|Clickable Index of Bacteria|
You have learned that living organisms have at least seven unique properties listed in the index above. As Bacteria are living organisms, then they must possess these properties as well.
What are Bacteria?
We have learned that Biology can be illustrated as a tree of life:
In this tree of life, one of the basal branches evolves into a group of organisms that is part of Domain Prokarya, and is the current extant Kingdom Bacteria. Eubacteria is a synonym used by some authors for the kingdom of the bacteria.
|Number of Kingdoms|
As you may recall, bacteria were first part of Kingdom Plantae (green) when there were only two kingdoms. The organisms had cell walls at least and so that was enough for them to end up in that kingdom. Once we understood that they lack a nucleus or chloroplast or mitochondrion, the bacteria needed to be moved into a new kingdom...Monera (yellow) was created for them in the 3 to 5 kingdom era. The Archaea were split out from Monera in the transition to 6 kingdoms, so Monera was renamed Bacteria (or Eubacteria).
Bacteria are found as fossils in rocks dating to 3.5 billion years before present time, so these are the most ancient forms of life that are known. These ancient bacteria were the archetype for prokaryotic organisms. For perhaps 1.5 billion years, these organisms were the only ones on planet earth. Living in the ocean waters that were rich in chemicals formed in the interface between weather and atmosphere. Over this long period of time, these organisms evolved very diverse structure and function!
|Bacteria Have Cellular Structure|
Bacteria evolved many cell sizes, but all are relatively small. The mycoplasmas are the smallest of the bacteria, and cyanobacteria are among the largest. Most bacteria fall somewhere between these extremes. However, all of these bacteria are small cells compared to a "typical" plant cell. The plant cell has a nucleus that is larger than any of the bacteria, making it eukaryotic. The plant cell has mitochondria for respiration, and its size is in the same size range as a "typical" bacterium. The plant cell has chloroplasts for photosynthesis, and its size is in the same size range as a "typical" cyanobacterium.
Bacteria have also evolved into many cell shapes. Four major shape categories are shown here. In three dimensions the coccus form is a sphere; the bacillus is an elongated, rounded, cube; the spirillium is a worm-like helix; the vibrio is a bent bacillus or a partial gyre of a helical form.
Vibrio cholerae: http://www.designthatmatters.org/proto_portfolio/cholera_treatment/multimedia/vibrio_cholerae.jpg
Helicobacter pylori: http://helico.gsnu.ac.kr/
Another structural feature of bacteria is that the cells form various associations within a population of cells. Some species separate completely after cell division and are unicellular. Some remain connected for awhile after one cell division and until the next; this association is given the prefix diplo- as the cells appear to be paired. When cells form long chains by cell division, the prefix strepto- applies and we sometimes call these filamentous as well. Other species form clumps of cells and we use staphylo- as a prefix and sometimes call the association colonial too.
Given those ideas, what would you call this association?
If you guessed streptobacillus you would be most correct! Here is an image that has several artificial-color scanning electron microscope images of various bacteria. Perhaps you can classify each of the associations...
Bacterial cells come in three basic forms in terms of the cell boundary.
Mycoplasmas lack a cell wall and so have just a lipopolysaccharide bilayer for a cell membrane. The membrane includes transport proteins that regulate what comes in and goes out of the cell. But without any wall, these cells in a hypotonic environment would take in much water and burst open (lyse). So mycoplasma cells survive in isotonic environments such as in the cytoplasm of other cells, in body fluids, etc.
Other species of bacteria are called Gram positive because they can be sequentially stained with a crystal violet dye, iodine, and saffranin (red) dye. Because the purple dye gets in and complexed with iodine, but cannot be rinsed out before staining with red dye, they stain a dark purple color. The reason the purple dye-iodine complex cannot escape is the structure of the cell wall. Outside the lipopolysaccharide membrane is a thick wall made of murein. Murein is a peptidoglycan in which muramic acid is the peptide. This rigid wall is what allows these bacteria to live in a hypotonic environment without bursting. As water moves into these cells through the wall, turgor pressure develops inside them, but the wall prevents the bursting of the cells. Penicillin-related antibiotics kill these bacteria by inhibiting the synthesis of murein, so walls are thin and the cells explode by osmosis!
