Abstract
The vegetative cell of "Metabacterium polyspora" is "cucumber-shaped", about 21 x 5.7 microns, Gram-negative. Cylindrical endospores are best stained by Rakette and Ziehl-Neelsen staining. The bacterium reproduces by sporulation (2 to 8 endospores per cell) and by binary fission. Lateral, bow-like "hatching" of the endospores was seen. About 52% of guinea pigs harbor 5 x 10(6), 36% below 2 x 10(6) and 1% more than 1 x 10(8) "M. polyspora" in 1 g of caecal content. Dipicolinic acid was demonstrated using HPLC in the caecum homogenate from a guinea pig. The amount of it was proportional to the number of spores. Cultivation under strict anaerobic conditions did not succeed. It was possible to cultivate this giant endosymbiont in vitro in a heteroxenic culture incubated in a 5% CO2 atmosphere using liquid medium supplemented with cell-free filtrate of the caecum. The caecum filtrate containing undefined growth factor(s) is necessary for long-term culture. The replication rate was low. These findings suggest that the giant endosymbiont "M. polyspora" is a spore-forming prokaryote without the attributes of a strict anaerobe.