bacterial architecture

Figure 1. Electron micrograph of E. coli. This is an electron micrograph of a common Gram negative bacteria that resides in the intestines of most vertebrates on the planet. In size it is approximately 1 to 2 micrometers in length by 0.5 to 1.0 micrometers in width. These cells are actively growing as can be seen by the number of cells that are in the process of cell division or binary fission. Some of them look to be ready to separate and others appear to be just beginning to form their "cross walls". Their surface appears to be covered with a sort of fuzzy material. This "fuzz" is composed of lipopolysaccharide (LPS) and capsular material that covers the outer portion of the cell. These substances serve as a sort of armor to protect the cells.

Figure 2. Campylobacter. This is an electron micrograph of a spiral-shaped gram negative, bacterium, Campylobacter that is an important intestinal pathogen. Dr. M. Konkel in the WSU Microbiology department is investigating the pathogenic determinants in this bacteria.

The size, shape and arrangement of bacteria, and other microbes, is the result of their genes and thus is a defining characteristic called MORPHOLOGY. Bacteria come in a bewildering and exciting variety of size and shapes, with new ones being discovered all the time. Nature loves VARIETY in its life forms as you can see just looking at your fellow humans. The most common bacterial shapes are RODS, COCCI, and SPIRAL. However, within each of these groups are hundreds of unique variations. Rods may be long, short, thick, thin, have rounded or pointed ends, thicker at one end than the other etc. Cocci may be large, small, or oval shaped to various degrees. Spiral shaped bacteria may be fat, thin, loose spirals or very tight spirals. The GROUP ASSOCIATIONS of microbes, both in liquid on solid medium, are also defining. Bacteria may exists mainly as single cells or as common grouping such as chains, uneven clusters, pairs, tetrads, octads and other packets. They may exist as masses embedded within a capsule. There are square bacteria, star-shaped bacteria, stalked bacteria, budding bacteria that grow in net-like arrangements and many other morphologies. When observing bacteria one should describe as many of these characteristics as possible. Consider how you would describe a BLIND DATE to a friend.

For an exceedingly colorful view of bacterial size and shape visit the Molecular Probe site. Click on the GALLERY and then on BACTERIA STAINS. Compare the size of Bacillus cereus and Pseudomonas aeruginosa. Click here for artificially colored bacteria.

BACTERIAL CELL COMPONENTS

Composite Bacterial Cell

Figure 3. Composite cartoon showing the major structures found in bacteria. No one bacteria contains all of these different components, but most bacteria contain the majority of them. Another GENERIC bacterial cell.

A typical bacterial cell is composed of the following structures. You are required to learn the function of each.

The CYTOPLASM is the TOTAL OF EVERYTHING INSIDE of the cytoplasmic membrane. It has a gel-like consistency, but small molecules can move through it rapidly; that is it only takes a few microseconds for a molecule to move from one end of the cell to the other. The following components are in the cytoplasm.

Proteins, mostly enzymes. Each E. Coli cell contains approximately 1,000 different enzymes at any given time. There may be only a few molecules of an enzyme or 1,000s of copies. Proteins vary in size from 8,000 molecular weight to >1,000,000, with the average MW being ~40,000.
RIBOSOMES are composed of RNA and protein and are the FACTORIES upon which the proteins are made. There are 1,000s of ribosomes in each healthy growing cell.

UnknownMicrobe.gif (32607 bytes)
Figure 4. Unknown large bacterium found in forest pond. Note the large granules which could be sulfur or poly-beta- hydroxybutyric acid. Image taken using a phase contrast X40 objective by Steve Durr, [email protected]

A number of STORAGE GRANULES may be present in a cell depending on its physiology & nutritional environment. These may be STARCH, FAT, SULFUR, or PHOSPHATE. Bacteria exist in a very competitive environment where nutrients are usually in SHORT SUPPLY, so they tend to store up extra nutrients when possible.
PLASMIDS are small circular DNA molecules that can be thought of as carrying EXTRA GENES that can be used for special situations. They usually can be DISPENSED WITH when not required. There may be several different plasmids in one cell and the numbers of each may vary from only ONE to 100s in a cell .
Each cell contains a large CIRCULAR GENOME composed of DNA. This is referred to as the CHROMOSOME of a cell. This chromosome contains the basic genetic information needed by the cell to survive and produce daughter cells. One might think of it as the INSTRUCTION MANUAL for building a particular bacterium.
Each cell contains NUTRIENTS it has imported in from the outside, or made inside, and all the chemical intermediates the cell required to make new structures. In addition, cells contain waste materials that are subsequently excreted. For example, common waste materials of many microbes include alcohol, lactic and acetic acids; which some have called MICROBE PEE.

