Listeria monocytogenes cell-to-cell spread in epithelia is heterogeneous and dominated by rare pioneer bacteria

Pushing boundaries: mechanisms enabling bacterial pathogens to spread between cells

For multiple intracellular bacterial pathogens, the ability to spread directly into adjacent epithelial cells is an essential step for disease in humans. For pathogens such as Shigella, Listeria, Rickettsia, and Burkholderia, this intercellular movement frequently requires the pathogens to manipulate the host actin cytoskeleton and deform the plasma membrane into structures known as protrusions, which extend into neighboring cells. The protrusion is then typically resolved into a double-membrane vacuole (DMV) from which the pathogen quickly escapes into the cytosol, where additional rounds of intercellular spread occur. Significant progress over the last few years has begun to define the mechanisms by which intracellular bacterial pathogens spread. This review highlights the interactions of bacterial and host factors that drive mechanisms required for intercellular spread with a focus on how protrusion structures form and resolve.

Computed tomography and magnetic resonance imaging findings in central nervous system listeriosis

PURPOSE

To describe the imaging findings and determine the incidence of a characteristic worm-like pattern along the white matter tracts in neurolisteriosis on CT/MRI.

METHODS

An IRB-approved retrospective study in 21 consecutive neurolisteriosis cases during January 2002-July 2020. At least one of the following is required: (1) Positive Listeria monocytogenes (LM) in blood with clinical signs of meningeal irritation and/or abnormal CSF profile, (2) positive LM in blood with signs of encephalitis, (3) positive LM in CSF, (4) positive LM from brain biopsy/aspiration. Six cases were excluded due to the lack of contrast-enhanced images, leaving a total of 15 cases for analysis (mean age 53.5 years ± 18.8 SD). The imaging studies were independently reviewed by two blinded readers. Demographic data, imaging findings, and incidence of the worm-like pattern were reported. The Cohen's kappa was used to calculate interrater reproducibility.

RESULTS

Of the 12 patients with relevant imaging findings, nine cases (75%) had parenchymal lesions (eight cases in supratentorial compartment and one case in infratentorial compartment), four cases (33.3%) had leptomeningeal enhancement and two cases (16.7%) had hydrocephalus. Brain abscesses were found in eight cases and nodules evocative of abscess in one case. Restricted diffusion in the central area and hemosiderin deposition were observed in all cases. The involvement of white matter tract in a worm-like pattern was demonstrated in eight of nine patients with parenchymal lesions (88.9%).

CONCLUSION

Abnormal findings in brain CT/MRI images are common in neurolisteriosis. The incidence of worm-like spread along the white matter tracts is high and may help diagnose suspicious patients.

Vγ9+Vδ2+ T cell control of Listeria monocytogenes growth in infected epithelial cells requires butyrophilin 3A genes

Intracellular bacteria produce antigens, which serve as potent activators of γδ T cells. Phosphoantigens are presented via a complex of butyrophilins (BTN) to signal infection to human Vγ9+Vδ2+ T cells. Here, we established an in vitro system allowing for studies of Vγ9+Vδ2+ T cell activity in coculture with epithelial cells infected with the intracellular bacterial pathogen Listeria monocytogenes. We report that the Vγ9+Vδ2+ T cells efficiently control L. monocytogenes growth in such cultures. This effector function requires the expression of members of the BTN3A family on epithelial cells. Specifically, we observed a BTN3A1-independent BTN3A3 activity to present antigen to Vγ9+Vδ2+ T cells. Since BTN3A1 is the only BTN3A associated with phosphoantigen presentation, our study suggests that BTN3A3 may present different classes of antigens to mediate Vγ9+Vδ2+ T cell effector function against L. monocytogenes-infected epithelia.

