Potent hydroxamate-derived compounds arrest endodyogeny of Toxoplasma gondii tachyzoites

Toxoplasmosis is a zoonosis that is a worldwide health problem, commonly affecting fetal development and immunodeficient patients. Treatment is carried out with a combination of pyrimethamine and sulfadiazine, which can cause cytopenia and intolerance and does not lead to a parasitological cure of the infection. Lysine deacetylases (KDACs), which remove an acetyl group from lysine residues in histone and non-histone proteins are found in the Toxoplasma gondii genome. Previous work showed the hydroxamate-type KDAC inhibitors Tubastatin A (TST) and Vorinostat (Suberoylanilide Hydroxamic Acid, SAHA) were effective against T. gondii. In the present study, the effects of three hydroxamates (KV-24, KV-30, KV-46), which were originally designed to inhibit human KDAC6, showed different effects against T. gondii. These compounds contain a heterocyclic cap group and a benzyl linker bearing the hydroxamic acid group in para-position. All compounds showed selective activity against T. gondii proliferation, inhibiting tachyzoite proliferation with IC50 values in a nanomolar range after 48h treatment. Microscopy analyses showed that after treatment, tachyzoites presented mislocalization of the apicoplast, disorganization of the inner membrane complex, and arrest in the completion of new daughter cells. The number of dividing cells with incomplete endodyogeny increased significantly after treatment, indicating the compounds can interfere in the late steps of cell division. The results obtained in this work that these new hydroxamates should be considered for future in vivo tests and the development of new compounds for treating toxoplasmosis.

Dichloroacetate and Pyruvate Metabolism: Pyruvate Dehydrogenase Kinases as Targets Worth Investigating for Effective Therapy of Toxoplasmosis

Toxoplasmosis, a protozoan infection caused by Toxoplasma gondii, is estimated to affect around 2.5 billion people worldwide. Nevertheless, the side effects of drugs combined with the long period of therapy usually result in discontinuation of the treatment. New therapies should be developed by exploring peculiarities of the parasite's metabolic pathways, similarly to what has been well described in cancer cell metabolism. An example is the switch in the metabolism of cancer that blocks the conversion of pyruvate into acetyl coenzyme A in mitochondria. In this context, dichloroacetate (DCA) is an anticancer drug that reverts the tumor proliferation by inhibiting the enzymes responsible for this switch: the pyruvate dehydrogenase kinases (PDKs). DCA has also been used in the treatment of certain symptoms of malaria; however, there is no evidence of how this drug affects apicomplexan species. In this paper, we studied the metabolism of T. gondii and demonstrate that DCA also inhibits T. gondii's in vitro infection with no toxic effects on host cells. DCA caused an increase in the activity of pyruvate dehydrogenase followed by an unbalanced mitochondrial activity. We also observed morphological alterations frequently in mitochondria and in a few apicoplasts, essential organelles for parasite survival. To date, the kinases that potentially regulate the activity of pyruvate metabolism in both organelles have never been described. Here, we confirmed the presence in the genome of two putative kinases (T. gondii PDK [TgPDK] and T. gondii branched-chain α-keto acid dehydrogenase kinase [TgBCKDK]), verified their cellular localization in the mitochondrion, and provided in silico data suggesting that they are potential targets of DCA.IMPORTANCE Currently, the drugs used for toxoplasmosis have severe toxicity to human cells, and the treatment still lacks effective and safer alternatives. The search for novel drug targets is timely. We report here that the treatment of T. gondii with an anticancer drug, dichloroacetate (DCA), was effective in decreasing in vitro infection without toxicity to human cells. It is known that PDK is the main target of DCA in mammals, and this inactivation increases the conversion of pyruvate into acetyl coenzyme A and reverts the proliferation of tumor cells. Moreover, we verified the mitochondrial localization of two kinases that possibly regulate the activity of pyruvate metabolism in T. gondii, which has never been studied. DCA increased pyruvate dehydrogenase (PDH) activity in T. gondii, followed by an unbalanced mitochondrial activity, in a manner similar to what was previously observed in cancer cells. Thus, we propose the conserved kinases as potential regulators of pyruvate metabolism and interesting targets for new therapies.

