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The host cell profited from the chemical energy the mitochondrion produced, and the mitochondrion benefited from the protected, nutrient-rich environment surrounding it. When two become one. Since in all probability these hopanoids are of prokaryotic origin, this shows the enormous importance of prokaryotes in the formation of fossil fuel, in particular petroleum. However, horizontal genetic exchanges are possible despite these differences. Some may appear randomly distributed, others equally spaced.
Ribosomes in both prokaryotes and eukaryotes are an essential part of the translation in the synthesis of proteins machinery in cells. They act as the physical support on which the genetic information already transferred from the DNA to the messenger RNA is used to assemble amino acids in the correct order to make functional proteins.
In bacteria, ribosomes are smaller than in eukaryotic cells. They sediment less easily in the ultracentrifuge. They consist of about two-thirds ribonucleic acid and one-third protein and, following proper Chemical treatment, they dissociate into two subunits of different size and weight. To fulfill their roles in protein biosynthesis, ribosomal RNA molecules must contain several functionally different regions. The nucleotide sequences in some of these regions are conserved and in others are highly varied.
The smaller subunit of the prokaryotic ribosome contains a sequence 16S approximately nucleotides that is particularly suitable for genetic and phylogenetic comparison between different bacterial strains and also for studies of evolution. During the very long geologic eons, when bacteria evolved into what taxonomists traditionnally call groups, families, genera or species, changes were imprinted in the sequence of ribosomal RNAs.
These imprints or molecular signatures can be used to identify different bacteria and also to assess the most probable evolutionary distance between them Woese, Chloramphenicol, streptomycin, erythromycin and tetracyclines, for example, act as much more potent protein synthesis inhibitors in bacteria than in eukaryotic cells and are clinically useful antibacterial drugs although they are not entirely devoid of toxicity in eukaryotes.
Laboratory techniques can be used to stain the bodies of the bacterial cells with a cationic dye e. India ink. It can be visualized as a defined area surrounding blue dots bacterial bodies.
These transparent zones are usually called capsules and our knowledge of their chemistry is now extensive Bayer and Bayer, A large number of polysaccharides have been isolated from capsules and the slime material of bacteria.
Size, charge and composition of the capsular material are of primary importance in determining the roles and the usefulness of this structure for the bacteria. It seems that capsular material can be involved in giving some pathogenic bacteria a certain protection against phagocytosis by mononuclear cells of the animal body, and also in sticking adherence to the surfaces of the environment.
Capsule-producing bacteria such as Streptococcus pneumoniae and Klebsiella pneumoniae are more resistant to phagocytic white blood cells and can invade a tissue or an organ more rapidly than those deprived of capsules. The immune response to polysaccharidic capsular material is often less pronounced and also affords shorter protection than proteinic substances of bacterial origin.
They have a very high water content and they too seem to facilitate the attachment of the bacterial cells to solid surfaces. The sticky polysaccharide of the glycocalyx allows some oral bacteria to attach to tooth enamel and plays a role in the formation of dental plaque, the initial phase of tooth decay.
Polysaccharides excreted by other types of bacteria e. Xanthomonas, Pseudomonas find many applications as gelifying agents in shampoos, seasonings, lubricating agents, etc.
As noted previously, there is at least one small replicon in each prokaryotic cell and, in some cases, up to seventeen Fox, a. Borrelia burgdorferi a much larger proportion of the hereditary information may be found among the plasmids than was thought initially Fox, b. Also, it is now known that a few types of bacteria have a different distribution of their genes: two circular large replicons or even a linear one Jumas-Bilak et al. The similarity of the general genetic organization one large replicon and a variable stock of small replicons-associated genes across the entire prokaryotic super-kingdom after 3.
It lends further support to our view that the prokaryotic world is organized as a global superorganism whose constituent cells communicate and cooperate through easy and frequent genetic exchanges, with similar mechanisms and structures. Contrary to what we observe in eukaryotes, the free circulation of genes in prokaryotes is not compatible with the notion of species. Bacillus and Clostridium and mostly under adverse conditions, specialized and extremely resistant cells are formed.
