Plasma membrane protrusions mediate host cell-cell fusion induced by Burkholderia thailandensis

The Rickettsia actin-based motility effectors RickA and Sca2 contribute differently to cell-to-cell spread and pathogenicity

Rickettsia parkeri is an obligate intracellular, tick-borne bacterial pathogen that can cause eschar-associated rickettsiosis in humans. R. parkeri invades host cells, escapes from vacuoles into the cytosol, and undergoes two independent modes of actin-based motility mediated by effectors RickA or Sca2. Actin-based motility of R. parkeri enables bacteria to enter protrusions of the host cell plasma membrane that are engulfed by neighboring host cells. However, whether and how RickA and Sca2 independently contribute to cell-to-cell spread in vitro or pathogenicity in vivo has been unclear. Using live cell imaging of rickA::Tn and sca2::Tn mutants, we discovered both RickA and Sca2 contribute to different modes of cell-to-cell spread. Compared with Sca2-spread, RickA-spread involves the formation of longer protrusions that exhibit larger fluctuations in length and take a longer time to be engulfed into neighboring cells. We further compared the roles of RickA and Sca2 in vivo following intradermal (i.d.) infection of Ifnar1-/-; Ifngr1-/- mice carrying knockout mutations in the genes encoding the receptors for IFN-I (Ifnar1) and IFN-γ (Ifngr1), which exhibit eschars and succumb to infection with wild-type (WT) R. parkeri. We observed that RickA is important for severe eschar formation, whereas Sca2 contributes to larger foci of infection in the skin and dissemination from the skin to the internal organs. Our results suggest that actin-based motility effectors RickA and Sca2 drive two distinct forms of cell-to-cell spread and contribute differently to pathogenicity in the mammalian host.IMPORTANCERickettsia parkeri, a bacterium in the spotted fever group of Rickettsia species, can be transmitted from ticks to humans, leading to symptoms including fever, rash, muscle aches, and a lesion at the site of the tick bite. During Rickettsia parkeri infection, bacteria invade cells within the animal host, proliferate in the host cell's cytosol, move using a process called actin-based motility, and spread to neighboring host cells. Rickettsia parkeri is unusual in having two bacterial proteins that mediate actin-based motility. The significance of our research is to reveal that each of these bacterial actin-based motility proteins contributes differently to spread between cells and to the signs of infection in a mouse model of spotted fever disease. Our results are important for understanding the contribution of actin-based motility to mammalian infection by Rickettsia parkeri as well as to infection by other bacterial and viral pathogens that require this process to spread between cells and cause disease.

Symbiotic T6SS affects horizontal transmission of Paraburkholderia bonniea among Dictyostelium discoideum amoeba hosts

Three species of Paraburkholderia are able to form facultative symbiotic relationships with the amoeba, Dictyostelium discoideum. These symbiotic Paraburkholderia share a type VI secretion system (T6SS) that is absent in other close relatives. We tested the phenotypic and transcriptional effect of tssH ATPase gene disruption in P. bonniea on its symbiosis with D. discoideum. We hypothesized that the ∆tssH mutant would have a significantly reduced ability to affect host fitness or transmit itself from host to host. We found that the T6SS does not directly affect host fitness. Instead, wildtype P. bonniea had significantly higher rates of horizontal transmission compared to ∆tssH. In addition, we observed significant differences in the range of infection prevalence achieved by wildtype vs. ∆tssH symbionts over multiple host social stages in the absence of opportunities for environmental symbiont acquisition. Successful symbiont transmission significantly contributes to sustained symbiotic association. Therefore, the shared T6SS appears necessary for a long-term evolutionary relationship between D. discoideum and its Paraburkholderia symbionts. The lack of difference in host fitness outcomes was confirmed by indistinguishable host gene expression patterns between hosts infected by wildtype or ∆tssH P. bonniea in an RNA-seq time series. These data also provided insight into how Paraburkholderia symbionts may evade phagocytosis by its amoeba host. Most significantly, cellular oxidant detoxification and lysosomal hydrolase delivery appear to be subject to the push and pull of host-symbiont crosstalk.

