Flashcards in Chapter #3: Bacteria and Archaea Deck (74):
Many years it was thought that microbes were small because it increases the surface area to volume ratio. As this ratio increases, nutrient uptake and diffusion of molecules within the cell become more efficient, which in turn facilitates a rapid growth rate. Shape affects the S/V ratio. A rod with the same volume as a coccus has a higher S/V ratio than does the coccus. This means that a rod can have greater nutrient flux across its plasma membrane.
For bacteria to be very large, they must have other characteristics that max their S/V ratio, or their size must be of some selective value.
Selectively permeable barrier, mechanical boundary of cell, nutrient and waste transport, location of many metabolic processes (respiration, photosynthesis), detection of environmental cues for chemotaxis.
Protein synthesis. Cytoplasm of bacterial and archaeal cells often packed with them, and other ribosomes may be loosely attached to plasma membrane. Cytoplasmic ribosomes synthesize proteins destined to remain in cell, whereas plasma membrane ribosomes make proteins that will reside in the cell envelope or are transported to the outside.
Bacterial and archaeal ribosomes are called 70S ribosomes (80S in eukaryotes) and are constructed of a 50S and a 30S subunit. The S stands for Svedberg unit.
Localization of genetic material.
Provides shape and protection from osmotic stress, toxin substances. And in pathogens, it can contribute to pathogenicity.
Survival under harsh environmental conditions; only observed in Bacteria.
The plasma membrane and all the surrounding layers external to it. The cell envelopes of many bacteria consist of the plasma membrane, cell wall, and at least one additional layer.
Intracytoplasmic Membrane Systems
In addition to the plasma membrane, some bacteria have extensive intracytoplasmic membrane systems. These internal membranes and the plasma membrane have a common, basic design. However, they can differ significantly in terms of the lipids and proteins they contain.
Fluid Mosaic Model
Proposes that membranes are lipid bilayers within which proteins float.
Within the lipid bilayer, small globular particles are visible; these have been suggested to be membrane proteins lying within the membrane lipid bilayer.
Being structurally asymmetric, with polar and nonpolar ends. Most membrane-associated lipids are amphiphatic.
A type of membrane protein. Loosely connected to the membrane and can be easily removed. They are soluble in aqueous solutions and make up about 20-30% of total membrane protein.
Type of membrane protein. Not easily extracted from membranes and are insoluble in aqueous solutions when freed from lipids.
They are amphiphatic, their hydrophobic regions are buried in the lipid while the hydrophilic portions project from the membrane surface. Carbohydrates often are attached to the outer surface of plasma membrane proteins.
The integral proteins carry out some of the most important functions of the membrane. Many are transport proteins used to move materials in and out of cell. Some part of protein secretion systems. Other involved in energy conserving processes.
The plasma membrane is very dynamic. The lipid composition varies with environmental temperature in such a way that the membrane remains fluid during growth. For example, bacteria growing at lower temperatures have more unsaturated fatty acids in their membrane phospholipids-that is, there are one or more covalent bonds in the long hydrocarbon chain. At higher temperatures, there phopspholipids have more saturated fatty acids- those in which the carbon atoms are connected only with single covalent bonds.
Other factors also affect the lipid composition of membranes. For instance, some pathogens change the lipids in their plasma membranes in order to protect themselves from antimicrobial peptides produced by the immune system.
Bacterial membranes usually differ from eukaryotic membranes in lacking sterols (steroid-containing lipids) such as cholesterol. However, many bacterial membranes contain sterol-like molecules called hopanoids.
They are synthesized from the same precursors as steroids, and like the sterols in eukaryotic membranes, they probably stabilize the membrane. Hopanoids are also of interest to ecologists and geologists: the total mass of hopanoids in sediments is estimated to be around 10^11-12 tons-about as much as the total mass of organic carbon in all living organisms (10^12 tons)- and evidence exists that hopanoids have contributed significantly to the formation of petroleum.
