Both incompatible and compatible rhizobia inhabit the intercellular spaces of leguminous root nodules

In addition to rhizobia, many types of co-existent bacteria are found in leguminous root nodules, but their habitats are unclear. To investigate this phenomenon, we labeled Bradyrhizobium diazoefficiens USDA122 and Bradyrhizobium sp. SSBR45 with Discosoma sp. red fluorescent protein (DsRed) or enhanced green fluorescent protein (eGFP). USDA122 enhances soybean growth by forming effective root nodules, but SSBR45 does not form any nodules. Using low-magnification laser scanning confocal microscopy, we found that infected cells in the central zone of soybean nodules appeared to be occupied by USDA122. Notably, high-magnification microscopy after co-inoculation of non-fluorescent USDA122 and fluorescence-labeled SSBR45 also revealed that SSBR45 inhabits the intercellular spaces of healthy nodules. More unexpectedly, co-inoculation of eGFP-labeled USDA122 and DsRed-labeled SSBR45 (and vice versa) revealed the presence of USDA122 bacteria in both the symbiosomes of infected cells and in the apoplasts of healthy nodules. We then next inspected nodules formed after a mixed inoculation of differently-labeled USDA122, without SSBR45, and confirmed the inhabitation of the both populations of USDA122 in the intercellular spaces. In contrast, infected cells were occupied by single-labeled USDA122. We also observed Mesorhizobium loti in the intercellular spaces of active wild-type nodules of Lotus japonicus using transmission electron microscopy. Compatible intercellular rhizobia have been described during nodule formation of several legume species and in some mutants, but our evidence suggests that this type of colonization may occur much more commonly in leguminous root nodules.

Dissecting the antibacterial activity of oxadiazolone-core derivatives against Mycobacterium abscessus

Mycobacterium abscessus (M. abscessus), a rapidly growing mycobacterium, is an emergent opportunistic pathogen responsible for chronic bronchopulmonary infections in individuals with respiratory diseases such as cystic fibrosis. Most treatments of M. abscessus pulmonary infections are poorly effective due to the intrinsic resistance of this bacteria against a broad range of antibiotics including anti-tuberculosis agents. Consequently, the number of drugs that are efficient against M. abscessus remains limited. In this context, 19 oxadiazolone (OX) derivatives have been investigated for their antibacterial activity against both the rough (R) and smooth (S) variants of M. abscessus. Several OXs impair extracellular M. abscessus growth with moderated minimal inhibitory concentrations (MIC), or act intracellularly by inhibiting M. abscessus growth inside infected macrophages with MIC values similar to those of imipenem. Such promising results prompted us to identify the potential target enzymes of the sole extra and intracellular inhibitor of M. abscessus growth, i.e., compound iBpPPOX, via activity-based protein profiling combined with mass spectrometry. This approach led to the identification of 21 potential protein candidates being mostly involved in M. abscessus lipid metabolism and/or in cell wall biosynthesis. Among them, the Ag85C protein has been confirmed as a vulnerable target of iBpPPOX. This study clearly emphasizes the potential of the OX derivatives to inhibit the extracellular and/or intracellular growth of M. abscessus by targeting various enzymes potentially involved in many physiological processes of this most drug-resistant mycobacterial species.

Consequences of chronic bacterial infection in Drosophila melanogaster

Even when successfully surviving an infection, a host often fails to eliminate a pathogen completely and may sustain substantial pathogen burden for the remainder of its life. Using systemic bacterial infection in Drosophila melanogaster, we characterize chronic infection by three bacterial species from different genera - Providencia rettgeri, Serratia marcescens, and Enterococcus faecalis–following inoculation with a range of doses. To assess the consequences of these chronic infections, we determined the expression of antimicrobial peptide genes, survival of secondary infection, and starvation resistance after one week of infection. While higher infectious doses unsurprisingly lead to higher risk of death, they also result in higher chronic bacterial loads among the survivors for all three infections. All three chronic infections caused significantly elevated expression of antimicrobial peptide genes at one week post-infection and provided generalized protection again secondary bacterial infection. Only P. rettgeri infection significantly influenced resistance to starvation, with persistently infected flies dying more quickly under starvation conditions relative to controls. These results suggest that there is potentially a generalized mechanism of protection against secondary infection, but that other impacts on host physiology may depend on the specific pathogen. We propose that chronic infections in D. melanogaster could be a valuable tool for studying tolerance of infection, including impacts on host physiology and behavior.

Visualization and quantification of the cellular and extracellular components of Salmonella Agona biofilms at different stages of development

Salmonella is a major food-borne pathogen able to persist in food processing environments because of its ability to form biofilms. A Salmonella enterica serotype Agona isolate from poultry (S24) was grown at 37°C in biofilms for up to 144 hours (H144) in attachment to polystyrene surfaces. Biofilm structures were examined at different stages in their development (H3, H24, H48, H72, H96 and H144) using confocal laser scanning microscopy (CLSM) in conjunction with fluorescent dyes for live cells (SYTO 9), dead cells (propidium iodide), proteins (fluorescein isothiocyanate isomer I), lipids (DiD'oil), α-polysaccharides (concanavalin A, tetramethylrhodamine conjugate), and β-polysaccharides (calcofluor white M2R). Strain S24 developed a robust biofilm at H72 (biovolume of 166,852.5 ± 13,681.8 μm3 in the observation field of 16,078.2 μm2). The largest biovolume of live cells was also detected at H72 (128,110.3 ± 4,969.1 μm3), decreasing thereafter, which was probably owing to the detachment of cells prior to a new phase of colonization. The percentage of dead cells with regard to total cells in the biofilms increased throughout the incubation, ranging from 2.3 ± 1.1% (H24) to 44.2 ± 11.0% (H144). Proteins showed the greatest biovolume among the extracellular components within the biofilms, with values ranging from 1,295.1 ± 1,294.9 μm3 (H3) to 19,186.2 ± 8,536.0 μm3 (H96). Maximum biovolume values of 15,171.9 ± 660.7 μm3 (H48), 7,055.3 ± 4,415.2 μm3 (H144), and 2,548.6 ± 1,597.5 μm3 (H72) were observed for β-polysaccharides, α-polysaccharides and lipids, respectively. A strong (P < 0.01) positive correlation was found between the total biovolume of biofilm and the biovolume of live cells, proteins and β-polysaccharides, which may serve as useful markers of biofilm formation. The present work provides new insights into the formation of S. Agona biofilms. Our findings may contribute to the designing of reliable strategies for preventing and removing these bacterial communities.

The vascular plant-pathogenic bacterium Ralstonia solanacearum produces biofilms required for its virulence on the surfaces of tomato cells adjacent to intercellular spaces

The mechanism of colonization of intercellular spaces by the soil-borne and vascular plant-pathogenic bacterium Ralstonia solanacearum strain OE1-1 after invasion into host plants remains unclear. To analyse the behaviour of OE1-1 cells in intercellular spaces, tomato leaves with the lower epidermis layers excised after infiltration with OE1-1 were observed under a scanning electron microscope. OE1-1 cells formed microcolonies on the surfaces of tomato cells adjacent to intercellular spaces, and then aggregated surrounded by an extracellular matrix, forming mature biofilm structures. Furthermore, OE1-1 cells produced mushroom-type biofilms when incubated in fluids of apoplasts including intercellular spaces, but not xylem fluids from tomato plants. This is the first report of biofilm formation by R. solanacearum on host plant cells after invasion into intercellular spaces and mushroom-type biofilms produced by R. solanacearum in vitro. Sugar application led to enhanced biofilm formation by OE1-1. Mutation of lecM encoding a lectin, RS-IIL, which reportedly exhibits affinity for these sugars, led to a significant decrease in biofilm formation. Colonization in intercellular spaces was significantly decreased in the lecM mutant, leading to a loss of virulence on tomato plants. Complementation of the lecM mutant with native lecM resulted in the recovery of mushroom-type biofilms and virulence on tomato plants. Together, our findings indicate that OE1-1 produces mature biofilms on the surfaces of tomato cells after invasion into intercellular spaces. RS-IIL may contribute to biofilm formation by OE1-1, which is required for OE1-1 virulence.