They are called spores or endospores, have no metabolic activity of their own are fully dormant , but can be converted into vegetative growing and multiplying cells if favorable conditions prevail. This process is called germination. Various enzymes in spores are much more stable, particularly in relation to heat, than the corresponding enzymes in vegetative cells. The resistance of these prokaryotic endospores to elevated temperatures, to chemical substances and to radiations is much higher than that of any eukaryotic cell.
It appears that this striking molecular stability depends more on the internal environment of the sporulated cell than on intrinsic structural or other peculiar properties. Calcium dipicolinate stabilizes the DNA helices of the spores.
Dehydration, calcium dipicolinate and an impervious, bilayered protein keratin-like coat all appear to play major roles in protecting the prokaryotic spore. The distinct organization of the intracellular genome in practicaily all prokaryotic cells. E: Equator LR: The large replicon, also called genophore, nucleoid, and, erroneously, chromosome.
It represents the large majority of the cellular DNA, containing the essential and stable genes of prokaryotic cells. SR: Small replicon plasmids and prophages. Very small, self-replicating DNA molecules kept ususally as long as the few converting genes they carry are useful to their host cell.
Easily disposable and replaceable by other SR carrying temporarily better genetic supplements. There is at least one SR per prokaryotic cell but their number may reach seventeen. SR are visiting genes, each one able to multiply in many different prokaryotic strains. It can last almost forever as demonstrated by spores kept in glass containers sealed about years ago, and even more strikingly by Bacillus spores isolated from the gut of extinct bees, fossilized in amber, which apparently have been reactivated after several million years Cano, ; These communities are opportunistically rearranged and reorganized when necessary.
Each type of cell is highly specialized, nonetheless it can fit well with others in metabolically complementary ensembles similar to the ones of a multicellular eukaryotic organism animal or plant. However, they have the ability to modify or replace constituent cells if needed.
These cells practice an efficient sharing of their biochemical activities: exchange of partially modified substances, of enzymes, cross-feeding with metabolic end-products, etc. Such teams form giant ensembles in all fertile soils, ocean floors, the digestive tracts of all animals, etc.
Abundant populations can also be found at the surface of naturel waters. Sometimes these cells originate from far-away communities. As is the case for artistic mosaics, the smaller and more varied the stones, the easier it is to obtain a good result; in prokaryotic mixed communities, the cells fall rapidly in their respective, appropriate place.
In some cases it is amazing how rapidly new, well-balanced and successful communities of prokaryotic strains become established and persist practically unchanged for long periods of time. In the sterile digestive tract of any newborn animal a prokaryotic community will form, sometimes within a few days, and maintain a surprising resilience during the entire life-time of its host.
In such communities, different strains do not kill their local competitors but they eventually outbreed the less efficient and poorly adapted ones.
Nevertheless, biological solidarity and interdependance among the stable strains in these communities is enormous. In some cases less than one per cent of the strains from well established mixed groups can be grown when artificially isolated and fed in the laboratory. This shows that team partners offer each other support and assistance in the form of substances that we either fail to identify as growth or multiplication requirements or to supply in proper concentrations in laboratory culture media.
There are unsuspected similarities between the ways of functioning of our blood cells and those of prokaryotic cells. Both belong to their own multicellular organism and proceed from a common original cell egg for animals or first viable cell on earth for prokaryotes , they move freely in their liquid or viscous surroundings, have areas of high biochemical specialization and provide services and help e.
They too are so adapted and specialized for life in a multicellular organism that most of them cannot be easily cultivated as isolated cells in vitro. Interestingly, some bacterial teams create their own microclimate adjusted to their need. Sometimes, even the temperature is locally maintained at an optimal level. The compounded effects of prokaryotic communities play a major role in stabilizing the atmospheric nitrogen concentration, the acidity and alkalinity of their environment, etc.
This ensures favorable conditions and homeostasis for the entire planet. The significance and importance of prokaryotes as essential components and stabilizers of the biosphere cannot be overemphasized. The totality of their local mixed communities, acting as an ensemble, a single global superorganism, represents the most active and decisive element in the maintenance of planetary homeostasis. Soil fertility and its conservation depend mostly on large, mixed prokaryotic communities, often working in unison with fungi.