Live-cell imaging reveals single-cell and population-level infection strategies of Listeria monocytogenes in macrophages

Pathogens have developed intricate strategies to overcome the host's innate immune responses. In this paper we use live-cell microscopy with a single bacterium resolution to follow in real time interactions between the food-borne pathogen L. monocytogenes and host macrophages, a key event controlling the infection in vivo. We demonstrate that infection results in heterogeneous outcomes, with only a subset of bacteria able to establish a replicative invasion of macrophages. The fate of individual bacteria in the same host cell was independent from the host cell and non-cooperative, being independent from co-infecting bacteria. A higher multiplicity of infection resulted in a reduced probability of replication of the overall bacterial population. By use of internalisation assays and conditional probabilities to mathematically describe the two-stage invasion process, we demonstrate that the higher MOI compromises the ability of macrophages to phagocytose bacteria. We found that the rate of phagocytosis is mediated via the secreted Listeriolysin toxin (LLO), while the probability of replication of intracellular bacteria remained constant. Using strains expressing fluorescent reporters to follow transcription of either the LLO-encoding hly or actA genes, we show that replicative bacteria exhibited higher PrfA regulon expression in comparison to those bacteria that did not replicate, however elevated PrfA expression per se was not sufficient to increase the probability of replication. Overall, this demonstrates a new role for the population-level, but not single cell, PrfA-mediated activity to regulate outcomes of host pathogen interactions.

Wrapping dynamics and critical conditions for active nonspherical nanoparticle uptake

The cellular uptake of self-propelled nonspherical nanoparticles (NPs) or viruses by cell membrane is crucial in many biological processes, but its universal dynamics have yet to be elucidated. In this study, using the Onsager variational principle, we obtain a general wrapping equation for nonspherical self-propelled nanoparticles. Two analytical critical conditions are theoretically found, indicating a continuous full uptake for prolate particles and a snapthrough full uptake for oblate particles. They precisely capture the full uptake critical boundaries in the phase diagrams numerically constructed in terms of active force, aspect ratio, adhesion energy density, and membrane tension. It is found that enhancing activity (active force), reducing effective dynamic viscosity, increasing adhesion energy density, and decreasing membrane tension can significantly improve the wrapping efficiency of the self-propelled nonspherical nanoparticles. These results give a panoramic view of the uptake dynamics of active nonspherical nanoparticles, and may offer instructions for designing an effective active NP-based vehicle for controlled drug delivery.

Listeria monocytogenes-How This Pathogen Uses Its Virulence Mechanisms to Infect the Hosts

Listeriosis is a serious food-borne illness, especially in susceptible populations, including children, pregnant women, and elderlies. The disease can occur in two forms: non-invasive febrile gastroenteritis and severe invasive listeriosis with septicemia, meningoencephalitis, perinatal infections, and abortion. Expression of each symptom depends on various bacterial virulence factors, immunological status of the infected person, and the number of ingested bacteria. Internalins, mainly InlA and InlB, invasins (invasin A, LAP), and other surface adhesion proteins (InlP1, InlP4) are responsible for epithelial cell binding, whereas internalin C (InlC) and actin assembly-inducing protein (ActA) are involved in cell-to-cell bacterial spread. L. monocytogenes is able to disseminate through the blood and invade diverse host organs. In persons with impaired immunity, the elderly, and pregnant women, the pathogen can also cross the blood-brain and placental barriers, which results in the invasion of the central nervous system and fetus infection, respectively. The aim of this comprehensive review is to summarize the current knowledge on the epidemiology of listeriosis and L. monocytogenes virulence mechanisms that are involved in host infection, with a special focus on their molecular and cellular aspects. We believe that all this information is crucial for a better understanding of the pathogenesis of L. monocytogenes infection.

Listeria motility increases the efficiency of epithelial invasion during intestinal infection

Listeria monocytogenes (Lm) is a food-borne pathogen that causes severe bacterial gastroenteritis, with high rates of hospitalization and mortality. Lm is ubiquitous in soil, water and livestock, and can survive and proliferate at low temperatures. Following oral ingestion of contaminated food, Lm crosses the epithelium through intestinal goblet cells in a mechanism mediated by Lm InlA binding host E-cadherin. Importantly, human infections typically occur with Lm growing at or below room temperature, which is flagellated and motile. Even though many important human bacterial pathogens are flagellated, little is known regarding the effect of Lm motility on invasion and immune evasion. Here, we used complementary imaging and computer modeling approaches to test the hypothesis that bacterial motility helps Lm locate and engage target cells permissive for invasion. Imaging explanted mouse and human intestine, we showed that Lm grown at room temperature uses motility to scan the epithelial surface and preferentially attach to target cells. Furthermore, we integrated quantitative parameters from our imaging experiments to construct a versatile "layered" cellular Potts model (L-CPM) that simulates host-pathogen dynamics. Simulated data are consistent with the hypothesis that bacterial motility enhances invasion by allowing bacteria to search the epithelial surface for their preferred invasion targets. Indeed, our model consistently predicts that motile bacteria invade twice as efficiently over the first hour of infection. We also examined how bacterial motility affected interactions with host cellular immunity. In a mouse model of persistent infection, we found that neutrophils migrated to the apical surface of the epithelium 5 hours post infection and interacted with Lm. Yet in contrast to the view that neutrophils "hunt" for bacteria, we found that these interactions were driven by motility of Lm-which moved at least ~50x faster than neutrophils. Furthermore, our L-CPM predicts that motile bacteria maintain their invasion advantage even in the presence of host phagocytes, with the balance between invasion and phagocytosis governed almost entirely by bacterial motility. In conclusion, our simulations provide insight into host pathogen interaction dynamics at the intestinal epithelial barrier early during infection.

