Planctomycetes are a group of morphologically distinct bacteria predominantly classified into two orders: Planctomycetales and Brocadiales. These gram-negative bacteria exhibit unique features, including division by budding and the presence of stalks or appendages. Their cells are often found in rosette arrangements, and they are notable for possessing an S-layer in their cell envelope, which is relatively uncommon among bacteria. Additionally, Planctomycetes frequently exhibit intracellular compartments reminiscent of eukaryotic organelles.
While prokaryotes and eukaryotes typically differ significantly in their cellular structures, Planctomycetes challenge this distinction by demonstrating cellular compartmentalization. Eukaryotic cells have a membrane-enclosed nucleus, whereas prokaryotic DNA is compacted into a nucleoid within the cytoplasm. However, Planctomycetes produce specialized structures known as pirellulosomes, which are non unit membrane-enclosed compartments containing the nucleoid, ribosomes, and other cytoplasmic components.
The life cycle of Planctomyces involves budding. Motile swarmer cells attach to a surface, develop a stalk at the attachment site, and produce a new cell at the opposite pole through budding. This daughter cell then develops a flagellum, detaches from the mother cell, and initiates a new cycle. Physiologically, Planctomyces species are facultative aerobes capable of growing through either respiration or fermentation of sugars. They inhabit various aquatic environments, including freshwater and marine ecosystems. The genus Isosphaera, a relative of Planctomyces, is a filamentous, gliding bacterium found in hot springs. Isolation of Planctomyces and its relatives typically requires low-nutrient media.
In certain genera, such as Gemmata, the nucleoid is further enclosed by invaginations of the cell membrane, forming a structure surrounded by a true unit membrane. Despite this structural complexity, the DNA remains in a covalently closed, circular, and supercoiled state typical of bacterial genomes.
An additional noteworthy compartment, the anammoxosome, is found in members of the order Brocadiales, such as Brocadia anammoxidans. This structure facilitates the anaerobic oxidation of ammonia and is protected by a unique lipid membrane that prevents damage from toxic intermediates generated during this metabolic process.
Planctomyces is one of the most thoroughly studied genera within the phylum. Similar to the stalked bacterium Caulobacter, Planctomyces features a stalk, but its structure is distinctly composed of protein without any cell wall or cytoplasm. The stalk primarily functions as an attachment mechanism and is more slender than the prosthecal stalk of Caulobacter.
Planctomycetes are classified into two orders — Planctomycetales and Brocadiales.
These Gram-negative bacteria exhibit unique features. They divide by budding. Some have stalks or appendages, while others form rosette-like cell arrangements. Certain species possess an S-layer in their cell envelope.
All Planctomycetes show intracellular compartments resembling eukaryotic organelles.
They produce specialized structures known as pirellulosomes, which are non-unit membrane-enclosed compartments containing the nucleoid, ribosomes, and other cytoplasmic components.
In the genus Gemmata, the nucleoid is further enclosed by invaginations of the cell membrane, forming a structure surrounded by a true unit membrane.
The genus Isosphaera is a filamentous, gliding bacterium found in hot springs.
An additional noteworthy compartment, the anammoxosome, is found in members of the order Brocadiales, such as Brocadia anammoxidans.
The anammoxosome facilitates the anaerobic oxidation of ammonia — a form of chemolithoautotrophy. The toxic hydrazine intermediate of this reaction is contained within its unique lipid membrane, protecting cytoplasmic components.
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