Ultrastructural description of new Rickettsia-like organisms in the commercial abalone Haliotis tuberculata (Gastropoda: Haliotidae) from the NW of Spain

Rickettsia-like organisms (RLOs) were found in the commercially farmed abalone Haliotis tuberculata in the northwestern region of the Atlantic Coast of Spain and are described from light and transmission electron microscopy observations. The RLOs measured approximately 1.6 x 0.9 microm and were found in intracytoplasmic, spherical to ellipsoidal vacuoles (up to 8 microm) in the epithelial cells of the digestive diverticulae. The morphological ultrastructure of these organisms was typically prokaryotic, including a plasmalemma and a thin Gram-negative type cell wall. Several ultrastructural changes were observed in the epithelial cells of the host containing the RLOs. The nuclei became pycnotic and several basophilic dense inclusions appeared in the cytoplasm. In addition, the host cell appeared lysed and was ruptured in advanced stages of infection. It was impossible to ascertain whether the RLOs are responsible for this disease, as a haplosporidian infection was also present. We can only conclude that the presence of RLOs simultaneously with a haplosporidian parasite may contribute to the mortality of the abalone host.     

Observations on eosinophilic granule cells in peritoneal exudates of eels, Anguilla amtmlis

Eosinophilic granule cells (EGC) are reported among peritoneal exudate cells (PEC) of unstimulated and carrageenan-, zymosan-, latex- and Aeromonas hydrophila -stimulated eels, Anguilla australis . At the light microscopy level EGC are large (< 50 μm diameter) and have strongly to moderately basophilic cytoplasm, eosinophilic granules, and often striated colourless cytoplasmic inclusions. Ultrastructurally, the cytoplasm is rich in ribosomes, and contains characteristic granules, parallel tubular structures 46–50 nm in diameter, and parallel rod-like arrays (PRLA) 14–15 nm in diameter that are angular or hexagonal in cross section. EGC contain granule-associated, partially or completely cyanide inhibited, but azide and aminotriazole resistant, peroxidase, acid phosphatase, esterases and weak cytoplasmic periodic acid-Schiff positivity. PRLA lack enzyme content. A few (< 10%) unstimulated and carrageenan- and zymosan-stimulated EGC contain granule-associated and diffuse β-galactosidase, and EGC in eels injected i.p. with filtered carrageenan-stimulated PEC supernatants show heterogeneity in glucosaminidase content.     

Acrosome differentiation in the ascidians Clavelina lepadiformis and Ciona intestinalis

The spermatozoa of both Clavelina lepadiformis and Ciona intestinalis have architectural features characteristic of ascidian spermatozoa that have been previously described. They have an elongated head (6 microm and 3 microm long, respectively) and a single mitochondrion that is closely applied laterally to the nucleus; they lack a midpiece. The acrosome of Clavelina lepadiformis spermatozoa is a moderately electron-dense, pear-shaped flattened vesicle, approx. 300 nm x 200 nm x 40 nm in length, width, and height, respectively. The acrosome of Ciona intestinalis spermatozoa is a moderately electron-dense, round flattened vesicle with an electron-dense plate in its central region. It is approx. 200 nm x 200 nm x 50 nm in length, width, and height, respectively. During spermiogenesis in both ascidians, several proacrosomal vesicles (50-70 nm in diameter) appear in a blister at the future apex of the spermatids. These vesicles appear to be associated with the inner surface of the plasma membrane enclosing the blister. They come into contact with each other along the inner surface of the plasma membrane and fuse to form a horseshoe-shaped acrosomal vesicle, which becomes a round, flattened vesicle during further differentiation. Some speculations about the mechanism of acrosome differentiation, the possible role of the acrosome during fertilization, and in the speciation of ascidians are presented.     