Fatal Listeria monocytogenes septicemia and meningitis complicated by Candida glabrata fungemia: a case report

Listeria monocytogenes is a Gram-positive bacteria and etiological agent of listeriosis. It has the ability to colonize the intestinal lumen and cross the intestinal, blood-brain, and placental barriers, leading to invasive listeriosis responsible for septicemia and meningitis in subjects at risk such as patients with diabetes mellitus, the elderly, and immunocompromised individuals and, for maternal-neonatal infection in pregnant women. We report a rare case of L. monocytogenes septicemia and meningitis complicated by Candida glabrata fungemia on a patient with a history of type 2 diabetes mellitus, hypothyroidism, hypertension, chronic kidney failure, chronic ischemic vascular encephalopathy, and atrial fibrillation. Although adequate therapy was rapidly started with an initial partial clinical improvement, the patient suddenly experienced clinical worsening concomitantly with Candida septicemia resulting in a fatal outcome. To our knowledge, this is the first described case of an invasive L. monocytogenes infection complicated by Candida sepsis. We hypothesize that concomitant Candida infection may play a significant role in the pathogenesis and virulence of L. monocytogenes.

Manipulation of epithelial cell architecture by the bacterial pathogens Listeria and Shigella

Subversion of the host cell cytoskeleton is a virulence attribute common to many bacterial pathogens. On mucosal surfaces, bacteria have evolved distinct ways of interacting with the polarised epithelium and manipulating host cell structure to propagate infection. For example, Shigella and Listeria induce cytoskeletal changes to induce their own uptake into enterocytes in order to replicate within an intracellular environment and then spread from cell-to-cell by harnessing the host actin cytoskeleton. In this review, we highlight some recent studies that advance our understanding of the role of the host cell cytoskeleton in the mechanical and molecular processes of pathogen invasion, cell-to-cell spread and the impact of infection on epithelial intercellular tension and innate mucosal defence.

Force-induced wrapping phase transition in activated cellular uptake

Intracellular pathogens, including all viruses and many bacteria, enter a host cell through either passive endocytosis or active self-propulsion. Though the cellular uptake of passive particles via endocytic process has been studied extensively, little work has been done on the active entry of self-propelled pathogens, such as Listeria monocytogenes. Here, we present a theoretical model to investigate the adhesive wrapping of a self-propelled particle by a plasma membrane, and find a type of first-order wrapping transition from a small partial wrapping state to a large partial wrapping state triggered by the active force. The phase diagram displays more complex behaviors compared with the passive wrapping mediated merely by adhesion. We also find that a tubular protrusion can be formed if the active force exceeds a force barrier. These results may provide a useful guidance to the study of activity-driven cellular entry of active particles into cells.

Encapsulated bacteria deform lipid vesicles into flagellated swimmers

Swimming bacterial pathogens can penetrate and shape the membranes of their host cells. We study an artificial model system of this kind comprising Escherichia coli enclosed inside vesicles, which consist of nothing more than a spherical membrane bag. The bacteria push out membrane tubes, and the tubes propel the vesicles. This phenomenon is intriguing because motion cannot be generated by pushing the vesicles from within. We explain the motility of our artificial cell by a shape coupling between the flagella of each bacterium and the enclosing membrane tube. This constitutes a design principle for conferring motility to cell-sized vesicles and demonstrates the universality of lipid membranes as a building block in the development of new biohybrid systems.