A third group of bacterial species are called Gram negative. These cells typically have a lipopolysaccharide bilayer membrane on each side of a very thin murein wall. So there are two membranes regulating transport into the cell and just a thin wall between. Because there is much less wall material, the purple dye-iodine complex inside the cell rinses right out with the alcohol rinse. So these bacteria appear pink rather than purple after the Gram staining procedure.
Further boundary defense found in some species is the production of a capsule or sheath. This layer is composed of mucoid polysaccharides that are secreted through the wall. This sticky sheath can cause bacterial cells to clump together, provide defense against desiccation (drying out), and provide a mechanical/chemical barrier to reduce influx of antibiotics or other molecules into the cell. The very famous transformation experiments involving antibiotic resistance of "rough" (no sheath) and "smooth" (with sheath) bacteria revolved around this cellular feature. The smooth forms were resistant to the antibiotics. DNA taken into the rough cells could transform them into smooth forms. This experiment was part of the evidence that DNA is the hereditary material for living cells.
Another defense against the vagaries of the environment can be production of an endospore. When certain species are stressed by the environment, they encapsulate their DNA and other cellular materials with a spore coat. The exterior cell around the spore can be lost completely. The remaining spore coat is very resistant to drying out...and protects the DNA and other cell material inside. The spore can be frozen or boiled or dried out for long periods of time...years...and can then germinate into a new full cell when returned to favorable conditions. An example organism with this feature is Bacillus anthracis. You probably remember the news a few years ago about spores of this bacterium being loaded into mail envelopes and a woman here in CT actually died from anthrax, the disease!
|Bacteria Have Homeostasis|
Bacteria have metabolism and can maintain their internal environment...in other words they have homeostasis. Much of what happens inside the cell is determined genetically. This means that the expression of DNA is involved. The DNA of the nucleoid of the cell is where much of the genome is observable. When an environmental signal causes a DNA gene to be expressed a cascade of events happens.
RNA polymerase, an enzyme, attaches to a promoter region on the genome. It transcribes an mRNA molecule based upon the sequence of bases in the DNA template. Metabolic enzymes provide supplies of nucleotide tri-phosphates (ATP, GTP, CTP, UTP) for this transcription process.
The mRNA, produced by enzymes working on the genome, moves away from the nucleoid region and encounters the subunits of a 70S ribosome. The ribosome uses the sequence of nucleotides in the mRNA to construct a corresponding polypeptide sequence of Amino Acids. To do this, ribosomal RNA (rRNA) interacts with the mRNA. Each one of about 25 amino acids is brought to the ribosome by a particular transfer RNA (tRNA). Between adjacent tRNA-AA units in the ribosome, enzymes and ribozymes catalyze the formation of a peptide bond between the amino acids...forming a chain of amino acids. Thus the sequence of nucleotides in DNA instructions are converted to an amino acid sequence in a protein.
The conversion of DNA instructions in the genome to active proteins responsible for cell behavior is sometimes called the Central Dogma of Genetics:
ATP GTP CTP UTP
~20 × tRNA-AA
The proteins produced in this process catalyze specific reactions in the cell or provide structural materials, such as materials for murein. Suites of proteins are involved with metabolic pathways for digesting food materials into subunits (sugars, fatty acids, amino acids, other small molecules). Some of these processes yield energy and others require energy overall. Some require carbon-based material, others produce carbon-based materials. In the 3.5 billion years of evolution, bacteria have come up with a bewildering array of biochemical pathways. The metabolism of the bacteria is more diverse than in any other kingdom.
