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Gram-positive Bakterien: Bazillen und Mykoplasmen
Der Stamm Tenericutes, der die einzige Klasse Mollicutes umfasst, besteht aus Bakterien, die keine Zellwand besitzen. Der Begriff „Mollicutes“ stammt…
Der Stamm Tenericutes — allgemein bekannt als Mykoplasmen — umfasst eine einzige Klasse, Mollicutes. Diese Bakterien haben keine Zellwände und gehören zu den kleinsten bekannten Organismen.
Mykoplasmen leben oft in enger Verbindung mit tierischen oder pflanzlichen Wirten in unterschiedlichen Umgebungen.
Trotz des Fehlens von Zellwänden sind Mykoplasmenmembranen resistent gegen osmotische Lyse, teilweise aufgrund des Vorhandenseins von Sterolen und Lipoglykanen.
Mykoplasmen weisen pleomorphe Formen auf, wobei Kulturen oft kleine kokzoide Zellen, große geschwollene Zellen und filamentöse Formen aufweisen, die verzweigt sein können.
Gattung Spiroplasma ist spiralförmig. Es fehlen Zellwände und Flagellen, es zeigt aber Beweglichkeit durch schraubenartige Bewegungen oder langsame Wellen.
Spiroplasma-Spezies kommen an verschiedenen Stellen vor, von der Außenseite von Insekten und Pflanzen bis hin zu ihrem inneren Gewebe.
Eine bemerkenswerte Art, Spiroplasma citri, verursacht bei Zitruspflanzen die hartnäckige Zitruskrankheit.
Andere Spezies, wie z. B. Mycoplasma pneumoniae, Ureaplasma urealyticum, und Metamycoplasma hominis, sind für den Menschen pathogen.
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Q1: Why do mycoplasmas lack cell walls despite being bacteria?
Mycoplasmas are members of the phylum Tenericutes, class Mollicutes, and have lost their cell walls entirely. This absence of peptidoglycan, confirmed through electron microscopy, distinguishes them from typical bacteria. The loss may provide advantages in their obligate symbiotic lifestyle with animal or plant hosts, allowing closer association with host cells.
Q2: How do mycoplasmas survive without cell walls?
Despite lacking cell walls, mycoplasmas resist osmotic lysis through sterols and lipoglycans embedded in their cytoplasmic membrane. Sterols stabilize the membrane, and many species require sterols in growth media for survival. Lipoglycans, long-chain heteropolysaccharides covalently bound to membrane lipids, further enhance membrane stability and facilitate attachment to host cell receptors.
Q3: What are the different morphological forms of mycoplasmas?
Mycoplasmas exhibit pleomorphic forms, appearing as small coccoid cells (0.2–0.3 μm), large swollen cells, and filamentous branched forms. In agar cultures, they form characteristic fried egg colonies with a dense central core embedded in the medium and a lighter surrounding area. This morphological diversity reflects their flexible cell structure.
Q4: How does Spiroplasma move without flagella?
Spiroplasma, a helical or spiral-shaped genus within Mollicutes, lacks both cell walls and flagella yet exhibits motility through rotary screw-like motion or slow undulations. This movement is believed to involve intracellular fibrils that enable locomotion. Spiroplasma species inhabit diverse environments, from insect hemolymph and plant vascular fluids to flower and leaf surfaces.
Q5: What diseases do pathogenic mycoplasmas cause?
Several mycoplasma species are pathogenic to humans and plants. Spiroplasma citri causes citrus stubborn disease and corn stunt disease in plants. Human pathogens include Mycoplasma pneumoniae, Ureaplasma urealyticum, and Metamycoplasma hominis. Some Spiroplasma species also cause insect diseases such as honeybee spiroplasmosis.
Q6: What are the genome characteristics of mycoplasmas?
Mycoplasmas possess small genomes ranging from 0.6 to 2.2 megabase pairs, a feature typical of obligate symbionts. Their reduced genome size reflects their dependence on host cells for survival. Most species require complex media containing fresh serum or ascitic fluid to provide essential sterols, unsaturated fatty acids, vitamins, amino acids, and nucleotide precursors.
Q7: How are mycoplasmas classified within bacterial phyla?
Mycoplasmas belong to the phylum Tenericutes, which contains a single class, Mollicutes, derived from Latin mollis meaning soft. Though they lack gram-positive cell walls, mycoplasmas are phylogenetically related to the bacterial phylum Firmicutes. Understanding their classification requires methods of classification and identification that account for their unique structural absence and molecular relationships.