A new virus associated with the parasitoid Cotesia marginiventris (Hymenoptera: Braconidae): Replication in noctuid host larvae

A nonoccluded filamentous baculo-like virus occurred in scattered hypodermal and tracheal matrix cells of five species of noctuid larvae (Spodoptera frugiperda, S. exigua, S. ornithogalli, Heliothis zea, and Trichoplusia ni) parasitized by Cotesia (= Apanteles) marginiventris. Infected cells and their nuclei were moderately hypertrophied. Infected cells contained prominent virogenic stromata, fibrous cytoplasmic inclusions, and moderate numbers of a filamentous virus. Virions were 50–98 nm in diameter, ≥865 nm in length, and straight to slightly flexuous. Nucleocapsids were 30–39 nm in diameter with a core 21–24 nm in diameter, and were enclosed by one or two envelopes. Nucleocapsids occurred in the nuclei and in the cytoplasm. Enveloped nucleocapsids were restricted to the cytoplasm.     

Characterization of intracytoplasmic prokaryote infections in Dreissena sp. (Bivalvia: Dreissenidae)

This study characterizes intracytoplasmic infections with prokaryote microorganisms in Dreissena sp. (near Dreissena polymorpha) from northeastern Greece and represents the first report of such infections in freshwater bivalves. Light microscope observations of stained tissues revealed basophilic, cytoplasmic inclusion bodies in 87.5% (28/32) of the mussels sectioned. Inclusions in epithelial cells and connective tissues were noted, respectively, in 34.4 and 71.9% of the sample, with 5 mussels (15.6%) having both tissue types infected. Epithelial cell infections were observed in histological sections only in digestive gland tubules and ducts; within tubules, inclusions were present more often in secretory than digestive cells. Connective tissue infections, however, were systemic; among the 32 mussels sectioned, inclusions were found in the gills (65.6%), foot (12.5%), mantle (9.4%), labial palps (6.3%), digestive gland (6.3%), stomach (6.3%), and gonads (3.1%). Cytoplasmic inclusions (maximum dimension, 138 microm) were prominent enough in the gills to be visible in 17.0% of the 247 mussels dissected. Ultrastructurally, prokaryote cells in gill connective tissues were clearly characteristic of Chlamydiales-like organisms, with each intracytoplasmic inclusion containing a loosely packed mixture of elementary, reticulate, intermediate bodies, and blebs. Prokaryote colonies in digestive gland epithelial cells exclusively contained 1 of 4 morphological cell types and were considered Rickettsiales-like. Hexagonal, virus-like particles were present in the cytoplasm of the largest of these Rickettsiales-like prokaryotes. Although host stress was evident from localized cell necrosis and dense hemocyte infiltration, overall infection was fairly benign, with no major, adverse impact on body condition evident among sectioned or dissected mussels. A possible negative effect was partial constriction of gill water tubes, but at the infection intensity observed (typical range 1 to 7 inclusion bodies per section), significant interference with respiration and other metabolic functions of the gills was highly unlikely.     

Ultrastructure of the Lamellated Cytoplasmic Inclusions in Schwann Cells of the Marine Nematode,Deontostoma californicum

Electron microscopy of the photoreceptors in the marine nematode, Deontostoma californicum, revealed numerous lamellated inclusions in the Schwann cells ensheathing the lateral cephalic nerves. Immediately after the axons from the modified bipolar neurons of the photoreceptors enter the lateral nerves, these spherical-to-oval lamellated bodies are observed in the surrounding Schwann cell cytoplasm. These previously undescribed Schwann cell inclusions, approximately 500 nm long and 320 nm in diameter, are lamellated and characterized by the presence of an electron-dense stalk-like process, 80-280 nm long. The lamellated inclusions are bound by a single limiting membrane, 6-7 nm thick, which shows occasional interruptions. The internal structure of the inclusions is characterized by the presence of electron-dense lamellae or bands, 11-16 nm thick, which assume various complex patterns ranging from arrays of parallel linear densities to a reticulate appearance. In addition, the nematode Schwann cell cytoplasm contains the usual organelles, gliosome- and lysosome-like inclusions. Their relationship with lipofuscin pigments is briefly discussed.     

