Do mycobacteria produce endospores?
Endospores are unique among bacterial spores in that they are produced inside of another cell (the mother cell) and, upon maturation, are released as free spores by lysis of the mother cell. They are readily recognized under phase-contrast microscopy by their phase bright (refractile) appearance. They also exhibit diagnostic features under electron microscopy, such as a protein shell consisting of an inner coat and an electron dense, outer coat. Endospores are composed of numerous molecules found, thus far, only in bacterial endospores. These molecules include most of the proteins that encase the spore in a protective shell (called the coat), a family of DNA-protective proteins known as SASP that are bound to the chromosome, and a unique small molecule, dipicolinic acid. All previously known examples of endospore-forming bacteria are members of the low G+C group of Gram-positive bacteria (Firmicutes) belonging either to Bacilli or to Clostridia, and in all cases in which a genome sequence is available, orthologs of genes involved in endospore formation are readily seen. The Mycobacterium genus is a member of the high G+C group of Gram-positive bacteria (Actinobacteria) for which there are no prior claims of endospore formation. Certain members of the group, such as Streptomyces, do produce spores, but spores of a fundamentally different kind that are not produced inside a mother cell.
A recent publication in PNAS reported that M. marinum and M. bovis bacillus Calmette–Guérin produce a type of spore known as an endospore, which had been observed only in the low G+C group of Gram-positive bacteria. Evidence was presented that the spores were similar to endospores in ultrastructure, in heat resistance and in the presence of dipicolinic acid. This paper reports that the genomes of Mycobacterium species and those of other high G+C Gram-positive bacteria lack orthologs of many, if not all, highly conserved genes diagnostic of endospore formation in the genomes of low G+C Gram-positive bacteria. It also failed to detect the presence of endospores by light microscopy or by testing for heat-resistant colony-forming units in aged cultures of M. marinum. Finally, we failed to recover heat-resistant colony-forming units from frogs chronically infected with M. marinum. It concludes that it is unlikely that Mycobacterium is capable of endospore formation.
Do mycobacteria produce endospores? PNAS USA December 22 2009 doi: 10.1073/pnas.0911299107
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Tags: Bacteria, Biology, Health, Medicine, Microbiology, Science, Tuberculosis
