Afipia felis induces uptake by macrophages directly into a nonendocytic compartment

Identification of a novel Afipia species isolated from an Indian flying fox

An old world fruit bat Pteropus giganteus, held in captivity and suffering from necrosis of its wing digits, failed to respond to antibiotic therapy and succumbed to the infection. Samples submitted to the National Centre for Foreign Animal Disease were tested for viral infection. Vero E6 cells exhibited minor but unique cytopathic effects on second blind passage, and full CPE by passage four. Utilizing an unbiased random amplification technique from cell culture supernatant, we identified a bacterium belonging to the Bradyrhizobiaceae. Purification of cell culture supernatant on TY media revealed a slow growing bacterial isolate. In this study using electron microscopy, 16S rRNA gene analysis and whole genome sequencing, we identify a novel bacterial species associated with the site of infection belonging to the genus Afipia. This genus of bacteria is very diverse, with only a limited number of species characterized. Afipia felis, previously described as the etiological agent to cause cat scratch disease, and Afipia septicemium, most recently shown to cause disease in humans, highlight the potential for members of this genus to form a branch of opportunistic pathogens within the Bradyrhizobiaceae. Increased utilization of next generation sequencing and genomics will aid in classifying additional members of this intriguing bacterial genera.

Lipid-labeling facilitates a novel magnetic isolation procedure to characterize pathogen-containing phagosomes

Here we describe a novel approach for the isolation and biochemical characterization of pathogen-containing compartments from primary cells: We developed a lipid-based procedure to magnetically label the surface of bacteria and visualized the label by scanning and transmission electron microscopy (SEM, TEM). We performed infection experiments with magnetically labeled Mycobacterium avium, M. tuberculosis and Listeria monocytogenes and isolated magnetic bacteria-containing phagosomes using a strong magnetic field in a novel free-flow system. Magnetic labeling of M. tuberculosis did not affect the virulence characteristics of the bacteria during infection experiments addressing host cell activation, phagosome maturation delay and replication in macrophages in vitro. Biochemical analyses of the magnetic phagosome-containing fractions provided evidence of an enhanced presence of bacterial antigens and a differential distribution of proteins involved in the endocytic pathway over time as well as cytokine-dependent changes in the phagosomal protein composition. The newly developed method represents a useful approach to characterize and compare pathogen-containing compartments, in order to identify microbial and host cell targets for novel anti-infective strategies.

The Afipia toolbox and its use to isolate flagellar mutants

Afipia felis, a Gram-negative alphaproteobacterium, has been implicated as one of the causative agents of cat scratch disease. To identify and begin to examine the virulence traits of this organism, we developed and tested a highly efficient transposon delivery system and a stable plasmid vector expressing green fluorescent protein. The transposome system is based on a Tn5-derived transposon and a phage restriction endonuclease type I inhibitor. Electroporation of this construct produced a library of >2600 mutants, which were screened for flagella biosynthesis mutants using a monoclonal antibody to Afipia flagellin. Insertion loci for two selected mutants were located in the genes for flagellin and flagellin biosynthesis FlhA, confirming the validity of the approach.

Isolation and identification of amoeba-resisting bacteria from water in human environment by using an Acanthamoeba polyphaga co-culture procedure

Amoeba-resisting bacteria (ARB) such as Legionella spp. are currently regarded as potential human pathogens living in the environment. To detect ARB from both human and environmental samples, co-culture with amoebae has been demonstrated as an efficient tool. However, using this procedure, mostly water from cooling towers and hospital water supplies have been investigated as the possible reservoir of ARB. In the present study, we studied ARB population in 77 environmental water samples including rivers, fountains, lakes and domestic wells in the south of France. As a result, a total of 244 isolates corresponding to 89 different species of ARB, but not Legionella spp., were identified. Ability to grow within and/or to be lytic for amoebae was revealed for the first time for several human pathogens. Six isolates are likely to be the members of a new or uncharacterized genus/species. An anaerobic bacterium, Clostridium frigidicarnis was demonstrated to be lytic for amoebae. This preliminary work demonstrates that the water environment in the vicinity of humans is a reservoir of ARB, including well-known pathogens for which amoebae and/or water was not recognized earlier as a possible reservoir.

