Thesis advisor: Mary F. Roberts / Thesis advisor: Jianmin Gao / The research reported in this thesis focused on three proteinaceous virulence factors of the intracellular bacterial pathogen Listeria monocytogenes: listeriolysin O (LLO), broad-range phospholipase C (PC-PLC), and phosphatidylinositol-specific phospholipase C (PI-PLC). Based on sequence homology of LLO with other cholesterol-dependent cytolysins (CDC), the protein has four domains of which domain 4 is thought to anchor the protein to cholesterol-containing surfaces while domain 3 mediates protein-protein binding on the membrane and contributes α-helices that convert to two β-strands that form the large β-barrel pore. It was previously assumed that the sequential and cooperative behaviors of domain 3 in each LLO monomer required D4 to bind to cholesterol-enriched membranes. By cloning and expressing a separate protein containing domains 1, 2, and 3 (D123) and the isolated domain 4 (D4) of LLO, I could uncouple some of the events in its membrane binding and pore-formation. Flow cytometry, used to investigate protein binding to vesicles and to red blood cells, showed that D123 had no membrane affinity on its own, but became membrane-bound when sub-lytic amounts of LLO were added. D123, not membrane-lytic by itself, became hemolytic when trace amounts of LLO were present to provide a membrane anchor for D123 proteins. FRET and fluorescence correlation spectroscopy were used to show that D123 and LLO formed oligomers at nanomolar concentration and could also associate with one another in the solution. These results suggest that D4 provides an initial membrane attachment but need not be present on all monomers to trigger the cooperative conformational change that leads to membrane insertion and pore formation. The gene for L. monocytogenes PC-PLC was obtained, expressed in E. coli and the product protein purified and characterized. The zinc content of this metalloenzyme was analyzed with ICP-MS. The dissociation constants of the three zinc ions proposed as necessary for PC-PLC activity ranged from 0.05 to 60 μM. Enzymatic activities of PC-PLC were analyzed for various substrates, include long-chain phospholipid in vesicles (LUVs, SUVs) and micelles (Triton X-100), and short-chain lipids (diC4PC, diC6PC, diC7PC) mono-dispersed in solutions. Key results include the following: (1) the L. monocytogenes PC-PLC has an acidic pH optimum (in contrast to other bacterial PC-PLC enzymes) consistent with its role in vacuole lysis upon acidification; (2) the preference of PC-PLC for longer chain monomeric substrates is not because of a higher kcat but a reduced Km suggesting some amount of hydrophobicity is important for substrate binding in the active site; (3) the apparent Kd of PC-PLC for Zn2+ derived from kinetics at pH 6.0 (1.94 ± 0.22 μM) is lower that that from ICP-MC; and (4) PC-PLC enzymatic activity is not enhanced by added LLO that generates pores in vesicles (likewise, PC-PLC does not affect the membrane lytic activity of LLO) indicating no synergism between the two virulence factors. These results should aid in understanding the function of PC-PLC in L. monocytogenes pathogenicity. The L. monocytogenes PI-PLC and a variant with reduced catalytic activity were expressed and are currently used in a collaborative project with the Portnoy laboratory at the University of California at Berkeley. / Thesis (PhD) — Boston College, 2014. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

The pathogenicity of food-borne intracellular bacterium Listeria monocytogenes is greatly associated with its abilities to invade non-phagocytic cells, counteract the host innate immune system, resist bactericidal antibiotic-mediated killing, and breaking the physical barriers. In the last 30 years of research on L. monocytogenes, several virulence factors, such as Listeriolysin O (LLO), InlA, InlB, ActA, PI-PLC, and PC-PLC have already been characterized as important players that help this bacterium to achieve the key stage of infection. There are approximately 3,000 open reading frames in Listeria’s genome; however, only few virulence factors are functionally characterized. Thus, it is important to identify new virulence factors and understand how new virulence factors in Listeria help this opportunistic pathogen to counteract the host innate immune system, resist antibiotic-mediated killing, colonize vital organs, and finally successfully develop life-threatening listeriosis. In this study, inrame deletion mutagenesis was used to generate the deletion mutants of novel listerial virulence factors and a series of biochemical, in vitro and in vivo experiments were conducted to characterize the roles of these virulence factors during the infection process. In the first part of this study, an AlkD-like protein (Adlp, LmoF2365_0220) was identified and the protein is associated with oxidant tolerance and aminoglycoside antibiotic resistance. In the second part of this study, a new internalin-like protein (LmoH7858_0369) was shown to be involved in invasion of Hep-G2 cells and organ colonization in mice. The third part of this study showed that listeriolysin O (LLO) mediates cytotoxicity on brain endothelial cells, suggesting that LLO may contribute to the invasion of the central nervous system by L. monocytogenes. In summary, we identified and characterized two novel virulence factors, Adlp and LmoH7858_0369 that contributed to bacterial infection and revealed a new invasion mechanism of CNS cells that is mediated by LLO. Results from these studies provide a better understanding on the pathogenicity of L. monocytogenes and can be used as therapeutical targets or vaccine candidates