Goussia girellae n. sp. (Apicomplexa: Eimeriorina) in the opaleye, Girella nigricans

Goussia girellae n. sp. is described from the opaleye fish, Girella nigricans. Merogonic stages were observed in the apices of intestinal epithelial cells, in the lamina propria, and in extra-intestinal sites including liver, gills, and spleen. Gamonts were observed in the intestinal epithelial cells. Only unsporulated oocysts were detected in the intestine, and sporulation occurred when feces containing oocysts were incubated for 48 h in seawater at 21 degrees C. Oocysts are elongated (24.8 x 14.7 micron) with a wall about 200 nm thick and have no residuum, micropyle, or polar granule. Sporocysts are ellipsoid (8.5 x 4.5 micron), have a thin two-layered wall approximately 30 nm thick, and consist of two valves joined by a suture. Although moribund opaleye were also infected with Gyrodactylus sp., Cryptobia sp., Cardicola sp., and epitheliocystis organisms (chlamydia), all fish were heavily infected with G. girellae and morbidity was thus attributed to the coccidium.     

Exploitation of host signal transduction pathways and cytoskeletal functions by invasive bacteria

Many bacteria that cause disease have the capacity to enter into and live within eukaryotic cells such as epithelial cells and macrophages. The mechanisms used by these organisms to achieve and maintain this intracellular lifestyle vary considerably, but most mechanisms involve subversion and exploitation of host cell functions. Entry into non-phagocytic cells involves triggering host signal transduction mechanisms to induce rearrangement of the host cytoskeleton, thereby facilitating bacterial uptake. Once inside the host cell, intracellular pathogens either remain within membrane bound inclusions or escape to the cytoplasm. Those living in the cytoplasm can further pirate the host actin system, using actin as a mechanism to facilitate movement within and between host cells. Organisms remaining within the vacuole have specialized mechanisms for intracellular survival and growth which involve additional communication with the host cell. Some of the processes involved in the various steps of facultative intracellular parasitism are discussed in the context of subverting the host cell cytoskeleton and signal transduction pathways for bacterial benefit.     

Dynamics of intracellular granules with CD63-GFP in rat basophilic leukemia cells

CD63 is located on the basophilic granule membranes in resting basophils, mast cells, and platelets, and is also located on the plasma membranes of the cells. We constructed a CD63-GFP (green fluorescent protein) plasmid and introduced it into rat basophilic leukemia (RBL-2H3) cells to observe the movements of CD63 on degranulation. The movements of CD63-GFP were studied in living RBL cells by confocal laser scanning microscopy (CLSM). CD63-GFP, in which GFP was conjugated to the C-terminus of CD63, was located on both the granule membranes and the plasma membranes of RBL cells. The diameter of the fluorescent granules in the cytoplasm varied from 0.5 to 1.5 microm. Before antigen stimulation most granules with CD63-GFP hardly moved in RBL cells. However, after antigen stimulation the plasma membranes ruffled violently and the granules moved dramatically. They reached the plasma membranes in a few minutes and fused with them instantaneously. Analysis of the movement of each granule provided a new insight into the elementary process of degranulation. The velocity of the granule movement toward the plasma membranes on antigen stimulation was calculated to be 0.1+/-0.02 microm/s. This shows that the granules are able to reach the plasma membranes in 2 or 3 min if the diameter of the cells is 20 microm.     