Biogenesis of Afipia-containing phagosomes in non-professional phagocytes

Afipia felis is a Gram-negative alpha-proteobacterium, a rare cause of human cat scratch disease (CSD), and likely a pathogen of amoeba. Here, we show that various members of the genus Afipia attach to and are taken up by various non-professional phagocytic mammalian cells (epithelial CHO, endothelial EA.hy926, epithelial HeLa, epithelial INT407 cells, endothelial HMEC-1, endothelial HUVEC, and fibroblast L929 cells). However, only A. felis was able to do this efficiently. Invasion depended on a functional actin cytoskeleton and much less on microtubule dynamics. Bacteria were slowly taken up into HMEC-1 (and HUVEC) via pocket-like structures and they resided within membrane-surrounded phagosomes. While A. felis was found in a non-canonical endocytic compartment in macrophage cells, Afipia-containing phagosomes in HMEC-1 were transiently positive for early endosomal EEA1 and then became and remained positive for lysosome-associated membrane protein-1 (LAMP1) and the proton-pumping ATPase, suggesting undisturbed, albeit slowed, phagosome biogenesis in these cells. Similarly, at 24h of infection, most phagosomes in HeLa, INT407, HUVEC and in EA.hy926 cells were positive for LAMP1. In summary, A. felis enters various non-professional phagocytes and its compartmentation differs between macrophages and non-professional phagocytes.

The phagosome: compartment with a license to kill

Phagosomes are fascinating subcellular structures. After all, there are only a few compartments that are born before our very eyes and whose development we can follow in a light microscope until their contents disintegrate and are completely absorbed. Yet, some phagosomes are taken advantage of by pathogenic microorganisms, which change their fate. Research into phagosome biogenesis has flourished in recent years - the purpose of this review is to give a glimpse of where this research stands, with emphasis on the cell biology of macrophage phagosomes, on new model organisms for the study of phagosome biogenesis and on intracellular pathogens and their interference with normal phagosome function.

Lipid Microdomain-Dependent Macropinocytosis Determines Compartmentation of Afipia felis

Phagocytic compartments are specialized endocytic organelles and usually mature along the degradative pathway into phagolysosomes. The rare human pathogen Afipia felis localizes to a compartment that is different from canonical phagocytic compartments. Here, we present evidence that internalization of Afipia by macrophages and unusual phagosome development are considerably decreased by attachment of cholera toxin B subunit to macrophage ganglioside GM1 or by extraction or oxidation of plasma membrane cholesterol. Amiloride (an inhibitor of Na(+)/H(+) exchanger and macropinocytosis) strongly inhibited uptake of A. felis at a late step, i.e. the closure of macropinocytic structures rather than the production of membrane ruffles. Ultrastructural evidence showed that A. felis was taken up by macrophages via macropinocytosis. In contrast, A. felis opsonized with a monoclonal IgG antibody was ingested by a zipper-like mechanism, resulting in normal phagosome maturation. Hence, while the preferred path of A. felis uptake is dependent on the integrity of lipid microdomains and on macropinocytosis, and while this uptake leads to an unusual phagosome and to intracellular survival of A. felis, those bacteria that enter using Fcgamma receptors are delivered to a late endocytic compartment.