<p>Gram-positive pathogens, such as <italic>Streptococcus pyogenes</italic> and <italic>Staphylococcus aureus</italic>, are etiological agents of a large array of human diseases. Unfortunately, our ability to treat these infections is increasingly limited due to the development of bacterial resistance to many existing therapies. Thus, novel targets for antimicrobial development are urgently needed. An attractive candidate for a new class of anti-virulence chemotherapeutics is the sortase class of enzymes. Sortases are extracellular transpeptidases unique to Gram-positive bacteria. Their function is to covalently attach secreted virulence factors to the bacterial cell wall. Deletion or inhibition of sortases results in severe attenuation of bacteria for infection. In order to develop novel effective antimicrobial agents, a robust understanding of the biological and chemical mechanisms of the target are required. To this end, this dissertation endeavors to further illuminate the biochemical mechanism of sortase enzymes and to extend the current knowledge of the roles of sortases and their substrates during infection.</p><p>Through steady-state kinetics, active site reactivity measurements, three-dimensional structure determination via X-ray crystallography, and computational modeling of substrate binding, the basic enzyme mechanism of <italic>S. pyogenes</italic> sortase A (SrtA) has been revealed. In general, <italic>S. pyogenes</italic> SrtA displays many of the same mechanistic characteristics as previously studied sortases, including a reverse protonation mechanism, a conserved tertiary structure arrangement, and utilization of similar substrate binding interfaces and conserved active site residue functions. These findings suggest a general sortase mechanism, conserved among classes and species.</p><p>Initial steps have also been taken to characterize <italic>S. pyogenes</italic> sortase C (SrtC). SrtC enzymes are unique in that they covalently polymerize secreted proteins, rather than attach them to peptidoglycan. Full length and truncation mutant constructs of SrtC and its substrate, T3, and peptide substrate mimics have been produced in soluble form for use in kinetic assays. Additionally, initial crystallization conditions have been identified for <italic>S. pyogenes</italic> SrtC towards the goal of three-dimensional structure determination. A homology model of the structure has also been produced, displaying many of the general features observed for other sortase enzymes.</p><p>Additionally, a computational analysis of the mechanism of isopeptide bond formation in <italic>S. pyogenes</italic> SPy0128, a substrate of <italic>S. pyogenes</italic> SrtC, has been performed. Isopeptide bonds have previously been found in structural studies of Gram-positive bacterial adhesins in each domain of these multi-domain proteins. The bonds are typically formed between conserved lysine and asparagine residues, and formation is likely catalyzed by adjacent conserved glutamates. A direct nucleophilic attack mechanism, starting from an inverse protonation state, is supported in this study. Of note, there appears to be temporal regulation of isopeptide bond formation in the different domains of <italic>S. pyogenes</italic> SPy0128, with the C-terminal domain isopeptide bond forming prior to or simultaneously with the N-terminal domain isopeptide bond.</p><p>Previous studies suggest that SrtA activity is required for <italic>S. aureus</italic> to survive phagocytosis by a macrophage. The production of reactive oxygen species by professional phagocytes could lead to inhibition of SrtA via oxidation of a conserved nucleophilic cysteine residue in the active site. Through determination of inhibition kinetics, identification of oxidative modifications, reduction potential measurements, and analyses of SrtA in vivo activity in the presence of reactive oxygen species, it has been demonstrated that <italic>S. aureus</italic> SrtA is resistant to oxidative inhibition. These findings support SrtA activity inside the phagolysosome of a professional phagocyte and likely contribute to the ability of <italic>S. aureus</italic> to evade the innate immune system.</p><p>The roles of sortases and their substrates during <italic>S. aureus</italic> survival inside professional phagocytes have not been thoroughly investigated. Through analysis of the regulation of these surface proteins under phagolysosomal conditions and macrophage phagocytosis survival assays, initial characterization of the functions of sortases and their substrates in this environment has been completed. Previous studies have suggested a role for SrtA and its substrate, Protein A, and these genes and two other sortase-substrates were upregulated in response to phagolysosomal conditions. However, neither sortases nor their substrates demonstrated a direct function in phagocytosis survival. These findings imply a complex interplay between <italic>S. aureus</italic> and professional phagocytes. Further studies are necessary to delineate the direct activities of surface anchored proteins during phagocytosis of <italic>S. aureus</italic> by professional phagocytes.</p> / Dissertation