This activity is one of the most evident confirmations of the Gaia hypothesis Lovelock and Margulis, which presents the entire biosphere as a life-supporting entity, modified and maintained in a form most favorable to life, just as an animal maintains a stable internal environment favorable to all its cells: homeostasis. The latter has been created for our biosphere and improved by prokaryotes alone over the first two billion years of life on our planet.
As he gradually acquired a better understanding of the prokaryotes, man has recently begun to copy and exploit this potential of prokaryotic communities in sewage treatment, sludge digestion processes and oil spill clean-ups, etc. Mycorrhizae in particular and other types of fungi are often participants in favorable associations with bacteria.
Together, by their complex common activities, they favor the metabolic exchanges and growth of the associated plants. Many prokaryotic mixed communities contain in their surroundings a variety of eukaryotic species. The simplest example are the protoctists Margulis, represented by unicellular organisms or by only slightly differentiated multicellular entities.
Often they carry intracellular prokaryotic endosymbionts, and more often ectosymbionts on their surface. The much more complex animals are always associated with prokaryotic endo or ectosymbionts or commensals. For many hosts prokaryotes are essential for survival Margulis and Fester, We carry in our own cells descendants of Precambrian bacteria as tiny power plants: mitochondria, and photosynthetic eukaryotic cells carry bacteria or former bacteria, e.
Many prokaryotic cells which became endosymbionts seem incapable of independent life and behave as organelles of the eukaryotic host cell. By contrast, there are several important examples of associations between communities of interdependent bacterial types and higher organisms which are absolutely essential to the survival of both under natural conditions Margulis and Fester, One such association that has been and still is, as already stated, intensively studied is the ruminant-bacterial flora ecosystem Hungate, , ; Baldwin, It is now well established that grazing animals like ruminants must resort to microorganisms as digestive agents to subsist on grasses and leafy plants, an enormous source of renewable nutrients.
Ruminants are herbivorous animals in whom a special organ, the rumen, has evolved. It accomodates, physically and physiologically, the billions of bacteria and, in smaller numbers, protozoa that break down the insoluble polysaccharide of plants. They supply the host with essential nutrients and energy derived from plant carbohydrates that would otherwise be unavailable to it. Under natural conditions, ruminants obtain most of their essential nutritive and growth factors as a result of the microbial activity in the rumen Hobson, After regurgitation, the massive microbial flora enters a true stomach abomasum and the small intestine, where it serves as a nutrient.
Thus, in many regions of the world, the human food supply depends to a large extent on baeterial cellulose decomposers and sugar fermenters that have adapted to life in symbiosis with the ruminants. Recent progress in molecular biology has also improved our knowledge of the diversity of ruminal strains and lead to necessary renaming and reclassification.
It is certain that the number of different strains that are normal inhabitants of the rumen greatly exceeds Krause and Russell, the figure between 20 and 30 that was the accepted estimate some decades ago.
The extent of interdependence and collaboration in this ecosystem is amazing. It also illustrates how successful and efficient different strains, practicing labor and resource sharing, can be.
Bacteroides succinogenes, Ruminococcus albus , starch e. Bacteroides spp. Lachnospira spp. For example, a rumen inhabitant, Selenomonas ruminantium, which is non-cellulolytic, grows well in vitro on a cellulose substrate, providing it is associated with Bacteroides succinogenes, whereas on its own it will not grow.
This is but one example that illustrates the complexity and the collaborative nature of the interrelations within the rumen bacterial community. Constancy of the rumen ecosystem is essential for the good health of the host animal.
Under ordinary circumstances it is maintained within narrow limits. The equilibrium between the members of the microscopic flora on the one hand, and the two symbiosis partners on the other, can be broken and this generally results in severe illness as it happens when the animals are switched abruptly from a diet composed predominantly of forage to one that is rich in grain.
A protein spore coat then forms around the cortex while the DNA of the mother cell disintegrates. Further maturation of the endospore occurs with the formation of an outermost exosporium. The endospore is released upon disintegration of the mother cell, completing sporulation. Figure 7. The forespore becomes surrounded by a double layer of membrane, a cortex, and a protein spore coat, before being released as a mature endospore upon disintegration of the mother cell.