The diversity of metabolism can be divided into four nutritional modes...with many variations within each of them! As you can see in the chart below, the different modes are characterized by what the energy source and carbon sources are for each mode.
|Nutrition Mode||Energy Source||Carbon Source|
Bacteria that are photoautotrophic use light energy to convert carbon dioxide into useful molecules for the cell. A good example photoautotrophic pathway is photosynthesis. There are two basic models of photosynthesis, one using water as the source of electrons, and the other using hydrogen sulfide:
|Photosynthesis with water: CO2 + H2O||light|
|O2 + CH2O|
|Photosynthesis with hydrogen sulfide: CO2 + H2S||light|
|Ss + CH2O|
Below is a micrograph of a slice through Anabaena a cyanobacterium. In this photo you can see the cell wall, cytoplasm, nucleoid areas, and many curved internal membranes. These membranes are called thylakoid membranes. The green color here is artificial but it indicates where chlorophyll would be located. So you can observe how structure relates to function. If you observe a chloroplast, you will see that it too is filled with thylakoid lamellae. Both cyanobacteria and chloroplasts have nucleoids with naked circular DNA. Both have 70S ribosomes. It is little wonder that the chloroplast is believed to have evolved from an endosymbiotic cyanobacterium.
Chemoautotrophic bacteria generally use inorganic chemicals as their source of energy. This energy is achieved by oxidation/reduction reactions often involving metal ions. The energy is used to convert carbon dioxide into the molecules needed for life. Some of these bacteria are part of our answer for controlling heavy metal pollution and so on. They can be responsible for biologically-driven corrosion of metals. But a good example is Nitrosomonas which has evolved a nitrification pathway. This organism gets its energy by converting ammonium to nitrite to convert carbon dioxide into carbohydrates for growth. The nitrites released from this metabolism are taken up by root hairs nearby in the soil. So this bacterium increases the fertility of soils and contributes to plant growth in our crop fields.
|Nitrification: CO2 + NH4+||→→→→||NO2- + CH2O|
Photoheterotrophs use light energy to convert organic molecules from one form to another to drive their biochemistry. Rhodospirillium rubrum is an example bacterium with this type of metabolism. This purple bacterium has a carotenoid pigment, spirillaxanthin assisting as a major antenna pigment for this metabolism.
|Photoheterotrophic pathway: C2H4O2- + H2O|| |
|O2 + CH2O|
Chemoheterotrophism is the style of our own metabolism. We derive our energy from organic chemicals we eat from the environment. We convert these food molecules into other molecules for our own human biochemistry. So we are a species that is chemoheterotrophic. Our metabolism is characterized by the pathway known as respiration.
|Respiration: CH2O + O2
||CO2 + H2O + ATP!
Many bacteria have this mode of nutrition too. Of course they have no mitochondrion to carry out this pathway, so they do glycolysis and the Kreb's cycle in their cytosol and the electron transport system coupled to oxidative phosphorylation using mesosomes (infoldings of the cell membrane) instead. About 2 billion years ago symbiotic bacteria of this type became endosymbionts in eukaryotic cells and evolved into mitochondria! Both have naked circular DNA and 70S ribosomes. Of course, like humans, chemoheterotrophic bacteria are dependent upon a food supply. No wonder some of these bacteria live on or inside us and take advantage of our skills in finding food to feed them within us. As humans we are actually an entire ecosystem with perhaps as many as 200 species of other organisms in and on us! Most of these are chemoheterotrophs.
Of course not all chemoheterotrophs live in aerobic (oxygen containing) environments. So there are various pathways by which large organic food molecules are converted to smaller ones without the uptake of oxygen. These pathways are generically called fermentation. For common sugars, fermentation can produce either ethyl alcohol and carbon dioxide gas or lactic acid as a fermentation product. When we make sauerkraut or yogurt, the bacteria in those foods produce acids that preserve the cabbage or the milk. When we use Zymomonas bacteria in the juice of Agave (a tropical plant), the bacteria use the sugar to make alcohol to preserve the pulque (~beer).
