Procaryotic Domains

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Procaryotic Domains

Single cells or simple associations of similar cells

(usually 0.210.0 m in smallest dimension,

although some are much larger) forming a group

defined by cellular, not organismal, properties(i.e., by the structure and components of the cells

of an organism rather than by the properties of

the organism as a whole)


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The nucleoplasm (genophore) is, with a few exceptions, not separated

from the cytoplasm by a unit-membrane system (nuclear membrane).

Cell division is not accompanied by cyclical changes in the texture or staining properties ofeither nucleoplasm or cytoplasm; a microtubular (spindle) system is not formed.

The plasma membrane (cytoplasmic membrane) is frequently complex in topology andforms vesicular, lamellar, or tubular intrusions into the cytoplasm; vacuoles and replicatingcytoplasmic organelles independent of the plasma membrane system (chlorobium vesicles,gas vacuoles) are relatively rare and are enclosed by nonunit membranes.

Respiratory and photosynthetic functions are associated with the plasma-membranesystem in those members possessing these physiological attributes, although in thecyanobacteria there may be an independence of plasma and thylakoid membranes.

Ribosomes of the 70S type (except for one groupthe Archaeawith slightly higher Svalues) are dispersed in the cytoplasm; an endoplasmic reticulum with attached ribosomesis not present.

The cytoplasm is immobile; cytoplasmic streaming, pseudopodial movement, endocytosis,and exocytosis are not observed. Nutrients are acquired in molecular form.

Enclosure of the cell by a rigid wall is common but not universal. The cell may benonmotile or may exhibit swimming motility (mediated by flagella of bacterial type) orgliding motility on surfaces.


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In organismal terms, these ubiquitousinhabitants of moist environments are

predominantly unicellular microorganisms, butfilamentous, mycelial, or colonial forms alsooccur.

Differentiation is limited in scope (holdfaststructures, resting cells, and modifications in cellshape).

Mechanisms of gene transfer and recombinationoccur, but these processes never involvgametogenesis and zygote formation.


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Although procaryotic organisms can usually be readily differentiated

from eucaryotic microorganisms, in some instances it may be difficult,especially with procaryotes that exhibit some attributes similar to thoseof microscopic eucaryotes.

For instance,

The hyphae formed by actinomycetes might be confused with thehyphae formed by molds;

A fascicle of bacterial flagella could give the misleading impression ofbeing a single eucaryotic flagellum;

The ability of spirochetes to twist and contort their shape is suggestive

of the flexibility exhibited by certain protozoa;

some eucaryotic cells are as small as bacteria, and some bacteria areas large as eucaryotic cells (see footnote to Table 1).

,


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The most reliable approach is probably the demonstrationof the absence of a nuclear membrane in procaryotes.

Other procaryotic cell features range from those that arerelatively easy to determine to the molecular characteristicsthat require sophisticated methods.

Fluorescent- labeled gene probes that can easily

distinguish between procaryotic and eucaryotic cells havebeen developed.

Some characteristics that may help to differentiate betweenprocaryotes and eucaryotes are listed in Table 1.


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ARCHAEA VS. BACTERIA

The two fundamentally different groups (domains) thatcomprise the procaryotes are the Bacteriaand the Archaea.

Recent phylogenetic analyses of the Bacteria, Archaea, andEucaryausing conserved protein sequences have shownthat the majority of trees support a closer relationshipbetween the Archaeaand Eucaryathan between Archaea

and Bacteria.

Although the possibility of interdomain horizontal genetransfer complicates this picture, the apparent ArchaeaEucaryasisterhood raises interesting questions about the

phylogenetic relationships between procaryotes andeucaryotes and the root of the universal tree of life.

Table 2 provides some characteristics differentiating thesetwo procaryotic groups. A general description of each groupfollows.


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Bacteria

For practical purposes the Bacteriamay be divided into three phenotypicsubgroups:

(1) those that are Gram-negative and have a cell wall,

(2) those that are Gram-positive and have a cell wall, and

(3) those that lack any cell wall.

