Formicine ants swallow their highly acidic poison for gut microbial selection and control.

Animals continuously encounter microorganisms that are essential for health or cause disease. They are thus challenged to control harmful microbes while allowing the acquisition of beneficial microbes. This challenge is likely especially important for social insects with respect to microbes in food, as they often store food and exchange food among colony members. Here we show that formicine ants actively swallow their antimicrobial, highly acidic poison gland secretion. The ensuing acidic environment in the stomach, the crop, can limit the establishment of pathogenic and opportunistic microbes ingested with food and improve the survival of ants when faced with pathogen contaminated food. At the same time, crop acidity selectively allows acquisition and colonization by Acetobacteraceae, known bacterial gut associates of formicine ants. This suggests that swallowing of the poison in formicine ants acts as a microbial filter and that antimicrobials have a potentially widespread but so far underappreciated dual role in host-microbe interactions.

Activity of Tracheal Cytotoxin of Bordetella pertussis in a Human Tracheobronchial 3D Tissue Model.

Bordetella pertussis is a highly contagious pathogen which causes whooping cough in humans. A major pathophysiology of infection is the extrusion of ciliated cells and subsequent disruption of the respiratory mucosa. Tracheal cytotoxin (TCT) is the only virulence factor produced by B. pertussis that has been able to recapitulate this pathology in animal models. This pathophysiology is well characterized in a hamster tracheal model, but human data are lacking due to scarcity of donor material. We assessed the impact of TCT and lipopolysaccharide (LPS) on the functional integrity of the human airway mucosa by using in vitro airway mucosa models developed by co-culturing human tracheobronchial epithelial cells and human tracheobronchial fibroblasts on porcine small intestinal submucosa scaffold under airlift conditions. TCT and LPS either alone and in combination induced blebbing and necrosis of the ciliated epithelia. TCT and LPS induced loss of ciliated epithelial cells and hyper-mucus production which interfered with mucociliary clearance. In addition, the toxins had a disruptive effect on the tight junction organization, significantly reduced transepithelial electrical resistance and increased FITC-Dextran permeability after toxin incubation. In summary, the results indicate that TCT collaborates with LPS to induce the disruption of the human airway mucosa as reported for the hamster tracheal model.

Distribution of the obligate endosymbiont Blochmannia floridanus and expression analysis of putative immune genes in ovaries of the carpenter ant Camponotus floridanus.

The bacterial endosymbiont Blochmannia floridanus of the carpenter ant Camponotus floridanus contributes to its hosts' ontogeny via nutritional upgrading during metamorphosis. This primary endosymbiosis is essential for both partners and vertical transmission of the endosymbionts is guaranteed by bacterial infestation of oocytes. Here we present a detailed analysis of the presence and localisation of B. floridanus in the ants' ovaries obtained by FISH and TEM analyses. The most apical part of the germarium harbouring germ-line stem cells (GSCs) is not infected by the bacteria. The bacteria are detectable for the first time in lower parts of the germarium when cystocytes undergo the 4th and 5th division and B. floridanus infects somatic cells lying under the basal lamina surrounding the ovarioles. With the beginning of cystocyte differentiation, the endosymbionts are exclusively transported from follicle cells into the growing oocytes. This infestation of the oocytes by bacteria very likely involves exocytosis-endocytosis processes between follicle cells and the oocytes. Nurse cells were never found to harbour the endosymbionts. Furthermore we present first gene expression data in C. floridanus ovaries. These data indicate a modulation of immune gene expression which may facilitate tolerance towards the endosymbionts and thus may contribute to their transovarial transmission.

An antibiotic target ranking and prioritization pipeline combining sequence, structure and network-based approaches exemplified for Serratia marcescens.

We investigate a drug target screening pipeline comparing sequence, structure and network-based criteria for prioritization. Serratia marcescens, an opportunistic pathogen, serves as test case. We rank according to (i) availability of three dimensional structures and lead compounds, (ii) not occurring in man and general sequence conservation information, and (iii) network information on the importance of the protein (conserved protein-protein interactions; metabolism; reported to be an essential gene in other organisms). We identify 45 potential anti-microbial drug targets in S. marcescens with KdsA involved in LPS biosynthesis as top candidate drug target. LpxC and FlgB are further top-ranked targets identified by interactome analysis not suggested before for S. marcescens. Pipeline, targets and complementarity of the three approaches are evaluated by available experimental data and genetic evidence and against other antibiotic screening pipelines. This supports reliable drug target identification and prioritization for infectious agents (bacteria, parasites, fungi) by these bundled complementary criteria.