Maturation of Rhodococcus equi-containing vacuoles is arrested after completion of the early endosome stage

Rhodococcus equi is a facultative intracellular bacterium that can cause bronchopneumonia in foals and AIDS patients. Here, we have analyzed R. equi-containing vacuoles (RCVs) in murine macrophages by confocal laser scanning microscopy, by transmission electron microscopy and by immunochemistry upon purification. We show that RCVs progress normally through the early stages of phagosome maturation acquiring PI3P, early endosome antigen-1, and Rab5, and loosing all or much of them within minutes. Although mature RCVs possess the normally late endocytic markers, lysosome-associated membrane proteins, lysobisphosphatidic acid and Rab7, they lack other hallmark features of late endocytic organelles such as possession of cathepsin D, acid beta-glucuronidase, proton-pumping ATPase and the ability to fuse with prelabeled lysosomes. Bacterial strains possessing a virulence-associated plasmid maintain a nonacidified compartment for 48 h, whereas isogenic strains lacking such plasmids acidify progressively. In summary, RCVs represent a novel phagosome maturation stage positioned after completion of the early endosome stage and before reaching a fully mature late endosome compartment. In addition, vacuole biogenesis can be influenced by bacterial plasmids.

Unusual trafficking pattern of Bartonella henselae -containing vacuoles in macrophages and endothelial cells

Bartonella henselae, the agent of cat-scratch disease and vasculoproliferative disorders in humans, is a fastidious facultative intracellular pathogen, whose interaction with macrophages and endothelial cells (ECs) is crucial in the pathogenesis of these diseases. However, little is known about the subcellular compartment in which B. henselae resides. Two hours after infection of murine macrophages and human ECs, the majority of B. henselae-containing vacuoles (BCVs) lack typical endocytic marker proteins, fail to acidify, and do not fuse with lysosomes, suggesting that B. henselae resides in a non-endocytic compartment. In contrast to human umbilical vein endothelial cells, bacterial death and lysosomal fusion with BCVs is apparent in J774A.1 macrophages at 24 h. This phenomenon of delayed lysosomal fusion requires bacterial viability, and is confined to the BCV itself. Using magnetic selection, we enriched for transposon-mutagenized B. henselae trapped in lysosomes of macrophages 2 h after infection. Genes affected appear to be relevant to the intracellular lifestyle in macrophages and ECs and include some previously implicated in Bartonella pathogenicity. We conclude that B. henselae has a specific capacity to actively avoid the host endocytic pathway after entry of macrophages and ECs, from within a specialized non-endocytic membrane-bound vacuole.

Forced retraction of spinal root injury enhances activation of p38 MAPK cascade in infiltrating macrophages

Root-rupture injury is a type of preganglionic brachial plexus injury resulting from traction force, where a small section of the spinal root is usually left behind. We have established experimental models of both root-rupture injury with traction force and rhizotomy without traction force in rats and we examined the activation of microglia/ macrophages in both conditions. LGP107 and LGP96, which are rat homologs of lysosome-associated membrane proteins, were most useful as immunohistochemical markers of mononuclear phagocytes. The metabolic activation of macrophages was analyzed by immunohistochemistry with a series of antibodies against tumor necrosis factor-alpha (TNF-alpha), cathepsin B, p38 mitogen-activated protein kinase (MAPK), and mitogen-activated kinase kinase 3 (MKK3). Both root-rupture injury and rhizotomy rapidly induced the aggregation of numerous macrophages from the injured dorsal root to the dorsal funiculus and TNF-alpha was highly expressed by the macrophages in the injured dorsal root at 48 h. Activation of p38 MAPK was preferentially observed in the macrophages at the ruptured dorsal root; however, only slight activation of p38 MAPK was observed at the rhizotomized dorsal root. These findings suggest that traction injury of the spinal root might induce activation of the p38 MAPK cascade and production of TNF-alpha in the infiltrating macrophages, both of which might participate in aggravation of the root injury.

Endothelial cells from bovine pulmonary microvasculature respond to Mycoplasma bovis preferentially with signals for mononuclear cell transmigration