Figure 5. Lipid bilayer. The red balls represent phosphate groups and the bent-lines the fatty acid molecules. Each half is a phospholipid layer and the two together comprise the BILAYER typical of biological membranes. The central portion is hydrophobic and dislikes water whereas the outermost portions are hydrophilic and love water. Click here for a discussion of lipid bilayers with a few basic figures.

The CYTOPLASMIC MEMBRANE is the structure that makes the cell possible as it is a SELECTIVE BARRIER that separates the highly organized machinery of a cell from the frightful chaos on the exterior. The cytoplasmic membrane is composed of a PHOSPHOLIPID BILAYER in which are embedded the various proteins that control what goes in and out of a cell (Views of membranes and another view that can be manipulated and still another view that takes a lot of memory but shows much detail). The cytoplasmic membrane is fluid (like a soap bubble) and usually very DELICATE and easily ruptured if the supporting cell wall is removed. The inner portion of the cytoplasmic membrane is composed of lipid or fat molecules that make it impermeable to molecules that dissolve in water, but water itself can MOVE FREELY through the cytoplasmic membrane. This inner region is said to be HYDROPHOBIC or "water hating". Because the cytoplasmic membrane is a bilayer, the lipid molecules face each other, placing the phosphate groups on the outer edges of the cytoplasmic membrane. Phosphate is HYDROPHILIC and likes to associate with water molecules. Many of the proteins embedded in the lipid bilayer are TRANSPORT proteins that BIND specific molecules and carry them into or out of the cell as required. The proteins allow the cell to live in very dilute nutrient solutions because they soak up the nutrients like a selective "sponge" and bring in the rare nutrient molecules. Click here to look at some lipid molecules, chose the "gif" forms.

Cell Membrane
Figure 6. Cartoon of cytoplasmic membrane, showing its complex composition of protein, carbohydrates and lipids. The proteins "float" in the lipid "sea". Some proteins span the membrane, while others are attached to only one side or the other, or to other proteins embedded in the cytoplasmic membrane. Water can pass freely through the cytoplasmic membrane, but most other molecules can not. One important characteristic of the cytoplasmic membrane is that PROTONS (H+) are unable to cross the cytoplasmic membrane. Click here to learn about the lipid bilayer. Click here to see more pictures of membranes along with a good discussion.

Gram Positive and Gram Negative CW
Figure 7. Cartoon illustrating the relative structure of gram negative (top) and gram positive (bottom) cell walls. The major differences lie in the thickness of the rigid peptidoglycan layer and in the presence of an outer membrane in gram negative cells. In gram negative cells the peptidoglycan layer is very thin, being only a few molecules thick, whereas in gram positive cells this layer is very thick.

The CELL WALL is the total structure that defines the exterior of the cytoplasm. It includes the cytoplasmic membrane and one or two other layers in most prokaryotes. For a more detailed discussion of the bacterial cell wall composition click here.

In GRAM NEGATIVE cells the cell wall consists of three components; the cytoplasmic membrane, a thin layer of a rigid sugar MOLECULE NET immediately exterior to the cytoplasmic membrane that covers the entire cytoplasmic membrane and an outermost lipid bilayer called the OUTER MEMBRANE. The rigid layer is called PEPTIDOGLYCAN and is the SHAPE-FORMING component of the cell. The outer membrane also functions as a SELECTIVE GATE but its selectivity is much LESS specific than that of the cytoplasmic membrane. The outer layer of the outer membrane in Gram negative cells is composed of a molecule called LPS. This molecule is medically significant as it is HIGHLY TOXIC to humans and death from infections of G- bacteria are often the result of poisoning by the LPS.

Gram positive cells have only two layers, the cytoplasmic membrane and a THICK LAYER of peptidoglycan as the outermost component.

Several protein rod-like structures pass through the cell wall. These include the following:

PILI, are relatively short, HOLLOW PROTEIN RODS that are important in binding the cell to solid surfaces. Because of this characteristic pili are very important in pathogens. Often, if a pathogen loses its ability to produce pili it also loses its ability to cause disease. There are usually 100s of pili per cell. Pili are important in producing biofilms, which are the slimy layers covering your teeth, tongue, the bottoms of ships, trickling filter sewage treatment plants and the rocks in lakes and streams.

Pili Cartoon Em of Piliu

Figure 8. Pili. These fine hair-like, protein structures on the cell wall are pili. There are usually several 100/cell. In most cases they have special binding proteins at the end of the stiff rods. These types of pili are often important in adhesion of the cell to surfaces, such as teeth.

SEX PILI are longer (than the adhesion-pili described above), hollow protein rods that are mainly found on G- cells. These structures are involved in the TRANSFER OF GENETIC MATERIAL from one cell to another. The DNA that is transferred may either be plasmid or chromosomal DNA. Cells which carry the genes for making sex pili are said to be MALE or F+ cells. Usually the genes for sex pili formation are carried by SEX PLASMIDS. The process of DNA transfer is called CONJUGATION.