Physicochemical characteristics and microbial community evolution of biofilms during the start-up period in a moving bed biofilm reactor

This study aimed to investigate biofilm properties evolution coupled with different ages during the start-up period in a moving bed biofilm reactor system. Physicochemical characteristics including adhesion force, extracellular polymeric substances (EPS), morphology as well as volatile solid and microbial community were studied. Results showed that the formation and development of biofilms exhibited four stages, including (I) initial attachment and young biofilm formation, (II) biofilms accumulation, (III) biofilm sloughing and updating, and (IV) biofilm maturation. During the whole start-up period, adhesion force was positively and significantly correlated with the contents of EPS, especially the content of polysaccharide. In addition, increased adhesion force and EPS were beneficial for biofilm retention. Gram-negative bacteria mainly including Sphaerotilus, Zoogloea and Haliscomenobacter were predominant in the initial stage. Actinobacteria was beneficial to resist sloughing. Furthermore, filamentous bacteria were dominant in maturation biofilm.

Escherichia coli and Candida albicans induced macrophage extracellular trap-like structures with limited microbicidal activity

The formation of extracellular traps (ETs) has recently been recognized as a novel defense mechanism in several types of innate immune cells. It has been suggested that these structures are toxic to microbes and contribute significantly to killing several pathogens. However, the role of ETs formed by macrophages (METs) in defense against microbes remains little known. In this study, we demonstrated that a subset of murine J774A.1 macrophage cell line (8% to 17%) and peritoneal macrophages (8.5% to 15%) form METs-like structures (METs-LS) in response to Escherichia coli and Candida albicans challenge. We found only a portion of murine METs-LS, which are released by dying macrophages, showed detectable killing effects on trapped E. coli but not C. albicans. Fluorescence and scanning electron microscopy analyses revealed that, in vitro, both microorganisms were entrapped in J774A.1 METs-LS composed of DNA and microbicidal proteins such as histone, myeloperoxidase and lysozyme. DNA components of both nucleus and mitochondrion origins were detectable in these structures. Additionally, METs-LS formation occurred independently of ROS produced by NADPH oxidase, and this process did not result in cell lysis. In summary, our results emphasized that microbes induced METs-LS in murine macrophage cells and that the microbicidal activity of these METs-LS differs greatly. We propose the function of METs-LS is to contain invading microbes at the infection site, thereby preventing the systemic diffusion of them, rather than significantly killing them.

Killing machines: three pore-forming proteins of the immune system

The evolution of early multicellular eukaryotes 400-500 million years ago required a defensive strategy against microbial invasion. Pore-forming proteins containing the membrane-attack-complex-perforin (MACPF) domain were selected as the most efficient means to destroy bacteria or virally infected cells. The mechanism of pore formation by the MACPF domain is distinctive in that pore formation is purely physical and unspecific. The MACPF domain polymerizes, refolds, and inserts itself into bilayer membranes or bacterial outer cell walls. The displacement of surface lipid/carbohydrate molecules by the polymerizing MACPF domain creates clusters of large, water-filled holes that destabilize the barrier function and provide access for additional anti-bacterial or anti-viral effectors to sensitive sites that complete the destruction of the invader via enzymatic or chemical attack. The highly efficient mechanism of anti-microbial defense by a combined physical and chemical strategy using pore-forming MACPF-proteins has been retargeted during evolution of vertebrates and mammals for three purposes: (1) to kill extracellular bacteria C9/polyC9 evolved in conjunction with complement, (2) to kill virus infected and cancer cells perforin-1/polyperforin-1 CTL evolved targeted by NK and CTL, and (3) to kill intracellular bacteria transmembrane perforin-2/putative polyperforin-2 evolved targeted by phagocytic and nonphagocytic cells. Our laboratory has been involved in the discovery and description of each of the three pore-formers that will be reviewed here.

Apoplastic immunity and its suppression by filamentous plant pathogens

Microbial plant pathogens have evolved a variety of strategies to enter plant hosts and cause disease. In particular, biotrophic pathogens, which parasitize living plant tissue, establish sophisticated interactions in which they modulate the plant's metabolism to their own good. The prime decision, whether or not a pathogen can accommodate itself in its host tissue, is made during the initial phase of infection. At this stage, the plant immune system recognizes conserved molecular patterns of the invading microbe, which initiate a set of basal immune responses. Induced plant defense proteins, toxic compounds and antimicrobial proteins encounter a broad arsenal of pathogen-derived virulence factors that aim to disarm host immunity. Crucial regulatory processes and protein-protein interactions take place in the apoplast, that is, intercellular spaces, plant cell walls and defined host-pathogen interfaces which are formed between the plant cytoplasm and the specialized infection structures of many biotrophic pathogens. This article aims to provide an insight into the most important principles and components of apoplastic plant immunity and its modulation by filamentous microbial pathogens.

Gene expression in gut symbiotic organ of stinkbug affected by extracellular bacterial symbiont

The bean bug Riptortus pedestris possesses a specialized symbiotic organ in a posterior region of the midgut, where numerous crypts harbor extracellular betaproteobacterial symbionts of the genus Burkholderia. Second instar nymphs orally acquire the symbiont from the environment, and the symbiont infection benefits the host by facilitating growth and by occasionally conferring insecticide resistance. Here we performed comparative transcriptomic analyses of insect genes expressed in symbiotic and non-symbiotic regions of the midgut dissected from Burkholderia-infected and uninfected R. pedestris. Expression sequence tag analysis of cDNA libraries and quantitative reverse transcription PCR identified a number of insect genes expressed in symbiosis- or aposymbiosis-associated patterns. For example, genes up-regulated in symbiotic relative to aposymbiotic individuals, including many cysteine-rich secreted protein genes and many cathepsin protease genes, are likely to play a role in regulating the symbiosis. Conversely, genes up-regulated in aposymbiotic relative to symbiotic individuals, including a chicken-type lysozyme gene and a defensin-like protein gene, are possibly involved in regulation of non-symbiotic bacterial infections. Our study presents the first transcriptomic data on gut symbiotic organ of a stinkbug, which provides initial clues to understanding of molecular mechanisms underlying the insect-bacterium gut symbiosis and sheds light on several intriguing commonalities between endocellular and extracellular symbiotic associations.

Neutrophil extracellular traps: double-edged swords of innate immunity

Spectacular images of neutrophils ejecting nuclear chromatin and bactericidal proteins, in response to microbes, were first reported in 2004. As externalized chromatin could entangle bacteria, these structures were named neutrophil extracellular traps (NETs). Subsequent studies identified microorganisms and sterile conditions that stimulate NETs, as well as additional cell types that release extracellular chromatin. The release of NETs is the most dramatic stage in a cell death process called NETosis. Experimental evidence suggests that NETs participate in pathogenesis of autoimmune and inflammatory disorders, with proposed involvement in glomerulonephritis, chronic lung disease, sepsis, and vascular disorders. Exaggerated NETosis or diminished NET clearance likely increases risk of autoreactivity to NET components. The biological significance of NETs is just beginning to be explored. A more complete integration of NETosis within immunology and pathophysiology will require better understanding of NET properties associated with specific disease states and microbial infections. This may lead to the identification of important therapeutic targets.

Innate immune dysfunctions in aged mice facilitate the systemic dissemination of methicillin-resistant S. aureus

Elderly humans show increased susceptibility to invasive staphylococcal disease after skin and soft tissue infection. However, it is not understood how host immunity changes with aging, and how that predisposes to invasive disease. In a model of severe skin infection, we showed that aged mice (16- to 20-month-old) exhibit dramatic bacterial dissemination compared with young adult mice (2-month-old). Bacterial dissemination was associated with significant reductions of CXCL1 (KC), polymorphonuclear cells (PMNs), and extracellular DNA traps (NETs) at the infection site. PMNs and primary skin fibroblasts isolated from aged mice showed decreased secretion of CXCL2 (MIP-2) and KC in response to MRSA, and in vitro analyses of mitochondrial functions revealed that the mitochondrial electron transport chain complex I plays a significant role in induction of chemokines in the cells isolated from young but not old mice. Additionally, PMNs isolated from aged mice have reduced ability to form NETs and to kill MRSA. Expression of nuclease by S. aureus led to increased bacterial systemic dissemination in young but not old mice, suggesting that defective NETs formation in elderly mice permitted nuclease and non-nuclease expressing S. aureus to disseminate equally well. Overall, these findings suggest that gross impairment of both skin barrier function and innate immunity contributes to the propensity for MRSA to disseminate in aged mice. Furthermore, the study indicates that contribution of bacterial factors to pathogenicity may vary with host age.

