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Batteri Gram-positivi: Bacilli e Mycoplasma
Il phylum Tenericutes, che comprende la sola classe Mollicutes, include batteri privi di parete cellulare. Il termine Mollicutes deriva dal latino mol…
Il phylum Tenericutes, comunemente noto come micoplasma, comprende un'unica classe, i Mollicutes. Questi batteri mancano di pareti cellulari e sono tra i più piccoli organismi conosciuti.
I micoplasmi vivono spesso in stretta associazione con ospiti animali o vegetali in ambienti diversi.
Nonostante la mancanza di pareti cellulari, le membrane dei micoplasmi sono resistenti alla lisi osmotica in parte a causa della presenza di steroli e lipoglicani.
I micoplasmi mostrano forme pleomorfe, con colture che spesso mostrano piccole cellule coccoidi, grandi cellule gonfie e forme filamentose che possono essere ramificate.
Il genere Spiroplasma è elicoidale o a forma di spirale. Manca di pareti cellulari e flagelli, ma mostra motilità attraverso movimenti a vite o ondulazioni lente.
Le specie di Spiroplasma si trovano in vari luoghi, dalle superfici esterne di insetti e piante ai loro tessuti interni.
Una specie notevole, lo Spiroplasma citri, causa la malattia ostinata degli agrumi nelle piante di agrumi.
Altre specie, come il Mycoplasma pneumoniae, l'Ureaplasma urealyticum e il Metamycoplasma hominis, sono patogene per l'uomo.
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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.