In the guts of more than 25 species of arthropods we observed filaments containing refractile inclusions previously discovered and named ''Arthromitus'' in 1849 by Joseph Leidy [Leidy, J. (1849) Proc. Acad. Nat. Sci. Philadelphia 4, 225-233]. We cultivated these microbes from boiled intestines of 10 different species of surface-cleaned soil insects and isopod crustaceans. Literature review and these observations lead us to conclude that Arthromitus are spore-forming, variably motile, cultivable bacilli. As long rod-shaped bacteria, they lose their flagella, attach by fibers or fuzz to the intestinal epithelium, grow filamentously, and sporulate from their distal ends. When these organisms are incubated in culture, their life history stages are accelerated by light and inhibited by anoxia. Characterization of new Arthromitus isolates from digestive tracts of common sow bugs (Porcellio scaber), roaches (Gromphodorhina portentosa, Blaberus giganteus) and termites (Cryptotermes brevis, Kalotermes flavicollis) identifies these flagellated, spore-forming symbionts as a Bacillus sp. Complete sequencing of the 16S rRNA gene from four isolates (two sow bug, one hissing roach, one death's head roach) confirms these as the low-G؉C Gram-positive eubacterium Bacillus cereus. We suggest that B. cereus and its close relatives, easily isolated from soil and grown on nutrient agar, enjoy filamentous growth in moist nutrient-rich intestines of healthy arthropods and similar habitats.

An endospore-forming rod-shaped filamentous bacterium was taken from boiled intestines of common sow bugs (Porcellio scaber, isopod crustaceans). The bacteria were grown on peptone – yeast extract medium. As many as 180 cells per filament were counted in culture; filament length was a function of time after germination and oxic conditions. Cultures continued to grow filamentously after 10 successive transfers. The development of spores was inhibited by strict anaerobiosis for 3 months. Spore-forming filaments over 100 μm long in fresh intestinal material were observed only in guts taken from sow bugs cultivated in darkness. Phenotypic tests presented here show this isolate to be a member of the genus Bacillus, most closely resembling B. cereus.Key words: Arthromitus, motile bacilli, isopod, sow bug, intestinal filaments.