Über Glykoproteideinschlüsse in den Trophoblastzellen der menschlichen Plazenta und die Frage ihres Zusammenhangs mit der Bildung von Gonadotropin

Summary In some of the basophilic trophoblastic cells of the basal plate and islands of the mature human placenta there are glycoprotein inclusions, many of which are within vacuoles. As established by the histochemical results these inclusions are composed of a neutral mucopolysaccharide and a basophilic protein with SH groups. They gradually increase in number from the 4th month of pregnancy and are not influenced by labor. From the first to the fourth month of gestation small vacuoles, initially infra-nuclear but later supranuclear, may be seen in the syncitial cells of the villi; these likewise contain glycoprotein granules. The same inclusions are to be found in some trophoblastic cells of the chorionic plate and of the chorion laeve, and in an occasional epithelial cell of the amnion.On the basis of the histochemical behavior of these inclusions, their similarity with the -granules of the hypophysis, and the obvious high activity of the trophoblastic cells which bear them, it is postulated that these inclusions contain gonadotropin. Their occurrence during pregnancy parallels the biochemical concentration and immunofluorescent histological demonstration of gonadotropin in the placenta. Thus, at the end of pregnancy the basal trophoblastic cells of the human placenta appear to form at least two different hormones: relaxin, probably represented morphologically by the acidophilic protein inclusions (described elsewhere), and gonadotropin most likely represented by the basophilic glycoprotein inclusions.     

A whole range of fine structural criteria for immunohistochemically identified LH cells in rats

Summary Pituitaries from normal, young and adult male rats were fixed either in sublimate-formalin or in glutaraldehyde-osmium. In adjacent Paraplast sections, almost all the gonadotrophs were immunostained with both LH and FSH antisera. The rat LH and FSH antisera used were shown to be highly specific by the absorption test and by double antibody radioimmunoassay. Thin and thick adjacent Epon sections were prepared for EM and immunohistochemical examination. Cells stained with the rat LH antiserum were identified by LM, and then observed in detail by EM. On the basis of these observations we suggest that the LH cells are arranged in a sequence of basophils, i.e., Types II/III, III, III/IV and IV: Type II/III basophils are elongate with a cytoplasmic process and less vesiculated. They have morphological features of Type II (classical thyrotrophs) and also of Type III basophils. Type III basophils are oval in shape and moderately vesiculated. Both Types II/III and III basophils can be divided into two classes of cell characterized mainly by the existence of only small secretory granules (150–220 nm in diameter) (Type A) or by the coexistence of small and large (350–500 nm) (Type B). Type III/IV basophils are cells intermediate between types III and IV basophils, and moderately vesiculated with an abundance of secretory granules (150–300 nm in diameter). Type IV basophils are large, spherical or oval cells whose RER cisternae are conspicuously dilated; they contain less numerous secretory granules (150–300 nm in diameter). It is concluded that LH cells are not a single cell type, but include a wide range of subtypes.     

Ultrastructural investigation on a strain of a cytoplasmic-polyhedrosis virus with nuclear inclusions

A strain of a cytoplasmic-polyhedrosis virus causes the formation of crystalline inclusions almost entirely in the nucleus, and only rarely in the cytoplasm, of the midgut epithelial cells of the silkworm Bombyx mori. It also differs from the typical strain in causing the hypertrophy of the nucleoli and the formation of dense reticulum and spherical bodies in the nucleus. The virus particles and the virogenic stromata of the new strain resemble those of the typical strain. The cytoplasmic inclusions contain virus particles, while the nuclear inclusions do not. When the infected larvae are kept at 30°C for 15 hr or at 35°C for 3–15 hr, the nuclear inclusions break up into particles of 70–250 nm in diameter. The particles are dispersed in the cells but not present in the spaces previously occupied by the decomposed inclusions.     

Histology, ultrastructure, and morphogenesis of a rickettsia-like organism causing disease in the oyster, Crassostrea ariakensis Gould