Differential regulation of Sciaenops ocellatus viperin expression by intracellular and extracellular bacterial pathogens

Viperin is an antiviral protein that has been found to exist in diverse vertebrate organisms and is involved in innate immunity against the infection of a wide range of viruses. However, it is largely unclear as to the potential role played by viperin in bacterial infection. In this study, we identified the red drum Sciaenops ocellatus viperin gene (SoVip) and analyzed its expression in relation to bacterial challenge. The complete gene of SoVip is 2570 bp in length and contains six exons and five introns. The open reading frame of SoVip is 1065 bp, which is flanked by a 5'-untranslated region (UTR) of 34 bp and a 3'-UTR of 350 bp. The deduced amino acid sequence of SoVip shares extensive identities with the viperins of several fish species and possesses the conserved domain of the radical S-adenosylmethionine superfamily proteins. Expressional analysis showed that constitutive expression of SoVip was relatively high in blood, muscle, brain, spleen, and liver, and low in kidney, gill, and heart. Experimental challenges with poly(I:C) and bacterial pathogens indicated that SoVip expression in liver was significantly upregulated by poly(I:C) and the fish pathogen Edwardsiella tarda but down-regulated by the fish pathogens Listonella anguillarum and Streptococcus iniae. Similar differential induction patterns were also observed at cellular level with primary hepatocytes challenged with E. tarda, L. anguillarum, and S. iniae. Infection study showed that all three bacterial pathogens could attach to cultured primary hepatocytes but only E. tarda was able to invade into and survive in hepatocytes. Together these results indicate that SoVip is involved in host immune response during bacterial infection and is differentially regulated at transcription level by different bacterial pathogens.

The monofunctional catalase KatE of Xanthomonas axonopodis pv. citri is required for full virulence in citrus plants

Background

Xanthomonas axonopodis pv. citri (Xac) is an obligate aerobic phytopathogen constantly exposed to hydrogen peroxide produced by normal aerobic respiration and by the plant defense response during plant-pathogen interactions. Four putative catalase genes have been identified in silico in the Xac genome, designated as katE, catB, srpA (monofunctional catalases) and katG (bifunctional catalase).

Methodology/Principal Findings

Xac catalase activity was analyzed using native gel electrophoresis and semi-quantitative RT-PCR. We demonstrated that the catalase activity pattern was regulated in different growth stages displaying the highest levels during the stationary phase. KatE was the most active catalase in this phase of growth. At this stage cells were more resistant to hydrogen peroxide as was determined by the analysis of CFU after the exposition to different H₂O₂ concentrations. In addition, Xac exhibited an adaptive response to hydrogen peroxide, displaying higher levels of catalase activity and H₂O₂ resistance after treatment with sub-lethal concentrations of the oxidant. In the plant-like medium XVM2 the expression of KatE was strongly induced and in this medium Xac was more resistant to H₂O₂. A XackatE mutant strain was constructed by insertional mutagenesis. We observed that catalase induction in stationary phase was lost meanwhile the adaptive response to peroxide was maintained in this mutant. Finally, the XackatE strain was assayed in planta during host plant interaction rendering a less aggressive phenotype with a minor canker formation.

Conclusions

Our results confirmed that in contrast to other Xanthomonas species, Xac catalase-specific activity is induced during the stationary phase of growth in parallel with the bacterial resistance to peroxide challenge. Moreover, Xac catalases expression pattern is modified in response to any stimuli associated with the plant or the microenvironment it provides. The catalase KatE has been shown to have an important function for the colonization and survival of the bacterium in the citrus plant during the pathogenic process. Our work provides the first genetic evidence to support a monofunctional catalase as a virulence factor in Xac.

Caught with their PAMPs down? The extracellular signalling actions of molecular chaperones are not due to microbial contaminants

In recent years, it has been hypothesised that a new signalling system may exist in vertebrates in which secreted molecular chaperones form a dynamic continuum between the cellular stress response and corresponding homeostatic physiological mechanisms. This hypothesis seems to be supported by the finding that many molecular chaperones are released from cells and act as extracellular signals for a range of cells. However, this nascent field of biological research seems to suffer from an excessive criticism that the biological activities of molecular chaperones are due to undefined components of the microbial expression hosts used to generate recombinant versions of these proteins. In this article, a number of the proponents of the cell signalling actions of molecular chaperones take this criticism head-on. They show that sufficient evidence exists to support fully the hypothesis that molecular chaperones have cell-cell signalling actions that are likely to be part of the homeostatic mechanism of the vertebrate.

The olive fly endosymbiont, "Candidatus Erwinia dacicola," switches from an intracellular existence to an extracellular existence during host insect development

As polyphagous, holometabolous insects, tephritid fruit flies (Diptera: Tephritidae) provide a unique habitat for endosymbiotic bacteria, especially those microbes associated with the digestive system. Here we examine the endosymbiont of the olive fly [Bactrocera oleae (Rossi) (Diptera: Tephritidae)], a tephritid of great economic importance. "Candidatus Erwinia dacicola" was found in the digestive systems of all life stages of wild olive flies from the southwestern United States. PCR and microscopy demonstrated that "Ca. Erwinia dacicola" resided intracellularly in the gastric ceca of the larval midgut but extracellularly in the lumen of the foregut and ovipositor diverticulum of adult flies. "Ca. Erwinia dacicola" is one of the few nonpathogenic endosymbionts that transitions between intracellular and extracellular lifestyles during specific stages of the host's life cycle. Another unique feature of the olive fly endosymbiont is that unlike obligate endosymbionts of monophagous insects, "Ca. Erwinia dacicola" has a G+C nucleotide composition similar to those of closely related plant-pathogenic and free-living bacteria. These two characteristics of "Ca. Erwinia dacicola," the ability to transition between intracellular and extracellular lifestyles and a G+C nucleotide composition similar to those of free-living relatives, may facilitate survival in a changing environment during the development of a polyphagous, holometabolous host. We propose that insect-bacterial symbioses should be classified based on the environment that the host provides to the endosymbiont (the endosymbiont environment).

Specific in situ visualization of the pathogenic endophytic fungus Aciculosporium take, the cause of witches' broom in bamboo

The endophytic fungus Aciculosporium take (Ascomycota; Clavicipitaceae) causes continuous shoot growth in bamboo. The colonized shoot eventually results in witches' broom formation but maintains normal leaf arrangement and branching pattern. To analyze the mechanism of well-regulated symptom development, the location of the fungal endophytic hyphae in host tissues was visualized. A colorimetric in situ hybridization technique using a species-specific oligonucleotide probe targeting the 18S rRNA of A. take was used. In situ hybridization was performed on tissue sections of diseased shoots with or without external signs of fungal colonization. Specific signals were detected in intercellular spaces of the bamboo tissues. Most signals were detected in the shoot apical meristem and the leaf primordia. In addition, fewer signals were detected in the lateral buds, juvenile leaves, and stems. These results indicate that A. take grows endophytically, particularly in the shoot apical meristem of the host. The location of A. take hyphae suggests that the mechanism of symptom development can be explained by the action of exogenous fungal auxin, which continuously induces primordium initiation within the host.