Draft Genome Sequence of the Moderately Heat-Tolerant Lactococcus lactis subsp. lactis bv. diacetylactis Strain GL2 from Algerian Dromedary Milk.

Lactococcus lactis subsp. lactis bv. diacetylactis GL2 is a moderately thermotolerant lactic acid bacterium isolated from dromedary raw milk. Here, we present the draft genome sequence of this potential new dairy starter strain, which combines thermotolerance and the capacity to metabolize lactose, casein, and citrate.

Scrutinizing the immune defence inventory of Camponotus floridanus applying total transcriptome sequencing.

BACKGROUND: Defence mechanisms of organisms are shaped by their lifestyle, environment and pathogen pressure. Carpenter ants are social insects which live in huge colonies comprising genetically closely related individuals in high densities within nests. This lifestyle potentially facilitates the rapid spread of pathogens between individuals. In concert with their innate immune system, social insects may apply external immune defences to manipulate the microbial community among individuals and within nests. Additionally, carpenter ants carry a mutualistic intracellular and obligate endosymbiotic bacterium, possibly maintained and regulated by the innate immune system. Thus, different selective forces could shape internal immune defences of Camponotus floridanus. RESULTS: The immune gene repertoire of C. floridanus was investigated by re-evaluating its genome sequence combined with a full transcriptome analysis of immune challenged and control animals using Illumina sequencing. The genome was re-annotated by mapping transcriptome reads and masking repeats. A total of 978 protein sequences were characterised further by annotating functional domains, leading to a change in their original annotation regarding function and domain composition in about 8% of all proteins. Based on homology analysis with key components of major immune pathways of insects, the C. floridanus immune-related genes were compared to those of Drosophila melanogaster, Apis mellifera, and other hymenoptera. This analysis revealed that overall the immune system of carpenter ants comprises many components found in these insects. In addition, several C. floridanus specific genes of yet unknown functions but which are strongly induced after immune challenge were discovered. In contrast to solitary insects like Drosophila or the hymenopteran Nasonia vitripennis, the number of genes encoding pattern recognition receptors specific for bacterial peptidoglycan (PGN) and a variety of known antimicrobial peptide (AMP) genes is lower in C. floridanus. The comparative analysis of gene expression post immune-challenge in different developmental stages of C. floridanus suggests a stronger induction of immune gene expression in larvae in comparison to adults. CONCLUSIONS: The comparison of the immune system of C. floridanus with that of other insects revealed the presence of a broad immune repertoire. However, the relatively low number of PGN recognition proteins and AMPs, the identification of Camponotus specific putative immune genes, and stage specific differences in immune gene regulation reflects Camponotus specific evolution including adaptations to its lifestyle.

Transcriptional profiling of Bordetella pertussis reveals requirement of RNA chaperone Hfq for Type III secretion system functionality.

Bordetella pertussis, the causative agent of human whooping cough (pertussis) produces a complex array of virulence factors in order to establish efficient infection in the host. The RNA chaperone Hfq and small regulatory RNAs are key players in posttranscriptional regulation in bacteria and have been shown to play an essential role in virulence of a broad spectrum of bacterial pathogens. This study represents the first attempt to characterize the Hfq regulon of the human pathogen B. pertussis under laboratory conditions as well as upon passage in the host and indicates that loss of Hfq has a profound effect on gene expression in B. pertussis. Comparative transcriptional profiling revealed that Hfq is required for expression of several virulence factors in B. pertussis cells including the Type III secretion system (T3SS). In striking contrast to the wt strain, T3SS did not become operational in the hfq mutant passaged either through mice or macrophages thereby proving that Hfq is required for the functionality of the B. pertussis T3SS. Likewise, expression of virulence factors vag8 and tcfA encoding autotransporter and tracheal colonization factor, respectively, was strongly reduced in the hfq mutant. Importantly, for the first time we demonstrate that B. pertussis T3SS can be activated upon contact with macrophage cells in vitro.

An engineered 3D human airway mucosa model based on an SIS scaffold.