Moribund specimens of the oyster, Crassostrea ariakensis Gould, aged 2-3 years were collected from Hailing Bay in Yangxi County of Guangdong Province from February to May and November to December in the years 2001, 2002, and 2003. A massive infection by an obligate intracellular prokaryote, specifically a rickettsia-like organism (RLO), was found. Here we report investigations of this RLO in the tissues of the oyster C. ariakensis Gould and describe the histology, ultrastructure, and morphogenesis of this pathogen in C. ariakensis Gould. Light microscopic observations of stained tissues revealed cytoplasmic inclusion bodies typical of prokaryote infection in about 87% (26/30) of the oysters. Most inclusions were observed in epithelial cells and connective tissues of the gill, mantle, and digestive gland of most of the infected oysters. The shape, size, and color of inclusions from different tissues were polymorphic. Electron microscopic examination of digestive gland, gill, and mantle tissues showed that the RLOs were intracytoplasmic. RLOs were often round, dumb-bell-shaped (undergoing binary fission), or occasionally rod-shaped and ranged from approximately 0.58 to 1.20microm in size. The organisms exhibited an ultrastructure characteristic of prokaryotic bacteria-like cells, including a trilaminar cell wall, electron-dense periplasmic ribosome zone, and a DNA nucleoid. Reproductive stages, including transverse binary fission, were observed by TEM. These stages were frequently observed within membrane-bound cytoplasmic vacuoles. Hexagonal phage-like particles in the cytoplasm of RLOs were also observed.     

Fine structure of the early stages of spermatogenesis in the Pacific oyster, Crassostrea gigas (Mollusca, Bivalvia)

The aim of this study is to describe the early stages of spermatogenesis of the Pacific oyster Crassostrea gigas using both light and electron microscopy. The gonad is formed by gonadal tubules invaginated in a connective tissue constituting a storage tissue. Myoepithelial cells surround each gonadal tubule and are associated with an acellular matrix delimiting the outer part of the tubule, the inner part is composed by intragonadal somatic cells associated with germinal lineage. Two types of spermatogonia are identified, where type I spermatogonia (Spg I) are large, scarce and pale cells leaned against the base of the tubule (nuclear diameter: 5.5+/-0.5 microm). Type II spermatogonia (Spg II) are clustered and dark cells which appear smaller than type I (nuclear diameter: 4.3+/-0.3 microm). The aspect of nuage-like material in cytoplasm is described from pale spermatogonia to primary spermatocytes (nuclear diameter: pachytene 3.6+/-0.3 microm, diplotene 3.4+/-0.3 microm), while no structure related to a chromatoid body was observed in oyster spermatocytes and spermatids.     

A rickettsiales-like infection in the Pacific razor clam, Siliqua patula

An intracellular procaryotic rickettsiales-like organism was discovered in the digestive absorptive epithelium of the Pacific razor clam, Siliqua patula, from the Pacific Coast of Washington State. Of 80 animals examined, 30% contained the organisms, and all positive animals were between 1.5 mm and 3.0 cm shell length. The organisms appeared as basophilic inclusions averaging 5.0 μm in diameter. Ultrastructurally, the inclusions were membrane-bound vacuoles containing procaryotic forms which averaged 0.8 μm by up to 1.8 μm in length. These organisms had a double layer of trilaminar membrane which appeared to form blebs and outer membrane vesicles. The organisms were pleomorphic by both histological and electron microscopical examination. The host response to infection appeared to consist only of the rounding and detachment of the infected cells. Specific immunofluorescent tests for Rickettsia rickettsia, R. prowazekii, Coxiella burnetti, Chlamydia psittaci, and Legionella pneumophila were all negative when tested on impression smears of infected clam digestive gland.     

Two novel alphaherpesviruses associated with fatal disseminated infections in Atlantic bottlenose dolphins

Two immature female bottlenose dolphins (Tursiops truncatus) were found stranded on the Atlantic coast of the USA. Necropsy and histopathologic examination of both dolphins demonstrated acute necrotizing lesions in multiple organ systems. Commonly seen in these lesions were cells with enlarged nuclei that contained single 4 to 6 microm diameter homogeneous eosinophilic inclusion bodies that were often surrounded by a clear halo. Ultrastructural examination revealed that intranuclear inclusions contained 90 to 110 nm diameter viral particles with electron-dense cores and hexagonal profiles. Viral particles were also present in the cytoplasm, and these were surrounded by variably electron-dense envelopes. Enveloped virions were 140 nm in diameter. Polymerase chain reactions targeting the DNA polymerase and terminase genes of herpesviruses were carried out on unfixed tissues of both animals, and analysis of the DNA products indicated the presence of two novel alphaherpesviruses. The gross, histologic, ultrastructural, and molecular genetic findings indicate disseminated herpesviral infections, and support the conclusion that the alphaherpesviruses caused the deaths of the two dolphins. This is the first report of disseminated herpesviral infection in cetaceans.     