Location of pathogenic bacteria during persistent infections: insights from an analysis using game theory

Bacterial persistent infections are responsible for a significant amount of the human morbidity and mortality. Unlike acute bacterial infections, it is very difficult to treat persistent bacterial infections (e.g. tuberculosis). Knowledge about the location of pathogenic bacteria during persistent infection will help to treat such conditions by designing novel drugs which can reach such locations. In this study, events of bacterial persistent infections were analyzed using game theory. A game was defined where the pathogen and the host are the two players with a conflict of interest. Criteria for the establishment of Nash equilibrium were calculated for this game. This theoretical model, which is very simple and heuristic, predicts that during persistent infections pathogenic bacteria stay in both intracellular and extracellular compartments of the host. The result of this study implies that a bacterium should be able to survive in both intracellular and extracellular compartments of the host in order to cause persistent infections. This explains why persistent infections are more often caused by intracellular pathogens like Mycobacterium and Salmonella. Moreover, this prediction is in consistence with the results of previous experimental studies.

Endophytic colonization of olive roots by the biocontrol strain Pseudomonas fluorescens PICF7

Confocal microscopy combined with three-dimensional olive root tissue sectioning was used to provide evidence of the endophytic behaviour of Pseudomonas fluorescens PICF7, an effective biocontrol strain against Verticillium wilt of olive. Two derivatives of the green fluorescent protein (GFP), the enhanced green and the red fluorescent proteins, have been used to visualize simultaneously two differently fluorescently tagged populations of P. fluorescens PICF7 within olive root tissues at the single cell level. The time-course of colonization events of olive roots cv. Arbequina by strain PICF7 and the localization of tagged bacteria within olive root tissues are described. First, bacteria rapidly colonized root surfaces and were predominantly found in the differentiation zone. Thereafter, microscopy observations showed that PICF7-tagged populations eventually disappeared from the root surface, and increasingly colonized inner root tissues. Localized and limited endophytic colonization by the introduced bacteria was observed over time. Fluorescent-tagged bacteria were always visualized in the intercellular spaces of the cortex region, and no colonization of the root xylem vessels was detected at any time. To the best of our knowledge, this is the first time this approach has been used to demonstrate endophytism of a biocontrol Pseudomonas spp. strain in a woody host such as olive using a nongnotobiotic system.

[Cyanobacterium Nostoc paludosum Kütz as a basis of creation of agriculturally useful microbial associations by the example of bacteria of the genus Rhizobium]

Different species of Rhizobium were successfully introduced into the extracellular slime of Nostoc paludosum (Kütz) Elenk, strain 18; cyanobacteria did not eliminate them and exhibited no specificity to the introduced species. Both partners were shown to exist in a self-sufficient manner in an artificial consortium, the stability of which is determined by the technology of growing the cultures in collections. Cyanobacteria act as carriers of introduced satellites, providing contact with the inoculated material through the slime, and increase the nitrogen-fixing ability of legume plants due to the increase of the number and activity of nodules. The fact of penetration of cyanobacterial hormogonia into the nodules has been noted. The treatment of seeds by the consortium resulted in an increase of the harvest as compared with the standard methods of nitragin treatment of legumes.

Salmonella induces a switched antibody response without germinal centers that impedes the extracellular spread of infection

T-dependent Ab responses are characterized by parallel extrafollicular plasmablast growth and germinal center (GC) formation. This study identifies that, in mice, the Ab response against Salmonella is novel in its kinetics and its regulation. It demonstrates that viable, attenuated Salmonella induce a massive early T-dependent extrafollicular response, whereas GC formation is delayed until 1 mo after infection. The extrafollicular Ab response with switching to IgG2c, the IgG2a equivalent in C57BL/6 mice, is well established by day 3 and persists through 5 wk. Switching is strongly T dependent, and the outer membrane proteins are shown to be major targets of the early switched IgG2c response, whereas flagellin and LPS are not. GC responses are associated with affinity maturation of IgG2c, and their induction is associated with bacterial burden because GC could be induced earlier by treating with antibiotics. Clearance of these bacteria is not a consequence of high-affinity Ab production, for clearance occurs equally in CD154-deficient mice, which do not develop GC, and wild-type mice. Nevertheless, transferred low- and high-affinity IgG2c and less efficiently IgM were shown to impede Salmonella colonization of splenic macrophages. Furthermore, Ab induced during the infection markedly reduces bacteremia. Thus, although Ab does not prevent the progress of established splenic infection, it can prevent primary infection and impedes secondary hemogenous spread of the disease. These results may explain why attenuated Salmonella-induced B cell responses are protective in secondary, but not primary infections.

Differential requirement of P2X7 receptor and intracellular K+ for caspase-1 activation induced by intracellular and extracellular bacteria

Interleukin-1beta (IL-1beta) is a pro-inflammatory cytokine that plays an important role in host defense and inflammatory diseases. The maturation and secretion of IL-1beta are mediated by caspase-1, a protease that processes pro-IL-1beta into biologically active IL-1beta. The activity of caspase-1 is controlled by the inflammasome, a multiprotein complex formed by NLR proteins and the adaptor ASC, that induces the activation of caspase-1. The current model proposes that changes in the intracellular concentration of K(+) potentiate caspase-1 activation induced by the recognition of bacterial products. However, the roles of P2X7 receptor and intracellular K(+) in IL-1beta secretion induced by bacterial infection remain unknown. Here we show that, in response to Toll-like receptor agonists such as lipopolysaccharide or infection with extracellular bacteria Staphylococcus aureus and Escherichia coli, efficient caspase-1 activation is only triggered by addition of ATP, a signal that promotes caspase-1 activation through depletion of intracellular K(+) caused by stimulation of the purinergic P2X7 receptor. In contrast, activation of caspase-1 that relies on cytosolic sensing of flagellin or intracellular bacteria did not require ATP stimulation or depletion of cytoplasmic K(+). Consistently, caspase-1 activation induced by intracellular Salmonella or Listeria was unimpaired in macrophages deficient in P2X7 receptor. These results indicate that, unlike caspase-1 induced by Toll-like receptor agonists and ATP, activation of the inflammasome by intracellular bacteria and cytosolic flagellin proceeds normally in the absence of P2X7 receptor-mediated cytoplasmic K(+) perturbations.

Yersinia has a tropism for B and T cell zones of lymph nodes that is independent of the type III secretion system

Pathogenic Yersinia have a pronounced tropism for lymphatic tissues and harbor a virulence plasmid that encodes a type III secretion system, pTTSS, that transports Yops into host cells. Yops are critical virulence factors that prevent phagocytosis by macrophages and neutrophils and Yersinia mutants lacking one or more Yops are defective for survival in lymphatic tissues, liver, and gastrointestinal tract. However, here we demonstrate that Y. pseudotuberculosis (Yptb) mutants lacking the pTTSS survived as well as or better than wild-type (WT) Yptb in the mesenteric lymph nodes (MLN). Infection with pTTSS mutants caused lymphadenitis with little necrosis, whereas infection with WT Yptb provoked lymphadenitis with multiple necrotic suppurative foci. Gentamicin protection assays and microscopic examination of the MLN revealed that pTTSS mutants resided extracellularly adjacent to B and T lymphocytes in the cortex and paracortex. WT Yptb was found extracellularly adjacent to neutrophils and macrophages in necrotic areas and adjacent to B and T lymphocytes in less-inflamed areas. To determine whether lymphocytes protected pTTSS mutants from phagocytic cells, Rag1^−/− mice were infected with pTTSS mutants or WT Yptb. pTTSS mutants but not WT, were impaired for survival in MLN of Rag1^−/− mice, suggesting that lymphocyte-rich regions constitute a protective niche for pTTSS mutants. Finally, we show that invasin and the chromosomally encoded TTSS were not required for Yptb survival in MLN. In summary, chromosomally encoded factors are sufficient for Yptb replication in the cortex and paracortex of MLN; the pTTSS enables Yersinia to survive within phagocyte-rich areas of lymph nodes, and spread to other tissues.

