Relaxed specificity of BcpB transporters mediates interactions between Burkholderia cepacia complex contact-dependent growth inhibition systems

Belonging to the two-partner secretion family of proteins, contact-dependent growth inhibition (CDI) systems mediate interbacterial antagonism among closely related Gram-negative bacteria. The toxic portion of a large surface protein, BcpA/CdiA, is delivered to the cytoplasm of neighboring cells where it inhibits growth. Translocation of the antibacterial polypeptide out of the producing cell requires an associated outer membrane transporter, BcpB/CdiB. Some bacteria, including many Burkholderia species, encode multiple distinct CDI systems, but whether there is interaction between these systems is largely unknown. Using Burkholderia cepacia complex species as a model, here we show that related BcpB transporters exhibit considerable secretion flexibility and can secrete both cognate and non-cognate BcpA substrates. We also identified an additional unique Burkholderia dolosa CDI system capable of mediating interbacterial competition and demonstrated that its BcpB transporter has similar relaxed substrate specificity. Our results showed that two BcpB transporters (BcpB-2 and BcpB-3) were able to secrete all four of the B. dolosa BcpA toxins, while one transporter (BcpB-1) appeared unable to secrete even its cognate BcpA substrate under the tested conditions. This flexibility provided a competitive advantage, as strains lacking the full repertoire of BcpB proteins had decreased CDI activity. Similar results were obtained in Burkholderia multivorans, suggesting that secretion flexibility may be a conserved feature of Burkholderia CDI systems. Together these findings suggest that the interaction between distinct CDI systems enhances the efficiency of bacterial antagonism. IMPORTANCE The Burkholderia cepacia complex (Bcc) is a group of related opportunistic bacterial pathogens that occupy a diverse range of ecological niches and exacerbate disease in patients with underlying conditions. Contact-dependent growth inhibition (CDI) system proteins, produced by Gram-negative bacteria, contain antagonistic properties that allow for intoxication of closely related neighboring bacteria via a secreted protein, BcpA. Multiple unique CDI systems can be found in the same bacterial strain, and here we show that these distinct systems interact in several Bcc species. Our findings suggest that the interaction between CDI system proteins is important for interbacterial toxicity. Understanding the mechanism of interplay between CDI systems provides further insight into the complexity of bacterial antagonism. Moreover, since many bacterial species are predicted to encode multiple CDI systems, this study suggests that interactions between these distinct systems likely contribute to the overall competitive fitness of these species.

Burkholderia multivorans requires species-specific GltJK for entry of a contact-dependent growth inhibition system protein

Interbacterial antagonism and communication are driving forces behind microbial community development. In many Gram‐negative bacteria, contact‐dependent growth inhibition (CDI) systems contribute to these microbial interactions. CDI systems deliver the toxic C‐terminus of a large surface exposed protein to the cytoplasm of neighboring bacteria upon cell−contact. Termed the BcpA‐CT, import of this toxic effector domain is mediated by specific, yet largely unknown receptors on the recipient cell outer and inner membranes. In this study, we demonstrated that cytoplasmic membrane proteins GltJK, components of a predicted ABC‐type transporter, are required for entry of CDI system protein BcpA‐2 into Burkholderia multivorans recipient cells. Consistent with current CDI models, gltJK were also required for recipient cell susceptibility to a distinct BcpA‐CT that shared sequences within the predicted “translocation domain” of BcpA‐2. Strikingly, this translocation domain showed low sequence identity to the analogous region of an Escherichia coli GltJK‐utilizing CDI system protein. Our results demonstrated that recipient bacteria expressing E. coli gltJK were resistant to BcpA‐2‐mediated interbacterial antagonism, suggesting that BcpA‐2 specifically recognizes Burkholderia GltJK. Using a series of chimeric proteins, the specificity determinant was mapped to Burkholderia‐specific sequences at the GltK C‐terminus, providing insight into BcpA transport across the recipient cell cytoplasmic membrane.