Rapid development of antimicrobial resistance (AMR) among bacteria, combined with diminished new antibiotic discovery rates, is an increasing threat to human health. Bacterially derived antimicrobial peptides (AMP) hold excellent potential as potent novel therapeutics. This study embraces traditional natural AMP discovery methods and the newer in silico genome mining tool BAGEL 3 to facilitate identification of novel antimicrobial agents. The traditional screening efforts led to the discovery of two promising antimicrobial producer strains; Bacillus pumilus J1 producing two AMPs, peptides NI03 and NI04, and Klebsiella pneumoniae A7, which produced peptide NI05. In silico mining of the B. pumilus J1 and K. pneumoniae A7 genomes and those from under exploited anaerobic bacteria using BAGEL 3 yielded 18 putative bacteriocin structures that were associated with multiple known and relevant bacteriocin accessory genes and/or carried significant homologies to known bacteriocins. Peptide NI04 proved to be active against Gram positive species only, including meticillin resistant Staphylococcus aureus and vancomycin resistant enterococci and peptide NI03, in addition to these pathogens, showed activity against E. coli. Peptide NI05 was active against Gram-negative pathogens including extended spectrum β-lactamase producing E. coli. All isolated peptides were observed to be proteinaceous in nature and highly heat stable. Peptides were purified or partially purified using solid phase extraction followed by RP-HPLC. The mass of the peptides was determined using ESI or MALDI-TOF mass spectrometry. Tandem MS fragmentation of peptide NI04 generated several sequence tags. Draft genome sequences of the B. pumilus J1 and K. pneumoniae A7 producer strains were obtained using the Illumina MiSeq platform. This allowed identification of the genes encoding peptide NI04, which was confirmed to be novel and was named pumicin NI04. Further characterisation of pumicin NI04 demonstrated it was non-toxic to keratinocytes, Vero cells and non-haemolytic up to at least 18x the minimum inhibitory concentration. The discovery revealed that pumicin NI04 belongs to the WXG-100 peptide superfamily, having homology with the mycobacterial and staphylococcal virulence factor EsxA. This represents the first report of antimicrobial activity in a WXG-100 peptide and has intriguing evolutionary implications. Although it was not possible to fully characterise peptides NI03 and NI05, when BAGEL 3 was used to mine the B. pumilus J1 genome, a promising putative bacteriocin candidate was identified that was homologous to Enterocin AS-45, which also confers anti Gram-negative activity and may be related to the activity observed for NI03, however more evidence is required. Investigations of the K. pneumoniae A7 bacteriocin on the other hand helped establish that the K. pneumoniae microcin E492 pathway was present and highly conserved in strain A7, and is likely to be responsible for the activity observed indicating that NI05 was not a novel peptide.

Fusarium head blight is a devastating fungal disease of cereals caused by the pathogen Fusarium graminearum that leads to important economic losses due to diminished yields and grain downgrading. F. graminearum deploys several secondary metabolites known as virulence factors to facilitate its invasion of host tissues. These include the gramillins, a group of bicyclic lipopeptide ionophores that cause cell death and increased virulence in Arabidopsis, maize, and barley, but not wheat. Ionophores are involved in many plant-microbe interactions, but current knowledge of the molecular mechanisms governing host response to these molecules is limited. Susceptibility to gramillin varies among cultivars of affected species, but the basis for insensitivity has not yet been described, nor has the function of gramillin during infection. Here, we establish ion leakage as a method to survey Canadian barley for sensitivity, demonstrate that insensitivity to gramillin is likely mediated by a plant protease rather than inducible immune responses, and suggest a possible function of gramillin in positively regulating the expression of other fungal virulence factors during infection. This contributes to deepening our understanding of cyclic lipopeptide ionophores and their role during plant-microbe interactions.

