Bacterial Structure and Growth
Name and describe the three types of spiral bacteria.
1. Curved rods, called vibrio 2. Spirilla, which are helical, corkscrew shaped, rigid and have flagella 3. Spirochete, which is helical, highly twisted, flexible and moves by means of an axial filament.
Describe endospores
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Name the 3 classical bacterial shapes.
1. Coccus (pl. cocci) - round-shaped 2. Bacillus (pl. bacilli) - rod-shaped 3. Spiral - spiral-shaped
Describe the general characteristics of bacteria.
1. Have 1,1.5 or 2 circular chromosomes that are not in a nucleus. The chromosomes are usually attached to the cell wall in several places to prevent entangling and to ensure proper division. 2. Have no specific organelles such as mitochondria 3. Many have flagella, fimbriae and various capsules, though not all bacteria have this 4. Divide by binary fission 5. Do not have histone proteins. 6. Have cell walls made of peptidoglycan, which is a polymer that keeps the cell walls rigid.
What are some examples of inclusions?
1. Metachromatic granules (volutin) - act as phosphate reserves 2. Carboxysomes - ribulose 1,5 diphosphate carboxylase for CO2 fixation. 3. Sulfur granules - energy reserves 4. polysaccharide granules - energy reserves 5. lipid inclusions - energy reserves 6. magnetosomes - iron oxide, which destroyes H2O2 7. Gas vacuoles - protein-covered cylinders. MERRY POPULAR LLAMAS STRUM GREAT CURLY MANDOLINS.
What are the functions of glycocalyx?
1. Protection: from phagocytosis by macrophages. Macrophages have difficulty attaching to the bacterium because of the glycocalyx. Therefore, glycocalyx is a virulence factor. 2. Attachment: to various surfaces such as teeth (dental plaque), hospital equipment (catheters), plant roots, water pipes, rocks. 3. Formation of biofilms. Bacteria secrete this extracellular polysaccharide, encapsulating themselves in it. The result of this encapsulation is the formation of a biofilm, which has many layers of bacteria in it. The biofilm protects the bacteria from dehydration, environmental stressors and antibiotics. It is dangerous when this biofilm forms on hospital equipment such as catheters or on artificial joints because mechanical stressors cause parts of the biofilm to break, circulating viable bacteria through the bloodstream. These bacteria may be killed by antibiotics, but the biofilm remains to cause reinfection. The biofilm can only be removed by removing the infected part and replacing it with a sterile one.
What is an axial filament?
An axial filament is also called an endoflagella. Axial filaments are attached at one or both (usually both) ends of the cell and spiral around the cell. It functions, just like other flagella, to provide motility to the bacterium by rotating in a clockwise or counterclockwise direction, causing the bacterium to spiral and move rapidly. Motility is important for getting to a source of nutrition, action or the host cell. Motility, chemotaxis and flagella are all virulence factors that aid microorganisms in causing disease in humans and animals. Spirilla and spirochetes especially can have endoflagella that wrap themselves around the cells and that are then covered by a flagella sheath to hold the flagella to the cell body.
Describe bacteria's ability to sense and respond to their environment in terms of chemoreceptors.
Bacteria are able to sense their environment. They can sense and respond to stimuli, such as light, temperature and chemicals. Bacteria have a sensory organ, which they use to do this. In some cases, bacteria have a rudimentary brain to analyze and detect their environment. Sometimes, they have a rudimentary memory, whereby they can remember things they have previously sensed. Sensory molecules called "chemoreceptors" are complex and attach to different chemicals in order to assess the environment. They are formed into a sensory organ. The chemoreceptors are rods, part of which sense chemicals in the bacteria's environment and part of which are in the bacteria itself and function in signal transduction. The signal is sent to the flagella motor, which turns in a clockwise or anticlockwise direction, depending on the stimulus detected by the chemoreceptors. When viewing the chemoreceptors through a microscope, you will be able to see a highly ordered, hexagonal assembly of sensory proteins.
Explain the bacterial cell wall.
Bacteria have a cell wall made of peptidoglycan, which is a macromolecular polymer. Peptidoglycan is made of repeating units of disaccharide sugars linked by polypeptides. The sugars are either N-acetylglucosamine or N-acetylmuramic acid. These sugars are assembled into long chains of 10-65 sugars and form a sugar (glycan) backbone of the molecule. The function of the bacterial cell wall is to hold all the cellular machinery in place. It is also a very important criteria by which we recognize different cells, classify them and segregate them. The cell wall is also very important for diagnostic purposes because you can sometimes use a basic stain to detect if bacteria in a sample have a specific type of membrane with a specific arrangement to identify the bacteria (e.g. gram negative diplococcus). This information is important, not only to identify bacteria, but also to treat disease. The polypepetides may contain different amino acids. All of them, however, have side chains, each of which are 4 amino acids long. The peptidoglycan is layer upon layer of long glycans all connected by the short tetrapeptides.
