MONERAN DIVERSITY
Introduction -
The history of procaryotic life is a success story spanning at least 3.5 billion years. Prokaryotes were the earliest organisms, and they lived and evolved all alone on Earth for 2 billion years. They have continued to adapt and flourish on an evolving Earth, and in turn they have helped change the Earth. In terms of metabolic impact and numbers, prokaryotes still dominate the biosphere, outnumbering all eukaryotes combined. More prokaryotes inhabit the human mouth than the total number of people who have ever lived. Prokaryotes are also the most pervasive of organisms thriving in habitats too hot, cold, salty, acidic or basic for any eukaryote. Although prokaryotes are individually microscopic, their collective impact is gigantic. What do you think of when you think of bacteria? We often think of disease when we think of bacteria but this is unfair. Only a minority of prokaryotes cause disease. The great majority of prokaryotic species are essential to all life on Earth. If for some reason all prokaryotes were suddenly to perish, the chemical cycles that sustain life would halt, and all other forms of life would also be doomed. Animals, plants, fungi, and protists probably evolved from symbiotic associations of ancestral prokaryotes.
describe the basic structure of a prokaryotic cellMost prokaryotic cells are relatively small (200nanometers - 500micrometers). They lack membrane enclosed organelles. Their DNA is located in the cytoplasm and arranged in a single circular chromosome [eukaryotes have several linear molecules of DNA].
Some monerans have plasmids, smaller circular strands of DNA that are capable of replicating independently. In most environments , bacteria can survive without their plasmids because all essential functions are programmed by the chromosome.However, plasmids endow the cell with genes for resistance to antibiotics, for metabolism of unusual nutrients not present in the normal environment, and for other special contingencies. Most have cell walls, but they differ in molecular composition and construction from those of plants, protists and fungi. The cell wall maintains the shape of the cell, affords physical protection, and prevents the cell from bursting in a hypoosmotic environment.
examine members of the Kingdom Monera and describe characteristics that unify them- Monerans have smaller, simpler genomes (1/1000) than eukaryotes and differ in some details of genetic replication and protein synthesis.
- Prokaryotes reproduce only asexually by the mode of cell division called binary fission. The bacteria copies its DNA and then splits in two. The process of conjugation occurs in a small number of bacteria. During this process a donor and a recipient directly transfer genes from one bacteria to another.
- Many monerans can produce capsules, protective layers of polysaccharides around their cell walls. Many prokaryotes also produce a net of polysaccharides called the glycocalyx that helps them stick to the surface of rocks, teeth, and host cells. Some monerans attach themselves to objects with proteins strands called pili.
use examples to illustrate moneran diversity with respect to the following: form, distribution, motility, ecological role, nutrition, and human diseasesform - Cocci or spherical bacteria occur singly or in pairs (diplococci), in chains of many cells (streptococci), and in clusters resembling bunches of grapes (staphylococci). Rod-shaped bacilli are most commonly solitary, but there are also forms with the rods arranged in chains. Spirilla are spiral shaped bacteria reminiscent of corkscrews. The majority of monerans are single-celled in nature, however, some species tend to aggregate reversibly in two-celled to several celled groups. Others have the form of true colonies (permanent aggregates of identical cells)
Distribution - Prokaryotes are the most pervasive of organisms thriving in all habitats of the earth including habitats too hot, cold, salty, acidic or basic for any eukaryote. Some prokaryotes even thrive and grow rapidly in environments as hot as 100 degrees celcius and as acidic as pH 2 [thermoacidophiles]. The bacteria living in the Dead Sea tolerate 10 times the salt of normal sea water [extreme halophiles] Methanogens - live in swamps and marshes where other microbes have consumed all the oxygen. The methane that bubbles out is known as marsh gas.
Under adverse conditions, many monerans encase their DNA and some of their cytoplasm in a tough envelope. This structure called an endospore can lay dormant for years until favorable conditions return and the coating breaks. Anthrax can survive in endospore form for 60 years. Boiling water is not hot enough to kill viral endospores so an autoclave must be used.
[Even without the ability to form endospores, some bacteria can endure harsh conditions. A recent investigation has uncovered 11,000 year old bacteria of the genus enterobacter in the intestinal contents of the partially preserved remains of a mastadon. This genus does not form endospores, so the cell must have been slowly reproducing over the years to remain viable.]
In an environment without limiting resources, bacterial growth is exponential. Most have generation times of 1 to 3 hours, but some are as short as 20 minutes. At that rate 1 cell would be a billion in just 24 hours. Luckily in the lab and in nature growth of bacteria stops when the cells use up some nutrient or the colony poisons itself with an accumulation of metabolic waste.
motility - Monerans have three different ways of moving.
- Many move by rotating stiff flagella that are bent into s-curves. When the flagella are rotated in one direction, the monerans move in a straight line. When they are rotated in the other direction they tumble.
- Spirochetes use filaments that wrap around the cell to move them like a corkscrew through the viscous fluids they inhabit.
- Some bacteria secrete slimy chemicals and move by a gliding motion.