HDAC inhibitors Tubastatin A and SAHA affect parasite cell division and are potential anti-Toxoplasma gondii chemotherapeutics

The redirectioning of drugs in the pharmaceutical market is a well-known practice to identify new therapies for parasitic diseases. The histone deacetylase inhibitors Tubastatin A (TST) and Suberoylanilide Hydroxamic Acid (SAHA), firstly developed for cancer treatment, are effective against protozoa parasites. In this work, we aimed to demonstrate the activity of these drugs as potential agents against Toxoplasma gondii, the causative agent of toxoplasmosis. TST and SAHA were active against different genotypes of Toxoplasma gondii, such as, RH (type I), EGS (I/III) and ME49 (type II) strains. The IC₅₀ values for the RH strain were 19 ± 1 nM and 520 ± 386 nM for TST and 41 ± 3 nM and 67 ± 36 nM for SAHA, for 24 and 48 h, respectively. Both compounds were highly selective for T. gondii and their anti-proliferative effect was irreversible for 8 days. The calculated selectivity indexes (39 for TST and 30 for SAHA) make them lead compounds for the future development of anti-Toxoplasma molecules. Western blotting showed TST led to a significant increase of the nuclear histone H4 and a decrease of H3 acetylation levels. Treatment with 1 μM TST and 0.1 μM SAHA for 48 h decreased the amount of global α-tubulin. Fluorescence and electron microscopy showed that both drugs affected the endodyogeny process impairing the budding of daughter cells. The drugs led to the formation of large, rounded masses of damaged parasites with several centrosomes randomly dispersed and incorrect apicoplast division and positioning. TST-treated parasites showed a rupture of the mitochondrial membrane potential and led to a failure of the IMC assembling of new daughter cells. SAHA and TST possibly inhibit HDAC3 and other cytoplasmic or organelle targeted HDACs involved in the modification of proteins other than histones.

The life-cycle of Toxoplasma gondii reviewed using animations

Toxoplasma gondii is a protozoan parasite that is the causative agent of toxoplasmosis, an infection with high prevalence worldwide. Most of the infected individuals are either asymptomatic or have mild symptoms, but T. gondii can cause severe neurologic damage and even death of the fetus when acquired during pregnancy. It is also a serious condition in immunodeficient patients. The life-cycle of T. gondii is complex, with more than one infective form and several transmission pathways. In two animated videos, we describe the main aspects of this cycle, raising questions about poorly or unknown issues of T. gondii biology. Original plates, based on electron microscope observations, are also available for teachers, students and researchers. The main goal of this review is to provide a source of learning on the fundamental aspects of T. gondii biology to students and teachers contributing for better knowledge and control on this important parasite, and unique cell model. In addition, drawings and videos point to still unclear aspects of T. gondii lytic cycle that may stimulate further studies.

Identification of the alpha-enolase P46 in the extracellular membrane vesicles of Bacteroides fragilis

BACKGROUND

Members of the Bacteroides fragilis group are the most important components of the normal human gut microbiome, but are also major opportunistic pathogens that are responsible for significant mortality, especially in the case of bacteraemia and other severe infections, such as intra-abdominal abscesses. Up to now, several virulence factors have been described that might explain the involvement of B. fragilis in these infections. The secretion of extracellular membrane vesicles (EMVs) has been proposed to play a role in pathogenesis and symbiosis in gram-negative bacteria, by releasing soluble proteins and other molecules. In B. fragilis, these vesicles are known to have haemagglutination and sialidosis activities, and also contain a capsular polysaccharide (PSA), although their involvement in virulence is still not clear.

OBJECTIVE

The aim of this study was to identify proteins in the EMV of the 638R B. fragilis strain by mass spectrometry, and also to assess for the presence of Bfp60, a surface plasminogen (Plg) activator, previously shown in B. fragilis to be responsible for the conversion of inactive Plg to active plasmin, which can also bind to laminin-1.

METHODS

B. fragilis was cultured in a minimum defined media and EMVs were obtained by differential centrifugation, ultracentrifugation, and filtration. The purified EMVs were observed by both transmission electron microscopy (TEM) and immunoelectron microscopy (IM). To identify EMV constituent proteins, EMVs were separated by 1D SDS-PAGE and proteomic analysis of proteins sized 35 kDa to approximately 65 kDa was performed using mass spectrometry (MALDI-TOF MS).