The pathogenic bacteria, Yersinia, synthesize an apparatus called a type III secretion system, which transports bacterial proteins, Yops, from the bacteria into important immune cells, such as macrophages and neutrophils. Normally, macrophages and neutrophils control bacterial infections by ingesting the bacteria; however, the Yops inactivate these immune cells, which in turn, enable Yersinia to replicate extracellularly and cause disease in many types of tissues. Pathogenic Yersinia are frequently found in lymph nodes of infected hosts, and the Yops are important for the bacteria to replicate and cause disease in lymph nodes since Yersinia mutants that lack Yops do not colonize lymph nodes efficiently. Surprisingly, the authors found that Yersinia pseudotuberculosis lacking the type III secretion system colonizes the mesenteric lymph nodes and survives extracellularly next to lymphocytes. However, in mice lacking lymphocytes, the type III secretion mutants did not survive although wild-type Yersinia did. The authors' findings reveal that other bacterial factors are sufficient for mesenteric lymph node (MLN) colonization of Yersinia and that lymphocytes provide a protective niche for Yersinia strains lacking the type III secretion system. Potentially, these avirulent mutant strains, which persist for at least 5 d in the mesenteric lymph nodes, could be used as live attenuated vaccines to protect against Yersinia infections, or as carriers of other antigens.

Identification, recombinant expression, immunolocalization in macrophages, and T-cell responsiveness of the major extracellular proteins of Francisella tularensis

A safer and more effective vaccine than the previously developed live attenuated vaccine is needed for combating Francisella tularensis, a highly infectious bacterial pathogen. To search for potential candidates for inclusion in a new vaccine, we characterized the proteins present in the culture filtrates of a virulent recent clinical isolate and the attenuated live vaccine strain of F. tularensis using a proteomic approach. We identified a total of 12 proteins; among these, catalase-peroxidase was much more abundant in the culture filtrate of the virulent clinical isolate, whereas bacterioferritin was more abundant in the culture filtrate of the live vaccine strain. Streptolysin O treatment of infected human macrophages indicated that catalase-peroxidase and the heat shock protein GroEL are released intracellularly by actively growing F. tularensis. Mice immunized with F. tularensis developed significant cell-mediated immune responses to catalase-peroxidase, the heat shock protein GroEL, and bacterioferritin as measured by splenic lymphocyte proliferation and gamma interferon production. Finally, we expressed the major culture filtrate proteins that are promising vaccine candidates in Escherichia coli at high levels in soluble form to facilitate study of their immunobiology and potential role in vaccines.

Macrophages Acquire Neutrophil Granules for Antimicrobial Activity against Intracellular Pathogens

A key target of many intracellular pathogens is the macrophage. Although macrophages can generate antimicrobial activity, neutrophils have been shown to have a key role in host defense, presumably by their preformed granules containing antimicrobial agents. Yet the mechanism by which neutrophils can mediate antimicrobial activity against intracellular pathogens such as Mycobacterium tuberculosis has been a long-standing enigma. We demonstrate that apoptotic neutrophils and purified granules inhibit the growth of extracellular mycobacteria. Phagocytosis of apoptotic neutrophils by macrophages results in decreased viability of intracellular M. tuberculosis. Concomitant with uptake of apoptotic neutrophils, granule contents traffic to early endosomes, and colocalize with mycobacteria. Uptake of purified granules alone decreased growth of intracellular mycobacteria. Therefore, the transfer of antimicrobial peptides from neutrophils to macrophages provides a cooperative defense strategy between innate immune cells against intracellular pathogens and may complement other pathways that involve delivery of antimicrobial peptides to macrophages.

Alternative methods to limit extracellular bacterial activity for enumeration of intracellular bacteria

The gentamicin survival assay, a method routinely used to estimate bacterial infection of eukaryotic host cells, depends on the presumed limited penetration of gentamicin across the eukaryotic cell membrane. However, some studies have suggested that gentamicin may in fact enter eukaryotic cells and kill intracellular bacteria. In this study we devised alternative methods to enumerate intracellular Salmonellae using a lytic bacteriophage, SP6, and an amino acid auxotroph, Pro- mutant, which replicates selectively within host cells in the presence of its uptake inhibitor, 3,4-dehydro-L-proline. The conventional gentamicin survival assay was systematically compared with the alternative methods for the enumeration of intracellular Salmonellae. We found that gentamicin decreases the survival of intracellular Salmonellae when added to extracellular media at concentrations above 20 microg/ml. The alternative methods do not suffer from this disadvantage, suggesting that they should be used to replace the gentamicin survival assay. In addition, the proline auxotroph method could be applied to detect bacterial release from host cells.

FliC-specific CD4+ T cell responses are restricted by bacterial regulation of antigen expression

Salmonella typhimurium, a facultatively intracellular pathogen, regulates expression of virulence factors in response to distinct environments encountered during the course of infection. We tested the hypothesis that the transition from extra- to intracellular environments during Salmonella infection triggers changes in Ag expression that impose both temporal and spatial limitations on the host T cell response. CD4(+) T cells recovered from Salmonella immune mice were propagated in vitro using Ag derived from bacteria grown in conditions designed to emulate extra- or intracellular environments in vivo. Extracellular phase bacteria supported a dominant T cell response to the flagellar subunit protein FliC, whereas intracellular phase bacteria were unable to support expansion of FliC-specific T cells from populations known to contain T cells with reactivity to this Ag. This result was attributed to bacterial regulation of FliC expression: transcription and protein levels were repressed in bacteria growing in the spleens of infected mice. Furthermore, Salmonella-infected splenocytes taken directly ex vivo stimulated FliC-specific T cell clones only when intracellular FliC expression was artificially up-regulated. Although it has been suggested that a microanatomical separation of immune T cells and infected APC exists in vivo, we demonstrate that intracellular Salmonella can repress FliC expression below the T cell activation threshold. This potentially provides a mechanism for intracellular Salmonella at systemic sites to avoid detection by Ag-specific T cells primed at intestinal sites early in infection.

Nod1-mediated endothelial cell activation by Chlamydophila pneumoniae

Seroepidemiological and animal studies, as well as demonstration of viable bacteria in atherosclerotic plaques, have linked Chlamydophila pneumoniae infection to development of chronic vascular lesions and coronary heart disease. Inflammation and immune responses are dependent on host recognition of invading pathogens. The recently identified cytosolic Nod proteins are candidates for intracellular recognition of bacteria, such as the obligate intracellular chlamydia. In the present study, mechanisms of endothelial cell activation by C. pneumoniae via Nod proteins were examined. Viable, but not heat-inactivated, chlamydia activated human endothelial cells, suggesting that invasion of these cells is necessary for their profound activation. Endothelial cells express Nod1. Nod1 gene silencing by small interfering RNA reduced C pneumoniae-induced IL-8 release markedly. Moreover, in HEK293 cells, overexpressed Nod1 or Nod2 amplified the capacity of C pneumoniae to induce nuclear factor kappaB (NF-kappaB) activation. Interestingly, heat-inactivated bacteria were still able to induced a NF-kappaB reporter gene activity via Nod proteins when transfected intracellularly, but not when provided from the extracellular side. In contrast, TLR2 sensed extracellular heat-inactivated chlamydia. In conclusion, we demonstrated that C pneumoniae induced a Nod1-mediated and Nod2-mediated NF-kappaB activation in HEK293 cells. In endothelial cells, Nod1 played a dominant role in triggering a chlamydia-mediated inflammatory process.

Recognition of bacterial products by toll-like receptors

Over the past 5 years, our knowledge about how the immune system senses the microbial world has changed fundamentally. It has been known for decades that microbial products such as lipopolysaccharide or bacterial DNA have a profound activity on human cells. Whereas the molecular structure of many different pathogenic microbial compounds has been extensively studied and characterized, the molecular basis of their recognition by the immune system remained elusive for a long time. It was the late Charles Janeway who developed the idea of microbial structures forming pathogen-associated molecular patterns (PAMP) that would be recognized by pattern-recognition receptors [1]. Even if the notion of pattern recognition is challenged today, the discovery of the family of Toll receptors in species as diverse as Drosophila and humans, and the identification of their role in distinguishing molecules and structures that are common to microorganisms has led to a renewed appreciation of the innate immune system. This review focuses on the current knowledge about the different molecules that are recognized by Toll receptors in mammalian cells.