Bacteria also evolved relationships with each other, and with later-evolving organisms such as Homo sapiens. These relationships are generally called symbiosis that can take on one of several forms:
There are many good examples of these relationships with humans. There are small mites that live on our skin and feed on the sloughing dead cells. They are saprobes. A bacterial parasite that we all have is Streptococcus salivarius that lives on our teeth and produce acids from metabolism of sugar; these acids cut cavities into our teeth that can ultimately result in loss of a tooth or even death by endocarditis! There are many species of bacteria on our hands that cause no infection or any other kind of problem for us; they are commensals. Finally, if we are healthy, we can thank one of several mutualists that live inside our body cavities. We all have stable digestive systems thanks to a thriving culture of Escherichia coli in our intestines. They benefit from the food passing through our body, and we benefit by their elimination of gas and diarrhea that would result from their absence. Another good mutualist is Lactobacillus vaginalis which provide the acidic pH needed to keep yeasts and other nasties from infecting the vaginal canal in females. The bacteria get a nice warm and moist environment for their life, but both the female and her life partners benefit from lack of irritating infections.
Obviously using antibiotics to control bacteria can be an effective way to alleviate disease, but it also impacts our beneficial bacteria in negative ways. So antibiotics have side effect of gastric upset and, often yeast infection in females. What is amazing is the when people get a virus disease (colds, flu, etc.) they ask doctors for and actually get antibiotics that have no effect on the virus disease, but can cause these nasty side effects on top of the cold or flu! We need to be careful in the choices we make in drug therapies.
Growth, defined as an irreversible change in size, can be a function of two different processes: cell size increase and cell number increase. For unicellular organisms obviously growth of the individual is a cell size process only...but growth of the population by cell division can be an important parameter in the extent of a disease! And since bacteria include a number of diseases, we need to realize that "bacterial growth" can be more interestingly a population growth phenomenon.
Bacterial cells are generally very small so growth at the individual cell level is difficult for a large animal such as Homo sapiens. However, from the time of cell division until the next cell division, a bacterial cell may double in size. This growth is achieved by the combination of two functions. One of these is synthesis of new material inside the cell from food materials taken into the cell and osmosis of water caused by the increase of solutes inside the cell. The increase in pressure as molecules are brought in actively (feeding) or passively (osmosis) is what expands the wall to increase the internal volume of the cell.
Because cell expansion is thus a minor component of growth, we generally think of bacterial growth as a result of increases in the number of cells in a local population. And this too will be more important for those bacteria that form chains or clumps of cells in association.
Bacterial growth by cell division can be very rapid under good growing conditions. The number of cells in a local population can double in 20 minutes or even less under ideal conditions. Some quick calculations would reveal that bacteria could quickly take over planet Earth if conditions could remain ideal. Of course that is a big "if" and obviously cannot be achieved or it would already have happened. However, bacteria are very competitive in ideal environments and even in less-than-ideal environments. They are the ultimate survivors, having been on this planet for over 3.5 billion years!
I once had a bacterial skin infection that appeared as a small pimple. It grew far more than typical acne bacteria. Within two days I had a fever of 103.5F and went to the hospital. The fever hung around for four days while I was given intravenous antibiotics to control the bacterium. Keflex (cephalexin) is one of our most potent cell division inhibiting antibiotics. This had no effect upon my bacteria! After some microbiology culture tests, it was found that the bacterium I had was a dangerous "hospital infection"; rare in nature but has evolved inside hospitals to be resistant to most of our antibiotics. I was close to death, but the testing revealed that this bacterium (MERSA=MRSA=Methacillin Resistant Staphylococcus Aureus) was sensitive to sulfanilimide ("sulfa"). This antibiotic does not work on cell division, but starves out the bacteria by blocking their ability to process folic acid (an important vitamin). So within a day of sulfa treatment my fever broke, and after two weeks I was back to normal. So these diseases can attack quickly and cell division can be so rapid that it can be life threatening.
