Prokaryotes come in have many different types and shapes, but the most common shapes are cocci and rods. Cocci are spherical cells that can exist in different arrangements; their shape can be useful in identification. For example, diplococcus is a pair of cocci that arises when cocci divide and remain together to form pairs. Long chains of cocci that result when cells stick after repeated division are characteristics of Streptococcus, and irregular shaped, grape -like clumps are characteristics of Staphylococcus.
Rod –shaped bacteria are called bacilli. The shape of the rod's end often varies between species and may be flat, rounded, cigar-shaped, or bifurcated.
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Another common shape for bacteria is the vibrio. Vibrios are comma-shaped bacteria that resemble rods. Spirilla are rigid, spiral- shaped bacteria. Spirochetes are another type of spiral-shaped bacteria, but they are flexible and have a unique internal flagella arrangement.
Prokaryotic cells exhibit a highly ordered intracellular organization. This organization is needed in order for the prokaryote cells to respond to the exterior environment and to respond to other cells. Also, prokaryotic cells must be able to transport materials from their surrounding environment into the cell and vice versa. In addition, they must protect themselves from the osmotic pressure, which is caused by movement of water freely across the cell membrane in response to concentration gradients. The cell wall, and the cell membrane fulfill these roles. Internal structures are also responsible for the growth and reproduction of bacteria. The cell wall is a structure that surrounds the plasma membrane; the periplasm, found only in gram-negative bacteria is the space that lies between the cell wall and the cell membrane. The cell wall gives the bacterial cell its shape and provides protection. In some bacteria, the cell wall is strong enough to withstand 375 pounds per square inch of internal osmotic pressure.
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The cell walls and cytoplasmic membranes of bacteria contain peptidoglycan. It is an enormous molecule composed of amino acids and carbohydrates. There are two types of walls found in bacteria and easily identified using a method called Gram staining. Gram positive bacteria are bacterial cells that have a very thick peptidoglycan cell wall. It is about (25nm) and it is the thickness of this material that allows the cell wall to retain the crystal violet dye used in the Gram stain.
Gram-negative bacteria have a cell wall with a different structure. The cell wall is much thinner and there is an outer lipid layer. This layer can be dissolved with alcohol during Gram staining, which removes the dye and makes the bacteria appear as pink upon counterstaining with Safranin solution. It is important to mention that in addition to peptidoglycan, most cell walls of Gram-positive bacteria contain teichoic acids, large molecules composed of repeating units of sugar and phosphates ( McKane and Kandel, 1996) which gives the cell a negative charge determining the type of substances attracted to and transported into the cell.
The cell membrane of bacteria encloses the cytoplasm. It is about 5nm thick, and consists of 40 percent phospholipids and 60 percent protein. The mosaic of phospholipids and protein are not cemented. The phospholipids are arranged in two parallel layers and represent the barrier function of the membrane. Proteins are embedded in this phospholipid bilayer. These proteins carry out important functions such as cell wall synthesis, and energy metabolism. Another significant function for the membrane protein is transportation of charged solutes such as sugar, ions, amino acids and nitrogenous bases. The cell wall transport system is highly specific, and it may require energy when it allows different concentrations of solutes to be established outside or inside of the cell against its concentration gradient.
The cytoplasm consists of the cytosol, a semi-fluid mass of amino acids, proteins sugar, vitamins, salt, and ions. Suspended in the cytoplasm of all bacteria is a region of chromosomal material called the nucleoid. The genetic material of prokaryotes is carried on a single circular molecule of DNA that constitutes the cell's nucleoid. Most cells have one copy of the chromosomes. However, the chromosomes divide before cell division.
The size of the chromosome varies according to the species. For example, Mycoplasma, the smallest bacteria contains the smallest strand of DNA, which directs the synthesis of fewer than 1000 cell products, while Escherichia coli, which is found in the gastrointestinal tract of mammals has a chromosome that contain information for the production of about 4000 products.