Explain differential staining with regards to cell walls.
Because of the structural differences between gram positive and gram negative cell membranes, differential stains may be used to distinguish between gram positive and gram negative bacteria. Differential stains are stains that can differentiate between two structures. In this case, the structure is the cell wall, and the stain is the gram stain. Gram staining is the first step to identifying bacteria. The first step in gram staining is to apply crystal violet (purple dye). The crystal violet is the primary stain, which stains all cells. The second step is to apply iodine (mordant). The iodine forms crystals with the crystal violet, which may become trapped in the peptidoglycan layers. The third step is to perform an alcohol wash. The crystals in the gram negative bacteria will wash out easily as the peptidoglycan layer is thinner. This will decolourize the gram negative bacteria. The crystals in the gram positive bacteria will not wash out as easily because they will get caught in the thick peptidoglycan layer. The fourth step is to apply safranin or carbolfuchsin (counterstain). The result is that gram negative cells will be stained pink (safranin), and gram positive cells will be stained purple (crystal violet). Gram staining does not always work, however, as some cells have neither a gram negative or a gram positive cell membrane. There are atypical cell walls - for example the cell walls of mycobacteria and nocardia. These cell walls have mycolic acid, which gives them waxy, lipid properties. They have a waxy layer attached to their peptidoglycan, which prevents the stain from penetrating. Instead, these cells must be stained with acid alcohol and exposed to heat to dissolve their waxy layer. These cells are called "acid-fast" cells. A perfect example of acid fast cells is mycobacterium tuberculosis. The cell walls of acid-fast cells are more similar to gram positive cell walls than gram negative.
What are some structures that all bacteria have?
Cell membrane, 70S ribosomes, nucleoid region, cytoplasm
Describe fimbriae and pili in terms of structure and function.
Fimbriae and pili are built by bacteria on their surface. They are structurally similar to flagella, except that they are shorter and finer. They function in attachment, particularly to host cells and to environmental surfaces. They are a virulence factor. For example, E. coli can make bundle-forming pili, which makes them pathogenic because it allows them to attach to and disturb epithelial cells, causing disease. There are types of bacteria that rely on pili and fimbriae to attach to the particular area of the human or animal's body and not to other areas. For example, gonococcus attaches to columnar epithelial cells to cause disease. Pili are also used in the transfer of genetic information between cells. Sex pili are contractile structures produced by bacteria that target another bacterium with a different type of plasmid. The sex pili attach to the cell, pull it closer and then exchange genetic information with it. Plasmids are extrachromosomal bits of DNA that are supercoiled, can be present in large numbers and encode specific genes not included in the chromosome.
Describe the structure of flagella.
Flagella are long, filamentous appendages that propel bacteria, functioning in locomotion. They are complex structures made of many molecules that form mechanical-like structures. These structures are anchored into the cell membrane. Flagella have three parts - filament, hook and basal body. The filament is the long, outmost region made of subunits of a single, polymerized protein - flagellin. The flagellin is arranged as helical chains around a hollow core. When new flagellin molecules are formed, they are passed through the flagella, through the filament and deposited next to the growing chain of flagellin molecules. Sometimes, this process of filament assembly is used to secrete various proteins and virulence factors. Some bacteria deposit molecules on the tip of their flagella which function in adherence to structures such as host cells or catheters. The filament is sealed after its formation. The filament is attached to the hook, which is similar in structure to the filament except that it is wider and is made of hook protein rather than flagella protein. The hook is attached to the basal body, which is embedded in the cell. The basal body consists of a small rod, which is inserted through a series of discs. These discs are embedded in the cell membrane and cell wall. Gram positive bacteria only have discs embedded in their plasma membrane. Gram negative bacteria have a more complex membrane and also have discs in their outer membrane. To move the flagella and therefore propel the cell, the basal body rotates. The basal body is rotated by the flagella motor.
Define and describe glycocalyx.
Glycocalyx (or "capsule") is a covering around some bacteria usually made from carbohydrates that have been excreted from the bacteria. It can be compact and tight (capsule) or can be loosely fitting (slime layer). It is a very important antigen. An antigen is a structure (polysaccharide, sugar, protein, lipid or any kind of molecule) to which an immune response can be mounted. It is important in the interaction between the animal and human immune systems. Capsule is a tight polymer, sometimes made of repeating sugars, sometimes made of random sugars. A loose gelatinous coating is a slime layer. Glycocalyx is sometimes referred to as extracellular polysaccharide because it is made from excreted polymerized sugars. The composition of the glycocalyx varies from species to species and sometimes within a species. Bacteria can vary the sugars used to produce glycocalyx, changing the antigenic presentation to the immune system. This hides the bacteria from the immune system and is part of the microorganism's virulence arsenal.