We study a synthetic system of motile Escherichia coli bacteria encapsulated inside giant lipid vesicles. Forces exerted by the bacteria on the inner side of the membrane are sufficient to extrude membrane tubes filled with one or several bacteria. We show that a physical coupling between the membrane tube and the flagella of the enclosed cells transforms the tube into an effective helical flagellum propelling the vesicle. We develop a simple theoretical model to estimate the propulsive force from the speed of the vesicles and demonstrate the good efficiency of this coupling mechanism. Together, these results point to design principles for conferring motility to synthetic cells.

Time-Resolved Fluorescence Microscopy Screens on Host Protein Subversion During Bacterial Cell Invasion

Intracellular bacterial pathogens have evolved a plethora of strategies to invade eukaryotic cells. By manipulating host signaling pathways, in particular vesicular trafficking, these microbes subvert host functions to promote their internalization and to establish an intracellular niche. During these events, host endomembrane compartments are dynamically reorganized. Shigella flexneri, the causative agent of bacillary dysentery, recruits components of the host recycling pathway and the exocyst of non-phagocytic enterocytes in the vicinity of its entry site to facilitate its access to the host cytosol. These factors are either dynamically tethered to in situ formed macropinosomes or to the bacteria-containing vacuole itself. The underlying interactions cannot readily be monitored as individual bacterial infection events take place without synchronicity using cellular infection models. Therefore, time-resolved screens by fluorescence microscopy represent a powerful tool for the study of host subversion. Such screens can be performed with libraries of fluorescently tagged host factors. Using the cytosolic pathogenic agent Shigella flexneri as a model, we provide detailed protocols for such medium-to-high throughput multidimensional imaging screening of the dynamic host-pathogen cross talk. Our workflow is designed to be easily adapted for the study of different host factor libraries and different pathogen models.

Mechanical Forces Govern Interactions of Host Cells with Intracellular Bacterial Pathogens

To combat infectious diseases, it is important to understand how host cells interact with bacterial pathogens. Signals conveyed from pathogen to host, and vice versa, may be either chemical or mechanical. While the molecular and biochemical basis of host-pathogen interactions has been extensively explored, relatively less is known about mechanical signals and responses in the context of those interactions. Nevertheless, a wide variety of bacterial pathogens appear to have developed mechanisms to alter the cellular biomechanics of their hosts in order to promote their survival and dissemination, and in turn many host responses to infection rely on mechanical alterations in host cells and tissues to limit the spread of infection. In this review, we present recent findings on how mechanical forces generated by host cells can promote or obstruct the dissemination of intracellular bacterial pathogens. In addition, we discuss how in vivo extracellular mechanical signals influence interactions between host cells and intracellular bacterial pathogens. Examples of such signals include shear stresses caused by fluid flow over the surface of cells and variable stiffness of the extracellular matrix on which cells are anchored. We highlight bioengineering-inspired tools and techniques that can be used to measure host cell mechanics during infection. These allow for the interrogation of how mechanical signals can modulate infection alongside biochemical signals. We hope that this review will inspire the microbiology community to embrace those tools in future studies so that host cell biomechanics can be more readily explored in the context of infection studies.

A Motile Doublet Form of Salmonella Typhimurium Diversifies Target Search Behaviour at the Epithelial Surface

The behaviors of infectious bacteria are commonly studied in bulk. This is effective to define the general properties of a given isolate, but insufficient to resolve subpopulations and unique single‐microbe behaviors within the bacterial pool. We here employ microscopy to study single‐bacterium characteristics among Salmonella enterica serovar Typhimurium (S.Tm), as they prepare for and launch invasion of epithelial host cells. We find that during the bacterial growth cycle, S.Tm populations switch gradually from fast planktonic growth to a host cell‐invasive phenotype, characterized by flagellar motility and expression of the Type‐three‐secretion‐system‐1. The indistinct nature of this shift leads to the establishment of a transient subpopulation of S.Tm “doublets”—waist‐bearing bacteria anticipating cell division—which simultaneously express host cell invasion machinery. In epithelial cell culture infections, these S.Tm doublets outperform their “singlet” brethren and represent a hyperinvasive subpopulation. Atop both glass and enteroid‐derived monolayers, doublets swim along markedly straighter trajectories than singlets, thereby diversifying search patterns and improving the surface exploration capacity of the total bacterial population. The straighter swimming, combined with an enhanced cell‐adhesion propensity, suffices to account for the hyperinvasive doublet phenotype. This work highlights bacterial cell length heterogeneity as a key determinant of target search patterns atop epithelia.