No / Mycobacterium tuberculosis remains the predominant bacterial scourge of mankind. Understanding of its biology and pathogenicity has been greatly advanced by the determination of whole genome sequences for this organism. Bacterial lipoproteins are a functionally diverse class of membrane-anchored proteins. The signal peptides of these proteins direct their export and post-translational lipid modification. These signal peptides are amenable to bioinformatic analysis, allowing the lipoproteins encoded in whole genomes to be catalogued. This review applies bioinformatic methods to the identification and functional characterisation of the lipoproteins encoded in the M. tuberculosis genomes. Ninety nine putative lipoproteins were identified and so this family of proteins represents ca. 2.5% of the M. tuberculosis predicted proteome. Thus, lipoproteins represent an important class of cell envelope proteins that may contribute to the virulence of this major pathogen.

The pathogen Streptococcus pyogenes is a significant cause of human morbidity and mortality. Most of the work in this thesis is focused on streptococcal virulence factor IdeS, but the thesis also features work on SpeB, another streptococcal virulence factor. Both IdeS and SpeB are secreted cysteine proteases and both have previously been shown to degrade human IgG. IgG is the only known substrate for IdeS while SpeB is a more promiscuous protease with a larger number of identified substrates. A significant part of the data presented in this thesis is the result of designing and optimizing methods to detect and accurately measure the proteolytic degradation of IgG. Methods aimed at measuring the binding interactions between enzyme and substrate have also been frequently utilized. I show that IdeS is a monomeric protease, as opposed to previously published data that suggested it to be dimeric. IdeS cleaves the two heavy chains of IgG in a two-step reaction and I demonstrate that the first cleavage is magnitudes faster than the second one. This means that IdeS is a more efficient enzyme than previously thought. The difference in rate cannot completely be explained by a loss of affinity between IdeS and IgG after the cleavage of the first heavy chain. The velocity of IdeS is further increased by the presence of human Cystatin C, via an unknown mechanism. Cystatin C is normally a protease inhibitor and it having an opposite effect is puzzling.The synthesis and evaluation of novel inhibitors are also described. Peptide analogues mimicking the sequence surrounding the scissile bond on IgG - with an amino acid replaced with a more rigid motif - act as specific, but low-affinity, inhibitors of IdeS. The peptide analogues’ inhibitory capacity for SpeB and papain was also assayed.When it comes to SpeB, I show that it does not have IgG as a substrate under physiological conditions, in contrast to what was previously thought. This thesis does not only present findings on the IgG degrading capacity of IdeS and SpeB but also include data on fundamental enzymatic properties for these proteases.

Introduction : En 2011, un arbovirus émergent appelé virus Schmallenberg (SBV) et appartenant à la famille des Bunyaviridae a été identifié en Allemagne et s’est répandu en Europe. Le SBV infecte les ruminants domestiques et sauvages. Chez l’adulte, la virémie est transitoire et l’infection est souvent inapparente. En revanche, chez les femelles gestantes, le SBV peut franchir la barrière transplacentaire et infecter le fœtus, pouvant provoquer des avortements et des malformations du système nerveux central. Parmi les protéines virales synthétisées par le SBV, la protéine non-structurale NSs est un facteur de virulence majeur. Elle entraîne notamment la dégradation de sa sous-unité Rpb1 de l’ARN polymérase II pour inhiber la transcription cellulaire. Ce travail a pour but d’étudier les propriétés biochimiques et fonctionnelles de NSs et d’identifier les déterminants moléculaires régissant ses principales activités.Méthodes et résultats: L’analyse in silico de la séquence peptidique de NSs réalisée à l’aide d’algorithmes de prédiction a permis de designer plusieurs de mutants de délétion de la protéine. L’observation de la localisation cellulaire des mutants dans plusieurs modèles humains et ovins confirme la prédiction d’une distribution principalement nucléaire pour NSs. De façon intéressante, une séquence interne à la protéine (33-51) sert de motif d’adressage spécifique au niveau des nucléoles (NoLS) et nous avons pu démontrer la co-localisation de NSs avec plusieurs protéines nucléolaires. De plus, l’infection de cellules humaines et ovines par le SBV entraîne la translocation de protéines nucléolaires (B23 et fibrillarine) vers le nucléoplasme, témoignant d’un stress nucléolaire viro-induit. Pour évaluer l’impact de la localisation nucléolaire de NSs sur ce phénomène, un virus recombinant dont NSs a été délétée de son motif d’adressage nucléolaire (SBVΔNoLS) a été produit par génétique inverse. Le SBVΔNoLS n’induit plus de redistribution de B23 confirmant le rôle de NSs dans l’induction d’un stress nucléolaire au cours de l’infection. Ces résultats ont été confirmés dans des cellules souches neurales humaines, qui constituent un modèle pertinent par rapport aux lésions provoquées par le SBV