The endospores have been visualized using Malachite Green spore stain. Endospores of certain species have been shown to persist in a dormant state for extended periods of time, up to thousands of years.
After germination, the cell becomes metabolically active again and is able to carry out all of its normal functions, including growth and cell division. Not all bacteria have the ability to form endospores; however, there are a number of clinically significant endospore-forming gram-positive bacteria of the genera Bacillus and Clostridium.
These include B. Pathogens such as these are particularly difficult to combat because their endospores are so hard to kill. Special sterilization methods for endospore-forming bacteria are discussed in Control of Microbial Growth. Structures that enclose the cytoplasm and internal structures of the cell are known collectively as the cell envelope.
In prokaryotic cells, the structures of the cell envelope vary depending on the type of cell and organism. Most but not all prokaryotic cells have a cell wall , but the makeup of this cell wall varies. All cells prokaryotic and eukaryotic have a plasma membrane also called cytoplasmic membrane or cell membrane that exhibits selective permeability, allowing some molecules to enter or leave the cell while restricting the passage of others. The structure of the plasma membrane is often described in terms of the fluid mosaic model , which refers to the ability of membrane components to move fluidly within the plane of the membrane, as well as the mosaic-like composition of the components, which include a diverse array of lipid and protein components Figure 8.
The plasma membrane structure of most bacterial and eukaryotic cell types is a bilayer composed mainly of phospholipids formed with ester linkages and proteins. These phospholipids and proteins have the ability to move laterally within the plane of the membranes as well as between the two phospholipid layers. Figure 8. The bacterial plasma membrane is a phospholipid bilayer with a variety of embedded proteins that perform various functions for the cell. Note the presence of glycoproteins and glycolipids, whose carbohydrate components extend out from the surface of the cell.
The abundance and arrangement of these proteins and lipids can vary greatly between species. Archaeal membranes are fundamentally different from bacterial and eukaryotic membranes in a few significant ways. First, archaeal membrane phospholipids are formed with ether linkages, in contrast to the ester linkages found in bacterial or eukaryotic cell membranes.
Second, archaeal phospholipids have branched chains, whereas those of bacterial and eukaryotic cells are straight chained.
Finally, although some archaeal membranes can be formed of bilayers like those found in bacteria and eukaryotes, other archaeal plasma membranes are lipid monolayers. Membrane proteins and phospholipids may have carbohydrates sugars associated with them and are called glycoproteins or glycolipids, respectively. These glycoprotein and glycolipid complexes extend out from the surface of the cell, allowing the cell to interact with the external environment Figure 8.
Glycoproteins and glycolipids in the plasma membrane can vary considerably in chemical composition among archaea, bacteria, and eukaryotes, allowing scientists to use them to characterize unique species. Plasma membranes from different cells types also contain unique phospholipids, which contain fatty acids. As described in Using Biochemistry to Identify Microorganisms , phospholipid-derived fatty acid analysis PLFA profiles can be used to identify unique types of cells based on differences in fatty acids.
Archaea, bacteria, and eukaryotes each have a unique PFLA profile. One of the most important functions of the plasma membrane is to control the transport of molecules into and out of the cell. Internal conditions must be maintained within a certain range despite any changes in the external environment. The transport of substances across the plasma membrane allows cells to do so. Cells use various modes of transport across the plasma membrane. For example, molecules moving from a higher concentration to a lower concentration with the concentration gradient are transported by simple diffusion , also known as passive transport Figure 9.
Figure 9. Simple diffusion down a concentration gradient directly across the phospholipid bilayer. Some small molecules, like carbon dioxide, may cross the membrane bilayer directly by simple diffusion. However, charged molecules, as well as large molecules, need the help of carriers or channels in the membrane.
These structures ferry molecules across the membrane, a process known as facilitated diffusion Figure Figure Facilitated diffusion down a concentration gradient through a membrane protein. Active transport occurs when cells move molecules across their membrane against concentration gradients Figure Active transport against a concentration gradient via a membrane pump that requires energy. Group translocation also transports substances into bacterial cells.