One important feature of the cell envelope of gram-negative bacteria is a space that is frequently seen between the plasma membrane and the outer membrane in electron micrographs. It also is sometimes observed between the plasma membrane and the wall in gram-positive bacteria. This space is called the periplasmic space.
The subtance that occupies it is the periplasm. The nature of the periplasmic space and periplasm differs in gram+ and gram- bacteria.
In gram-negative bacteria, contains hydrolytic enzymes and binding proteins for nutrient processing and uptake; in gram-positive bacteria and archaeal cells, may be smaller of absent.
Bacteria Cell Wall
The game-positive cell wall consists of a single, 20-80 nm thick, homogenous layer of peptidoglycan (murein) lying outside the plasma membrane.
In contrast, the gram-negative cell wall is quite complex. It has a 2-7-nm thick peptidoglycan layer covered by a 7-8 nm thick outer membrane. THe walls of gram-positive cells are more resistant to osmotic pressure because of the thicker peptidoglycan layer.
Enormous, meshlike polymer composed of many identical subunits. Each subunit contains two sugar derivatives, N-acetyl-glucosamine (NAG) and N-acetylmuramic acid (NAM), and several different amino acids.
The amino acids form a short peptide consisting of four alternating D- and L-amino acids; the petide is conencted to the carboxyl group of NAM. The presence of D-amino acids protects against degradation by most peptidases, which recognize only the L-isomers of amino acid residues.
The meshlike peptidoglycan polymer is formed by linking the petidoglycan subunits together to form a peptidoglycan strand. These strands are then cross-linked to each other. The backbone of a peptidoglycan strand is thus composed of alternating NAG and NAM residues. The peptidoglycan strands are joined by cross-links between the peptides of adjacent strands.Each peptidoglycan strand is helical, and the peptides extend out from the backbone at right angles to each other. Thus each strand can become crosslinked to strands at each side, above, and below.
Many bacteria cross-link peptidoglycan by connecting the carboxyl group of the terminal D-alanine (position 4) directly to the amino group of diaminopimelic acid (position 3). Some other bacteria use a peptide interbridge instead. With or without a peptide interbridge, cross-linking results in an enormous peptidoglycan sac that's actually one dense, interconnected network. These sacs have been isolated from gram+ bacteria and are strong enough to retain their shape and integrity, yet they're relatively porous and elastic.
Gram+ Cell Walls
Gram+ bacteria belong to the phyla Firmicutes and Actinobacteria. Most of these bacteria have thick cell walls composed primarily of peptidoglycan. In addition, their cell walls usually contain large amounts of secondary cell wall polymers, including teichoic acids.
The periplasmic space of gram+ bacteria lies between plasma membrane and cell wall, and is smaller compared to gram- bacteria. The periplasm has few proteins; probably because the peptidoglycan sac is porous and any proteins secreted by the cell usually pass through it.
Polymers of glycerol or ribitol joined by phosphate groups. Amino acids such as D-alanine or sugars such as glucose are attached to the glycerol and ribitol groups. The teichoic acids are covalently connected to peptidoglycan or to plasma membrane lipids; in the latter case, they are called lipoteichoic acids. Teichoic acids appear to extend to the surface of the peptidoglycan. Because they're negatively charged, they help give the gram+ cell wall its negative charge. Teichoic acids aren't present in gram- bacteria.
Function not clear, but many theories. One is to help create and maintain the structure of the cell envelope and to protect the cell from harmful substances in the environment. Some of them may be involved in binding pathogenic species to host tissues, thus initiating the infectious disease process.
Enzymes secreted by gram+ bacteria.
Exoenzymes often serve to degrade polymeric nutrients that would otherwise be too large for transport across the plasma membrane. Those proteins that remain in the periplasmic space are usually attached to the plasma membrane.
Gram+ Cell Walls: Proteins on Surface
Staphylococci and most other gram+ bacteria have a layer of proteins on the surface of the peptidoglycan. These proteins are involved in interactions of the cell with its environment.
Some are noncovalently attached by binding to teichoic acids or other cell wall polymers. For example, S-layer proteins bind noncovalently to polymers scattered throughout the cell wall. Enzymes involved in peptidoglycan synthesis and turnover also seem to interact noncovalently with the cell wall.