Perhaps you have heard of "flesh eating E. coli". Normally Escherichia coli is a wonderful organism that lives inside our intestines to help us digest our food. So in the right place the bacterium is very helpful to us. If you have ever had a round of antibiotics and got explosive diarrhea as a result, you can thank the loss of this organism from your digestive system! We need them. But there are strains of this bacterium that produce some amazing toxins and digestive enzymes... so that getting them on another location of your body can be dangerous. My previous physician went to Florida, got a small wound that became infected. He arrived in Connecticut with his arm swollen, hot, and angry. The tissue inside was digesting! Within a few days he was dead!
Some bacteria are immotile (do not move) but depend upon water currents or other environmental factors to move them around. But this has to do with locomotion which is only one form of movement. Inside the cell materials are in motion as well. We call this kind of movement either passive "Brownian movement" or active "cytoplasmic streaming" or "cyclosis". Intracellular movement is found in all of the living bacteria whether they locomote or not!
Other species, including myxobacteria and cyanobacteria, appear to use the force of slime secretion to jet them around. This kind of motility is sometimes called gliding motility. The precise mechanism for this is not clearly understood at present.
Some of the spirochetes have internal microtubules that can flex the cell, altering its shape and position so that the helical cells spin on their long axis. These are the only bacteria known to have microtubules. Some people believe that the microtubules and perhaps even the flagella of the eukaryotes evolved from a symbiosis of a host cell with a spirochete in ancient times. The gut parasites of termites that assist in cellulose digestion have a spirochete symbiosis...making the hypothesis that much more interesting.
Most of the motile bacteria move because they have flagella. This structure evolved from membrane proteins that have become a system of basal rings and rod shapes that penetrate the membrane and anchor the flagellum. The proteins are H+ ATPases, meaning that they use ATP energy to pump hydrogen ions across the membrane. This causes the proteins to rotate at rates from 200 to 1700 revolutions per second (12,000 rpm!). The external form of the motor proteins form a hook shaped structure that surrounds the flagellum. The hook apparently may, itself, be rotated to direct the movement. But the bulk of the movement is caused by the spinning of the flagellum inside the hook. The flagellum is made of a protein called flagellin to make a stiff helical structure. As the motor proteins rotate this helical pole, it creates the motive force.
The locomotion of bacteria can be directed toward or away from a stimulus. The gliding cyanobacteria move toward light for example. Growth toward a stimulus is often called positive taxis, while growth away from a stimulus is negative taxis. In the case of cyanobacteria, then, the movement observed if positive phototaxis.
Rhodospirillium rubrum also has positive phototaxis as it uses light in its metabolism...but it also can locomote toward food sources. The latter movement is known as positive chemotaxis.
It is also true that the direction of movement can be directed by where the flagella are located. So the cell can have the flagella near one end where they either pull or push the cell through the environment. This gives the organism an anterior or posterior end. Below is an example of a bacterium with the flagella only at one end of the cell, this condition is called lophotrichous.
It is also possible for bacteria to have flagella at both the anterior and posterior ends...amphitrichous. Someone needs to study how that works in terms of coordination...does one push while the other pulls?... do they take turns?...how is movement coordinated? Then there are bacteria with flagella coming out all around their perimeter...peritrichous.
There are recent reports of bacteria that are magnetotactic (move in ways organized with a magnetic field such as that of Earth). These bacteria have internal crystals of iron compounds that appear to be aligned with their movement in a magnetic field. So perhaps these magnetized crystals work like the needle in a compass to keep the bacteria moving ever the same direction in the magnetic field. Below is a transmission electron micrograph of such a bacterium.
As we discussed previously, bacteria can divide rapidly to produce more cells. This is a kind of reproduction that does not involve sex. You can start with just one bacterium and have a huge number of cells with a doubling time of under 20 minutes! So that is very fast reproduction.
Cell division in bacteria is not mitosis...anything you know about mitosis such as prophase, metaphase, anaphase, or telophase is irrelevant! The process of cell division in bacteria is completely different and called binary fission instead. Remember there is no nucleus in a prokaryotic cell!