In this case, as a molecule moves into a cell against its concentration gradient, it is chemically modified so that it does not require transport against an unfavorable concentration gradient. A common example of this is the bacterial phosphotransferase system, a series of carriers that phosphorylates i. Since the phosphorylation of sugars is required during the early stages of sugar metabolism, the phosphotransferase system is considered to be an energy neutral system.
Some prokaryotic cells, namely cyanobacteria and photosynthetic bacteria , have membrane structures that enable them to perform photosynthesis. These structures consist of an infolding of the plasma membrane that encloses photosynthetic pigments such as green chlorophylls and bacteriochlorophylls.
In cyanobacteria, these membrane structures are called thylakoids; in photosynthetic bacteria, they are called chromatophores, lamellae, or chlorosomes. The primary function of the cell wall is to protect the cell from harsh conditions in the outside environment. When present, there are notable similarities and differences among the cell walls of archaea, bacteria, and eukaryotes.
The major component of bacterial cell walls is called peptidoglycan or murein ; it is only found in bacteria. Structurally, peptidoglycan resembles a layer of meshwork or fabric Figure The structure of the long chains has significant two-dimensional tensile strength due to the formation of peptide bridges that connect NAG and NAM within each peptidoglycan layer.
In gram-negative bacteria, tetrapeptide chains extending from each NAM unit are directly cross-linked, whereas in gram-positive bacteria, these tetrapeptide chains are linked by pentaglycine cross-bridges.
Peptidoglycan subunits are made inside of the bacterial cell and then exported and assembled in layers, giving the cell its shape. This provides the cell wall with tensile strength in two dimensions. Since peptidoglycan is unique to bacteria, many antibiotic drugs are designed to interfere with peptidoglycan synthesis, weakening the cell wall and making bacterial cells more susceptible to the effects of osmotic pressure see Mechanisms of Antibacterial Drugs.
The Gram staining protocol see Staining Microscopic Specimens is used to differentiate two common types of cell wall structures Figure 13 [5]. Gram-positive cells have a cell wall consisting of many layers of peptidoglycan totaling 30— nm in thickness. These peptidoglycan layers are commonly embedded with teichoic acids TAs , carbohydrate chains that extend through and beyond the peptidoglycan layer. Bacteria contain two common cell wall structural types.
Gram-positive cell walls are structurally simple, containing a thick layer of peptidoglycan with embedded teichoic acid external to the plasma membrane. Gram-negative cell walls are structurally more complex, containing three layers: the inner membrane, a thin layer of peptidoglycan, and an outer membrane containing lipopolysaccharide. TA is thought to stabilize peptidoglycan by increasing its rigidity. TA also plays a role in the ability of pathogenic gram-positive bacteria such as Streptococcus to bind to certain proteins on the surface of host cells, enhancing their ability to cause infection.
In addition to peptidoglycan and TAs, bacteria of the family Mycobacteriaceae have an external layer of waxy mycolic acids in their cell wall; as described in Staining Microscopic Specimens , these bacteria are referred to as acid-fast, since acid-fast stains must be used to penetrate the mycolic acid layer for purposes of microscopy Figure Acid-fast cells are stained red by carbolfuschin.
The outer membrane of a gram-negative bacterial cell contains lipopolysaccharide LPS , a toxin composed of Lipid A embedded in the outer membrane, a core polysaccharide, and the O side chain. Gram-negative cells have a much thinner layer of peptidoglycan no more than about 4 nm thick [7] than gram-positive cells , and the overall structure of their cell envelope is more complex.
In gram-negative cells , a gel-like matrix occupies the periplasmic space between the cell wall and the plasma membrane, and there is a second lipid bilayer called the outer membrane , which is external to the peptidoglycan layer Figure This outer membrane is attached to the peptidoglycan by murein lipoprotein.
The outer leaflet of the outer membrane contains the molecule lipopolysaccharide LPS , which functions as an endotoxin in infections involving gram-negative bacteria, contributing to symptoms such as fever, hemorrhaging, and septic shock.
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