Other surface proteins are covalently attached to the peptidoglycan. Many covalently attached proteins have roles in virulence. For example, the M protein of pathogenic streptococci aids in adhesion to host tissues and interferes with host defenses In staphylococci, these surface proteins are covalently joined to the pentaglycine interbridge of the peptidoglycan.
An enzymes called sortase catalyzes the attachment of these surface proteins to the peptidoglycan. Sortases are attached to the plasma membrane of the cell.
Gram+ Cell Walls: Acid-Fast Bacteria
The acid-fast bacteria, a taxon within the phylum Actinobacteria, have somewhat different cell wall structure than the typical gram+ wall just described. These bacteria include members of the genus Mycobacterium. Their cell walls are unique among the gram+ bacteria because they consist of other layers in addition to peptidoglycan. An important component of their walls is a group of fatty acids called mycolic acids. Some have proposed that the mycolic acids form a bilayer structure that's analogous to the outer membrane of gram- bacteria.
Gram- Cell Walls
Much more complex than gram+ walls. The thin peptidoglycan layer next to the plasma membrane and bounded on either side by the periplasmic space usually constitutes only 5-10% of the wall weight.
The periplasmic space of gram- bacteria is also strikingly different from that of gram+ bacteria. It ranges in width from 1-71 nm. May constitute about 20-40% of total cell volume. When cell walls are disrupted carefully or removed without disturbing the underlying plasma membrane, periplasmic enzymes and other proteins are released and may be easily studied. Some periplasmic proteins participate in nutrient acquisition. Others involved in energy conservation.
The outer membrane lies outside the thin peptidoglycan layer and is thought to be linked to the cell in two ways.
- First is by Braun's lipoprotein, the most abundant protein in the outer membrane. This small lipoprotein is covalently joined to the underlying peptidoglycan and is embedded in the outer membrane by its hydrophobic end.
- The second linking mechanism not as widely accepted. It involves contact sites that appear to join the outer membrane and the plasma membrane. These contact sites not observed when cells are fixed by rapid freezing, and it's argued that they may be artifacts of chemical fixation.
Most unusual constituents of the outer membrane are its lipopolysaccharides.
Lipopolysaccharides (LPSs) Structure
Large, complex molecules that contain both lipid and carbohydrate, and consist of three parts. (1) Lipid A, (2) the core polysaccharide, and (3) the O side chain.
The lipid A region contains two glucosamine sugar derivatives, each with 3 fatty acids and phosphate or pyrophosphate attached. The fatty acids of lipid A are embedded in the outer membrane, while the remainder of the LPS molecule projects from the surface.
The core polysaccharide is joined to lipid A. The O side chain or O antigen is a polysaccharide chain extending outward from the core. It has several peculiar sugars and varies in composition between bacterial stains.
Lipopolysaccharides (LPSs) Function
Many important functions.
1) Contributes to the negative charge on the bacterial surface because the core polysaccharide usually contains charged sugars and phosphate.
2) Helps stabilize outer membrane structure because lipid A is a major constituent of the exterior leaflet of the outer membrane.
3) May contribute to bacterial attachment to surfaces and biofilm formation.
4) Helps create a permeability barrier. The geometry of LPS and interactions between neighboring LPS molecules are thought to restrict the entry of bile salts, antibiotics, and other toxic substances that might kill or injure the bacterium.
5) Also plays a role in protecting pathogenic gram- bacteria from host defenses. The O side chain of LPS is also called the O antigen because it elicits an immune response by an infected host. This response involves the production of antibodies that bind the strain-specific form of LPS that elicited the response. However, many gram- bacteria can rapidly change the antigenic nature of their O side chains, thus thwarting host defenses.
6) Importantly, the lipid A portion of LPS is toxic; as a result, LPS can act as an endotoxin and cause some of the symptoms that arise in gram- bacterial infections. If LPS or lipid A enters the bloodstream, a form of septic shock develops, for which there's no direct treatment.