dans le système nerveux.En parallèle de ce travail, nous avons recherché des partenaires cellulaires de NSs par la méthode du double-hybride en levures. Huit partenaires cellulaires de NSs ont été découverts, dont la chaîne légère de la dynéine de type 1 (Tctex-1) et la Major Vault Protein (MVP), qui sont toutes les deux impliquées dans le transport de protéines grâce à leur association aux microtubules. Une des hypothèses avancées est que ces protéines pourraient servir de cargos pour promouvoir le transport nucléo-cytoplasmique de NSs.Conclusions et perspectives : Ce travail de thèse a permis de démontrer que la protéine NSs du SBV est localisée principalement dans le noyau cellulaire et dans les nucléoles, grâce à une séquence d’adressage spécifique. L’infection virale induit un stress nucléolaire dépendant de NSs, qui a pu être reproduit dans un modèle de cellules souches neurales humaines. Les perturbations nucléolaires induites par NSs pourraient contribuer au blocage de la transcription cellulaire observé au cours de l’infection et, de manière subséquente, moduler la réponse antivirale de la cellule et/ou induire la mort cellulaire en lien avec la pathogenèse virale. Ainsi, ces perturbations des nucléoles pourraient être à l’origine d’une dégénérescence des neurones et des anomalies développementales observées chez les fœtus infectés. Au niveau moléculaire, nous souhaitons préciser l’implication de la protéine nucléolaire B23, relocalisée vers le nucléoplasme en cours d’infection, et/ou d’autres composants du nucléole dans l’initiation de ce processus. Enfin, l’hypothèse d’un transport rétrograde actif de NSs du cytoplasme vers le noyau médié par son interaction avec MVP ou Tctex1 est en cours d’investigation. / Introduction: In 2011, an emerging arbovirus named Schmallenberg virus (SBV), and belonging to the Bunyaviridae family, was discovered in Germany. Then, SBV has rapidly spread to Europe infecting wild and domestic ruminants. Adult infection is basically mild and associated with a short viremia (2-5 days). However, in case of pregnant females’ infection, SBV has the ability to cross the placental barrier to infect the foetuses, which can lead to stillbirth and central nervous system developmental abnormalities (arthrogyposis, hydranencephaly). Among bunyavirus-encoded proteins, the non-structural protein NSs has been shown to be an important virulence factor. Indeed, it is able to degrade the Rpb1 subunit of RNA polymerase II, leading to the inhibition of cellular transcription. The work of my thesis aimed to study biochemical and functional properties of NSs and to identify the molecular patterns ruling its main activities.Methods and results: An in silico amino acids sequence analysis was used to predict some common features of NSs and to help the design of several NSs mutants. As predicted by several algorithms, NSs and its mutants are mainly localised to cell nucleus in different cell types (from human and ovine origin). Interestingly, we highlighted an internal sequence (residues 33 to 51) containing a nucleolar localisation signal (NoLS), and have shown that NSs co-localises with several nucleolar proteins. Moreover, infections of human and ovine cell lines with SBV lead to re-localisation of nucleolar proteins to nucleoplasm (B23 and fibrillarin), demonstrating a viral-induced nucleolar stress. To assess the role of the NSs nucleolar localisation in this phenomenon, a recombinant virus, with a mutated version of NSs devoid of its NolS motif (SBVΔNoLS), was constructed by reverse genetic. Infection with SBVΔNoLS does not induce nucleolar stress, suggesting that the nucleolar stress induced by SBV occurs only if NSs is addressed to the nucleolus. Moreover, these results have been confirmed in human neural stem cells, which coud be a more relevant cellular model to mimic SBV infection in foetuses.Another way to study NSs functions was to identify its cellular partners by means of a yeast-two hybrid screen and using NSs as bait. Eight putative interactors of NSs have been discovered, including the dynein light chain type 1 (Tctex-1) and the Major Vault protein (MVP). These proteins are involved in cellular protein transport, notably by their associations with the microtubules network. Thus, NSs might interact with Tctex-1 and MVP to favour its shuttling from cell cytoplasm to the nucleus.Conclusions and perspectives: Altogether, these data indicate that SBV-NSs protein is mainly localised into cell nucleus and nucleolus, by means of its internal NoLS contained in the 33-51 NSs domain. SBV infection induces a nucleolar stress, particularly in human neural stem cells. NSs-induced nucleolar disruption could promote NSs inhibitory function on cellular transcription, and subsequently modulate cellular antiviral state and/or induce cell death. Regarding the pathogenesis in SBV-infected foetuses, nucleolar stress could be responsible for neurons degeneration and subsequent developmental abnormalities. At molecular level, our aim is to define the role of nucleolar protein B23 on viral replication, which is strongly relocalised to the nucleoplasm during SBV infection. Finally, hypothesis of NSs retrograde transport from cell cytoplasm to nucleus and the possible contributions of MVP and/or Tctex-1 needs to be further investigate.

