Bacteria; Firmicutes; Clostridia; Clostridiales; Metabacterium
Description and Significance
Metabacterium polyspora (size 15-20μm) is an exceptionally large bacteria.  It is one of the closest known relatives of the largest bacteria, Epulopiscium spp.  M. polyspora is found in the upper digestive tract of guinea pigs.  M. polyspora is unique because it produces multiple viviparous offspring. M. polyspora produces multiple endospores and rarely reproduces by binary fission. The various stages of development of the endospores take place over the course of passage through the guinea pig gastrointestinal tract. 
Structure, Metabolism, and Life Cycle
Metabacterium polyspora is a gram positive bacteria that reproduces in a unique way within its guinea pig host . Guinea pigs are coprophagous, so they ingest their feces which contains M. polyspora endospores. The mature endospores are the only ones that make it through the mouth of the guinea pig and into the gastrointestinal tract. All of the endospores germinate in the small intestine.  Rather than dividing by cell binary fission, the M. polyspora usually form forespores at both poles of the cell. The forespores multiply within the mother cell to produce multiple endospores. These endospores are then released into the colon and excreted with the guinea pig defecation.   The life cycle continues when the guinea pigs ingest the feces again.
Ecology and Pathogenesis
Metabacterium polyspora is found within the gastrointestinal tract of its guinea pig host. M. polyspora shares in a symbiotic relationship with guinea pigs. The coprophagous guinea pig allows for M. polyspora spores to continue to be generated within the gastrointestinal tract. 
 Angert, Esther R., Austin E. Brooks, and Norman R. Pace. 1996. Phylogenetic analysis of Metabacterium polyspora: clues to the evolutionary origin of daughter cell production in Epulopiscium species, the largest bacteria. Journal of bacteriology 178.51451-1456.
 Angert, Esther R., and Richard M. Losick. 1998. Propagation by sporulation in the guinea pig symbiont Metabacterium polyspora. Proceedings of the National Academy of Sciences 95.17: 10218-10223.
 Slonczewski, Joan L., and John W. Foster. 2009. Microbiology: An Evolving Science. Canada: W.W. Norton & Company, Inc. 687 p.
Page authored by Alyssa Schroeder, student of Mandy Brosnahan, Instructor at the University of Minnesota-Twin Cities, MICB 3301/3303: Biology of Microorganisms.