Type I IFN Does Not Promote Susceptibility to Foodborne Listeria monocytogenes

Type I IFN (IFN-α/β) is thought to enhance growth of the foodborne intracellular pathogen Listeria monocytogenes by promoting mechanisms that dampen innate immunity to infection. However, the type I IFN response has been studied primarily using methods that bypass the stomach and, therefore, fail to replicate the natural course of L. monocytogenes infection. In this study, we compared i.v. and foodborne transmission of L. monocytogenes in mice lacking the common type I IFN receptor (IFNAR1(-/-)). Contrary to what was observed using i.v. infection, IFNAR1(-/-) and wild-type mice had similar bacterial burdens in the liver and spleen following foodborne infection. Splenocytes from wild-type mice infected i.v. produced significantly more IFN-β than did those infected by the foodborne route. Consequently, the immunosuppressive effects of type I IFN signaling, which included T cell death, increased IL-10 secretion, and repression of neutrophil recruitment to the spleen, were all observed following i.v. but not foodborne transmission of L. monocytogenes. Type I IFN was also previously shown to cause a loss of responsiveness to IFN-γ through downregulation of the IFN-γ receptor α-chain on macrophages and dendritic cells. However, we detected a decrease in surface expression of IFN-γ receptor α-chain even in the absence of IFN-α/β signaling, suggesting that in vivo, this infection-induced phenotype is not type I IFN-dependent. These results highlight the importance of using the natural route of infection for studies of host-pathogen interactions and suggest that the detrimental effects of IFN-α/β signaling on the innate immune response to L. monocytogenes may be an artifact of the i.v. infection model.

Caspase-3 mediates the pathogenic effect of Yersinia pestis YopM in liver of C57BL/6 mice and contributes to YopM's function in spleen

The virulence protein YopM of the plague bacterium Yersinia pestis has different dominant effects in liver and spleen. Previous studies focused on spleen, where YopM inhibits accumulation of inflammatory dendritic cells. In the present study we focused on liver, where PMN function may be directly undermined by YopM without changes in inflammatory cell numbers in the initial days of infection, and foci of inflammation are easily identified. Mice were infected with parent and ΔyopM-1 Y. pestis KIM5, and effects of YopM were assessed by immunohistochemistry and determinations of bacterial viable numbers in organs. The bacteria were found associated with myeloid cells in foci of inflammation and in liver sinusoids. A new in-vivo phenotype of YopM was revealed: death of inflammatory cells, evidenced by TUNEL staining beginning at d 1 of infection. Based on distributions of Ly6G⁺, F4/80⁺, and iNOS⁺ cells within foci, the cells that were killed could have included both PMNs and macrophages. By 2 d post-infection, YopM had no effect on distribution of these cells, but by 3 d cellular decomposition had outstripped acute inflammation in foci due to parent Y. pestis, while foci due to the ΔyopM-1 strain still contained many inflammatory cells. The destruction depended on the presence of both PMNs in the mice and YopM in the bacteria. In mice that lacked the apoptosis mediator caspase-3 the infection dynamics were novel: the parent Y. pestis was limited in growth comparably to the ΔyopM-1 strain in liver, and in spleen a partial growth limitation for parent Y. pestis was seen. This result identified caspase-3 as a co-factor or effector in YopM's action and supports the hypothesis that in liver YopM's main pathogenic effect is mediated by caspase-3 to cause apoptosis of PMNs.

Cyclic di-GMP-dependent signaling pathways in the pathogenic Firmicute Listeria monocytogenes

We characterized key components and major targets of the c-di-GMP signaling pathways in the foodborne pathogen Listeria monocytogenes, identified a new c-di-GMP-inducible exopolysaccharide responsible for motility inhibition, cell aggregation, and enhanced tolerance to disinfectants and desiccation, and provided first insights into the role of c-di-GMP signaling in listerial virulence. Genome-wide genetic and biochemical analyses of c-di-GMP signaling pathways revealed that L. monocytogenes has three GGDEF domain proteins, DgcA (Lmo1911), DgcB (Lmo1912) and DgcC (Lmo2174), that possess diguanylate cyclase activity, and three EAL domain proteins, PdeB (Lmo0131), PdeC (Lmo1914) and PdeD (Lmo0111), that possess c-di-GMP phosphodiesterase activity. Deletion of all phosphodiesterase genes (ΔpdeB/C/D) or expression of a heterologous diguanylate cyclase stimulated production of a previously unknown exopolysaccharide. The synthesis of this exopolysaccharide was attributed to the pssA-E (lmo0527-0531) gene cluster. The last gene of the cluster encodes the fourth listerial GGDEF domain protein, PssE, that functions as an I-site c-di-GMP receptor essential for exopolysaccharide synthesis. The c-di-GMP-inducible exopolysaccharide causes cell aggregation in minimal medium and impairs bacterial migration in semi-solid agar, however, it does not promote biofilm formation on abiotic surfaces. The exopolysaccharide also greatly enhances bacterial tolerance to commonly used disinfectants as well as desiccation, which may contribute to survival of L. monocytogenes on contaminated food products and in food-processing facilities. The exopolysaccharide and another, as yet unknown c-di-GMP-dependent target, drastically decrease listerial invasiveness in enterocytes in vitro, and lower pathogen load in the liver and gallbladder of mice infected via an oral route, which suggests that elevated c-di-GMP levels play an overall negative role in listerial virulence.