The genome of the bacterium is held (usually) in a single circular DNA molecule. This DNA is not associated with histone proteins, so the word chromosome is used, but generally irrelevant because without protein association there is no typical staining (chromo- colored). The genome is attached to the cell membrane at one point.
Binary fission starts with the replication of the genome by the action of a range of proteins including DNA polymerase. The genome and the copy are identical. They are separated by movement of the proteins that attach them in the fluid mosaic of the cell membrane. By some as yet undetermined mechanism, the genome and its copy end up at opposite ends of the cell. The cell divides into two by the process of furrowing, a kind of pinching apart of one cell into two cells.
In binary fission, there is no synapsis of homologous chromosomes, because bacteria are fundamentally haploid (they have only the one molecule of DNA). During this process there is no genetic recombination (there is no exchange of DNA between one molecule and another). So, except for chance mutation, there is little chance for anything but clonal reproduction here.
Since there is no synapsis to diploidy, there is no meiosis either! So, there is no sexual reproduction of any kind in bacteria. However bacteria do have some interesting mechanisms for recombination however. These processes are called parasexual because the have at least some limited parallels to sexual recombination.
In species that undergo conjugation, the bacteria exist in two mating types. One mating type can produce a tubular appendage called a pilus. The pilus attaches to a nearby cell of the opposite mating type. Through the tube, some DNA can be sent from the pilus-forming type to its mate. The DNA then crosses-over (recombines) into the genome of the recipient. Sometimes this is a simple addition of new DNA, but other times a section of genomic DNA is lost. After conjugation the cells separate. The donor bacterium is virtually unchanged, but the recipient is genetically altered by the conjugation process. Here are two cells caught "in the act."
Bacteria can also receive foreign DNA in two other ways. In transformation naked circular DNA can be taken into bacterial cells as a plasmid. This plasmid is similar then to having a second chromosome...a second DNA molecule. Plasmids are used in molecular biology as vectors to take foreign genes into bacteria for amplification and purification within small engineered plasmids. This is a case of biotechnology harnessing a natural process. The famous 1928 experiment by Frederick Griffith involved moving a plasmid with genes for sheath production from the smooth strain to the rough strain, transforming it into an antibiotic resistant strain.
Bacteria can also receive foreign (and engineered) DNA by means of a virus. Bacterial viruses are known as phages. The 1958 experiment by Hershey and Chase showed that viruses inject their DNA into the bacteria and this DNA is used to make more viruses. Their study used radioisotopes of sulfur and phosphorus to sort out which materials went where. As a followup to Griffith's experiment, it validated that DNA rather than protein is the genetic material. Today we use viruses as vectors to move engineered plasmids from one organism to another. If in biotechnology courses you read about "screening a library" this process involves the use of viruses to amplify small segments of a eukaryotic genome to identify particular genes. This process is also at the root of the genome sequence discovery projects. This kind of DNA uptake from a virus is known as transduction.
Time for a structural quiz. If you have followed all of this, perhaps you can explain how this cluster of cells is showing two different shapes (see at ? marks). You should also be able to name what is being shown by the three arrows at the upper left, and what is going on at the two arrows in the lower right.
Throughout the material above, you have read about bacteria responding to the environment, to the presence of chemicals, light, or other organisms. I don't think we need to amplify this too much right here. But in your course on genetics, you will learn a bit more about how those genes are turned on that result in responses!
All of the structural and functional features allow the 10,000 or so species of Kingdom Bacteria to be related into a cladogram.
Mycoplasmas are Gram + bacteria that lack a cell wall and so are some of the smallest organisms on earth. Their cells are 0.1 μm in diameter. Mycoplasmas are involved in walking pneumonia...and this disease is sometimes antibiotic resistant because many antibiotics target cell wall synthesis. So to control this bacterium, we have to use antibiotics that target other aspects of metabolism.