FINDINGS

TEM micrographs proved the presence of spherical vesicles and IM confirmed the presence of Bfp60 protein on their surface. Mass spectrometry identified 23 proteins with high confidence. One of the proteins from the B. fragilis EMVs was identified as an enolase P46 with a possible lyase activity.

MAIN CONCLUSIONS

Although the Bfp60 protein was not detected by proteomics, α-enolase P46 was found to be present in the EMVs of B. fragilis. The P46 protein has been previously described to be present in the outer membrane of B. fragilis as an iron-regulated protein.

Development of dual fluorescent stage specific reporter strain of Toxoplasma gondii to follow tachyzoite and bradyzoite development in vitro and in vivo

Toxoplasma gondii is a protozoan that infects 30% of humans as intermediate hosts. T Sexual reproduction can occur only within the intestinal tract of felines, however, infection in other mammals and birds is associated with asexual replication and interconversion between the tachyzoite and bradyzoite stages. Bradyzoites are slow growing forms found in tissue cysts in latent infection. Recently, our group described the biological behavior of the EGS strain that forms thick walled cysts spontaneously in tissue culture, constituting a useful tool for examining the developmental biology of T. gondii. To further improve the usefulness of this model, we constructed genetically modified EGS parasites that express fluorescent tags under the control of stage specific promoters. The promoter regions for SAG-1 (tachyzoite specific), BAG-1 and LDH-2 (bradyzoite specific) were amplified by PCR and plasmids were constructed with mCherry (redT) and sfGFP (greenB) sequences, respectively. Strains of parasites were selected using FACS to arrive at single fluorescent and dual fluorescent strains of EGS expressing tags in a stage specific manner. In cell cultures, vacuoles labeled by immunofluorescence assay using anti-CST-1 a marker for T. gondii cyst wall contained parasites that were positive for BAG1-GFP and negative for SAG1-mCherry. Tachyzoites and bradyzoites harvested from the mice expressed stage specific mCherry and GFP proteins, respectively. These new dual fluorescent transgenic EGS strains are a promising tool to elucidate the mechanisms of T. gondii differentiation both in vitro and in vivo.

Inactivation of a fibronectin-binding TonB-dependent protein increases adhesion properties of Bacteroides fragilis

Bacteroides fragilis is the Gram-negative strictly anaerobic bacterium most frequently isolated from clinical infections, including intra-abdominal abscess and bacteraemia. A number of factors can contribute to its virulence, including the expression of adhesins. Some of them are already characterized and can recognize and bind to extracellular matrix components, such as fibronectin. One of the molecules responsible for fibronectin-binding is an outer-membrane protein previously described by our group, which belongs to the TonB-dependent family. The aim of the present work was to characterize this protein. Initially, it was confirmed by fluorescence and electron microscopy that the fibronectin-binding molecules were located in the bacterial surface, but the distribution of these molecules on the surface was not uniform. To further evaluate the role of this protein, the gene bf1991, responsible for encoding this protein, was inactivated by a suicide vector and the mutant strains generated were used in several experiments to verify possible phenotypical alterations. In adherence assays with fibronectin immobilized on latex beads an increased adhesion was observed with the mutant strains compared with the wild-type strain. Western blot analysis in the mutant strain revealed the absence of the 120 kDa TonB-dependent outer-membrane protein and an alteration in the expression of an unknown 30 kDa protein. Killing assays using peritoneal macrophages were performed to evaluate the role of this protein as a virulence attribute and it was observed that the mutant strains were more efficiently internalized than the wild-type strains, with more internalization in the samples covered with fibronectin than in the samples not covered with it.