Listeria monocytogenes is ubiquitously present in the environment, highly adaptable and tolerant to various stresses. L. monocytogenes is also a foodborne pathogen associated with the largest foodborne outbreaks in recent US history. Signaling pathways involving the second messenger c-di-GMP play important roles in increased stress survival of proteobacteria and mycobacteria, yet roles of c-di-GMP signaling pathways in L. monocytogenes have remained unexplored. Here, we identified and systematically characterized functions of the proteins involved in c-di-GMP synthesis, degradation and sensing. We show that elevated c-di-GMP levels in L. monocytogenes result in synthesis of a previously unknown exopolysaccharide that promotes cell aggregation, inhibits motility in semi-solid media, and importantly, enhances bacterial tolerance to commonly used disinfectants as well as desiccation. These properties of the exopolysaccharide may increase listerial survival in food processing plants as well as on produce during transportation and storage. Elevated c-di-GMP levels also grossly diminish listerial invasiveness in enterocytes in vitro, and impair bacterial accumulation in selected mouse organs during oral infection.

Temperature-dependence of yadBC phenotypes in Yersinia pestis

YadB and YadC are putative trimeric autotransporters present only in the plague bacterium Yersinia pestis and its evolutionary predecessor, Yersinia pseudotuberculosis. Previously, yadBC was found to promote invasion of epithelioid cells by Y. pestis grown at 37 °C. In this study, we found that yadBC also promotes uptake of 37 °C-grown Y. pestis by mouse monocyte/macrophage cells. We tested whether yadBC might be required for lethality of the systemic stage of plague in which the bacteria would be pre-adapted to mammalian body temperature before colonizing internal organs and found no requirement for early colonization or growth over 3 days. We tested the hypothesis that YadB and YadC function on ambient temperature-grown Y. pestis in the flea vector or soon after infection of the dermis in bubonic plague. We found that yadBC did not promote uptake by monocyte/macrophage cells if the bacteria were grown at 28 °C, nor was there a role of yadBC in colonization of fleas by Y. pestis grown at 21 °C. However, the presence of yadBC did promote recoverability of the bacteria from infected skin for 28 °C-grown Y. pestis. Furthermore, the gene for the proinflammatory chemokine CXCL1 was upregulated in expression if the infecting Y. pestis lacked yadBC but not if yadBC was present. Also, yadBC was not required for recoverability if the bacteria were grown at 37 °C. These findings imply that thermally induced virulence properties dominate over effects of yadBC during plague but that yadBC has a unique function early after transmission of Y. pestis to skin.

A mouse model of foodborne Listeria monocytogenes infection

Listeria monocytogenes causes foodborne disease in humans that ranges in severity from mild, self-limiting gastroenteritis to life-threatening systemic infections of the blood, brain, or placenta. The most commonly used animal model of listeriosis is intravenous infection of mice. This systemic model is highly reproducible, and thus, useful for studying cell-mediated immune responses against an intracellular bacterial pathogen, but it completely bypasses the gastrointestinal phase of L. monocytogenes infection. Intragastric inoculation of L. monocytogenes produces more variable results and may cause direct bloodstream invasion in some animals. The foodborne transmission model described here does not require specialized skills to perform and results in infections that more closely mimic human disease. This natural feeding model can be used to study both the host- and pathogen-derived factors that govern susceptibility or resistance to orally acquired L. monocytogenes.

Fecal transplantation does not transfer either susceptibility or resistance to food borne listeriosis in C57BL/6 and BALB/c/By mice

The composition of the intestinal microbiota has wide reaching effects on the health of an individual, including the development of protective innate immune responses.  In this report, a fecal transplantation approach was used to determine whether resistance to food borne listeriosis was dependent on the murine gut microbiota.  Transplantation of BALB/c/By feces did not increase the susceptibility of C57BL/6 mice to Listeria monocytogenes infection.   Likewise, transplantation of C57BL/6 fecal matter did not enhance the resistance of BALB/c/By mice.  Thus, intestinal microbiota composition is not a key factor that confers either susceptibility or resistance to food borne listeriosis in mice.