Contribution of folate biosynthesis to Ralstonia solanacearum proliferation in intercellular spaces

The vigorous proliferation of Ralstonia solanacearum OE1-1 in host intercellular spaces after the invasion of host plants is necessary for the virulence of this bacterium. A folate auxotroph, RM, in which a mini-Tn5 transposon was inserted into pabB encoding para-aminobenzoate synthase component I, lost its ability to vigorously proliferate in intercellular spaces along with its systemic infectivity and virulence after inoculation into roots and infiltration into leaves of tobacco plants. Complementation of RM with the pabB gene allowed the mutant to multiply in intercellular spaces and to cause disease. In tobacco plants that were pretreated with folate, RM was able to vigorously proliferate in the intercellular spaces and cause disease. Interestingly, when it was inoculated through cut stems, the mutant multiplied in the plants and was virulent. Moreover, the mutant multiplied well in stem fluids but not in intercellular fluids, suggesting that the folate concentration within intercellular spaces may be a limiting factor for bacterial proliferation. Therefore, folate biosynthesis contributes to the vigorous proliferation of bacteria in intercellular spaces and leads to systemic infectivity resulting in virulence.

Topographical localisation of cagA positive and cagA negative Helicobacter pylori strains in the gastric mucosa; an in situ hybridisation study

BACKGROUND

The cagA gene is a marker for the presence of the cag pathogenicity island, and the presence of cagA positive strains of Helicobacter pylori can identify individuals with a higher risk of developing gastrointestinal diseases.

AIMS

To study the interaction between H. pylori cagA(+) and cagA(-) strains and the gastric mucosa.

METHODS

Patients with H. pylori associated gastritis and peptic ulcers were studied. Biopsies were obtained from the antrum, corpus, fundus, and incisura for H pylori culture, and for in situ hybridisation studies. From each biopsy, multiple single H. pylori colonies were isolated and propagated for DNA isolation, and cagA was detected by the polymerase chain reaction (PCR). For in situ detection of H. pylori an oligonucleotide specific for an H. pylori common antigen and an oligonucleotide specific for cagA were used as probes. Biotinylated probes were incubated with biopsy sections, developed with streptavidin-horseradish peroxidase, and amplified with the tyramide system.

RESULTS

PCR results for cagA in isolated colonies confirmed the in situ hydridisation studies. In situ hybridisation identified cagA(+) bacteria in patients with cagA(+) isolates; cagA(-) bacteria in patients with cagA(-) isolates, and cagA(+) and cagA (-) bacteria in patients with both cagA(+) and cagA(-) isolates. CagA(-) bacteria usually colonised the mucous gel or the apical epithelial surface, whereas cagA(+) bacteria colonised the immediate vicinity of epithelial cells or the intercellular spaces.

CONCLUSIONS

These results document a different in vivo interaction between H. pylori cagA(+) or cagA(-) strains and the gastric mucosa.

Extracellular virulence factors of streptococci associated with animal diseases

A virulence factor denotes a bacterial product or strategy that contributes to virulence or pathogenicity. Streptococci produce a variety of protein toxins and enzymes that are capable of killing host cells and breaking down cell constituents, presumably to provide nutrients for the bacteria or to promote their spread. Some of these secreted products are hemolysins, streptokinases, hyaluronidases, exotoxins and proteases. In some cases, they play an important role in resistance to the host immune system, acting alone or in combination with cell-associated virulence factors (such as the capsule and surface proteins). Thus, the virulence of streptococci is considered as a multifactorial process. In contrast to well known human pathogens, and in spite of their veterinary importance, knowledge of virulence factors of most animal disease-associated streptococci is limited or almost inexistent. In the present article, the available information regarding the extracellular virulence factors of the most important animal disease-related streptococci is reviewed.

[Atypical cell forms overproducing extracellular substances in population of cycad cyanobionts]

The ultrastructure of the cyanobionts of the greenhouse-grown cycads Cycas circinalis, Ceratozamia mexicana, and Encephalartos villosus was studied. The cyanobiont microcolonies grown in the intercellular space of the cyanobacterial zone of cortical parenchyma in the cycad coralloid roots contained two specific forms of vegetative cells with a reduced cell wall, namely, protoplasts and spheroplasts. The protoplasts and spheroplasts exhibited ultrastructural changes indicating the overproduction of two extracellular substances, one of which resembled the mucilage polysaccharides and the other was proteinous. The substances were likely to be synthesized intracellularly and then be excreted with the aid of surface vesicles or by channels in the cytoplasmic membrane to form, respectively, a slimy extracellular matrix and an additional electron-opaque envelope around the cell. At the late developmental stages, the excretion of these substances was accompanied by degradative changes in the cells, leading eventually to cell death. The physiological role of these specific cell forms and the factors that induce their development and death in the cell populations of cyanobionts are discussed.

Survival, replication, and antibody susceptibility of Ehrlichia chaffeensis outside of host cells

Ehrlichia chaffeensis, an obligate intracellular, tick-transmitted bacterium, is susceptible to antibody-mediated host defense, but the mechanism by which this occurs is not understood. One possible explanation is that antibodies directly access the bacteria in the extracellular environment of the host, perhaps during bacterial intercellular transfer. Accordingly, we investigated whether bacteria could be found outside of host cells during infection. Host cell-free plasma obtained from infected mice was found to contain ehrlichiae, and the host cell-free ehrlichiae readily transferred disease to susceptible SCID recipients. The host cell-free ehrlichiae were found during infection of both immunocompetent BALB/c and immunocompromised BALB/c-scid mice and reached levels as high as 10(8)/ml in plasma during persistent infection in SCID mice. Approximately 10% of the blood-borne bacteria were found outside of host cells. Although it is generally accepted that replication of ehrlichiae occurs only within host cells, the cell-free bacteria were shown to undergo DNA replication and cell division in vitro for 3 to 5 days when incubated at 37 degrees C in plasma. Paradoxically, both infectivity and virulence were lost after 24 h of ex vivo culture. The data indicate that E. chaffeensis is exposed to the extracellular milieu during infection, presumably during intercellular transfer, and reveal that these intracellular bacteria do not require the environment of the host cell for replication. Our findings reveal a possible mechanism by which antibodies can access the intracellular bacteria upon their release into the extracellular milieu and mediate host defense and also have implications for understanding the replication and transmission of this vector-borne pathogen.

Preliminary evaluation of several disinfection/sterilization techniques for use with microdialysis probes

This study evaluated the suitability of some disinfection and sterilization methods for use with microdialysis probes. Disinfection or sterilization should minimize the tissue inflammatory reaction and improve the long-term health of rats on study and ensure the quality of data obtained by microdialysis sampling. Furthermore, the treatment should not negatively impact probe integrity or sampling performance. The techniques chosen for evaluation included two disinfection methods (70% ethanol and a commercial contact lens solution) and two sterilization methods (hydrogen peroxide plasma, and e-beam radiation). Linear microdialysis probes treated by these processes were compared to untreated probes removed from the manufacturer's packaging as if sterile (the control group). The probes were aseptically implanted in the livers of rats and monitored for 72 hours. The parameters chosen to evaluate probe performance were relative sample mass recovery and the relative in vivo extraction efficiency of the probe for caffeine. Post mortem bacterial counts and histopathology examination of liver tissue were also conducted. The probes remained intact and functional for the entire study period. The methods tested did not acutely alter the probes although hydrogen peroxide plasma and contact lens solution groups showed reduced extraction efficiencies. Minimal tissue damage was observed surrounding the probes and acute inflammatory reaction was mild to moderate. Low numbers of bacterial colonies from the implantation sites indicates that the health of animals in this study was not impaired. This was also true for the control group (untreated probe).

Science review: key inflammatory and stress pathways in critical illness - the central role of the Toll-like receptors

A pure reductionist approach can sometimes be used to solve an exceptionally complicated biologic problem, and sepsis is nothing if not complicated. A serious infection promptly leads to changes in many aspects of host physiology, including alterations in circulation, metabolism, renal, hepatic, and neuroendocrine function; all of these changes happen at once, and each influences one another. It is difficult to tease apart a problem of this sort, if only because the systems affected are so profoundly interactive. The key to understanding sepsis, insofar as we do understand it at present, was found in the use of genetic tools to study the very earliest events that take place at the interface of the pathogen and the host. The continued application of both forward and reverse genetic methods, in both mammals and insects, is steadily revealing the central biochemical events that occur during infection.

