The primary distinguishing characteristics of the prokaryotes are their relatively small size, usually on the order of 1 µm in diameter, and the absence of a nuclear membrane. The DNA of almost all bacteria is a circle which if extended linearly would be about 1 mM; this is the prokaryotic chromosome. Bacteria are haploid (if multiple copies of the chromosome are present they are all the same). Most prokaryotes have only a single large chromosome that is organized into a structure known as a nucleoid. The chromosomal DNA must be folded more than 1000-fold just to fit within the confines of a prokaryotic cell. Substantial evidence suggests that the folding may be orderly and may bring specified regions of the DNA into proximity. The nucleoid can be visualized by electron microscopy as well as by light microscopy after treatment of the cell to make the nucleoid visible. Thus, it would be a mistake to conclude that subcellular differentiation, clearly demarcated by membranes in eukaryotes, is lacking in prokaryotes. Indeed, some prokaryotes form membrane-bound subcellular structures with specialized function such as the chromatophores of photosynthetic bacteria.

Prokaryotic Diversity

 The small size and haploid organization of the prokaryotic chromosome limits the amount of genetic information it can contain. Recent data based on genome sequencing indicate that the number of genes within a prokaryote may vary from 468 in Mycoplasma genitalium to 7825 in Streptomyces coelicolor, and many of these genes must be dedicated to essential functions such as energy generation, macromolecular synthesis, and cellular replication. Any one prokaryote carries relatively few genes that allow physiologic accommodation of the organism to its environment. The range of potential prokaryotic environments is unimaginably broad, and it follows that the prokaryotic group encompasses a heterogeneous range of specialists, each adapted to a rather narrowly circumscribed niche.

The range of prokaryotic niches is illustrated by con sideration of strategies used for generation of metabolic energy. Light from the sun is the chief source of energy for life. Some prokaryotes such as the purple bacteria convert light energy to metabolic energy in the absence of oxygen production. Other prokaryotes, exemplified by the blue green bacteria (Cyanobacteria), produce oxygen that can provide energy through respiration in the absence of light. Aerobic organisms depend on respiration with oxygen for their energy. Some anaerobic organisms can use electron acceptors other than oxygen in respiration. Many anaerobes carry out fermentations in which energy is derived by metabolic rearrangement of chemical growth substrates. The tremendous chemical range of potential growth substrates for aerobic or anaerobic growth is mirrored in the diversity of prokaryotes that have adapted to their utilization.

Prokaryotic Communities

A useful survival strategy for specialists is to enter into consortia, arrangements in which the physiologic characteristics of different organisms contribute to survival of the group as a whole. If the organisms within a physically inter connected community are directly derived from a single cell, the community is a clone that may contain up to 108 or greater cells. The biology of such a community differs substantially from that of a single cell. For example, the high cell number virtually ensures the presence within the clone of at least one cell carrying a variant of any gene on the chromosome. Thus, genetic variability—the wellspring of the evolutionary process called natural selection—is ensured within a clone. The high number of cells within clones is also likely to provide physiologic protection to at least some members of the group. Extracellular polysaccharides, for example, may afford protection against potentially lethal agents such as antibiotics or heavy metal ions. Large amounts of polysaccharides produced by the high number of cells within a clone may allow cells within the interior to survive exposure to a lethal agent at a concentration that might kill single cells.

Many bacteria exploit a cell–cell communication mechanism called quorum sensing to regulate the transcription of genes involved in diverse physiologic processes, including bioluminescence, plasmid conjugal transfer, and the production of virulence determinants. Quorum sensing depends on the production of one or more diffusible signal molecules (eg, acetylated homoserine lactone [AHL]) termed autoinducers or pheromones that enable a bacterium to monitor its own cell population density (Figure 1). The cooperative activities leading to biofilm formation are controlled by quorum sensing. It is an example of multicellular behavior in prokaryotes.

Fig1. Quorum sensing. (Reproduced with permission from Nester EW, Anderson DG, Roberts CE, et al: Microbiology: A Human Perspective, 6th ed. McGraw-Hill, 2009, p. 181. © McGraw-Hill Education.)

Another distinguishing characteristic of prokaryotes is their capacity to exchange small packets of genetic information. This information may be carried on plasmids, small and specialized genetic elements that are capable of replication within at least one prokaryotic cell line. In some cases, plasmids may be transferred from one cell to another and thus may carry sets of specialized genetic information through a population. Some plasmids exhibit a broad host range that allows them to con vey sets of genes to diverse organisms. Of particular concern are drug resistance plasmids that may render diverse bacteria resistant to antibiotic treatment.

The survival strategy of a single prokaryotic cell line may lead to a range of interactions with other organisms. These may include symbiotic relationships illustrated by complex nutritional exchanges among organisms within the human gut. These exchanges benefit both the microorganisms and their human host. Parasitic interactions can be quite deleterious to the host. Advanced symbiosis or parasitism can lead to loss of functions that may not allow growth of the symbiont or parasite independent of its host.