Mechanism of Gram Staining
The difference between those bacteria that stain gram+ and those that stain gram- is due to the physical nature of their cell walls. If the cell wall is removed from gram+ bacteria, they stain gram-. Furthermore, bacteria that never make cell walls, such as the mycoplasmas, also stain gram-.
During the procedure, bacteria are first stained with crystal violet and next treated with iodine to promote dye retention. When bacteria are treated ethanol in the decolorization step, the alcohol is thought to shrink the pores of the thick peptidoglycan found in the cell walls of most gram+ bacteria, causing the peptidoglycan to act as a permeability barrier that prevents loss of crystal violet. Thus the dye-iodine complex is retained during the decolorization step and the bacteria remain purple.
In contrast, the peptidoglycan in gram- cell walls is very thin, not as highly crosslinked, and has larger pores. Alcohol treatment also may extract enough lipid from the outer membrane to increase the cell wall's porosity further. For these reasons, alcohol more readily removes the crystal violet-iodine complex from gram- bacteria. Thus gram- bacteria are easily stained red or pink by the counterstain safranin.
Despite role of LPS in creating a permeability barrier, the outer membrane is more permeable than the plasma membrane and permits the passage of small molecules such as glucose and other monosaccharides. This is due to porin proteins.
Most of them cluster together to form a trimer in the outer membrane. Each porin protein spans the outer membrane and is more or less tube shaped; its narrow, water-filled channel allows passage of molecules smaller than about 600 daltons. However, large molecules such as vitamin B12 also cross the outer membrane. Such large molecules don't pass through porins, but have specific carries transport them instead.
Microbes have several mechanisms to changes in osmotic pressure. This pressure arises whent he concentration of solutes inside the cell differs from that outside, and the responses work to equalize the solute concentrations. However, in certain situations, osmotic pressure can exceed the cell's ability to acclimate. In these cases, additional protection is provided by the cell wall. When cells are in hypotonic solutions-ones in which the solute concentration is less than that in the cytoplasm-water diffuses into the cell, causing it to swell. Without the cell wall, the pressure on the plasma membrane would become so great that the membrane would be disrupted and the cell would burst-a process called lysis. Conversely, in hypertonic solutions, water flows out and the cytoplasm shrivels up-a process called plasmolysis.
Cell Wall Protection
The protective nature of the cell wall is most clearly demonstrated when bacterial cells are treated with lysozyme or pencillin.
The enzyme lysozyme attacks peptidoglycan by hydrolyzing the glycosidic bond that connects N-acetylmuramic acid with N-acetylglucosamine.
Pencillin works by a different mechanism. It inhibits the enzyme transpeptidase, which is responsible for making the cross-links between peptidoglycan chains.
If bacteria are treated with either of these substances while in a hypotonic solution, they lyse. However, if in an isotonic solution, they survive and grow normally.
If they're gram+, treatment with lysozyme or penicillin results in the complete loss of the cell wall, and the cell becomes a protoplast.
When gram- bacteria are exposed to lysozyme or penicillin, the peptidoglycan layer is lost, but the outher membrane remains. These cells are called spheroplasts. Because they lack a complete cell wall, both protoplasts and spheroplasts are osomotically sensitive. If they are transferred to a hypotonic solution, they lyse.
Layers that are well organized and not easily washed off. Capsules are most often composed of polysaccharides, but some are constructed of other materials. Capsules are clearly visible in the light microscope when negative stains or specific capsule stains are employed. Also can be studied with electron microscope.
Although they're not required for growth and reproduction in lab cultures, they confer several advantages when bacteria grow in their normal habitats. They help pathogenic bacteria resist phagocytosis by host phagocytes.
Capsules contain a great deal of water and can protect against desiccation. They exclude viruses and most hydrophobic toxic materials such as detergents.
A zone of diffuse, unorganized material that's removed easily. It's usually composed of polysaccharides, but is not as easily observed by light microscopy. Gliding bacteria often produce slime, which in some cases has been shown to facilitate motility.