Mycoplasma bovis can cause arthritis or mastitis following pneumonia and mycoplasmemia in cattle. Interactions with pulmonary vascular endothelium have been recorded as localized vasculitis, perivascular mononuclear cell infiltrations, and accumulation of inflammatory cells in lesions. We compared adhesion mediators and cytokine gene expression as well as cytotoxicity of cultured primary bovine aortic and bovine pulmonary microvascular endothelial cells (BPMEC) challenged with M. bovis. We also tested if abscess-forming ability of strains of M. bovis is associated with changes on endothelial cells. Increased VCAM-1 surface expression was found in both cell types, while only infected BPMEC increased MCP-1 transcription, both mediators specific for mononuclear cell transmigration. Given no induction of ICAM-1 mRNA in either cell type, induction of IL-8 mRNA by BPMEC suggested that neutrophil transmigration was signaled in microvascular areas. Infected BPMEC showed early induction of IL-1beta and IL-6 mRNA. Excepting VCAM-1, differential strain effects were limited to BPMEC and not correlated with their abscess-forming capability. In addition, only strain DSA16 had minor cytotoxic effect on both cell types. We thus show that BPMEC are more susceptible than aortic cells to M. bovis-induced activation. Activation preferentially yielded signals for mononuclear cell transmigration, correlating well with in vivo observations of infiltrating cells at pulmonary sites.

Microscopic quantification of bacterial invasion by a novel antibody-independent staining method

Microscopic discrimination between extracellular and invasive, intracellular bacteria is a valuable technique in microbiology and immunology. We describe a novel fluorescence staining protocol, called FITC-biotin-avidin (FBA) staining, which allows the differentiation between extracellular and intracellular bacteria and is independent of specific antibodies directed against the microorganisms. FBA staining of eukaryotic cells infected with Gram-negative bacteria of the genus Neisseria or the Gram-positive pathogen Staphylococcus aureus are employed to validate the novel technique. The quantitative evaluation of intracellular pathogens by the FBA staining protocol yields identical results compared to parallel samples stained with conventional, antibody-dependent methods. FBA staining eliminates the need for cell permeabilization resulting in robust and rapid detection of invasive microbes. Taken together, FBA staining provides a reliable and convenient alternative for the differential detection of intracellular and extracellular bacteria and should be a valuable technical tool for the quantitative analysis of the invasive properties of pathogenic bacteria and other microorganisms.

Molecular analysis of shower curtain biofilm microbes

Households provide environments that encourage the formation of microbial communities, often as biofilms. Such biofilms constitute potential reservoirs for pathogens, particularly for immune-compromised individuals. One household environment that potentially accumulates microbial biofilms is that provided by vinyl shower curtains. Over time, vinyl shower curtains accumulate films, commonly referred to as "soap scum," which microscopy reveals are constituted of lush microbial biofilms. To determine the kinds of microbes that constitute shower curtain biofilms and thereby to identify potential opportunistic pathogens, we conducted an analysis of rRNA genes obtained by PCR from four vinyl shower curtains from different households. Each of the shower curtain communities was highly complex. No sequence was identical to one in the databases, and no identical sequences were encountered in the different communities. However, the sequences generally represented similar phylogenetic kinds of organisms. Particularly abundant sequences represented members of the alpha-group of proteobacteria, mainly Sphingomonas spp. and Methylobacterium spp. Both of these genera are known to include opportunistic pathogens, and several of the sequences obtained from the environmental DNA samples were closely related to known pathogens. Such organisms have also been linked to biofilm formation associated with water reservoirs and conduits. In addition, the study detected many other kinds of organisms at lower abundances. These results show that shower curtains are a potential source of opportunistic pathogens associated with biofilms. Frequent cleaning or disposal of shower curtains is indicated, particularly in households with immune-compromised individuals.

