Segmented filamentous bacteria associated with lymphoid tissues in the ileum of horses

Multifocal granulomatous jejunitis associated with an argyrophilic gram-positive segmented filamentous bacterium in a Holstein cow

Multifocal, raised, ulcerated firm nodules accompanied by an intussuscepted area were detected in the jejunum of an 8-year-old Holstein cow. The cut surfaces of the nodules were yellow-white. Microscopically, the lamina propria was expanded by an intense infiltration of epithelioid cells, multinucleate giant cells, macrophages, lymphocytes and neutrophils. Numerous bacteria were found within the granulomatous lesions. These were argyrophilic, gram-positive, periodic acid-Schiff-positive, segmented, rarely branched, elongate filamentous bacteria (2-28 μm in length, 0.2-0.35 μm in diameter). Ultrastructurally, a cell wall with an electron-transparent zone was detected. The present pathogen was clearly different from the argyrophilic, gram-negative, non-segmented, filamentous bacterium previously reported in a Holstein cow with jejunal granuloma. Comparative 16S rDNA gene sequencing analysis revealed that the organism was an unpublished species (GenBank accession number AB539875). This is the first report of bovine jejunal granuloma associated with an argyrophilic gram-positive segmented filamentous bacterium.

A comparative molecular study of the presence of "Candidatus arthromitus" in the digestive system of rainbow trout, Oncorhynchus mykiss (Walbaum), healthy and affected with rainbow trout gastroenteritis

Observations were made using histopathological techniques in conjunction with a nested polymerase chain reaction (PCR) protocol for the specific detection of "Candidatus arthromitus" on DNA extracted from wax-embedded tissues and fresh digestive contents of rainbow trout. Samples positive for "Candidatus arthromitus" DNA included fish with rainbow trout gastroenteritis (RTGE), clinically normal cohabiting fish, and apparently healthy controls from RTGE positive and RTGE negative sites. The results obtained from the PCR were confirmed by nucleotide sequencing. "Candidatus arthromitus" DNA was found in distal intestine as well as in sections of pyloric caeca, suggesting that both these locations are appropriate for molecular detection of "Candidatus arthromitus" DNA in trout. Furthermore, rainbow trout fry distal intestinal samples from two different hatcheries where RTGE had not been reported were also positive. Differences in "Candidatus arthromitus" DNA detection between paraffin wax-embedded and fresh digestive content samples from the same fish suggested that it may be predominantly epithelium-associated in healthy trout. Parallel histopathological observations indicated that pyloric caeca are the preferred site for visualizing segmented filamentous bacteria (SFB) in trout with RTGE. The results of this study showed that the presence of SFB was not invariably associated with clinical disease and that more information is required to understand the role of these organisms.

Histopathology and ultrastructure of segmented filamentous bacteria-associated rainbow trout gastroenteritis

Rainbow trout gastroenteritis (RTGE) is an emerging syndrome linked to the presence of large numbers of the segmented filamentous bacterium "Candidatus arthromitus" within the intestine. The present study examined the histopathological changes of the digestive tract of 152 trout with gross lesions typical of RTGE. Histopathology showed that 129 of 152 fish (85%) affected with RTGE had segmented filamentous bacteria in the distal intestine and/or pyloric caeca. The presence and number of segmented filamentous bacteria were always significantly higher (P < .001) in pyloric caeca, thereby suggesting the preferred site for these bacteria. Histopathological changes included enterocyte detachment and congestion of the lamina propria and adventitial layers. Samples from 6 RTGE-affected trout were examined using scanning and transmission electron microscopy, revealing a close interaction of segmented filamentous bacteria with the mucosa of distal intestine and pyloric caeca, with the presence of bacterial attachment sites, and with associated morphological changes of the apical membrane of enterocytes. Despite these interactions, segmented filamentous bacteria were not always adjacent to the areas with pathological changes, suggesting that if these organisms play a role in the pathogenesis of RTGE, extracellular products may be involved. Ultrastructural changes included loss of microvillar structure, membrane blebbing, hydropic mitochondrial damage, and basal hydropic degeneration of enterocytes, which frequently resulted in disruption of tight junctions and enterocyte detachment. The resulting exposure of large areas of lamina propria probably resulted in the compromise of the host osmotic balance and the facilitation of the entry of secondary pathogens.