Spontaneous cystogenesis in vitro of a Brazilian strain of Toxoplasma gondii

Conversion of Toxoplasma gondii tachyzoites to the bradyzoite stage and tissue cyst formation in the life cycle of the parasite have crucial roles in the establishment of chronic toxoplasmosis. In this work we investigated the in vitro cystogenesis and behavior of the EGS strain, isolated from human amniotic fluid. We observed that tachyzoites of the EGS strain converted to intracellular cysts spontaneously in LLC-MK2 epithelial cells, HSFS fibroblasts and C6 glial cell lineage. The peak of conversion occurred in the LLC-MK2 cells after 4days of infection, when 72.3±15.9 of the infected cells contained DBA positive cysts. Using specific markers against bradyzoite, tachyzoite and cyst wall components, we confirmed stage conversion and distinguished immature from mature cysts. It was also observed that the deposition of cyst wall components occurred before the total conversion of parasites. Transmission electron microscopy confirmed the fully conversion of parasites presenting the typical characteristics of bradyzoites as the posterior position of the nucleus and the presence of amylopectin granules. A thick cyst wall was also detected. Besides, the scanning microscopy revealed that the intracyst matrix tubules were shorter than those from the parasitophorous vacuole intravacuolar network and were immersed in a granular electron dense material. The EGS strain spontaneously forms high burden of cysts in cell culture without artificial stress conditions, and constitutes a useful tool to study this stage of the T. gondii life cycle.

The organization of the wall filaments and characterization of the matrix structures of Toxoplasma gondii cyst form

The encystation process is a key step in Toxoplasma gondii life cycle, allowing the parasite to escape from the host immune system and the transmission among the hosts. A detailed characterization of the formation and structure of the cyst stage is essential for a better knowledge of toxoplasmosis. Here we isolated cysts from mice brains and analysed the cyst wall structure and cyst matrix organization using different electron microscopy techniques. Images obtained showed that the cyst wall presented a filamentous aspect, with circular openings on its surface. The filaments were organized in two layers: a compact one, facing the exterior of the whole cyst and a more loosen one, facing the matrix. Within the cyst wall, we observed tubules and a large number of vesicles. The cyst matrix presented vesicles of different sizes and tubules, which were organized in a network connecting the bradyzoites to each other and to the cyst wall. Large vesicles, with a granular material in their lumen of glycidic nature were observed. Similar vesicles were also found associated with the posterior pole of the bradyzoites and in proximity to the cyst wall.

Esterprodrugs of ciprofloxacin as DNA-gyrase inhibitors: synthesis, antiparasitic evaluation and docking studies

Novel ester prodrugs of ciprofloxacin proved to be extremely efficient against Plasmodium falciparum and Toxoplasma gondii. Molecular modeling and computational calculations were used to understand the mechanisms of action of these drugs.

Evaluation of three novel azasterols against Toxoplasma gondii

Previous studies from our group have demonstrated the high susceptibility of Toxoplasma gondii tachyzoites to the sterol analogues 22,26-azasterol and 24,25-(R,S)-epiminolanosterol. In this work we present data on testing in vitro three novel azasterols as potential agents for the treatment of toxoplasmosis. The three compounds inhibited parasite growth at micromolar concentrations, in a dose-dependent manner. Electron microscopy analysis of intracellular tachyzoites after treatment with the most effective compound showed drastic mitochondrion swelling associated with the appearance of an electron-lucent matrix and disrupted cristae. Parasite lysis also took place. The appearance of electron dense cytoplasmic structures similar to amylopectin granules distributed throughout the parasite suggests that azasterols might be inducing differentiation of those tachyzoites which were not lysed to the bradyzoite stage.

The fine structure of the Acanthamoeba polyphaga cyst wall

Members of the genus Acanthamoeba are present in diverse environments, from freshwater to soil, and also in humans, causing serious brain and corneal infections. Their life cycle presents two stages: the dividing trophozoite and the quiescent cyst. The structures of these life stages have been studied for many years, and structural data have been used for taxonomy. The ultrastructural work on Acanthamoeba cysts was carried out previously by routine transmission electron microscopy (TEM), a process that requires the use of chemical fixation, a procedure that can cause serious artifacts in the ultrastructure of the studied material. In order to prevent fixation artifacts, we processed Acanthamoeba polyphaga cysts by ultrarapid freezing, followed by freeze-fracturing and deep-etching, in order to obtain a 3D visualization of the arrangements of the cyst wall. The exocyst presented an irregular surface, with vesicles located within or near this layer. The endocyst, instead, showed a biphasic arrangement with a more compact district in its innermost part, and a more loosened outer layer. For this reason, it was difficult to distinguish the filaments present in the intercyst space from those forming the endocyst. Surprisingly, the intercyst space was thinner when compared with samples processed by conventional TEM, evidencing the possible damage consequent to the use of chemical fixation.