To investigate interrelations of human obligate airway pathogens, such as Bordetella pertussis, and their hosts test systems with high in vitro/in vivo correlation are of urgent need. Using a tissue engineering approach, we generated a 3D test system of the airway mucosa with human tracheobronchial epithelial cells (hTEC) and fibroblasts seeded on a clinically implemented biological scaffold. To investigate if hTEC display tumour-specific characteristics we analysed Raman spectra of hTEC and the adenocarcinoma cell line Calu-3. To establish optimal conditions for infection studies, we treated human native airway mucosa segments with B. pertussis. Samples were processed for morphologic analysis. Whereas our test system consisting of differentiated epithelial cells and migrating fibroblasts shows high in vitro/in vivo correlation, hTEC seeded on the scaffold as monocultures did not resemble the in vivo situation. Differences in Raman spectra of hTEC and Calu-3 were identified in distinct wave number ranges between 720 and 1662 cm(-1) indicating that hTEC do not display tumour-specific characteristics. Infection of native tissue with B. pertussis led to cytoplasmic vacuoles, damaged mitochondria and destroyed epithelial cells. Our test system is suitable for infection studies with human obligate airway pathogens by mimicking the physiological microenvironment of the human airway mucosa.

Versatile roles of the chaperonin GroEL in microorganism-insect interactions.

The chaperonin 60 (Cpn60) is present in all three kingdoms of life and is one of the most conserved proteins in living organisms. The Escherichia coli Cpn60 (GroEL) is the best studied representative of the huge Cpn60 family. It is an essential protein because in conjunction with the chaperonin 10 (Cpn10 or GroES) it forms a protein-folding machine required for correct folding of many proteins and for recycling of misfolded proteins. As many other chaperones, GroEL and GroES are also known as heat-shock proteins (HSPs), since heat stress leads to a strong induction of their expression, a measure to counteract the increase in misfolded proteins as a result of a high nonphysiological temperature. A large amount of literature is available which is dedicated to the elucidation of how protein folding is assisted by this molecular chaperone. However, apart from this primary task, additional so-called 'moonlighting' functions of GroEL proteins unrelated to their folding activity have emerged in the past years. In fact, it becomes apparent that GroEL proteins have diverse functions in particular in mutualistic and pathogenic microorganism-host interactions. In this brief review, we describe some of these recent findings focusing on the importance of GroEL for microorganism-insect interactions.

Gene expression analysis of the endosymbiont-bearing midgut tissue during ontogeny of the carpenter ant Camponotus floridanus.

Insects have frequently evolved mutualistic relationships with extracellular and/or intracellular bacterial endosymbionts. Infection with endosymbionts seems to affect several cellular functions of the host such as immune pathways, oxidative stress regulation and autophagy. Our current knowledge about specific host factors leading to endosymbiont tolerance and/or control is still scarce and is based on very few associations between insect hosts and bacteria only. Camponotus floridanus ants harbour the obligate intracellular bacterium Blochmannia floridanus within specialized midgut cells called bacteriocytes. The number of Blochmannia endosymbionts within the midgut tissue increases strongly during host development and reaches a maximum at the late pupal stage, where the entire midgut is transformed into a symbiotic organ. After eclosion of workers the number of Blochmannia strongly decreases again. We chose 15 candidate genes from C. floridanus likely to be involved in host-symbiont interactions based on their significant homology to previously investigated symbiosis-relevant genes from other insects. We determined the expression of these genes in the endosymbiont-bearing midgut tissue in comparison to the residual body tissue at different developmental stages of C. floridanus in order to reveal changes in gene expression correlating with changes in endosymbiont number per host. Strikingly, two pattern recognition receptors (amidase PGRP-LB and PGRP-SC2) were highly expressed in the midgut tissue at the pupal stage, potentially down-modulating the IMD pathway to enable endosymbiont tolerance. Moreover, we investigated the immune gene expression in response to bacterial challenge at the pupal stage. Results showed that the midgut tissue differs in expression pattern in contrast to the residual body. Our results support a key role for amidase PGRPs, especially PGRP-LB, in regulation of the immune response towards endosymbionts in C. floridanus and suggest an involvement of the lysosomal system in control of Blochmannia endosymbionts.

Development and characterization of attenuated metabolic mutants of Bordetella bronchiseptica for applications in vaccinology.