Morphology of the air-breathing stomach of the catfish Hypostomus plecostomus

Histological and ultrastructural investigations of the stomach of the catfish Hypostomus plecostomus show that its structure is different from that typical of the stomachs of other teleostean fishes: the wall is thin and transparent, while the mucosal layer is smooth and devoid of folds. The epithelium lining the whole internal surface of the stomach consists of several types of cells, the most prominent being flattened respiratory epithelial cells. There are also two types of gastric gland cells, three types of endocrine cells (EC), and basal cells. The epithelial layer is underlain by capillaries of a diameter ranging from 6.1-13.1 microm. Capillaries are more numerous in the anterior part of the stomach, where the mean number of capillary sections per 100 microm of epithelium length is 4, compared with 3 in the posterior part. The cytoplasm of the epithelial cells, apart from its typical organelles, contains electron-dense and lamellar bodies at different stages of maturation, which form the sites of accumulation of surfactant. Small, electron-dense vesicles containing acidic mucopolysaccharides are found in the apical parts of some respiratory epithelial cells. Numerous gastric glands (2 glands per 100 microm of epithelium length), composed of two types of pyramidal cells, extend from the surface epithelium into the subjacent lamina propria. The gland outlets, as well as the apical cytoplasm of the cells are Alcian blue-positive, indicating the presence of acidic mucopolysaccharides. Zymogen granules have not been found, but the apical parts of cells contain vesicles of variable electron density. The cytoplasm of the gastric gland cells also contains numerous electron-dense and lamellar bodies. Gastric gland cells with electron-dense cytoplasm and tubulovesicular system are probably involved in the production of hydrochloric acid. Fixation with tannic acid as well as with ruthenium red revealed a thin layer of phospholipids and glycosaminoglycans covering the entire inner surface of the stomach. In regions of the epithelium where the capillaries are covered by the thin cytoplasmic sheets of the respiratory epithelial cells, a thin air-blood barrier (0.25-2.02 microm) is formed, thus enabling gaseous exchange. Relatively numerous pores closed by diaphragms are seen in the endothelium lining the apical and lateral parts of the capillaries. Between gastric gland cells, solitary, noninnervated endocrine cells (EC) of three types were found. EC are characterized by lighter cytoplasm than the surrounding cells and they contain dense core vesicles (DCV) with a halo between the electron-dense core and the limiting membrane. EC of type I are the most abundant. They are of an open type, reaching the stomach lumen. The round DCV of this type, with a diameter from 92-194 nm, have a centrally located core surrounded by a narrow halo. EC of type II are rarely observed and are of a closed type. They possess two kinds of DCV with a very narrow halo. The majority of them are round, with a diameter ranging from 88-177 nm, while elongated ones, 159-389 nm long, are rare. EC of type III are numerous and also closed. The whole cytoplasm is filled with large DCV: round, with a diameter from 123-283 nm, and oval, 230-371 nm long, both with a core of irregular shape and a wide, irregular halo. EC are involved in the regulation of digestion and probably local gas exchange. In conclusion, the thin-walled stomach of Hypostomus plecostomus, with its rich network of capillaries, has a morphology suggesting it is an efficient organ for air breathing.     