Microorganisms resistant to free-living amoebae

Free-living amoebae feed on bacteria, fungi, and algae. However, some microorganisms have evolved to become resistant to these protists. These amoeba-resistant microorganisms include established pathogens, such as Cryptococcus neoformans, Legionella spp., Chlamydophila pneumoniae, Mycobacterium avium, Listeria monocytogenes, Pseudomonas aeruginosa, and Francisella tularensis, and emerging pathogens, such as Bosea spp., Simkania negevensis, Parachlamydia acanthamoebae, and Legionella-like amoebal pathogens. Some of these amoeba-resistant bacteria (ARB) are lytic for their amoebal host, while others are considered endosymbionts, since a stable host-parasite ratio is maintained. Free-living amoebae represent an important reservoir of ARB and may, while encysted, protect the internalized bacteria from chlorine and other biocides. Free-living amoebae may act as a Trojan horse, bringing hidden ARB within the human "Troy," and may produce vesicles filled with ARB, increasing their transmission potential. Free-living amoebae may also play a role in the selection of virulence traits and in adaptation to survival in macrophages. Thus, intra-amoebal growth was found to enhance virulence, and similar mechanisms seem to be implicated in the survival of ARB in response to both amoebae and macrophages. Moreover, free-living amoebae represent a useful tool for the culture of some intracellular bacteria and new bacterial species that might be potential emerging pathogens.

Necrotic death of Rhodococcus equi-infected macrophages is regulated by virulence-associated plasmids

Rhodococcus equi is a gram-positive intracellular pathogen that can cause severe bronchopneumonia in foals and AIDS patients. It has been reported that advanced infection of foals is characterized by tissue necrosis, coinciding with the presence of degenerate bacteria-laden macrophages. Here, we report that the possession of the VapA-expressing plasmid, which has been previously correlated with a high level of virulence for foals and mice, strongly increases cytotoxicity of R. equi for murine macrophage-like (J774E) cells. Isolates containing different, VapB-expressing plasmids are less virulent and also have a lower cytotoxic potential. Isogenic strains lacking either plasmid are avirulent and have a very low cytotoxic potential. We show, using fluorescence-activated cell sorter analysis (annexin V/7-amino-actinomycin D and sub-G1-analysis), Western blotting [poly(ADP-ribose) polymerase processing analysis], and electron microscopy (macrophage and nucleus morphologies) that the deaths of murine macrophages are the result of necrotic rather than apoptotic events. We demonstrate that the bacteria must be alive in order to act cytotoxic. Therefore, one effect of the virulence-associated plasmids during infection with R. equi is the promotion of necrotic damage to the host.

Parachlamydia acanthamoebae enters and multiplies within human macrophages and induces their apoptosis [corrected]

Parachlamydia acanthamoebae is an obligately intracellular bacterium that naturally infects free-living amoebae. It is a potential human pathogen and may survive in human macrophages. We studied P. acanthamoebae entry into, and multiplication within, human monocyte-derived macrophages. After 8 h of incubation, 80% of macrophages were infected with a mean of 3.8 P. acanthamoebae organisms per cell. Electron microscopy demonstrated that parachlamydiae were in an intracellular vacuole. After infection with living organisms, the number of parachlamydiae per macrophage increased 4 times from day 0 to day 4, whereas heat-inactivated parachlamydiae were eliminated during the same period. Quantitative PCR confirmed that P. acanthamoebae replicates within macrophages. Transcriptional activity of P. acanthamoeba was detected by reverse transcription-PCR targeting the gene encoding ADP-ATP translocase (tlc). P. acanthamoebae exerted a cytopathic effect on macrophages. When macrophages were infected with living bacteria, their number decreased significantly from day 0 to day 4 due to apoptosis, as shown by annexin-V binding and electron microscopy. This study shows that P. acanthamoebae enters and multiplies within human macrophages before inducing their apoptosis.

Phagosome maturation: a few bugs in the system

Cells of the innate immune system ingest and destroy invading microorganisms by initially engulfing them into a specialized vacuole, known as the phagosome. The membrane of the forming phagosome is similar to the plasmalemma and its contents resemble the extracellular milieu. As such, the nascent phagosome is not competent to kill and eliminate the ingested microorganisms. However, shortly after sealing, the phagosome undergoes a series of rapid and extensive changes in its composition, the result of a sophisticated sequence of membrane fusion and fission reactions. Understanding the molecular basis of these events is of particular importance, since they are often the target of disruption by intracellular parasites such as Mycobacterium, Salmonella and Legionella. The objective of this review is to summarize the current knowledge of the molecular mechanisms underlying phagosomal maturation and its subversion by parasitic microorganisms.