A paracoccidioidomicose (PCM) é uma micose granulomatosa sistêmica, que tem como agentes etiológicos fungos dimórficos do gênero Paracoccidioides. A produção de melanina por vários fungos interfere no mecanismo da patogênese, como ocorre na paracoccidioidomicose. O presente projeto visou avaliar a produção de melanina pelos isolados de P. lutzii, e seus efeitos in vitro, avaliando o processo de fagocitose. Os resultados obtidos durante este trabalho mostraram que os isolados de P. lutzii (Pb01, Pb66, ED01, Pb1578 e Pb8334) melanizam de formas diferenciadas entre eles e quando comparados com isolados de P. brasiliensis (Pb60855, Pb18 e Pbcão). Ensaios de fagocitose, com os isolados Pb18, Pb60855 e Pb01, mostraram que a melanina reduziu a porcentagem de fagocitose das leveduras não tratadas de Pb18 e Pb60855, e o contrário aconteceu com Pb01. As análises dos perfis proteicos e enzimáticos dos isolados Pb18, Pb60855 e Pb01, permitiram determinar que o isolado Pb01 produz uma menor quantidade de proteínas que Pb18 e Pb60855 nas condições ensaiadas. / Paracoccidioidomycosis (PCM) is a granulomatous systemic mycosis, whose etiological agents are dimorphic fungi of the genus Paracoccidioides. Melanin production by various fungi interferes in the mechanism of pathogenesis, as in paracoccidioidomycosis. Thus, this project aimed to evaluate the production of melanin by isolates of P. lutzii and its in vitro effects by assessing the process of phagocytosis. The results obtained during this work showed that the isolates of P. lutzii (Pb01, Pb66, ED01, Pb1578 and Pb8334) produce melanin in different ways between them and also when compared with isolates of P. brasiliensis (Pb60855, Pb18 and Pbcão). Phagocytosis assays, with Pb18, Pb60855 and Pb01, showed that melanin reduced the percentage of phagocytosis of untreated yeast of Pb60855 and Pb18 and the opposite happened with Pb01. The analysis of the protein and enzymatic profiles, of the isolates Pb18, Pb60855 and Pb01, enabled to determine that the isolated Pb01 produces a smaller amount of proteins compared with Pb18 and Pb60855 in the tested conditions.

Burkholderia pseudomallei (Bp) is the causative agent of melioidosis, an opportunistic but serious human disease endemic to Southeast Asia and Northern Australia. The ‘Bp-group’ includes Bp and the closely-related organisms B. thailandensis (Bt) and B. oklahomensis (Bo), all of which are usually soil-dwelling saprophytes, and B. mallei (Bm) which is an equine-host-adapted pathogen. Bt is virulent in a number of invertebrate models but is generally non-pathogenic for mammals and is often used as a surrogate for the study of virulence mechanisms shared with Bp. Experiments to assess the potential of the Tobacco Hawkmoth Manduca sexta as a model host for Bp or Bt infection revealed surprising results. Bp, Bt and Bo were all lethal to M. sexta larvae. This is the first report of Bo virulence in an infection model. Additionally, the relative virulence of the three species was the reverse of that reported in humans and in larvae of the Greater Waxworm Galleria mellonella. Despite that, well-known hallmarks of Bp-group pathogenesis in mammalian hosts – intracellular survival and multiplication, actin remodelling and acute sepsis – were observed in M. sexta infection during a fluorescent confocal microscopy time-course study. M. sexta feeding experiments with Bt and Bo indicated that cultures of these bacteria are also pathogenic via the oral route, which is likely to be relevant for natural insect-bacteria interactions. Cell-free supernatant of Bo was as harmful to larvae as complete culture, supporting previous suggestions that Bp-group bacteria produce toxins or paralytic agents that are active against invertebrates. Finally, Rapid Virulence Annotation (RVA) was performed as a genome-wide screen for virulence determinants of Bp strain K96423, using three invertebrate bioassays with a recombinant expression library. In response to problems with the reproducibility of biologically active clones, a new statistical approach was devised which enabled quantitative identification of the most convincing RVA hits.