Oral transmission of Listeria monocytogenes in mice via ingestion of contaminated food

L. monocytogenes are facultative intracellular bacterial pathogens that cause food borne infections in humans. Very little is known about the gastrointestinal phase of listeriosis due to the lack of a small animal model that closely mimics human disease. This paper describes a novel mouse model for oral transmission of L. monocytogenes. Using this model, mice fed L. monocytogenes-contaminated bread have a discrete phase of gastrointestinal infection, followed by varying degrees of systemic spread in susceptible (BALB/c/By/J) or resistant (C57BL/6) mouse strains. During the later stages of the infection, dissemination to the gall bladder and brain is observed. The food borne model of listeriosis is highly reproducible, does not require specialized skills, and can be used with a wide variety of bacterial isolates and laboratory mouse strains. As such, it is the ideal model to study both virulence strategies used by L. monocytogenes to promote intestinal colonization, as well as the host response to invasive food borne bacterial infection.

Toward a molecular pathogenic pathway for Yersinia pestis YopM

YopM is one of the six "effector Yops" of the human-pathogenic Yersinia, but its mechanism has not been defined. After delivery to J774A.1 monocyte-like cells, YopM can rapidly bind and activate the serine/threonine kinases RSK1 and PRK2. However, in infected mice, effects of Y. pestis YopM have been seen only after 24-48 h post-infection (p.i.). To identify potential direct effects of YopM in-vivo we tested for effects of YopM at 1 h and 16-18 h p.i. in mice infected systemically with 10(6) bacteria. At 16 h p.i., there was a robust host response to both parent and ΔyopM-1 Y. pestis KIM5. Compared to cells from non-infected mice, CD11b(+) cells from spleens of infected mice produced more than 100-fold greater IFNγ. In the corresponding sera there were more than 100-fold greater amounts of IFNγ, G-CSF, and CXCL9, as well as more than 10-fold greater amounts of IL-6, CXCL10, and CXCL1. The only YopM-related differences were slightly lower CXCL10 and IL-6 in sera from mice infected 16 h with parent compared to ΔyopM-1 Y. pestis. Microarray analysis of the CD11b(+) cells did not identify consistent transcriptional differences of ≥4-fold at 18 h p.i. However, at 1 h p.i. mRNA for early growth response transcription factor 1 (Egr1) was decreased when YopM was present. Bone marrow-derived macrophages infected for 1 h also expressed lower Egr1 message when YopM was present. Infected J774A.1 cells showed greater expression of Egr1 at 1 h p.i. when YopM was present, but this pattern reversed at 3 h. At 6 h p.i., Cxcl10 mRNA was lower in parent-strain infected cells. We conclude that decreased Egr1 expression is a very early transcriptional effect of YopM and speculate that a pathway may exist from RSK1 through Egr1. These studies revealed novel early transcriptional effects of YopM but point to a time after 18 h of infection when critical transitional events lead to later major effects on cytokine gene transcription.

InlA promotes dissemination of Listeria monocytogenes to the mesenteric lymph nodes during food borne infection of mice

Intestinal Listeria monocytogenes infection is not efficient in mice and this has been attributed to a low affinity interaction between the bacterial surface protein InlA and E-cadherin on murine intestinal epithelial cells. Previous studies using either transgenic mice expressing human E-cadherin or mouse-adapted L. monocytogenes expressing a modified InlA protein (InlA(m)) with high affinity for murine E-cadherin showed increased efficiency of intragastric infection. However, the large inocula used in these studies disseminated to the spleen and liver rapidly, resulting in a lethal systemic infection that made it difficult to define the natural course of intestinal infection. We describe here a novel mouse model of oral listeriosis that closely mimics all phases of human disease: (1) ingestion of contaminated food, (2) a distinct period of time during which L. monocytogenes colonize only the intestines, (3) varying degrees of systemic spread in susceptible vs. resistant mice, and (4) late stage spread to the brain. Using this natural feeding model, we showed that the type of food, the time of day when feeding occurred, and mouse gender each affected susceptibility to L. monocytogenes infection. Co-infection studies using L. monocytogenes strains that expressed either a high affinity ligand for E-cadherin (InlA(m)), a low affinity ligand (wild type InlA from Lm EGDe), or no InlA (ΔinlA) showed that InlA was not required to establish intestinal infection in mice. However, expression of InlA(m) significantly increased bacterial persistence in the underlying lamina propria and greatly enhanced dissemination to the mesenteric lymph nodes. Thus, these studies revealed a previously uncharacterized role for InlA in facilitating systemic spread via the lymphatic system after invasion of the gut mucosa.