Segmented filamentous bacteria interact with intraepithelial mononuclear cells

Segmented filamentous bacteria (SFB) are found in multiple species and play an important role in the development of mucosal immunity. The mechanism by which the bacteria interact with the immune system has not been well defined. We provide morphologic evidence of direct interaction between SFB and intraepithelial mononuclear cells.

Identification by in situ hybridization of segmented filamentous bacteria in the intestine of diarrheic rainbow trout (Oncorhynchus mykiss)

Nonculturable segmented filamentous bacteria (SFB) have been described in the gut of rats, mice and chickens, and 16S rRNA sequences for these organisms are available. These organisms, peripherically related to Clostridium phylogenetic group I, have been provisionally named 'Candidatus Arthromitus'. This work reports the observation of similar bacteria in the intestinal content of the distal intestine, preferentially, in the adult rainbow trout (Oncorhynchus mykiss) that exhibited episodic acute diarrhea, usually during the summer. Abdominal distension, intestinal fluid-mucus content and epithelium detachment were observed in trout. The demonstration that the observed microorganisms are bacteria and belong in the 'Candidatus Arthromitus' group was achieved by in situ hybridization with, respectively, a eubacterial probe and an oligonucleotide probe designed to react specifically with SFB 16S rRNA (encoded by the rrs gene) sequences. The sequenced rrs gene was compared with published sequences and found to be closely related to (although distinct from) other SFB sequences. Implication of these bacteria in trout diarrheic illness remains hypothetical.

Transmission electron microscopic demonstration of phagocytosis and intracellular processing of segmented filamentous bacteria by intestinal epithelial cells of the chick ileum

Segmented filamentous bacteria (SFB) are autochthonous bacteria colonizing the ileum of many young animals by attaching to intestinal epithelial cells. These nonpathogenic bacteria strongly stimulate the mucosal immune system and induce intestinal epithelial cells to express major histocompatibility complex class II molecules. We tried to discover whether SFB are phagocytized and intracellularly processed by the host cells, which is indicative of antigen processing. The middle part of the ileum was extracted from 10- and 20-day-old broiler chicks (Gallus gallus domesticus). Samples were processed and examined by scanning and transmission electron microscopy (SEM and TEM, respectively). In SEM, no, few, medium, and dense SFB colonization levels were classified. In TEM of cells from animals with medium or dense SFB colonization levels, we could observe extracellular particles ranging from those only indenting the cell membrane to particles found in the cytoplasmatic area beyond the terminal web. These particles had a structural similarity with SFB that were floating freely in the intestinal lumen. Furthermore, we observed unlacing of the membrane and septum surrounding the extracellular particles and their incorporation into host cytoplasmatic components, which strongly suggests that these particles are phagocytized and intracellularly processed SFB. This conclusion is supported by TEM analysis of samples with no or few SFB, in which we failed to find these characteristic morphologies. The phagocytosis process described here could be an important trigger for the stimulating effect of SFB on the mucosal immune system.

Segmented filamentous bacteria in the bovine small intestine

Segmented filamentous bacteria (SFB) were observed in a 28-day-old calf, attached to the absorptive villi. Morphologically, they were similar to SFB described in other animal species. Because these organisms cannot be cultured, further characterization was not possible. The organisms were confined to the upper third of the absorptive villi and were not seen attached to the follicle-associated epithelium of the Peyer's patch, or observed in the caecum or colon. Although they were often associated with minor lesions, their pathological significance was doubtful. With this report, segmented filamentous bacteria have now been described in virtually all the commercially important livestock and poultry species, in other domestic animals, and in man.