Bordetella bronchiseptica is an important pathogen causing a number of veterinary respiratory syndromes in agriculturally important and food-producing confinement-reared animals, resulting in great economic losses annually amounting to billions of euros worldwide. Currently available live vaccines are incompletely satisfactory in terms of efficacy and safety. An efficient vaccine for livestock animals would allow reducing the application of antibiotics, thereby preventing the massive release of pharmaceuticals into the environment. Here, we describe two new potential vaccine strains based on the BB7865 strain. Two independent attenuating mutations were incorporated by homologous recombination in order to make negligible the risk of recombination and subsequent reversion to the virulent phenotype. The mutations are critical for bacterial metabolism, resistance to oxidative stress, intracellular survival and in vivo persistence. The resulting double mutants BB7865 risA aroA and BB7865 risA dapE were characterized as promising vaccine candidates, which are able to confer protection against colonization of the lower respiratory tract after sublethal challenge with the wild-type strain.

Molecular characterization of antimicrobial peptide genes of the carpenter ant Camponotus floridanus.

The production of antimicrobial peptides (AMPs) is a major defense mechanism against pathogen infestation and of particular importance for insects relying exclusively on an innate immune system. Here, we report on the characterization of three AMPs from the carpenter ant Camponotus floridanus. Due to sequence similarities and amino acid composition these peptides can be classified into the cysteine-rich (e.g. defensin) and glycine-rich (e.g. hymenoptaecin) AMP groups, respectively. The gene and cDNA sequences of these AMPs were established and their expression was shown to be induced by microbial challenge. We characterized two different defensin genes. The defensin-2 gene has a single intron, whereas the defensin-1 gene has two introns. The deduced amino acid sequence of the C. floridanus defensins is very similar to other known ant defensins with the exception of a short C-terminal extension of defensin-1. The hymenoptaecin gene has a single intron and a very peculiar domain structure. The corresponding precursor protein consists of a signal- and a pro-sequence followed by a hymenoptaecin-like domain and six directly repeated hymenoptaecin domains. Each of the hymenoptaecin domains is flanked by an EAEP-spacer sequence and a RR-site known to be a proteolytic processing site. Thus, proteolytic processing of the multipeptide precursor may generate several mature AMPs leading to an amplification of the immune response. Bioinformatical analyses revealed the presence of hymenoptaecin genes with similar multipeptide precursor structure in genomes of other ant species suggesting an evolutionary conserved important role of this gene in ant immunity.

Different evolutionary modifications as a guide to rewire two-component systems.

Two-component systems (TCS) are short signalling pathways generally occurring in prokaryotes. They frequently regulate prokaryotic stimulus responses and thus are also of interest for engineering in biotechnology and synthetic biology. The aim of this study is to better understand and describe rewiring of TCS while investigating different evolutionary scenarios. Based on large-scale screens of TCS in different organisms, this study gives detailed data, concrete alignments, and structure analysis on three general modification scenarios, where TCS were rewired for new responses and functions: (i) exchanges in the sequence within single TCS domains, (ii) exchange of whole TCS domains; (iii) addition of new components modulating TCS function. As a result, the replacement of stimulus and promotor cassettes to rewire TCS is well defined exploiting the alignments given here. The diverged TCS examples are non-trivial and the design is challenging. Designed connector proteins may also be useful to modify TCS in selected cases.

Endosymbiont Tolerance and Control within Insect Hosts.

Bacterial endosymbioses are very common in insects and can range from obligate to facultative as well as from mutualistic to pathogenic associations. Several recent studies provide new insight into how endosymbionts manage to establish chronic infections of their hosts without being eliminated by the host immune system. Endosymbiont tolerance may be achieved either by specific bacterial adaptations or by host measurements shielding bacteria from innate defense mechanisms. Nevertheless, insect hosts also need to sustain control mechanisms to prevent endosymbionts from unregulated proliferation. Emerging evidence indicates that in some cases the mutual adaptations of the two organisms may have led to the integration of the endosymbionts as a part of the host immune system. In fact, endosymbionts may provide protective traits against pathogens and predators and may even be required for the proper development of the host immune system during host ontogeny. This review gives an overview of current knowledge of molecular mechanisms ensuring maintenance of chronic infections with mutualistic endosymbionts and the impact of endosymbionts on host immune competence.

Immune response of the ant Camponotus floridanus against pathogens and its obligate mutualistic endosymbiont.