Comparative target site pharmacokinetics of immediate- and modified-release formulations of cefaclor in humans

Optimal dosing of beta-lactam antibiotics aims at maximizing the time at which drug levels in the interstitial space fluid (ISF)--the fluid that surrounds the causative microorganisms at the target site--exceed the minimal inhibitory concentration (MIC). One potentially attractive strategy to achieve this goal is to administer antibiotics as oral sustained-release formulations. The present study was designed to test the hypothesis that sustained-release formulations could lead to a more suitable pharmacokinetic profile in the ISF at the relevant target site. For this purpose, time versus cefaclor concentration profiles attained in the ISF were measured following administration of two formulations, an immediate- (500 mg IR) and a modified-release formulation in two different doses (500 mg MR and 750 mgMR) in a three-way crossover study of healthy male volunteers (n = 12). For the measurement of unbound cefaclor concentrations in the ISF of human skeletal muscle, the in vivo microdialysis technique was employed. For all three formulations, unbound cefaclor concentration in the ISF closely followed individual plasma concentration profiles in a dose-dependent pattern, with ISF to unbound plasma ratios ranging from 0.67 to 0.73. The mean residence time was found to be significantly longer for the MR formulations versus the IR formulation. The data of the present study indicate that time above MIC values at the target site can be substantially prolonged if an antibiotic is administered as a sustained-release product.

Human V gamma 2V delta 2 T cells produce IFN-gamma and TNF-alpha with an on/off/on cycling pattern in response to live bacterial products

Whereas cytokine production in alphabeta T cells is rapidly regulated by exposure to peptide Ag, the mechanisms regulating cytokine production by gammadelta T cells are unknown. In this study, we demonstrate that human Vgamma2Vdelta2 T cells produce IFN-gamma and TNF-alpha as early as 2 h after Ag exposure, and that they produce these cytokines in a dose- and time- dependent manner in response to stimulation with a live bacterial product, iso-butylamine (IBA), but not to dead bacteria or LPS. gammadelta T cells began, ceased, and then resumed IFN-gamma and TNF-alpha generation in an on/off/on cycling pattern, both in vitro and in vivo, depending on the presence or absence of IBA. IFN-gamma and TNF-alpha, whose optimum production was dependent on IBA-stimulated gammadelta T cells, were critical for monocyte-mediated killing of Escherichia coli. By limiting cytokine production to periods of direct contact with live bacteria, gammadelta T cells focus their resources at the site of infection, while limiting systemic immunopathology. Thus, human gammadelta T cells may mediate innate resistance to extracellular bacteria via tightly regulated cytokine production without necessarily expanding in number.

Effects of trimethoprim and co-trimoxazole on the morphology of Listeria monocytogenes in culture medium and after phagocytosis

The purpose of this study was to compare the extra- and intracellular activity of antifolates on Listeria monocytogenes. The fortuitous discovery of elongated bacteria in response to trimethoprim revealed a novel effect on the morphology of Listeria in cell culture medium and after phagocytosis. This phenomenon permitted the quantification of trimethoprim activity, revealing comparable activity intra- and extracellularly. Subinhibitory concentrations of trimethoprim resulted in bacterial elongation, which was reversed after removal of trimethoprim. We attribute this effect of trimethoprim to an inhibition of cell wall synthesis and/or cell separation of Listeria.

Culture at high density improves the ability of human macrophages to control mycobacterial growth

The mechanisms through which granuloma formation helps control mycobacterial infection are poorly understood, but it is possible that the accumulation of macrophages at high density at sites of infection promotes the differentiation of macrophages into cells with improved mycobactericidal activity. To test this possibility, varying numbers of monocytes were cultured in 96-well plates for 3 days, infected with Mycobacterium bovis bacillus Calmette-Guérin, and mycobacterial number was assessed 7 days after infection based on the measurement of luciferase activity expressed by a mycobacterial reporter strain or by counting CFU. Mycobacterial growth was optimal in cultures containing 5 x 10(4) cells/well, but increasing the number of cells to 2 x 10(5) cells/well resulted in complete inhibition of mycobacterial growth. This effect could not be explained by differences in mycobacterial uptake, multiplicity of infection, acidification of the extracellular medium in high density cultures, enhanced NO production, or paracrine stimulation resulting from secretion of cytokines or other proteins. The morphology of cells cultured at high density was strikingly different from that of monocytes cultured at 5 x 10(4) cells/well, including the appearance of numerous giant cells. The bacteriostatic activity of monocyte-derived macrophages was also dependent on cell number, but fewer of these more mature cells were required to control mycobacterial growth. Thus, the ability of human macrophages to control mycobacterial infection in vitro is influenced by the density of cells present, findings that may help explain why the formation of granulomas in vivo appears to be a key event in the control of mycobacterial infections.

Group B Streptococcus induces apoptosis in macrophages

Group B Streptococcus (GBS) is a pathogen that has developed some strategies to resist host immune defenses. Because phagocytic killing is an important pathogenetic mechanism for bacteria, we investigated whether GBS induces apoptosis in murine macrophages. GBS type III strain COH31 r/s (GBS-III) first causes a defect in cell membrane permeability, then at 24 h, apoptosis. Apoptosis was confirmed by several techniques based on morphological changes and DNA fragmentation. Cytochalasin D does not affect apoptosis, suggesting that GBS-III needs not be within the macrophage cytoplasm to promote apoptosis. Inhibition of host protein synthesis prevents apoptosis, whereas inhibition of caspase-1 or -3, does not. Therefore, GBS can trigger an apoptotic pathway independent of caspase-1 and -3, but dependent on protein synthesis. Inhibition of apoptosis by EGTA and PMA, and enhancement of apoptosis by calphostin C and GF109203X suggests that an increase in the cytosolic calcium level and protein kinase C activity status are important in GBS-induced apoptosis. Neither alteration of plasma membrane permeability nor apoptosis were induced by GBS grown in conditions impeding hemolysin expression or when we used dipalmitoylphosphatidylcholine, which inhibited GBS beta-hemolytic activity, suggesting that GBS beta-hemolysin could be involved in apoptosis. beta-Hemolysin, by causing membrane permeability defects, could allow calcium influx, which initiates macrophage apoptosis. GBS also induces apoptosis in human monocytes but not in tumor lines demonstrating the specificity of its activity. This study suggests that induction of macrophage apoptosis by GBS is a novel strategy to overcome host immune defenses.

Distribution and antimicrobial activity of fosfomycin in the interstitial fluid of human soft tissues

Fosfomycin is a broad-spectrum antibiotic which is established as therapy for uncomplicated lower urinary tract infections. In addition, preliminary data indicate that fosfomycin has a potential role in the treatment of soft tissue infections. However, the use of fosfomycin has not been established for this condition, and it is unclear whether the level of fosfomycin penetration into human soft tissues is high enough to eradicate relevant pathogens. To better characterize the antibiotic potential of fosfomycin, we applied a combined in vivo pharmacokinetic-in vitro pharmacodynamic model to human volunteers. For this purpose fosfomycin concentrations in vivo in the fluid of the interstitial space of human soft tissues were measured by microdialysis following intravenous infusion of 4 or 8 g of fosfomycin (n = 6). Subsequently, bacterial isolates with relevance for soft tissue infections were exposed to concentrations according to the in vivo pharmacokinetic profile in the interstitial space fluid obtained by microdialysis. Our experiments indicated a high degree of soft tissue penetration for fosfomycin, with ratios of the area under the concentration-time curve from 0 to 8 h for muscle (AUC(0-8(muscle)))/AUC(0-8(serum)) of 0.48+/-0.08 and 0.53+/-0.04 and ratios of AUC(0-8(adipose tissue))/AUC(0-8(serum)) of 0.74+/-0.12 and 0.71+/-0.11 following administration of 4 and 8 g, respectively. In corresponding in vitro simulation experiments with selected isolates of Staphylococcus aureus, Enterobacter cloacae, and Serratia marcescens for which MICs were 16 microg/ml, organisms were undetectable after a single dosing interval. Fosfomycin exhibits a strong ability to penetrate into the fluid of the interstitial space of soft tissues and reaches levels sufficient to substantially inhibit the growth of relevant bacteria at the target site. We therefore conclude that fosfomycin might qualify as an alternative candidate for the therapy of soft tissue infections.