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Many bacteria contain a plasmid- small circular piece of DNA that can replicate independently of the chromosome. Some plasmids are used to transfer genetic information between bacteria and are significant in Genetic engineering. Ribosomes are another component of the cytoplasm. These are hundreds of thousands of spherical particles and they are the sites for protein synthesis.
Outside the cell wall and membrane, many bacteria have a gel-like layer called a capsule. Most capsules are polysaccharides or polysaccharides-protein complexes. A thick capsule provides protection to certain bacteria, and prevents some types of bacteria from dehydrating. Also a capsule might protect bacteria from being engulfed and destroyed by the body's white blood cells. Some bacteria form thick –walled endospores around their chromosomes, and a small piece of their cytoplasm when the cell is exposed to harsh conditions such as heat, radiation, chemicals or lethal agents. This structure does not grow or reproduce. These endospores do not produce new cells; instead they can survive for thousands of years. Endospores are the most totally heat resistant form of life. They can survive in boiling water at 100 C for several hours. For example, spores of the bacterium that causes botulism, a fatal food poisoning, can survive in food that has been subjected to insufficient heating.
Finally, bacteria have several structures that project through the cell wall to form surface appendages. The most common are the flagella and the pili. The flagella consists of a body, a hook, and a filament. It resembles a rigid corkscrew that spins, much like the propeller of a boat. The rotation of a flagellum has been measured to be as high as 300 revolutions per second. Therefore, flagellated bacteria are capable of very rapid movement. Pili are protein tubes that extend from the cell; they are shorter and thinner than flagella. They are only found in certain species of gram-negative bacteria. Pili do not play a role in motility, but they help in conjugation between bacteria, and attachment of bacteria to other surfaces, such as tissues of an infected person.
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Movement
Many bacteria use flagella to move, the flagella turn and propel which make bacteria move. Bacteria may have a single flagellum or a clump of flagella. Other bacteria have flagella at both ends of the cell. Bacteria that lack flagella have other ways of movement. For example, myxobacteria produce a layer of slime; just like a slime trail which allows them to glide through it. Another kind of movement used by bacteria is corkscrew-like rotation; the spiral-shaped bacteria use this kind of movement. Bacterial motion is random, but sometimes bacteria that have flagella can move toward chemical nutrients, or away from a repellant such as poison. This behavior is called chemotaxis.
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One of the most amazing types of bacteria is the magnetotactic bacteria (MTB). Blakemore (1975) was able to isolate a bacterium that lived in marine mud. This bacterium moved toward the geomagnetic North Pole. Since that time, many MTB have been isolated, and most of them range from cocci to spirilla.
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These bacteria can migrate along the geomagnetic field toward their favorable habitat. They contain nanometer –sized crystals of iron minerals such as magnetite (Fe
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O
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) or greigite (Fe
3
S
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). These crystals are enclosed in a membrane and arranged in linear chains adjacent to the cell membrane. This magnetic moment causes motion that is parallel to geomagnetic field lines, helping the bacteria to swim toward high oxygen concentrations at the oxic-anoxic interface of water. Scientists have identified two different types of these bacteria: polar and axial. The polar variety swim in a preferred direction relative to the local field. Some polar bacteria in the northern hemisphere respond to high oxygen levels by swimming toward geomagnetic south. The axial varieties of MTB swim in both directions and rotate 180 degrees continuing to swim back and forth.
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Reproduction
Bacteria are active microorganisms, under ideal conditions they are constantly reproducing, metabolizing, and growing in number and in size. Bacterial growth is described as the increase in population size and it can occur in different ways. The predominant mode of bacterial reproduction is called binary fission, which is a form of asexual reproduction that produces two daughter cells. During binary fission, the cell increases in size and doubles in length, and the cytoplasmic volume increases since it is filled with new ribosomes and enzymes. The cell duplicates its genome and divides its resources in half. Each daughter cell gets the genetic instructions and other cellular constituents that are needed to continue the cycle. Once the cell wall is completed, the daughter cell becomes independent. Each time the cell divides by binary fission it forms a new generation of cells. Some bacteria reproduce by budding, in which smaller cells are produced from the surface of the parent bacteria.