Toxoplasma gondii: fluconazole and itraconazole activity against toxoplasmosis in a murine model

Toxoplasma gondii is an important opportunistic pathogen affecting immunocompromised patients with AIDS. Toxoplasmic encephalitis is responsible for high morbidity and mortality. In this study, we investigated the activity of the antifungals fluconazole (FLZ) and itraconazole (ITZ) against T. gondii in mice infected with the Me49 strain. As previously reported for ITZ, FLZ also demonstrated a selective effect against T. gondii in vitro; the IC(50) values obtained for FLZ were 8.9 microM and 3.1 microM after 24h and 48 h of treatment, respectively. A 10-day treatment of mice with orally or intraperitoneally administered 20mg/kg/day FLZ showed a significant survival difference compared to untreated mice. The administration of 20mg/kg/day ITZ significantly reduced the brain cyst burden compared to untreated mice but did not exert significant protection against death. The results obtained in this work are rather promising as ITZ and FLZ are safe and low-cost drugs available on the market.

Antiproliferative activities of two novel quinuclidine inhibitors against Toxoplasma gondii tachyzoites in vitro

OBJECTIVES

To study the antiproliferative effects of ER119884 and E5700, two quinuclidine-based inhibitors of squalene synthase (SQS), against Toxoplasma gondii tachyzoites in epithelial cells.

METHODS

The antiproliferative effects of the quinuclidine derivatives, alone or in combination with epiminolanosterol or antifolates, were analysed, resulting in the construction of isobolograms. The ultrastructure of treated tachyzoites was analysed by transmission electron microscopy.

RESULTS

The quinuclidine derivatives demonstrated selective anti-T. gondii activity, arresting parasite growth with IC50 values of 0.66 and 0.23 microM for ER119884 and E5700, respectively, after 24 h of interaction and 0.44 and 0.19 microM after 48 h of interaction. Both compounds induced remarkable alterations in the parasite ultrastructure, such as mitochondrial swelling and the presence of autophagosome-like structures, after 24 h of treatment. Combination of these quinuclidine derivatives with the antifolates sulfadiazine and pyrimethamine produced a synergic effect. When epiminolanosterol was combined with E5700, the effect observed was synergic, whereas the combination with ER119884 produced no interaction.

CONCLUSIONS

E5700 and ER119884 demonstrated selective activity against T. gondii tachyzoites and are a possible alternative to be used in association with the current therapy. The ultrastructural alterations observed suggest a possible interference with lipid metabolism.

Intravacuolar network may act as a mechanical support for Toxoplasma gondii inside the parasitophorous vacuole

The intravacuolar network inside the parasitophorous vacuole of Toxoplasma gondii consists of an intricate system of membrane-limited tubules of uncertain role in parasite development. We propose that it is an important structural support to the maintenance of the parasites in the characteristic rosette arrangement of parasites inside the vacuole, rather than being associated with the nutrient acquisition from the host cell, as previously suggested. We based our assumptions on observations made by field emission scanning electron microscopy of an epithelial cell line (LLCMK2) infected at various time intervals. Scraping the surface of infected monolayers with Scotch tape exposed the inner organization of the parasitophorous vacuole. Ultrathin sections and freeze-fracture replicas of analogous samples were correlated with field emission observations and added new data on tubular membranes and general organization of the parasitophorous vacuole.

Antiproliferative synergism of azasterols and antifolates against Toxoplasma gondii

The antiproliferative effects of two azasterols, 22,26-azasterol (20-piperidin-2-yl-5alpha-pregnan-3beta-20(R,S)-diol, AZA) and 24,25(R,S)-epiminolanosterol (EIL), in combination with sulphadiazine (SDZ) and pyrimethamine (PYR) were studied against Toxoplasma gondii tachyzoites growing in cultured mammalian cells. We had previously shown that AZA and EIL, two known inhibitors of Delta24(25)sterol methyl transferase in fungi and protozoa, have a potent and selective anti-T. gondii activity, although no 24-alkyl sterols have been detected in this parasite. We now report that when these sterol analogues were used in combination with the conventional SDZ/PYR treatment, potent synergistic effects were observed, ranging from 10- to 100-fold reductions of the IC50 values in the presence of sub-optimal doses of azasterols. When exposed to these drug combinations, intracellular T. gondii parasites displayed diverse subcellular alterations, including mitochondrial swelling, the arrest of the endodiogeny process with fragmented nuclei and subsequent cell lysis. These results suggest a potential new approach for the treatment of toxoplasmosis, which could significantly lower the required levels of antifolates and thus their adverse side effects.