A surface-focused biotinylation procedure identifies the Yersinia pestis catalase KatY as a membrane-associated but non-surface-located protein

This study identified major surface proteins of the plague bacterium Yersinia pestis. We applied a novel surface biotinylation method, followed by NeutrAvidin (NA) bead capture, on-bead digestion, and identification by liquid chromatography-tandem mass spectrometry (LC-MS-MS). The use of stachyose during biotinylation focused the reaction to the surface. Coupled with NA pulldown and immunoblot analysis, this method determined whether a protein was accessible to the surface. We applied the method to test the hypothesis that the catalase KatY is a surface protein of the plague bacterium Y. pestis. A rabbit serum recognized the catalase KatY as a major putative outer membrane-associated antigen expressed by Y. pestis cells grown at 37 degrees C. Similar findings by other groups had led to speculations that this protein might be exposed to the surface and might be a candidate for evaluation as a protective antigen for an improved plague vaccine. KatY was obtained only in the total membrane fraction, and stachyose greatly reduced its biotinylation as well as that of the periplasmic maltose binding protein, indicating that KatY is not on the bacterial surface. LC-MS-MS analysis of on-bead digests representing ca. 10(9) cells identified highly abundant species, including KatY, Pal, and OmpA, as well as the lipoprotein Pcp, all of which bound in a biotin-specific manner. Pla, Lpp, and OmpX (Ail) bound to the NA beads in a non-biotin-specific manner. There was no contamination from abundant cytoplasmic proteins. We hypothesize that OmpX and Pcp are highly abundant and likely to be important for the Y. pestis pathogenic process. We speculate that a portion of KatY associates with the outer membrane in intact cells but that it is located on the periplasmic side. Consistent with this idea, it did not protect C57BL/6 mice against bubonic plague.

Application of a Saccharomyces cerevisiae model to study requirements for trafficking of Yersinia pestis YopM in eucaryotic cells

YopM is a leucine-rich repeat (LRR) virulence protein that is delivered into host cells when any of the three human-pathogenic species of Yersinia binds to mammalian cells. It exhibits heterogeneity of size and sequence among the yersiniae, but the functional consequences of this variability are not yet known. Yersinia pestis YopM was previously shown to accumulate in the nuclei of infected HeLa cells by a mechanism that requires vesicular trafficking. In this study, we characterized the trafficking of Y. pestis YopM in a Saccharomyces cerevisiae model previously found to support nuclear localization of YopM from an enteropathogenic Yersinia strain (C. F. Lesser and S. I. Miller, EMBO J. 20:1840-1849, 2001). Y. pestis YopM was N-terminally fused to the yeast enhanced green fluorescent protein (yEGFP) and inducibly expressed in the cytoplasm. yEGFP-YopM localized to the yeast nucleus, showing that this property is conserved for YopMs so far tested and that infection and the presence of other Yops are not required for its trafficking. When expressed in S. cerevisiae that is temperature sensitive for vesicular transport, YopM failed to accumulate in the nucleus at the nonpermissive temperature but did accumulate when the permissive temperature was restored. This shows that vesicular trafficking also is required in yeast for normal localization of YopM. YopM consists of a 71-residue leader sequence, 15 LRRs, and a 32-residue tail. Deletion analysis revealed that the leader sequence or tail is alone insufficient to direct YopM to the nucleus, showing that the LRR structure is required. Both the N-terminal and C-terminal halves of YopM localized to the nucleus, indicating the possible presence of two nuclear localization signals (NLSs) in YopM or domains in YopM where an NLS-containing protein might bind; this fits with the presence of two highly conserved regions among Yersinia YopMs. yEGFP-YopM lacking LRRs 4 to 7 or 7 to 10 accumulated in the nucleus in yeast, and YopM lacking these LRRs concentrated normally in the HeLa cell nucleus after delivery by Yersinia infection, showing that these LRRs are not essential for YopM trafficking in eucaryotic cells. However, because Y. pestis carrying either of these YopMs is strongly compromised in virulence in mice, these findings revealed that LRRs 4 to 10 map a region of YopM or support a conformation of YopM that is necessary for a pathogenic effect.