Gram-negative Bacteria that have a cell wall

These have a Gram-negative type of cell-wall profile consisting of an outermembrane and an inner, relatively thin peptidoglycan layer (which contains muramicacid and is present in all but a few organisms that have lost this portion of wall; seefootnote to Table 1) as well as a variable complement of other components outsideor between these layers.

They usually stain Gram-negative, although the presence of a thickexopolysaccharide layer around the outer membrane may result in a Gram-positivestaining reaction, as seen in the cyst-like forms of some Azospirillumspecies.

Cell shapes may be spheres, ovals, straight or curved rods, helices, or filaments;some of these forms may be sheathed or capsulated. Reproduction is by binaryfission, but some groups show budding, and a rare group (Subsection II of thecyanobacteria) shows multiple fission.

Fruiting bodies and myxospores may be formed by the myxobacteria. Swimmingmotility, gliding motility, and nonmotility are commonly observed. Members may be

phototrophic or nonphototrophic (both lithotrophic and heterotrophic) bacteria andinclude aerobic, anaerobic, facultatively anaerobic, and microaerophilic species;


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Gram-positive Bacteria that have a cell wall

These Bacteriahave a cell-wall profile of the Gram-positive type; there is no outermembrane and the peptidoglycan layer is relatively thick.

Some members of the group have teichoic acids and/or neutral polysaccharides ascomponents of the wall. A few members of the group have cell walls that containmycolic acids.

Reaction with Grams stain is generally, but not always, positive; exceptions suchas Butyrivibrio, which has an unusually thin wall and stains Gram-negative, mayoccur. Cells may be spheres, rods, or filaments; the rods and filaments may be

nonbranching, but many show true branching.

Cellular reproduction is generally by binary fission; some produce spores as restingforms (endospores or spores on hyphae). The members of this division includesimple asporogenous and sporogenous bacteria, as well as the actinomycetes andtheir relatives.

Gram-positive Bacteriaare generally chemosynthetic heterotrophs and includeaerobic, anaerobic, facultatively anaerobic, and microaerophilic species;somemembers are obligate intracellular parasites.

Only one group, the heliobacteria, is photosynthetic and although these have a

Gram-positive type of cell wall they nevertheless stain Gram- negative.


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Bacteria lacking a cell wall

These Bacteriaare commonl called the mycoplasmas. They do not synthesize the precursors ofpeptidoglycan and are insensitive to b-lactam antibiotics or other antibiotics that inhibit cell wall synthesis.

They are enclosed by a unit membrane, the plasma membrane. The cells are highly pleomorphic and range insize from large deformable vesicles to very small (0.2 lm), filterable elements. Filamentous forms withbranching projections are common.

Reproduction may be by budding, fragmentation, and/or binary fission. Some groups show a degree ofregularity of form due to the placing of internal structures.

Usually, they are nonmotile, but some species show a form of gliding motility. No resting forms are known.Cells stain Gram-negative. Most require complex media for growth (high-osmotic-pressure surroundings) and

tend to penetrate the surface of solid media forming characteristic fried egg colonies.

The organisms resemble the naked L-forms that can be generated from many species of bacteria (notablyGrampositive Bacteria) but differ in that the mycoplasmas are unable to revert and make cell walls. Mostspecies are further distinguished by requiring both cholesterol and long-chain fatty acids for growth;unesterified cholesterol is a unique compo

nent of the membranes of both sterol-requiring and nonrequiring species if present in the medium. The mol%

G C content of rRNA is 4348 (lower than the 5054 mol% of walled Gramnegative and Gram-positiveBacteria); the mol% G C content of the DNA is also relatively low, 2346, and the genome size of themycoplasmas at 0.51.0 109 Da is less than that of other procaryotes.

The mycoplasmas may be saprophytic, parasitic, or pathogenic, and the pathogens cause diseases ofanimals, plants, and tissue cultures.

Procaryotic Domains

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