Bacteroides fragilis interferes with iNOS activity and leads to pore formation in macrophage surface

Bacteroides fragilis is the anaerobe most commonly recoverable from clinical specimens. The wide genetic diversity of this bacterium related with virulence potential is still an open question. In this study, we analyzed the morphological aspects and microbicide action of MØ during interactions with B. fragilis. A filamentous cytoplasm content release and a different actin organization colocalized with iNOS were detected. It was also possible to observe the reduction of NO production in the same conditions. The scanning electron microscopy showed the formation of pore-like structures in the surface of macrophages in the bacterial presence and by transmission electron microscopy we could observe the extrusion of cytoplasm contents as well as the condensation of chromatin in the nucleus periphery. These data suggest the existence of an inhibitory mechanism developed by B. fragilis strains for one of the macrophage microbicide actions.

Cultured Embryonic Retina Systems as a Model for the Study of Underlying Mechanisms ofToxoplasma gondiiInfection

PURPOSE

Toxoplasma gondii, the most common cause of retinochoroiditis in humans, is an obligate intracellular protozoan parasite that depends on te host cell's microenvironment to proliferate. Because congenital infection is associated with a higher risk of ocular involvement than a postnatally acquired infection, this study was conducted to investigate the ability of Toxoplasma gondii to infect retinal tissue during development, when cellular environmental changes normally occur.

METHODS

Retinas from 5- to 9-day-old chick embryos were used. Stationary cultures were prepared in 24-well cell culture dishes and maintained at 37 degrees C in DMEM plus 5% fetal bovine serum for 2 to 6 days. Then the wells were infected with 4 x 10(5) tachyzoites. Retina explants and aggregate cell cultures were maintained in DMEM under rotation at 37 degrees C. T. gondii proliferation was measured using [(3)H]-thymidine incorporation after 72 hours. Ornithine and arginine decarboxylase (ODC and ADC) activities were determined by measuring CO(2) production from [1-(14)C]-ornithine and [1-(14)C]-arginine, respectively.

RESULTS

The proliferation of tachyzoites was high in dense, stationary cultures expressing elevated ODC and ADC activity. The addition of ODC or ADC inhibitors reduced T. gondii proliferation by approximately 20% to 40%. As for cultured retina cells, retina explants also allowed T. gondii proliferation whenever ODC activity was high.

CONCLUSIONS

The data suggest a direct correlation between retinal polyamine biosynthesis and the proliferation of T. gondii, in agreement with the observation that individuals infected congenitally have a greater risk of development of toxoplasmic retinochoroiditis.

Selective anti-Toxoplasma gondii activities of azasterols

We report potent and selective inhibitory effects of 22,26-azasterol and 24,25-(R,S)-epiminolanosterol, known inhibitors of Delta24(25)-sterol methyltranferase (SMT) in fungi and protozoa, on the proliferation of Toxoplasma gondii in LLCMK2 cells. These compounds produced a dose-dependent reduction in parasite proliferation. 22,26-azasterol had an IC50 of 5.3 microM after 24 h and 4.5 microM after 48 h, while for 24,25-(R,S)-epiminolanosterol the IC50 values were 1 microM after 24 h and 0.12 microM after 48 h. The rapid reduction of parasite load suggested these compounds have selective cytotoxic effects against T. gondii. However, we were unable to detect 24-alkyl sterols in purified T. gondii tachyzoites using highly sensitive gas-liquid chromatography/mass spectrometry methods, a fact which indicated that the anti-proliferative effects of these azasterols were not mediated by inhibition of SMT. Transmission electron microscopy showed that the mitochondrion was the major target of drugs. Ultrastructural effects on plasma membrane, apicoplast and the formation of autophagosomal structures were also observed.