Listeria monocytogenes Invasion Into Sheep Kidney Epithelial Cells Depends on InlB, and Invasion Efficiency Is Modulated by Phylogenetically Defined InlB Isoforms

The facultative intracellular pathogen Listeria monocytogenes is of major veterinary importance in small ruminants. Nevertheless, details of L. monocytogenes interactions with cells of small ruminants are not fully established. To study the potential of L. monocytogenes to infect sheep cells, we used the finite sheep kidney cell line (shKEC), which was infected with the wild-type L. monocytogenes strain EGDe. The invasion efficiency was 0.015 ± 0.004%. The invasion factor InlB was critically important for invasion, and inlB gene deletion almost prevented L. monocytogenes invasion into shKEC cells. Comparison of the potential of phylogenetically defined InlB isoforms to restore the invasive phenotype of the EGDeΔinlB strain demonstrated that although all InlB isoforms restored invasion of the EGDeΔinlB strain into shKEC cells, the InlB isoforms typical of highly virulent ruminant strains of the clonal complexes CC1 and CC7 were more efficient than isoforms typical of CC2 and CC9 strains (which are less virulent toward ruminants) in supporting invasion. Listeria monocytogenes effectively multiplied with a doubling of time in about 90 min after they entered the sheep cells. Intracellular bacteria moved using the well-known actin polymerization mechanism. Cell-to-cell spreading was restricted to the infection of a few tens of neighboring cells for 7 days. Overall, the obtained results demonstrated that (i) InlB is required for invasion into sheep cells, (ii) InlB isoforms might be important for hypervirulence of certain clonal groups toward ruminants, and (iii) L. monocytogenes effectively multiplies in ovine cells once entered.

Mechanical competition triggered by innate immune signaling drives the collective extrusion of bacterially infected epithelial cells

Intracellular pathogens alter their host cells' mechanics to promote dissemination through tissues. Conversely, host cells may respond to the presence of pathogens by altering their mechanics to limit infection. Here, we monitored epithelial cell monolayers infected with intracellular bacterial pathogens, Listeria monocytogenes or Rickettsia parkeri, over days. Under conditions in which these pathogens trigger innate immune signaling through NF-κB and use actin-based motility to spread non-lytically intercellularly, we found that infected cell domains formed three-dimensional mounds. These mounds resulted from uninfected cells moving toward the infection site, collectively squeezing the softer and less contractile infected cells upward and ejecting them from the monolayer. Bacteria in mounds were less able to spread laterally in the monolayer, limiting the growth of the infection focus, while extruded infected cells underwent cell death. Thus, the coordinated forceful action of uninfected cells actively eliminates large domains of infected cells, consistent with this collective cell response representing an innate immunity-driven process.

Applications of bacteriophages against intracellular bacteria

Infectious diseases pose a significant threat to both human and animal populations. Intracellular bacteria are a group of pathogens that invade and survive within the interior of eukaryotic cells, which in turn protect them from antibacterial drugs and the host immune system. Limited penetration of antibacterials into host cells results in insufficient bacterial clearance and treatment failure. Bacteriophages have, over the decades, been proved to play an important role in combating bacterial infections (phage therapy), making them an important alternative to classical antibiotic strategies today. Phages have been found to be effective at killing various species of extracellular bacteria, but little is still known about how phages control intracellular infections. With advances in phage genomics and mechanisms of delivery and cell uptake, the development of phage-based antibacterial strategies to address the treatment of intracellular bacteria has general potential. In this review, we present the current state of knowledge regarding the application of bacteriophages against intracellular bacteria. We cover phage deployment against the most common intracellular pathogens with special attention to therapeutic and preventive strategies.