The rest of the Gram positive bacteria are quite diverse. Actinomycetes form branched chains and include the causative agents of tuberculosis and leprosy. Others such as Streptomyces live in soil and produce antibiotics against other bacteria; this genus is the source of streptomycin. The solitary bacteria, Bacillus and Clostridium are responsible for anthrax and botulism. Staphylococcus (clusters of spheres) and Streptococcus (chains of spheres) bacteria in this part of the phylogeny are responsible for "hospital infections." Some of these have, in the last 50 years, evolved resistance to antibiotics. I happened to get a MRSA (Methicillin Resistant Staphylococcus Aureus) infection which does not even respond to Keflex (Cephalexin) a few years ago. I nearly died, but once we had the microbiology report, doctors switched me to Sulfa drugs (a class of very old-fashioned antibiotic) which cured it. Phew!
The Spirochetes are helical heterotrophic cells that can grow to 0.25 mm long! So these are some of the longest cells in the kingdom, but are among the thinnest! These bacteria are unique in that they rotate along their length as they locomote. Some spirochetes are free-living and others are pathogens. Two famous ones are Treponema pallidum responsible for syphilis, and Borrelia burgdorferi responsible for Lyme disease.
The clade including the green sulfur, cyanobacteria, and prochlorophytes is interestingly different. While their cells may be unicellular, colonial, or filamentous, they are characterized by using photosynthesis. The green sulfur bacteria get their electrons for photosynthesis from H2S while cyanobacteria and prochlorophytes use H2O for their electrons. The former produces elemental sulfur as a byproduct; the latter produces oxygen gas (O2) as a byproduct. The 1.5 billion years of nothing but prokaryotic life on our planet resulted in the oxygen atmosphere that permitted eukaryotic life to evolve and thrive.
|Photosynthesis: CO2 + H2O||light|
|O2 + CH2O|
A key factor in this evolution was the ability to produce the green pigment, chlorophyll, which is able to absorb light energy and drive photosynthesis. These organisms also thrived by evolving the ability to convert nitrogen gas (N2) into ammonium (NH4+) using energy (ATP) from respiration. The enzyme that catalyzes nitrogen fixation, needs anaerobic conditions, so cyanobacteria have a specialized cell called a heterocyst (hetero- other, -cyst cell). in the colony or filament that is isolated from the oxygen being produced in photosynthesis in the adjacent cells.
The prochlorophytes, in addition to the chlorophyll a needed for photosynthesis have evolved chlorophyll b as well. This gives them the grassy green rather than the bluish green color of their relatives. The prochlorophytes include endosymbiotic (live inside host cells) species and it is believed that the chloroplast of green plants evolved from an endosymbiotic prochlorophyte.
The Purple sulfur, rhodopseudomonads, and purple non-sulfur bacteria use purple pigments to obtain their electrons from water or hydrogen sulfide, but do not contribute much to the global atmospheric oxygen. Some of the vibrios have very rapid locomotion, 100μm per second, and can penetrate prey bacteria at 100 revolutions per second!
Rikettsias have become obligate parasites. Related parasites include Legionella (responsible for Legionnaires' disease), and the enteric bacteria such as Escherichia coli that make your digestion possible. Other relatives of E. coli cause digestive upset: Salmonella causes food poisoning and Vibrio cholerae causes cholera. Rhizobium is a symbiont with legume plants and performs nitrogen fixation for them. The plant provides a special root nodule that blocks oxygen so that the bacteria can convert nitrogen gas to ammonium. Agrobacterium is a plant disease bacterium that is now used as a vector for much of the recombinant DNA experimentation in modern biology. Some of these proteobacteria are considered to be the source of the eukaryotic mitochondrion by endosymbiosis.
Nitrosomonas, a soil bacterium, has a metabolism that converts ammonium (NH4+)from nitrogen fixation or from animal urination into nitrite (NO2-) for uptake by plants. They thus play a major role in converting atmospheric nitrogen into forms useable by the rest of the organisms on the planet!
The myxobacteria are somewhat fungal in characteristics. The myxobacteria secrete slime, aggregate to an almost multicellular form, and can produce spores to move through the air! There are wonderful websites on Myxococcus for you to investigate.
This page © Ross E. Koning 1994.
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