Bacteria have several ways of transferring genetic material or DNA that do not involve growth. For example, two living bacteria can bind together and transfer genetic material in a process called conjugation. During conjugation, one bacterium must have a specialized plasmid and pilus. The specialized pilus can bind to the recipient bacterium and form a conjugation bridge, which is a passageway that enables the bacterium to transfer genetic information. In order for this process to occur, one copy of the plasmid passes through the bridge (pilus) to the recipient bacterium. The cells will detach after the transfer of DNA. Conjugation pili are longer and fewer than the pili used for attachment.
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Conjugation increases diversity in the population of bacteria. Other bacteria produce endospores that can remain dormant for years while waiting for conditions to improve. The ability of many bacteria to form spores make it possible for them to survive harsh conditions such as extreme heat, dryness, and lack of nutrients that would otherwise kill them.
Metabolic Diversity
All living organisms including bacteria need a constant supply of energy. Growth, movement, metabolism, and protein synthesis require a constant supply of energy. The processes of respiration (a.k.a. breathing) and fermentation, both release energy. Organisms that depend on the presence of oxygen in order to live are called obligate aerobes. For example, Mycobacterium tuberculosis, the bacteria that causes tuberculosis is an obligate aerobe. Some other bacteria cannot live in the presence of oxygen; in fact, they can be killed by it. This type of bacteria is classified as obligate anaerobes. Clostridium botulinum, which is found in soil is an obligate anaerobe. Another group of bacteria is classified as facultative anaerobes. These bacteria can live in the presence or absence of oxygen. The ability of these bacteria to switch between cellular respiration and fermentation means that they can live almost everywhere. For example, E.coli is a facultative anaerobe that can live in contaminated water, in sewage, or in the large intestine.
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Bacteriologists usually characterize an organism's nutritional source need according to the carbon source and the energy source it requires for growth. Depending on their source of energy, and their use of oxygen, prokayotes can be divided into several types. Heterotrophs utilize organic compounds to get the carbon, necessary for growth. Other prokaryotes are autotrophs; they make their own food/biomass from inorganic carbon or carbon dioxide (CO
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) molecules. Autotrophs can be further divided into two categories. The photoautotrophs derive energy from sunlight through the process of photosynthesis, using light energy to covert carbon dioxide and water to organic carbon compounds and oxygen. Therefore these bacteria are found in areas where light is plentiful such as the surfaces of lakes, streams, or oceans. Photosynthesis occurs in the green sulfur bacteria, the purple sulfur bacteria, and cyanobacteria. Cyanobacteria are the most diverse and largest group of photosynthetic bacteria. They have chlorophyll a, and use phycobiliproteins as accessory pigments.
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Cyanobacteria are found in some fresh water, salty water, or even on land.
The chemolithoautotrophs do not require light as a source of energy; instead they use chemical reactions that involve the oxidation of inorganic compounds or chemicals such as hydrogen sulfide, ammonium, nitrites, sulfur, hydrogen (as H
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) or iron. Oxidation is the loss of electrons or hydrogen atoms in a chemical reaction that result in the release of energy. Chemolithotrophic bacteria use oxygen or other electron acceptors in respiration in order to make ATP and get their energy. Other types of bacteria oxidize nitrogenous compounds to get their source of electrons. Nitrifying bacteria are an example of this group. These bacteria can live in soil or aquatic environments. They are significant because they play a role in the process of nitrification which oxidizes ammonia into nitrate. The chemoheterotrophs use organic compounds as a source of both energy and carbon. Many bacteria and some archaea are examples of this type.