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.

Burkholderia thailandensis uses a type VI secretion system to lyse protrusions without triggering host cell responses

To spread within a host, intracellular Burkholderia form actin tails to generate membrane protrusions into neighboring host cells and use type VI secretion system-5 (T6SS-5) to induce cell-cell fusions. Here, we show that B. thailandensis also uses T6SS-5 to lyse protrusions to directly spread from cell to cell. Dynamin-2 recruitment to the membrane near a bacterium was followed by a short burst of T6SS-5 activity. This resulted in the polymerization of the actin of the newly invaded host cell and disruption of the protrusion membrane. Most protrusion lysis events were dependent on dynamin activity, caused no cell-cell fusion, and failed to be recognized by galectin-3. T6SS-5 inactivation decreased protrusion lysis but increased galectin-3, LC3, and LAMP1 accumulation in host cells. Our results indicate that B. thailandensis specifically activates T6SS-5 assembly in membrane protrusions to disrupt host cell membranes and spread without alerting cellular responses, such as autophagy.

Cryo-electron tomography of stationary phase Burkholderia thailandensis

Burkholderia species belonging to the pseudomallei group include significant human and animal pathogens as well as the non-pathogenic species Burkholderia thailandensis . These bacteria co-opt the host cell machinery for their replication and spread between host cells. Thus, it is of interest to understand the structural features of these cells that contribute to host cell colonization and virulence. This study provides high-resolution cryo-electron tomograms of stationary phase Burkholderia thailandensis . It reveals the presence of compact nucleoids and storage granules, as well as examples of the type III secretion system and chemoreceptor arrays. The data can be used to investigate the near-atomic structure of stationary-phase bacterial macromolecules, such as ribosomes.

Burkholderia pseudomallei Complex Subunit and Glycoconjugate Vaccines and Their Potential to Elicit Cross-Protection to Burkholderia cepacia Complex

Burkholderia are a group of Gram-negative bacteria that can cause a variety of diseases in at-risk populations. B. pseudomallei and B. mallei, the etiological agents of melioidosis and glanders, respectively, are the two clinically relevant members of the B. pseudomallei complex (Bpc). The development of vaccines against Bpc species has been accelerated in recent years, resulting in numerous promising subunits and glycoconjugate vaccines incorporating a variety of antigens. However, a second group of pathogenic Burkholderia species exists known as the Burkholderia cepacia complex (Bcc), a group of opportunistic bacteria which tend to affect individuals with weakened immunity or cystic fibrosis. To date, there have been few attempts to develop vaccines to Bcc species. Therefore, the primary goal of this review is to provide a broad overview of the various subunit antigens that have been tested in Bpc species, their protective efficacy, study limitations, and known or suspected mechanisms of protection. Then, we assess the reviewed Bpc antigens for their amino acid sequence conservation to homologous proteins found in Bcc species. We propose that protective Bpc antigens with a high degree of Bpc-to-Bcc sequence conservation could serve as components of a pan-Burkholderia vaccine capable of protecting against both disease-causing groups.

Manipulation of host cell plasma membranes by intracellular bacterial pathogens

Manipulation of the host cell plasma membrane is critical during infection by intracellular bacterial pathogens, particularly during bacterial entry into and exit from host cells. To manipulate host cells, bacteria deploy secreted proteins that modulate or modify host cell components. Here, we review recent advances that suggest common themes by which bacteria manipulate the host cell plasma membrane. One theme is that bacteria use diverse strategies to target or influence host cell plasma membrane composition and shape. A second theme is that bacteria take advantage of host cell plasma membrane-associated pathways such as signal transduction, endocytosis, and exocytosis. Future investigation into how bacterial and host factors contribute to plasma membrane manipulation by bacterial pathogens will reveal new insights into pathogenesis and fundamental principles of plasma membrane biology.