A layer consisting of a network of polysaccharides extending from the surface of the cell. The term can encompass both capsules and slime layers because they usually are composed of polysaccharides. The glycocalyx aids in attachment to solid surfaces, including tissue surfaces in plant and animal hosts.
Many bacteria have this regularly structured layer. Has a pattern like floor titles and is composed of protein or glycoprotein. In gram- bacteria, the S-layer adheres directly to the outer membrane; it's associated with the peptidoglycan surface in gram+ bacteria.
BIological roles include protecting cell against ion and pH fluctuations, osmotic stress, enzymes, or predacious bacteria. Also helps maintain the shape and envelope rigidity of some cells, and it can promote cell adhesion to surfaces. Finally, the S-layer seems to protect some bacterial pathogens against host defenses, thus contributing to their virulence.
The potential use of S-layers in nanotechnology is due to the ability of S-layer proteins to self-assemble. That is, the S-layer proteins contain info required to spontaneously associate and form the S-layer without the aid of any additional enzymes or other factors. Thus, S-layer proteins could be used as building blocks for the creation of technologies such as drug-delivery systems and novel detection systems for toxic chemicals or bioterrorism agents.
The plasma membrane and everything within it.
The material bounded by the plasma membrane; thus the cytoplasm is a major part of the protoplast.
Homologues of all three types of eukaryotic proteins (microtubules, microfilaments, and intermediate filaments) have been identified in bacteria, and two have been in archaea. The bacterial cytoskeletal proteins are structurally similar to their eukaryotic counterparts and carry out similar functions: they particupate in cell division, localize proteins to certain sites in the cell, and determine cell shape. In addition, some cytoskeletal proteins appear to be unique in bacteria.
These can be extensive and complex in pohotosynthetic bacteria and in bacteria with very high respiratory activity, such as the nitrifying bacteria. The internal membranes of the photosynthetic cyanobacteria are called thylakoids and are analogous to the thylakoids of chloroplasts.
The internal membranous structures observed in bacteria may be aggregates of spherical vesicles, flatterned vesicles, or tubular membranes. They're often connected to the plasma membrane and are thought to arise from it by invagination. However, these internal membranes differ from the plasma membrane by being enriched for proteins and other molecules involved in energy conservation. For instance, the thylakoids of cyanobacteria contain the chlorophyll and photosynthetic reaction centers responsible for converting light energy into ATP, the energy currency used by cells. Thus the function of internal membranes may be to provide a larger membrane surface for greater metabolic activity.
Formed by aggregation of substances that may be either organic or inorganic.
Inclusions can take the form of granules, crystals, or globules; some are amorphous.
Some inclusions lie free in the cytoplasm. Other inclusions are enclosed by a shell or membrane that is single-layered and may consist of proteins or of both proteins and phospholipids. Some inclusions are surrounded by invaginations of the plasma membrane.
Many inclusions are used for storage or to reduce osomotic pressure by tying up molecules in particulate form. The quantity of inclusions used for storage varies with the nutritional status of the cell.
Some inclusions are so distinctive that they are increasingly being referred to as microcompartments.
Cells have a variety of storage inclusions. Many are formed when one nutrient is in ready supply but another nutrient is not. Some store end products of metabolic processes. In some cases these end products are used by the microbe when it is in different environmental conditions. The most common sotrage inclusions are glycogen inclusions, polyhydroxyalkonate granules, sulfur globules, and polyphosphate granules. Some storage inclusions, such as the cyanophycin granules in cyanobacteria, are observed only in certain organisms.
Longer than the length of the cell. Thus the chromosome is compacted in some way. It is thought that it is the result of supercoiling, which produces a dense, central core of DNA with loops extending out from the core. There's evidence that some of the proteins found in the nucleoid also contribute to packing the DNA into a smaller space.
In bacteria, the protein HU is thought to be imporant. An HU homologue is also found in some archaea. Most other archaea have histones associated with their chromosomes. These histones form nucleosomes that are similar to the nucleosomes observed in eukaryotes.