Human Placental Trophoblasts Infected by Listeria monocytogenes Undergo a Pro-Inflammatory Switch Associated With Poor Pregnancy Outcomes

The placenta controls the growth of the fetus and ensures its immune protection. Key to these functions, the syncytiotrophoblast (SYN) is a syncytium formed by fusion of underlying mononuclear trophoblasts. The SYN covers the placental surface and is bathed in maternal blood to mediate nutritional and waste exchanges between the mother and fetus. The bacterial pathogen Listeria monocytogenes breaches the trophoblast barrier and infects the placental/fetal unit resulting in poor pregnancy outcomes. In this work, we analyzed the L. monocytogenes intracellular lifecycle in primary human trophoblasts. In accordance with previous studies, we found that the SYN is 20-fold more resistant to infection compared to mononuclear trophoblasts, forming a protective barrier to infection at the maternal interface. We show for the first time that this is due to a significant reduction in L. monocytogenes uptake by the SYN rather than inhibition of the bacterial intracellular division or motility. We here report the first transcriptomic analysis of L. monocytogenes-infected trophoblasts (RNA sequencing). Pathway analysis showed that infection upregulated TLR2, NOD-like, and cytosolic DNA sensing pathways, as well as downstream pro-inflammatory circuitry (NF-κB, AP-1, IRF4, IRF7) leading to the production of mediators known to elicit the recruitment and activation of maternal leukocytes (IL8, IL6, TNFα, MIP-1). Signature genes associated with poor pregnancy outcomes were also upregulated upon infection. Measuring the release of 54 inflammatory mediators confirmed the transcriptomic data and revealed sustained production of tolerogenic factors (IL-27, IL-10, IL-1RA, TSLP) despite infection. Both the SYN and mononuclear trophoblasts produced cytokines, but surprisingly, some cytokines were predominantly produced by the SYN (IL-8, IL-6) or by non-fused trophoblasts (TNFα). Collectively, our data support that trophoblasts act as placental gatekeepers that limit and detect L. monocytogenes infection resulting in a pro-inflammatory response, which may contribute to the poor pregnancy outcomes if the pathogen persists.

Volume measurement and biophysical characterization of mounds in epithelial monolayers after intracellular bacterial infection

Mechanical forces are important in (patho)physiological processes, including how host epithelial cells interact with intracellular bacterial pathogens. As these pathogens disseminate within host epithelial monolayers, large mounds of infected cells are formed due to the forceful action of surrounding uninfected cells, limiting bacterial spread across the basal cell monolayer. Here, we present a protocol for mound volume measurement and biophysical characterization of mound formation. Modifications to this protocol may be necessary for studying different host cell types or pathogenic organisms.

For complete details on the use and execution of this protocol, please refer to Bastounis et al. (2021).

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Protocol allows for formation of mounds of extruded infected cells in cell monolayers

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Confocal microscopy and image processing to calculate volume of extruded domains

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Laser wounding protocol for tension estimation built around mounds

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TFM incorporated to measure traction stresses of infected mounders and surrounders

Listeria monocytogenes: review of pathogenesis and virulence determinants-targeted immunological assays

Listeria monocytogenes is one of the most invasive foodborne pathogens and is responsible for numerous outbreaks worldwide. Most of the methods to detect this bacterium in food require selective enrichment using traditional bacterial culture techniques that can be time-consuming and labour-intensive. Moreover, molecular methods are expensive and need specific technical knowledge. In contrast, immunological approaches are faster, simpler, and user-friendly alternatives and have been developed for the detection of L. monocytogenes in food, environmental, and clinical samples. These techniques are dependent on the constitutive expression of L. monocytogenes antigens and the specificity of the antibodies used. Here, updated knowledge on pathogenesis and the key immunogenic virulence determinants of L. monocytogenes that are used for the generation of monoclonal and polyclonal antibodies for the serological assay development are summarised. In addition, immunological approaches based on enzyme-linked immunosorbent assay, immunofluorescence, lateral flow immunochromatographic assays, and immunosensors with relevant improvements are highlighted. Though the sensitivity and specificity of the assays were improved significantly, methods still face many challenges that require further validation before use.