Numerous insect species harbor mutualistic endosymbionts that play a role in nutrient cycling or confer other fitness benefits to their hosts. Insect hosts face the problem of having to maintain such mutualistic bacteria while staging an immune response towards pathogens upon infection. In addition, hosts may regulate the number of endosymbionts present in their tissues via the innate immune system. Camponotus floridanus ants harbor the obligate endosymbiont Blochmannia floridanus in specialized midgut cells and ovaries. We identified genes transcriptionally induced in response to septic injury by suppression subtractive hybridization (SSH). Among these were genes involved in pathogen recognition (e.g. GNBP), signal transduction (e.g. MAPK-kinase), antimicrobial activity (e.g. defensin and hymenoptaecin), or general stress response (e.g. heat shock protein). A quantitative analysis of immune-gene expression revealed different expression kinetics of individual factors and also characteristic expression profiles after injection of gram-negative and gram-positive bacteria. Likewise, B. floridanus injected into the hemocoel elicited a comparable immune response of its host C. floridanus. Thus, the host immune system may contribute to controlling the endosymbiont population.

Bacteriocyte dynamics during development of a holometabolous insect, the carpenter ant Camponotus floridanus.

BACKGROUND: The carpenter ant Camponotus floridanus harbors obligate intracellular mutualistic bacteria (Blochmannia floridanus) in specialized cells, the bacteriocytes, intercalated in their midgut tissue. The diffuse distribution of bacteriocytes over the midgut tissue is in contrast to many other insects carrying endosymbionts in specialized tissues which are often connected to the midgut but form a distinct organ, the bacteriome. C. floridanus is a holometabolous insect which undergoes a complete metamorphosis. During pupal stages a complete restructuring of the inner organs including the digestive tract takes place. So far, nothing was known about maintenance of endosymbionts during this life stage of a holometabolous insect. It was shown previously that the number of Blochmannia increases strongly during metamorphosis. This implicates an important function of Blochmannia in this developmental phase during which the animals are metabolically very active but do not have access to external food resources. Previous experiments have shown a nutritional contribution of the bacteria to host metabolism by production of essential amino acids and urease-mediated nitrogen recycling. In adult hosts the symbiosis appears to degenerate with increasing age of the animals. RESULTS: We investigated the distribution and dynamics of endosymbiotic bacteria and bacteriocytes at different stages during development of the animals from larva to imago by confocal laser scanning microscopy. The number of bacteriocytes in relation to symbiont-free midgut cells varied strongly over different developmental stages. Especially during metamorphosis the relative number of bacteria-filled bacteriocytes increased strongly when the larval midgut epithelium is shed. During this developmental stage the midgut itself became a huge symbiotic organ consisting almost exclusively of cells harboring bacteria. In fact, during this phase some bacteria were also found in midgut cells other than bacteriocytes indicating a cell-invasive capacity of Blochmannia. In adult animals the number of bacteriocytes generally decreased. CONCLUSIONS: During the life cycle of the animals the distribution of bacteriocytes and of Blochmannia endosymbionts is remarkably dynamic. Our data show how the endosymbiont is retained within the midgut tissue during metamorphosis thereby ensuring the maintenance of the intracellular endosymbiosis despite a massive reorganization of the midgut tissue. The transformation of the entire midgut into a symbiotic organ during pupal stages underscores the important role of Blochmannia for its host in particular during metamorphosis.

Resemblance and divergence: the "new" members of the genus Bordetella.

Bordetella pertussis, the etiological agent of whooping cough, belongs to the bacterial pathogens first described in the so-called golden era of microbiology more than 100 years ago. In the course of the following decades, several other closely related pathogens were described which are nowadays classified in the genus Bordetella together with B. pertussis. These are the human and animal pathogens B. parapertussis, B. bronchiseptica and B. avium which are of high medical or veterinary interest, and which, together with B. pertussis, are referred to as the "classical" Bordetella species. Only in the past 15 years, several additional species were classified in the genus, frequently isolated from patients with underlying disease, animals or from the environment. Very little is known about most of these bacteria. In the present review, the current knowledge about these "new" Bordetella species is briefly summarized.

JANE: efficient mapping of prokaryotic ESTs and variable length sequence reads on related template genomes.