During cell division, bacterial chromosomes are further condensed by proteins called condensins. This extra level of packing is important for proper segregation of daughter chromosomes during cell division.
Irregularly shaped region that contains the cell's chromosomes and numerous proteins. The chromosomes of most bacteria and all known archaea are a single circle of double stranded DNA, but some bacteria have a linaer chromosome, and some bacteria have more than one chromosome.
For most bacteria and all known archaea, the nucleoid is simply a region in the cytoplasm; it's not separated with a membrane. However, there are a few exceptions being looked into.
Extra-chromosomal DNA molecules. They play important roles. Also proved to be invaluable to microbiologists and molecular geneticists in constructing and transferring new genetic combinations and in cloning genes.
Small, double-stranded DNA molecules that can exist independently of the chromosome. Both circular and linear plasmids have been documented, but most known plasmids are circular. Plasmids have relatively few genes, generally less than 30. Their genetic info is not essential to the host, and cells that lack them usually function normally. However, many plasmids carry genes that confer a selective advantage to their hosts in certain environments.
Plasmids are able to replicate autonomously. That is, plasmid and chromosomal replication are independent. Single-copy plasmids produce only one copy per host cell. Multicopy plasmids may be present at concentrations of 40 or more per cell.
Plasmids are inherited stably during cell division, but sometimes go through curing.
Some plasmids are able to integrate into the chromosome and thus are replicated with the chromosome. Such plasmids are called episomes.
Loss of a plasmid. Happens when plasmids are inherited during cell division but are not equally proportioned into daughter cells.
Can occur spontaneously or be induced by treatments that inhibit plasmid replication but not the host cell replication.
Can transfer copies of themselves to other bacteria during conjugation.
Confer antibiotic resistance to the cells that contain them. These are important in the spread of antibiotic resistance among bacteria.
May give the bacteria that harbor them a competitive advantage in the microbial world. Bacteriocins are proteins that destroy other, closely related bacteria.
Contain genes for the synthesis of bacteriocins known as colicins, which are produced by and directed against strains of E. colo.
Encode factors that make their hosts more pathogenic.
Carry genes for enzymes that degrade substances such as aromatic compounds (toluene), pesticides (2, 4-dichlorophenoxyacetic acid), and sugars (lactose).
Pili and Fimbriae
Many bacteria and archaea have short, fine, hairlike appendanges that are thinner than flagella. They're usually called fimbriae or pili. These terms are synonymous, although certain structures are always called one or the other. Other than that, terms are used interchangeably.
A cell may be covered with up to 1000 fimbriae, but they're only visible in an electron microscope. They're slender tubes composed of helically arranged protein subunits and are about 3-10 nm in diameter and up to several micrometers long. Several different types of fimbriae have been identified in gram- bacteria. Most function to attach bacteria to solid surfaces such as rocks in streams and host tissues. One type, called type IV pili, are involved in motility and the uptake of DNA during the process of bacterial transformation. Gram+ bacteria have at least two types of pili; both are involved in attaching the bacteria to surfaces.
Many bacteria have up to 10. These hairlike structures differ from other pili in the following ways. Often larger. They're genetically determined by conjugative plasmids and are required for conjugation. Some bacterial viruses attach specifically to receptors on sex pili at the start of their reproductive cycle.
Threadlike locomotor appendages extending outward from the plasma membrane and cell wall. Although the main function of flagella is motility, they can have other roles. They're important for certain types of swarming behavior. They can be involved in attachment to surfaces, and in some bacteria they're virulence factors.
Slender, rigid structures about 20nm across and up to 20 um long. Flagella are so thin they can't be observed directly without techniques designed to increase their thickness.
Bacterial species often differ distinctively in their patterns of flagella distribution, and these patterns are useful in identifying bacteria.
Composed of three parts. Filament, basal body, and hook. Some bacteria have sheaths surrounding their flagella.