Actin cables and comet tails organize mitochondrial networks in mitosis

Symmetric cell division requires the even partitioning of genetic information and cytoplasmic contents between daughter cells. While the mechanisms coordinating the segregation of the genome are well known, the processes which ensure organelle segregation between daughter cells remain less well-understood¹. Here, we identify multiple actin assemblies that play distinct but complementary roles in mitochondrial organization and inheritance in mitosis. First, we find a dense meshwork of subcortical actin cables assembled throughout the mitotic cytoplasm. This network scaffolds the endoplasmic reticulum and organizes three-dimensional mitochondrial positioning to ensure the equal segregation of mitochondrial mass at cytokinesis. Second, we identify a dynamic wave of actin filaments reversibly assembling on the surface of mitochondria through mitosis. Mitochondria sampled by this wave are enveloped within actin clouds that can spontaneously break symmetry to form elongated comet tails. Mitochondrial comet tails promote randomly directed bursts of movement that shuffle mitochondrial position within the mother cell to randomize inheritance of healthy and damaged mitochondria between daughter cells. Thus, parallel mechanisms mediated by the actin cytoskeleton ensure both equal and random inheritance of mitochondria in symmetrically dividing cells.

Candida albicans, a reservoir of Listeria monocytogenes?

Listeria monocytogenes is a pathogen causing serious or mortal infections in human risk populations. Its infectivity is in part due to its ability to infect diverse eukaryotic cells. Since several bacteria can enter into yeast cells, including Candida albicans, the aims of this work were to evaluate if L. monocytogenes was able to harbor, retaining its viability, within C. albicans cells and to evaluate the effect of temperature and an antibiotic as stressing factors in its rate of entry into yeast cells. Both microorganisms were co-incubated in BHI broth during 48 h and the entry of bacteria into yeast cells was evaluated at different times. Then, yeasts free of extracellular bacteria were obtained seeding samples of the co-culture on YGC agar, which contains chloramphenicol, to obtain extracellular bacteria-free yeasts. These extracellular bacteria free yeasts were used to search for bacterial DNA in total yeast DNA and to evaluate the viability of intra-yeast bacteria. Finally, the effect of temperature and of chloramphenicol as inducers of stress on the rate of bacterial entry into yeast cells were investigated. After co-culturing both microorganisms, wet mount optical microscopy showed the presence of moving bacteria within yeasts and transmission electron microscopy confirmed the presence of intra-yeast bacteria. PCR allowed to amplify L. monocytogenes iap gene in C. albicans total DNA obtained from yeasts free of extracellular bacteria. Moreover, the SYTO 9 green fluorescence observed in bacterial cells within vacuoles of yeasts suggests that intra-yeast bacteria remain viable. Furthermore, the entry of L. monocytogenes into yeasts cells was favored by the presence of stressing factors (chloramphenicol and temperature). Therefore, yeasts may be reservoirs of viable L. monocytogenes and might spread them to the following generations of yeasts.

Doppler imaging detects bacterial infection of living tissue

Living 3D in vitro tissue cultures, grown from immortalized cell lines, act as living sentinels as pathogenic bacteria invade the tissue. The infection is reported through changes in the intracellular dynamics of the sentinel cells caused by the disruption of normal cellular function by the infecting bacteria. Here, the Doppler imaging of infected sentinels shows the dynamic characteristics of infections. Invasive Salmonella enterica serovar Enteritidis and Listeria monocytogenes penetrate through multicellular tumor spheroids, while non-invasive strains of Escherichia coli and Listeria innocua remain isolated outside the cells, generating different Doppler signatures. Phase distributions caused by intracellular transport display Lévy statistics, introducing a Lévy-alpha spectroscopy of bacterial invasion. Antibiotic treatment of infected spheroids, monitored through time-dependent Doppler shifts, can distinguish drug-resistant relative to non-resistant strains. This use of intracellular Doppler spectroscopy of living tissue sentinels opens a new class of microbial assay with potential importance for studying the emergence of antibiotic resistance.

Honggu Choi et al. use biodynamic Doppler imaging to monitor bacterial infection of 3D living tissue and describe changes in the intracellular motions of living host tissue induced by early-stage infection. This work demonstrates the potential for the clinical use of this method to test for antibiotic-resistant infections.

