Metabacterium polyspora is a gastrointestinal (GI) symbiont of the guinea pig . These large bacteria (12 to 35 µm long) have the uncommon ability to produce as many as nine, phase-bright endospores per mother cell. Phylogenetic studies based on small subunit rRNA sequence comparisons have shown that M. polyspora is a low G+C Gram-positive bacterium and one of the closest known relatives of Epulopiscium spp. . For M. polyspora, sporulation is part of the normal life cycle, and its ability to form multiple endospores is an important means of reproduction.
Endospore development in M. polyspora is coordinated with passage of the bacterium through the gastrointestinal tract of its host . Only mature endospores survive passage through the mouth and stomach of the guinea pig. The spores then germinate in the small intestine. For a brief period in the life cycle of M. polyspora, just after spore germination, a cell may undergo binary fission. Although some cells do divide, many cells in a population appear to bypass this opportunity to reproduce by binary fission and instead immediately begin to form endospores. From the small intestine, M. polyspora pass into the cecum. The guinea pig cecum is a large blind sac at the junction between small and large intestine. The cecum is an important organ in the guinea pig digestive system. It harbors a variety of microorganisms that assist in the breakdown of food. In the cecum, M. polyspora endospores develop further. Eventually M. polyspora cells pass out of the cecum, through the colon and out of the guinea pig.
Like many herbivores that eat high roughage diets, guinea pigs are coprophagous. Recycling feces allows the guinea pig a second chance at extracting nutrients from its feed. It also provides M. polyspora a means of reentering in the GI tract. As a dormant and resilient endospore, M. polyspora can survive the harsh world outside the guinea pig and survive passage through the upper GI tract. Most endospore-forming bacteria produce a single endospore. Simple modifications of the core sporulation program allow for the formation of more than one endospore per mother cell in M. polyspora . At the start of sporulation, M. polyspora divides at both poles. Both forespores are engulfed by the mother cell and gain the capacity to undergo division. Through the combination of bipolar division and division of engulfed forespores, M. polyspora produces multiple forespores, which grow and mature into multiple phase-bright endospores. These spores share characteristics with other endospores including a mineralized core containing dipicolinic acid, a cortex and thick coat.
The life cycle of M. polyspora. Binary fission occurs infrequently. Instead, multiple endospores are formed by division at both cell poles, followed by fission of engulfed forespores.
The M. polyspora life cycle provides a novel paradigm for the role of endospore formation in the life of a bacterium. For most endospore-forming bacteria sporulation is a developmental program, called upon in times of stress, which allows the cell to become dormant and preserve its genetic material. For M. polyspora, sporulation is an important means of reproduction, which may help reinforce the symbiotic association of M. polyspora with its host.
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