The mycoplasmas, for example, are parasitic prokaryotes that have lost the ability to form a cell wall. Adaptation of these organisms to their parasitic environment has resulted in incorporation of a substantial quantity of cholesterol into their cell membranes. Cholesterol, not found in other prokaryotes, is assimilated from the metabolic environment provided by the host. Loss of function is exemplified also by obligate intracellular parasites, the chlamydiae and rickettsiae. These bacteria are extremely small (0.2–0.5 µm in diameter) and depend on the host cell for many essential metabolites and coenzymes. This loss of function is reflected by the presence of a smaller genome with fewer genes.

The most widely distributed examples of bacterial symbionts appear to be chloroplasts and mitochondria, the energy-yielding organelles of eukaryotes. Evidence points to the conclusion that ancestors of these chloroplasts and mitochondria were endosymbionts, essentially “domesticated bacteria” that established symbiosis within the cell mem brane of the ancestral eukaryotic host. The presence of multiple copies of these organelles may have contributed to the relatively large size of eukaryotic cells and to their capacity for specialization, a trait ultimately reflected in the evolution of differentiated multicellular organisms.

Classification of the Prokaryotes

An understanding of any group of organisms requires their classification. An appropriate classification system allows a scientist to choose characteristics that allow swift and accurate categorization of a newly encountered organ ism. This categorical organization allows prediction of many additional traits shared by other members of the category. In a hospital setting, successful classification of a pathogenic organism may provide the most direct route to its elimination. Classification may also provide a broad understanding of relationships among different organ isms, and such information may have great practical value. For example, elimination of a pathogenic organism will be relatively long-lasting if its habitat is occupied by a nonpathogenic variant.

The principles of prokaryotic classification are discussed in Chapter 3. At the outset, it should be recognized that any prokaryotic characteristic might serve as a potential criterion for classification. However, not all criteria are equally effective in grouping organisms. Possession of DNA, for example, is a useless criterion for distinguishing organisms because all cells contain DNA. The presence of a broad host range plasmid is not a useful criterion because such plasmids may be found in diverse hosts and need not be present all of the time. Useful criteria may be structural, physiologic, biochemical, or genetic. Spores—specialized cell structures that may allow survival in extreme environments—are useful structural criteria for classification because well-characterized subsets of bacteria form spores. Some bacterial groups can be effectively subdivided based upon their ability to ferment specified car bohydrates. Such criteria may be ineffective when applied to other bacterial groups that may lack any fermentative capability. A biochemical test, the Gram-stain, is an effective criterion for classification because response to the stain reflects fundamental differences in the bacterial cell envelope that divide most bacteria into two major groups.

Genetic criteria are increasingly used in bacterial classification, and many of these advances are made possible by the development of DNA-based technologies. It is now possible to design DNA probe or DNA amplification assays (eg, polymerase chain reaction [PCR] assays) that swiftly identify organisms carrying specified genetic regions with common ancestry. Comparison of DNA sequences for some genes has led to the elucidation of phylogenetic relationships among prokaryotes. Ancestral cell lines can be traced, and organisms can be grouped based on their evolutionary affinities. These investigations have led to some striking conclusions. For example, comparison of cytochrome c sequences suggests that all eukaryotes, including humans, arose from one of three different groups of purple photo synthetic bacteria. This conclusion in part explains the evolutionary origin of eukaryotes, but it does not fully take into account the generally accepted view that the eukaryotic cell was derived from the evolutionary merger of different prokaryotic cell lines.

Bacteria and Archaebacteria: The Major Subdivisions Within the Prokaryotes

A major success in molecular phylogeny has been the demonstration that prokaryotes fall into two major groups. Most investigations have been directed to one group, the bacteria. The other group, the archaebacteria, has received relatively little attention until recently, partly because many of its representatives are difficult to study in the laboratory. Some archaebacteria, for example, are killed by contact with oxygen, and others grow at temperatures exceeding that of boiling water. Before molecular evidence became available, the major subgroupings of archaebacteria had seemed disparate. The methanogens carry out an anaerobic respiration that gives rise to methane, the halophiles demand extremely high salt concentrations for growth, and the thermoacidophiles require high temperature and acidity for growth. It has now been established that these prokaryotes share biochemical traits such as cell wall or membrane components that set the group entirely apart from all other living organisms. An intriguing trait shared by archaebacteria and eukaryotes is the presence of introns within genes. The function of introns—segments of DNA that interrupts informational DNA within genes—is not established. What is known is that introns represent a fundamental characteristic shared by the DNA of archaebacteria and eukaryotes. This common trait has led to the suggestion that—just as mitochondria and chloroplasts appear to be evolutionary derivatives of the bacteria—the eukaryotic nucleus may have arisen from an archaebacterial ancestor.

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مواضيع ذات صلة

Protists

Microbial Metabolism: Focal Metabolites and Their Interconversion

Relationship of Biocide Concentration and Time on Antimicrobial Killing

Specific Actions of Selected Biocides

Flow cytometry

البروتوبلازم Protoplasm

غشاء الخلية The cell membrane

مملكة البدائيات (Monera)

كيس الصفار ( المح ) Yolk sac

اقسام البيئة البحرية

بيئة المياه البحرية

المياه الجارية