Because many components of the flagellum lie outside the cell wall, they must be transported across the plasma membrane and cell wall. Interestingly, evidence suggests that components of the basal body are evolutionarily related to a type of protein secretion system observed in gram- bacteria. This system, called a type III secretion system, has a needlelike structure through with proteins are secreted. The needle is thought to be analogous to the filament of the flagellum. Thus the flagellin subunits are transported by way of a type III-like secretion process through which the filament's hollow internal core. When the subunits reach the tip, they spontaneously aggregate under the direction of a protein called the filament cap; thus the filament grows at its tip rather than at the base. Filament synthesis is an example of self-assembly.
have one flagellum; if it's located at an end, it's said to be a polar flagellum.
have a single flagellum at each pole.
have a cluster of flagella at one or both ends.
Flagella are spread evenly over the whole surface of peritrichous bacteria.
Longest and most obvious portion of flagellum. Extends from the cell surface to the tip. The filament is a hollow rigid cylinder constructed of subunits of the protein flagellin. The filament ends with a capping protein.
Part of flagellum. Embedded in the cell. The most complex part of a flagellum. It appeared to have four rings, L, P, S and M ring, connected to a central rod. But now it's known the S and M ring are different portions of the same protein, and are referred to as the MS ring. The C ring was discovered, which is on the cytoplasmic side of the MS ring.
Gram+ bacteria have only two sings, an inner ring connected to the plasma membrane and an outer one probably attached to the peptidoglycan.
Links the filament to its basal body and acts as a flexible coupling. Made of different protein subunits.
Synthesis of Bacterial Flagella
Complex process involving at least 20-30 genes. Besides the gene for flagellin, 10 or more genes code for hook and basal body proteins; other genes are concerned with the control of flagellar construction or function. How the cell regulates or determines the exact location of flagella is not known.
Several structures outside the cell wall contribute to motility. 4 major methods of movement in bacteria are: the swimming movement conferred by flagella; the corkscrew movement of spirochetes; the twitching motiltiy associated with type IV pili; and gliding motility.
Motile bacteria and bacteria don't move aimlessly. Rather, motility us used to move toward nutrients such as sugars and amino acids and away from many harmful substances and bacterial waste products. Motile bacteria also can respond to environmental cues such as temperature (thermotaxis), light (phototaxis), oxygen (aerotaxis), osmotic pressure (osmotaxis), and gravity. Movement toward chemical attractants and away from repellents is known as chemotaxis. Motile archaea also exhibit a variety of different types of directed movements, including chemotaxis.
Although spirochetes have flagella, they work in a different manner. In many spirochetes, multiple flagella arise from each end of the cell and associate to form an axial fibril, which winds around the cell. The flagella don't extend outside the cell wall but rather remain in the periplasmic space and are covered by an outer sheath. The way in which axial fibrils propel the cell hasn't been fully established. They're thought to rotate like the external flagella of other bacteria, causing the corkscrew-shaped outer sheath to rotate and move the cell through the surrounding liquid. Flagellar rotation may also flex or bend the cell and account for the creeping or crawling movement observed when spirochetes are in contact with a solid surface.
Twitching and Gliding Moltility
Occurs when cells are on a solid surface. Cna involve type IV pili, the production of slime, or both. Thus they're considered together.
Type IV pili are present at one or both poles of some bacteria and are involved in twitching and gliding motility of some bacteria.
Characterized by short, intermittent, jerky motions of up to several micrometers in length and is normally seen on very moist surfaces. It occurs only when cells are in contact with each other; isolated cells rarely move by this mechanism. Considerable evidence exists that the pili alternately extend and retract to move bacteria during twitching motility. The extended pilus contacts the surface at a point some distance from the cell body. When the pilus retracts, the cell is pulled forward. Hydrolysis of ATP is thought to power the extension/retraction process.
Smooth and varies greatly in rate and in the nature of the motion. Although first observed over 100 years ago, the mechanism by which many bacteria remains a mystery. Some glide along in a direction parallel to the longitudinal axis of their cells. Others travel with a screwlike motion of even move in a direction perpendicular to the long axis of the cells. Still others rotate around their longitudinal axis while gliding. Such diversity in gliding movement correlates with the observation that more than one mechanism for gliding motility exists. Some types involve type IV pili, some involve slime, and some involve mechanisms that have not yet been elucidated.