Effects of menadione and hydrogen peroxide on glutathione status in growing Escherichia coli

Menadione (MD) and H2O2 caused distinct effects on glutathione status in growing Escherichia coli. Treatment of E. coli AB1157 with 1-25 mM H2O2 did not result in an appreciable decrease in intracellular total glutathione (reduced glutathione [GSH] + oxidized glutathione [GSSG]). Only when cells were treated with 25 mM H2O2 an increase in GSSG and a decrease in the GSH:GSSG ratio were observed. In cells deficient in catalase HPI, such effect was observed even at 10 mM H2O2. The exposure of E. coli AB1157 to MD caused a dose-dependent decrease in intracellular total glutathione, an increase in GSSG, and a decrease in the ratio of GSH:GSSG. In E. coli deficient in cytosolic superoxide dismutase activity, a decrease in total glutathione after incubation with 0.2 mM MD was not accompanied by an increase in GSSGin, and the ratio of GSHin:GSSGin was three times higher than in the wild-type cells. The changes in the redox status of extracellular glutathione under the action of both oxidants were similar. Although the catalase activity increased several times after exposure to both oxidants, there were little or no changes in the activity of enzymes related to glutathione metabolism. A possible role of changes in redox status of glutathione under oxidative stress is discussed.

Penetration of ciprofloxacin into the interstitial space of inflamed foot lesions in non-insulin-dependent diabetes mellitus patients

Interstitial ciprofloxacin concentrations were measured by microdialysis in inflamed foot lesions of non-insulin-dependent diabetes mellitus patients following intravenous administration of 0. 2 g of ciprofloxacin. Interstitial ciprofloxacin concentrations were significantly lower than corresponding serum concentrations. There was no significant difference in the penetration of ciprofloxacin into inflamed and unaffected tissue (area under the concentration-time curve(infection)/area under the concentration-time curve(unaffected tissue) = 0.99 +/- 0.15 [mean +/- standard error], n = 6). Thus, inflammation appears to have little or no effect on the penetration of ciprofloxacin into tissue.

An extracellular protease of Streptococcus gordonii hydrolyzes type IV collagen and collagen analogues

Streptococcus gordonii is a frequent cause of infective bacterial endocarditis, but its mechanisms of virulence are not well defined. In this study, streptococcal proteases were recovered from spent chemically defined medium (CDM) and fractionated by ammonium sulfate precipitation and by ion-exchange and gel filtration column chromatography. Three proteases were distinguished by their different solubilities in ammonium sulfate and their specificities for synthetic peptides. One of the enzymes cleaved collagen analogs Gly-Pro 4-methoxy-beta-naphthylamide, 2-furanacryloyl-Leu-Gly-Pro-Ala (FALGPA), and p-phenylazobenzyloxycarbonyl-Pro-Leu-Gly-Pro-Arg (pZ-peptide) and was released from the streptococci while complexed to peptidoglycan fragments. Treatment of this protease with mutanolysin reduced its 180- to 200-kDa mass to 98 kDa without loss of enzymatic activity. The purified protease cleaved bovine gelatin, human placental type IV collagen, and the Aalpha chain of fibrinogen but not albumin, fibronectin, laminin, or myosin. Enzyme activity was inhibited by phenylmethylsulfonyl fluoride, indicating that it is a serine-type protease. Maximum production of the 98-kDa protease occurred during growth of S. gordonii CH1 in CDM containing 0.075% total amino acids at pH 7.0 with minimal aeration. Higher initial concentrations of amino acids prevented the release of the protease without reducing cell-associated enzyme levels, and the addition of an amino acid mixture to an actively secreting culture stopped further enzyme release. The purified protease was stored frozen at -20 degreesC for several months or heated at 50 degreesC for 10 min without loss of activity. These data indicate that S. gordonii produces an extracellular gelatinase/type IV collagenase during growth in medium containing minimal concentrations of free amino acids. Thus, the extracellular enzyme is a potential virulence factor in the amino acid-stringent, thrombotic, valvular lesions of bacterial endocarditis.

Expression of listeriolysin O and ActA by intracellular and extracellular Listeria monocytogenes

Listeria monocytogenes requires listeriolysin O (LLO) and ActA, the products of hly and actA, respectively, to establish a productive intracellular infection. LLO is essential for vacuolar lysis and entry into the cytosol, while ActA is required for bacterial spread to adjacent cells. We have used a transcriptional reporter gene system to compare the expression of actA and hly during intracellular growth to that during growth in broth cultures. The hly and actA genes were transcriptionally fused to Escherichia coli lacZ and Bacillus pumilus cat-86 (cat), and the fusions were integrated in single copies into the L. monocytogenes chromosome. A chloramphenicol resistance assay indicated that the hly fusion but not the actA fusion was significantly activated in Luria-Bertani (LB) broth, and this finding correlated with LLO and ActA levels detectable in broth cultures. Quantitation of promoter activity on the basis of beta-galactosidase activity revealed up to 10-fold-higher level of expression of the hly fusion relative to the actA fusion in LB broth. In contrast, both fusions were active in the cytosol of J774 cells, and the activity of the actA fusion was approximately 3-fold higher than that of the hly fusion under these conditions. However, quantitative immunoprecipitation of ActA and LLO from infected J774 cells demonstrated approximately 70-fold more cytosolic ActA than cytosolic LLO. Finally, in comparison to induction in broth cultures, actA was highly induced (226-fold) and hly was moderately induced (20-fold) in J774 cells. Collectively, these results indicate that actA and hly are differentially regulated in response to the growth environment and that both genes are preferentially expressed during intracellular growth. Further, while the lower level of production of ActA than of LLO in broth can be accounted for by transcriptional regulation, the relative abundance of intracellular ActA compared to that of intracellular LLO is a function of additional, possibly host-mediated, factors.

Mycobacterium tuberculosis catalase and peroxidase activities and resistance to oxidative killing in human monocytes in vitro

Mycobacterium tuberculosis has a relatively high resistance to killing by hydrogen peroxide and organic peroxides. Resistance may be mediated by mycobacterial catalase-peroxidase (KatG) and possibly by alkyl hydroperoxide reductase (AhpC). To determine the interrelationship between sensitivity to H2O2, catalase and peroxidase activities, and bacillary growth rates measured both intracellularly in human monocytes and in culture medium, we examined one laboratory strain, two clinical isolates, and three recombinant strains of M. tuberculosis with differing levels of KatG and AhpC. Five of the mycobacterial strains had intracellular doubling times of 27 to 32 h, while one KatG-deficient clinical isolate (ATCC 35825) doubled in approximately 76 h. Killing of mycobacteria by exogenously added H2O2 was more pronounced for intracellular bacilli than for those bacilli derived from disrupted monocytes. Strains with no detectable KatG expression or catalase activity were relatively sensitive to killing (43 to 67% killing) by exogenous H2O2. However, once even minimal catalase activity was present, mycobacterial catalase activity over a 10-fold range (0.56 to 6.2 U/mg) was associated with survival of 85% of the bacilli. Peroxidase activity levels correlated significantly with resistance of the mycobacterial strains to H2O2-mediated killing. An endogenous oxidative burst induction by 4beta-phorbol 12beta-myristate 13alpha-acetate treatment of infected monocytes reduced the viability of the KatG null strain (H37Rv Inhr) but not the KatG-overexpressing strain [H37Rv(pMH59)]. These results suggest that mycobacterial resistance to oxidative metabolites (including H2O2 and other peroxides) may be an important mechanism of bacillary survival within the host phagocyte.