highlighting the importance of this lipid A modification in Klebsiella infection biology. Colistin, one of the last options to treat multidrug-resistant Klebsiella infections, triggers the in vivo lipid A pattern. Moreover, colistin-resistant isolates already express the in vivo lipid A pattern. In these isolates, LpxO-dependent lipid A modification mediates resistance to colistin. Deciphering the lipid A expressed in vivo opens the possibility of designing novel therapeutics targeting the enzymes responsible for the in vivo lipid A pattern.

Phagocytosis is a key process of the immune system. The human pathogen Klebsiella pneumoniae is a well known example of a pathogen highly resistant to phagocytosis. A wealth of evidence demonstrates that the capsule polysaccharide (CPS) plays a crucial role in resistance to phagocytosis. The amoeba Dictyostelium discoideum shares with mammalian macrophages the ability to phagocytose and kill bacteria. The fact that K. pneumoniae is ubiquitous in nature and, therefore, should avoid predation by amoebae, poses the question whether K. pneumoniae employs similar means to counteract amoebae and mammalian phagocytes. Here we developed an assay to evaluate K. pneumoniae-D. discoideum interaction. The richness of the growth medium affected the threshold at which the cps mutant was permissive for Dictyostelium and only at lower nutrient concentrations the cps mutant was susceptible to predation by amoebae. Given the critical role of bacterial surface elements on host-pathogen interactions, we explored the possible contribution of the lipopolysaccharide (LPS) and outer membrane proteins (OMPs) to combat phagoyctosis by D. discoideum. We uncover that, in addition to the CPS, the LPS O-polysaccharide and the first core sugar participate in Klebsiella resistance to predation by D. discoideum. K. pneumoniae LPS lipid A decorations are also necessary to avoid predation by amoebae although PagP-dependent palmitoylation plays a more important role than the lipid A modification with aminoarabinose. Mutants lacking OMPs OmpA or OmpK36 were also permissive for D. discoideium growth. Except the LPS O-polysaccharide mutants, all mutants were more susceptible to phagocytosis by mouse alveolar macrophages. Finally, we found a correlation between virulence, using the pneumonia mouse model, and resistance to phagocytosis. Altogether, this work reveals novel K. pneumoniae determinants involved in resistance to phagocytosis and supports the notion that Dictyostelium amoebae might be useful as host model to measure K. pneumoniae virulence and not only phagocytosis.