EM of Flagella flagella.gif (3452 bytes)
Figure 9. Bacterial flagella. Bacteria that move about using flagella always have the flagella arranged in a way that is descriptive of each bacteria.

FLAGELLA are the final structures that passes through the cell wall. Flagella are long, rigid protein rods that provide movement to many motile bacteria. At their base is a MOTOR that is driven by a flow of PROTONS from the outside of the cell inward; much like a turbine in a dam is driven by the flow of water through it. The number and arrangement of flagella on a cell is part of its GENETIC CHARACTERISTICS and is used to describe each species.

The most exterior components of bacterial cells are the CAPSULE and SLIME layers. These layers are usually composed of sugar polymers that are excreted by the cell under certain conditions. The term capsule usually applies to a DEFINED layer with a distinct outer edge, whereas a slime layer describes an ILL DEFINED concentration of polymeric material which just slowly gets less and less the further away from the cell. Although capsule production is a genetic characteristic, its production is STRONGLY influenced by the nutrient environment. For example, in a nutritionally poor medium a bacterium may produce little or no capsule/slime, but in the presence of a high concentration of sugar the capsule may be HUMONGOUS. The capsule has several roles.

	It protects the cell from DRYING.
	It serves as an extra source of NUTRITION. in times of need.
	It helps the cells STICK or attach to things because of its sticky (adhesive) nature and as such is part of biofilms.
	By sticking the cells to solid surfaces capsules/slime layers prevent them from washing away and provide a protective environment for the cells.
	It PROTECTS the cell from destruction by white blood cells.
	It may be TOXIC or inhibitory to a host's defense system and so aid in the disease process.

TAXIS

Magnetic Particles in Bacteria
Figure 10. Magnets inside magnetotaxic bacteria. Tiny iron magnets inside of MAGNETOTACTIC bacteria allow them to detect the earth's magnetic lines of force and move along them. What evolutionary advantage could this ability have that would improve the survival of such bacteria? Copied by permission of Dr. D.A. Bazylinski; ASM News 61 pg. 337(1995).

The ability of mobile cells to move in a desired direction is called TAXIS. Bacteria demonstrate several types of taxis. These include PHOTOTAXIS or the tendency to move TOWARDS or AWAY FROM LIGHT; CHEMOTAXIS. the ability to move TOWARDS a desired CHEMICAL or AWAY from a harmful one (POSITIVE or NEGATIVE taxis); MAGNETOTAXIS, the ability to follow the earth's MAGNETIC LINES OF FORCE. When you consider what these abilities REALLY MEAN you begin to achieve a true insight into the complexity of microbes. In the case of phototaxis the bacteria must have an EYE that detects light (i.e., they SEE light like you and I do). Furthermore different bacteria see different colored lights. In the case of chemotaxis, the bacteria have the equivalent of a NOSE in that they smell chemicals, identify them and then make a decision as to move away or towards them. That is, if they chemically detect a "nutrient" the motile ones move towards it, whereas if they "smell" a toxin, they move away from it. Magnetotactic bacteria have TINY MAGNETS in their cells that allow them to detect the north and south poles and then the ones in the northern hemisphere move north and those in the southern hemisphere move south. Which direction do you move in when you're hungry and smell pizza?

CRITICAL THINKING QUESTION: How does the taxis of bacteria relate to humans being bitten by mosquitoes?

SPORE FORMING BACTERIA

Figure 11. Spores from woodland pond. Image taken using a phase contrast X40 objective by Steve Durr, [email protected]

Some G+ bacteria form resistant structures called SPORES under adverse conditions. Spores are the most RESISTANT life form known. They are able to survive boiling in water at 100^oC for long periods. Spores are resistant to UV-light, to drying and many harmful chemicals. We know spores can live for 100s of yr. and recently spores several million yr. old have been revived from insects trapped in amber. Some disease organisms like anthrax and botulism form spores that reside in the soil. The size, shape, and location of a spore in the cell are all identifying genetic characteristics. For example, in the figure below, the spore on the left is TERMINAL, OVAL and SMALLER than the cell.

Figure 12. Spore structure and arrangements. The figure on the left shows the general structure of a bacterial ENDOSPORE. The figure on the right shows how the shape, location and the relative size of the formed-spore to the remains of the parent cell can be used to describe a bacterial spore-former. These characteristics are genetic and are like describing humans as being tall, blue eyed with blond hair. A = oval, terminal; B = rectangular, terminal; C = rectangular, subterminal, D = rectangular, central; E = circular, terminal; F = circular, central; G = terminal, club-shaped.

ALTERNATIVE INFORMATION

http://129.109.136.65/microbook/ch002.htm; Excellent chapter on bacterial structure and morphology.

Hosted by www.Geocities.ws