Describe the cell wall of gram negative bacteria.
Gram negative bacteria have a more complex cell wall than gram positive bacteria. The cell wall of gram negative bacteria has 3 components - inner membrane, peptidoglycan and outer membrane. The inner membrane is the plasma membrane. The peptidoglycan is not as thick as in gram positive bacteria, not having many layers. The outer membrane is similar to the plasma membrane. However, it has some extras in addition to the phospholipid bilayer. Between the outer and inner membranes is the periplasmic space, in which the peptidoglycan is connected to the outer membrane via lipoproteins. The cell walls of gram negative bacteria have lipooligosaccharides and lipopolysaccharides. These are lipids combined to long, repeated carbohydrate chains or sugars. The sugars can be long or short and usually consist of repeating units of 4-6 sugars. The carbohydrates can be 10-50 sugars long. Short sugars with only up to 10 sugars are lipooligosaccharides. The lipopolysaccharides and lipooligosaccharides are connected to a lipid moiety. The lipid moiety is called lipid A. Lipid A is present in all gram negative bacteria and is toxic to eukaryotic cells and organisms; it is an endotoxin. That is why treatment of bacteria with antibiotics is not always beneficial as lysis of the cells will release toxins, which can cause anaphylactic shock. Lipid A has different levels of toxicity in different organisms. For example, the lipopolysaccharide of neisseria meningitidis is so toxic that it can cause destruction of capillaries in the extremities to the extent that the tissues will necrotize. In the periplasmic space, bacteria harbour various proteins for transport, export, secretion, etc.
Describe the cell wall of gram positive cells.
Gram positive cell walls are simple with only one plasma membrane and a thick layer of peptidoglycan. The cell wall of gram positive cells has many layers of glycans, connected by tetrapeptides, making it very rigid. Molecules and proteins are found within the layers of glycans, but, most importantly, teichoic acids are found here. There are two types of teichoic acids - lipoteichoic and wall teichoic acids. Lipoteichoic acids are linked to the plasma membrane. Wall teichoic acids are connected to the peptidoglycan. These teichoic acids are antigens. They determine what immune response the bacteria generates. The teichoic acids are variable. Therefore, the type of immune response elicited in response to a bacteria or serovars (the type of antibodies elicited by a bacteria) can be used to identify them. Teichoic acids are negatively charged, regulating the movement of cations in and out of the cell.
What are the four categories of bacteria according to their flagella?
Monotrichous: have a single flagellum at one pole of the cell. Amphitrichous: have a single flagella at either pole of the cell. Lophotrichous: have two or more flagella at one pole of the cell Petritrichous: have flagella distributed over the entire cell.
What do you think of when you hear the words: mycoplasma, archaea, cell wall.
Mycoplasmas and archaea lack normal cell walls. Mycoplasmas lack cell walls and instead have sterols in the plasma membrane. Archaea lack cell walls or have cell walls made of pseudomurien (lack N-acetylmuramic acid and D-amino acids).
Describe the cellular membrane.
The cellular membrane is a phospholipid bilayer. The phospholipid bilayer is a fluid layer, which is capable of movement and is self-sealing. It contains many proteins such as channels, which allow non-specific permeability, and proteins involved in active uptake of molecules (influx) and expulsion of molecules (efflux). The plasma membrane often has various inclusions for different bacteria. Examples of inclusions are photosynthetic pigments, thylakoids and chromatophores. Photosynthetic pigments are found in foldings called thylakoids and chromatophores.
Describe the function of flagella and relate it to taxis.
The function of flagella is motility. Bacteria use their flagella to move from place to place by rotation of the flagella motor. Bacteria can sense their environment and move in response to stimuli such as light, chemicals and temperature. They can move toward "attractants" and away from "repellants." When there is no stimulus, bacteria do a random walk, which is a series of runs and tumbles. The four types of movement permitted by flagella are runs, tumbles, swarms and swimming. When bacteria want to move towards a stimulus, they move their flagella in an anticlockwise direction, folding them into a bundle to do a run. When bacteria want to change direction, they move their flagella in a clockwise direction, spreading them out before tumbling. Bacteria can detect even centimolar amounts of a chemical. They can travel up a concentration gradient to around six levels of logarithmic scale. They can sense an upward or downward concentration gradient up to six levels of logarithmic scale.
Define flagella.
a long filamentous appendage that propels bacteria and is required for locomotion.