Bacteria and archaea exhibit taxes to a variety of stimuli, including light and oxygen. However, the movement of cells toward chemical attractants or away from chemical repellents (chemotaxis) is the best studied type of taxis, and bacterial chemotaxis is best understood.
Attractants and repellents are detected by chemoreceptors.
The chemotactic behavior of bacteria has been studied using the tracking microscope. In the absence of a chemical gradient, bacteria move randomly, switching back and forth between a run and a tumble. During a run, the bacterium travels in a straight or slightly curved live. After a few seconds, the flagella "fly apart" and the bacterium stops and tumbles. The tumble randomly reorients the bacterium so that it often is facing in a different direction. Therefore when it begins the next run, it usually goes in a different direction. In contrast, when the bacterium is exposed to an attractant, it tumbles less frequently (or has longer runs) when traveling toward the attractant. Although the tumbles can still orient the bacterium away from the attractant, over time, the bacterium gets closer to the attractant. The opposite response occurs with a repellent. Tumbling frequency decreases (the run time lengthens) when the bacterium moves away from the repellent.
Clearly the bacterium must have some mechanism for sensing that it's getting closer to the attractant (or moving away from repellent). The behavior of the bacterium is shaped by temporal changes in chemical concentration. The bacterium moves toward the attractant because it senses that the concentration of the attractant is increasing. Likewise, it moves away from a repellent because it senses that the concentration of the repellent is decreasing. The bacterium's chemoreceptors play a critical role in this process.
Proteins that bind chemicals and transmit signals to other components of the chemosensing system. The chemosensing systems are very sensitive and allow the cell to respond to very low levels of attractants. In gram- bacteria, the chemoreceptor proteins are located in the periplasmic space or in the plasma membrane. Some receptors also participate in the initial stages of sugar transport into the cell.
Several genera of gram+ bacteria can form a resistant dormant structure called an endospore. Endospore-forming bacteria are common in soil, where they must be able to withstand fluctuating levels of nutrients. Endospore formation (sporulation) normally commences when growth ceases due to lack of nutrients. Thus it's a survival mechanism that allows the bacterium to produce a dormant cell that can survive until nutrients are again available and vegetative growth can resume. Interestingly, some bacteria have modified the sporulation process and use it to produce live offspring within themselves.
Endospores are extraordinarily resistant to environmental stresses such as heat, ultraviolet radiation, gamma radiation, chemical disinfectants, and desiccation. Inf act, some endospores have remained viable for around 100K years. They're of both practical and theoretical interest. Because of their resistance and the fact that several species of endospore-forming bacteria are dangerous pathogens, endospores are of great practical importance in food, industrial, and medical microbiology. In these areas, it's essential to be able to sterilize solutions and solid objects. Endospores often survive boiling for an hour or more; therefore autoclaves must be used to sterilize many materials. Endospores are of considerable theoretical interest to scientists studying the construction of complex biological structures. Bacteria manufacture these intricate structures in a very organized fashion over a period of a few hours.
Can be examined with both light and electron microscopes. Because endospores are impermeable to most stains, they often are seen as colorless areas in bacteria treated with methylene blue and other simple strains; staining procedures specific for endospores are used to make them clearly visible. Endospore position in the mother cell (sporangium) frequently differs among species, making it of value in identification. Endospores may be centrally located, close to one end (subterminal), or terminal. Sometimes an endospore is so large that it swells the sporangium.
Complex. The spore often is surrounded by a thin, delicate covering called the exosporium. A coat lies beneath the exosporium. It's composed of several protein layers and may be fairly thick. The cortex, which may occupy as much as half the spore volume, rests beneath the coat. It's made of a peptidoglycan that is less cross-linked than that in vegetative cells. The core wall is inside the cortex and surrounds the core. The core has normal cell structures such as ribosomes and a nucleoid but has a very low water content and is metabolically inactive.