Bacteroides fragilis adherence to Caco-2 cells

The ability of ten Bacteroides fragilis strains isolated from intestinal and non-intestinal infections, normal flora and environment to adhere to human colon carcinoma cells, Caco-2, was examined. The adherence capacity varied among the strains tested from strongly adherent (76-100%) to non- or weakly adherent (0-25%). Negative staining with Indian ink showed that all the strains were capsulated, although strain 1032 (strongly adherent and originated from bacteremia) had the highest rate of capsulated cells in the culture. All strains studied presented an electron-dense layer and no fimbrial structures in their surface after PTA negative staining. The analysis of the strains with ruthenium red showed the presence of an acidic polysaccharide and also surface vesicles in all of them. The strain 1032 presented an aggregative adherence pattern toward Caco-2 cells monolayers. It could be seen trapped by elongated microvilli and surrounded by extracellular material in the scanning electron microscope. Treatment with sodium periodate (100 mM/1 h) reduced significantly its adherence capacity and also the expression of an electron-dense layer and of the capsule, detected with PTA and Indian ink staining, respectively. We suggest that the capsular polysaccharide might mediate the adherence of the B. fragilis to Caco-2 cells.

Surface charge and surface carbohydrates of Cyptobia salmositica virulent and avirulent forms and of C. bullocki (Kinetoplastida: Cryptobiidae)

The surface charge and surface carbohydrate residues of the virulent (freshly isolated from the fish blood) and avirulent forms (from culture) of Cryptobia salmositica and one strain of C. bullocki were studied. Measurements of the zeta potential of parasites showed that C. bullocki and the virulent form of C. salmositica had a net negative surface charge of about -15 mV, whereas the attenuated form of C. salmositica showed a surface charge of -7.9 mV. Enzymatic treatments of parasites with neuraminidase, trypsin, or phospholipase C indicated the presence of sialic acid residues, phosphate groups, and protein glycoconjugates as components of the Cryptobia surface that accounted for their surface charge. Residues of alpha-D-man, alpha- and beta-D-gal, alpha-D-galNAc, alpha-L-fuc, and D-glcNAc could be detected on the surface of all parasites by specific fluorescein isothiocyanate (FITC)- and colloidal gold-labeled lectins. The cell surface of the avirulent form of C. salmositica showed the strongest reactivity to almost all lectins tested. A remarkable binding pattern of lectins in the anterior region of parasites was observed.

Computer aided three-dimensional reconstruction of the free-living protozoan Bodo sp. (Kinetoplastida:Bodonidae)

Computer aided three-dimensional (3-D) reconstructions of Bodo sp., a free-living kinetoplastid, were made from ultrathin (100 nm) and semithin (200 nm) serial sections, which were observed by transmission electron microscopy. Organelles and membrane systems were digitized into a computer and three-dimensional images were generated using the SYNU software package. It was observed that the internal disposition of structures like the contractile vacuole, the Golgi complex, the mitochondrion, and the nucleus maintain a constant relationship relative to each other and to the cytostome and flagellar pocket. The Golgi complex and the contractile vacuole elements are not apparently connected. The contractile vacuole fills a significant volume in the cell. Volume alterations occurring during systole/diastole cycles of the contractile vacuole are compensated by cytoskeletal adaptations. There is a subpellicular microtubule-free area adjacent to the contractile vacuole. Morphological evidence indicates that the flagellar pocket may not be the only site of vacuolar content elimination. A better view of the cytoskeleton was obtained in detergent-extracted cells, where a set of curved microtubules was observed separating the flagellar pocket from the cytostome and surrounding the cytopharynx.

Freeze-fracture study of Bodo sp. (Kinetoplastida: Bodonidae)

Freeze-fracture technique was used to analyse the structure of conventionally fixed and quickly frozen Bodo sp., a free-living kinetoplastid. In the former method, chemically fixed and cryopreserved cells presented a corrugated membrane pattern in the flagella and cell body surfaces. In the latter, however, replicas from quickly frozen unfixed flagellates showed membranes with a smoother aspect, allowing the observation of intramembranous particles (IMPs) on the fracture faces, hardly detectable in previously fixed samples. The IMPs were randomly distributed throughout the cell surface, except in the sparsely seen short IMP rows.