Listeria monocytogenes Infection of Bat Pipistrellus nathusii Epithelial cells Depends on the Invasion Factors InlA and InlB

L. monocytogenes is a widespread facultative intracellular pathogen. The range of natural hosts that supporting L. monocytogenes persistence in the environment has not been fully established yet. In this study, we were interested in the potential of L. monocytogenes to infect cells of bats, which are being increasingly recognized as a reservoir for microorganisms that are pathogenic to humans and domestic animals. A stable epithelial cell line was developed from the kidneys of Pipistrellus nathusii, a small bat widely distributed across Europe. The wild-type L. monocytogenes strain EGDe infected this cell line with an invasion efficiency of 0.0078 ± 0.0009%. Once it entered bat cells, L. monocytogenes doubled within about 70 min. When L. monocytogenes lacked either of the major invasion factors, InlA and InlB, invasion efficiency decreased by a factor of 10 and 25 respectively (p < 0.000001). The obtained results suggest that bat epithelial cells are susceptible to L. monocytogenes infection and that L. monocytogenes invasion of bat cells depends on the major invasion factors InlA and InlB. These results constitute the first report on in vitro studies of L. monocytogenes infection in bats.

Molecular Mechanisms of Intercellular Dissemination of Bacterial Pathogens

Several intracellular bacterial pathogens, including Listeria monocytogenes, Shigella flexerni, and Rickettsia spp. use an actin-based motility process to spread in mammalian cell monolayers. Cell-to-cell spread is mediated by protrusive structures that contain bacteria encased in the host cell plasma membrane. These protrusions, which form in infected host cells, are internalized by neighboring cells. In this review, we summarize key findings on cell-to-cell spread, focusing on recent work on mechanisms of protrusion formation and internalization. We also discuss the dynamic behavior of bacterial populations during spread, and highlight recent findings showing that intercellular spread by an extracellular bacterial pathogen.

The impact of long-range dispersal on gene surfing

Range expansions lead to distinctive patterns of genetic variation in populations, even in the absence of selection. These patterns and their genetic consequences have been well studied for populations advancing through successive short-ranged migration events. However, most populations harbor some degree of long-range dispersal, experiencing rare yet consequential migration events over arbitrarily long distances. Although dispersal is known to strongly affect spatial genetic structure during range expansions, the resulting patterns and their impact on neutral diversity remain poorly understood. Here, we systematically study the consequences of long-range dispersal on patterns of neutral variation during range expansion in a class of dispersal models which spans the extremes of local (effectively short-ranged) and global (effectively well-mixed) migration. We find that sufficiently long-ranged dispersal leaves behind a mosaic of monoallelic patches, whose number and size are highly sensitive to the distribution of dispersal distances. We develop a coarse-grained model which connects statistical features of these spatial patterns to the evolution of neutral diversity during the range expansion. We show that growth mechanisms that appear qualitatively similar can engender vastly different outcomes for diversity: Depending on the tail of the dispersal distance distribution, diversity can be either preserved (i.e., many variants survive) or lost (i.e., one variant dominates) at long times. Our results highlight the impact of spatial and migratory structure on genetic variation during processes as varied as range expansions, species invasions, epidemics, and the spread of beneficial mutations in established populations.

Listeria monocytogenes exploits host exocytosis to promote cell-to-cell spread

The facultative intracellular pathogen Listeria monocytogenes uses an actin-based motility process to spread within human tissues. Filamentous actin from the human cell forms a tail behind bacteria, propelling microbes through the cytoplasm. Motile bacteria remodel the host plasma membrane into protrusions that are internalized by neighboring cells. A critical unresolved question is whether generation of protrusions by Listeria involves stimulation of host processes apart from actin polymerization. Here we demonstrate that efficient protrusion formation in polarized epithelial cells involves bacterial subversion of host exocytosis. Confocal microscopy imaging indicated that exocytosis is up-regulated in protrusions of Listeria in a manner that depends on the host exocyst complex. Depletion of components of the exocyst complex by RNA interference inhibited the formation of Listeria protrusions and subsequent cell-to-cell spread of bacteria. Additional genetic studies indicated important roles for the exocyst regulators Rab8 and Rab11 in bacterial protrusion formation and spread. The secreted Listeria virulence factor InlC associated with the exocyst component Exo70 and mediated the recruitment of Exo70 to bacterial protrusions. Depletion of exocyst proteins reduced the length of Listeria protrusions, suggesting that the exocyst complex promotes protrusion elongation. Collectively, these results demonstrate that Listeria exploits host exocytosis to stimulate intercellular spread of bacteria.