Development and fine structure of the yolk nucleus of previtellogenic oocytes in the medaka Oryzias latipes

The development and fine structure of yolk nuclei in the cytoplasm of previtellogenic oocytes were examined by electron microscopy during several stages of oogenesis in the medaka, Oryzias latipes. Shortly after oogenesis starts, oocytes 20-30 microm in diameter have much electron-dense (basophilic) cytoplasm, within which a continuous or discontinuous, irregular ring-shaped lower electron-dense area of flocculent appearance (LF) begins to emerge around the nucleus. The yolk nucleus is first recognized within an LF area as a few fragments of dense granular thread measuring 20-25 nm in width. The threads consist of two rows of very dense granules resembling ribosomes or ribonucleoprotein (RNP)-like particles in size and electron density. These thread-like fragments gradually increase in number and length until they assemble into a compact, spherical mass of complicated networks. Analysis of serial sections suggests that the yolk nucleus is a complicated mass of numerous, small deformed vacuoles composed of a single lamella with double layers of ribosomes or RNP-like granules, rather than a mass of granular threads. When oocytes develop to greater than 100 microm in diameter, the yolk nucleus begins to fragment before dispersing throughout the surrounding cytoplasm, concomitantly with the disappearance of LF areas. At this stage of oogenesis, a restricted region of the granulosa cell layer adjacent to the yolk nucleus becomes somewhat columnar in morphology, fixing the vegetal pole region of the oocyte.     

Electron Microscopy of an Isometric Caulimo-Like Virus from Sweet Potato (Ipomoea batatas)

A newly recognised virus with isometric particles 50 nm in diameter was detected in Ipomoea batatas (sweet potato) by graft-transmission to I. setosa. Virus particles and intracellular inclusions, although not seen in infected I. batatus, were found in the cytoplasm of most cell types of I. setosa. The caulimo-like particles were most abundant with vesicles immediately adjacent to inclusions. The ovoid or spherical inclusions, which differ markedly from those of caulimovi, ruses, have a large central lacuna and usually several smaller peripheral lacunae. Infected vascular parenchyma cells sometimes protrude into, and occasionally completely occlude, adjacent xylem vessels, observations possibly explaining the sudden wilting and premature senescence of infected leaves.     

Metal selectivity of in situ microcolonies in biofilms of the Elbe river.

The ultrastructure of natural complex biofilm communities of the Elbe river grown in situ on microscopic glass coverslips was studied by using transmission electron microscopy and energy-dispersive x-ray (EDX) analysis. Characteristic microcolonies which measured between 3.3 and 9.3 microm in diameter were frequently observed. They had an outer envelope and harbored 6 to 30 cells. The cells formed short rods measuring 1.09 +/- 0.28 microm (n = 10) in length and 0.55 + 0.07 microm (n = 21) in width. They were surrounded by a thick layer of electron-transparent, nonosmicated matter, 120 to 300 nm thick. Individual cells exhibited a unique ultrastructural trait, namely, a concentric membrane stack which completely surrounded the cytoplasm. It consisted of three membrane doublets, which showed an overall thickness of 57 to 66 nm. The center-to-center spacing between two membrane doublets was 22.2 +/- 1.0 nm (n = 12). The bacterial cell wall seemed to be of the gram-negative type. The fact that upon shrinkage hexagonal clefts appeared proved the cells to be tightly packed, and septum formation by binary fissions was observed. All of these morphological details indicate that the cells within these microcolonies were actively growing and did not represent spore-like states. EDX analysis showed that only the electron-dense surface deposit of the microcolonies contained Mn and Fe in significant amounts, while these two elements were absent from the intercellular space and the cytoplasm of the microorganisms. In contrast, aluminum ions were able to penetrate the outer envelope of the microcolonies and were detected in the intercellular space. They were, however, completely absent from the microbial cytoplasm, indicating a filter cascade with respect to aluminum. From the ultrastructural data together with the deposition of iron and manganese on the microcolony surface, it appears that these organisms may belong to the genus Siderocapsa or Nitrosomonas. They do not precisely match any of the described species and may therefore represent a new species.