BACKGROUND: ESTs or variable sequence reads can be available in prokaryotic studies well before a complete genome is known. Use cases include (i) transcriptome studies or (ii) single cell sequencing of bacteria. Without suitable software their further analysis and mapping would have to await finalization of the corresponding genome. RESULTS: The tool JANE rapidly maps ESTs or variable sequence reads in prokaryotic sequencing and transcriptome efforts to related template genomes. It provides an easy-to-use graphics interface for information retrieval and a toolkit for EST or nucleotide sequence function prediction. Furthermore, we developed for rapid mapping an enhanced sequence alignment algorithm which reassembles and evaluates high scoring pairs provided from the BLAST algorithm. Rapid assembly on and replacement of the template genome by sequence reads or mapped ESTs is achieved. This is illustrated (i) by data from Staphylococci as well as from a Blattabacteria sequencing effort, (ii) mapping single cell sequencing reads is shown for poribacteria to sister phylum representative Rhodopirellula Baltica SH1. The algorithm has been implemented in a web-server accessible at http://jane.bioapps.biozentrum.uni-wuerzburg.de. CONCLUSION: Rapid prokaryotic EST mapping or mapping of sequence reads is achieved applying JANE even without knowing the cognate genome sequence.

Immunity and symbiosis.

The invertebrate immune system, which has become a major research focus, shares basic features of innate immunity with vertebrates and men. A special feature apparently found only in invertebrates is their close association with vertically heritable symbiotic microorganisms. The validity of the simple view of symbiosis as a mutually beneficial interaction between two uneven partners mainly improving the nutritional state of the two companions has been challenged, however, as symbiotic interactions might involve more partners, and symbiotic functions of the microorganisms are much more diverse than previously assumed. Likewise, microorganisms considered to be mostly harmful to their hosts have been shown to enhance host fitness under some circumstances. The role of a symbiont itself might change between environments or life stages of the host and symbionts might have features previously thought to be specific for pathogens. Understanding symbiotic interactions requires the comprehension of the cross-talk between the symbiotic companions, and the dissection of how long-lasting infections are established without eliminating the symbiont by host immune responses. Fascinating new findings in this field revealed that symbiosis might contribute to defence against pathogens or natural enemies. New symbiont-based approaches to defeat agricultural pests or pathogen transmission by arthropod vectors are becoming conceivable.

Genomic island excisions in Bordetella petrii.

BACKGROUND: Among the members of the genus Bordetella B. petrii is unique, since it is the only species isolated from the environment, while the pathogenic Bordetellae are obligately associated with host organisms. Another feature distinguishing B. petrii from the other sequenced Bordetellae is the presence of a large number of mobile genetic elements including several large genomic regions with typical characteristics of genomic islands collectively known as integrative and conjugative elements (ICEs). These elements mainly encode accessory metabolic factors enabling this bacterium to grow on a large repertoire of aromatic compounds. RESULTS: During in vitro culture of Bordetella petrii colony variants appear frequently. We show that this variability can be attributed to the presence of a large number of metastable mobile genetic elements on its chromosome. In fact, the genome sequence of B. petrii revealed the presence of at least seven large genomic islands mostly encoding accessory metabolic functions involved in the degradation of aromatic compounds and detoxification of heavy metals. Four of these islands (termed GI1 to GI3 and GI6) are highly related to ICEclc of Pseudomonas knackmussii sp. strain B13. Here we present first data about the molecular characterization of these islands. We defined the exact borders of each island and we show that during standard culture of the bacteria these islands get excised from the chromosome. For all but one of these islands (GI5) we could detect circular intermediates. For the clc-like elements GI1 to GI3 of B. petrii we provide evidence that tandem insertion of these islands which all encode highly related integrases and attachment sites may also lead to incorporation of genomic DNA which originally was not part of the island and to the formation of huge composite islands. By integration of a tetracycline resistance cassette into GI3 we found this island to be rather unstable and to be lost from the bacterial population within about 100 consecutive generations. Furthermore, we show that GI3 is self transmissible and by conjugation can be transferred to B. bronchiseptica thus proving it to be an active integrative and conjugative element CONCLUSION: The results show that phenotypic variation of B. petrii is correlated with the presence of genomic islands. Tandem integration of related islands may contribute to island evolution by the acquisition of genes originally belonging to the bacterial core genome. In conclusion, B. petrii appears to be the first member of the genus in which horizontal gene transfer events have massively shaped its genome structure.