In an environment that is uniform, motile bacteria (about half) wander randomly. In a heterogeneous environment many bacteria are capable of taxis, movement oriented toward or away from a stimulus. Ex.s chemotaxis, phototaxis
ecological role - The great majority of prokaryotic species are essential to all life on Earth. If for some reason all prokaryotes were suddenly to perish, the chemical cycles that sustain life would halt, and all other forms of life would also be doomed. The carbon, nitrogen, and other elements would become locked in the organic molecules of corpses and feces.
Prokaryotes rarely work alone instead groups of bacteria work together towards a common goal.
Prokaryotes also often depend on close relationships with members of the other kingdoms. This is called symbiosis. There are three categories of symbiotic relationships.
1) Mutualism - both benefit Ex. Plants of the legume family have lups on their roots called nodules which are home to nitrogen fixing bacteria.The bacteria provide the plant with nitrogen in exchange for a constant supply of sugar and organic nutrients.
2) Commensalism - one benefits the other does not but is not harmed. These include most of the bacteria that are found on the skin of humans.
3) Parasitism - the parasite benefits at the expense of the host.These mostly include the disease causing bacteria.
Cyanobacteria synthesize food and restore oxygen to the atmosphere. Some species of bacteria inhabit the guts of animals, playing an important role in the nutrition of cattle, termites, and other herbivores that subsist mainly on a diet of cellulose.
nutrition - Monerans may be heterotrophic or autotrophic.
- Most are heterotrophs - that is they use food produced by other organisms.
- Monerans that produce their own food are called autotrophs.
Photoautotrophs - from the Sun. Chemoautotrophs - use the energy of chemical reactions to synthesize food. [Some chemoautotrophs can fix nitrogen (convert gaseous nitrogen into ammonia compounds) Plants need nitrogen to synthesize proteins but cannot use gaseous nitrogen.]
human diseases - (Handout: Bad News Bacteria pg. 299 Modern Biology)
Approximately half of all human disease is caused by bacteria. Louis Pastuer, Joseph Lister, and other scientists began linking disease to pathogenic microbes in the late 1800's. The first to actually connect certain diseases to specific bacteria was Robert Koch, a German physician who determined the bacteria responsible for anthrax and tuberculosis. His methods established four criteria now called Koch's postulates, which are still the guidelines for medical microbiology. To substantiate a specific pathogen as the cause of a disease, the researcher must (1) find the same pathogen in each diseased individual investigated, (2) isolate the pathogen from a diseased subject and grow it in culture, (3) induce the disease in experimental animals by transferring the pathgen from the culture, and (4) isolate the same pathogen from the experimental animals after the disease develops.]
One of the most valuable tools for identifying bacteria is the Gram stain, which can be used to seperate many bacteria into two groups based on a difference in their cell walls. (Handout: photocopy of page 517 Campbell)
- Gram-poitive bacteria have simpler walls, with a relatively large amount of peptidoglycan.
- The walls of gram-negative bacteria have less peptidoglycan and are more complex in structure. An outer membrane on the gram-negative cell wall contains lipopolysaccharides (carbohydrates bonded to lipids).
Among pathogenic (disease-causing bacteria, gram-negative bacteria are generally more threatening than gram-positive species. The lipopolysaccharides on the walls of gram negative bateria are often toxic, and the outer membrane helps protect the pathogens against the defenses of their hosts. Furthermore, gram-negative bacteria are commonly more resistant to antibiotics because the outer membrane impedes entry of the drugs.
Some pathogens are opportunistic. They are normal residents of the body that inflict illness only when defenses have been weakened by such factors as poor nutrition or a recent bout with the flu. For example, Streptococcus pneumoniae lives in the throats of most healthy people, but this opportunist can multiply and cause pneumonia when the host's defenses are down.
Some bacteria disrupt the physiology of the host by their actual growth and invasion of tissues but more commonly, they cause illness by producing toxins.
- Exotoxins are proteins secreted by the cell Ex. When Clostridium botulinum grows anaerobically in poorly canned foods, one of the by-products causes the potentially fatal disease botulism.]
- Endotoxins are components of the outer membranes of certain gram negative bacteria. Ex. Samonella thyphi causes typhoid fever and several other species of salmonella, some of which are commonly found in poultry cause food poisoning.
With the discovery in the nineteenth century that "germs" cause disease, public health officials took steps to upgrade hygiene. Sanitation measures played a significant role in reducing infant mortality and extended the life expectancy dramatically in developed countries.
differentiate among fermentation, aerobic respiration, and photosynthesis in monerans- Many monerans are obligate anaerobes (cannot survive in the presence of oxygen).
- Some are facultative anaerobes (can live with or without oxygen).
- The rest are obligate aerobes (cannot survive without oxygen).
When oxygen is absent, both obligate and facultative anaerobes obtain energy by fermentation. The products of fermentation are CO2 and alcohol and a wide array of organic compounds depending on the bacteria involved.
In photosynthesis, photoautotrophic bacteria convert CO2 and water into carbohydrates by using the energy from sunlight
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contrast the ways in which bacterial decomposers and parasites obtain their foodA heterotroph that feeds on another living animal is also called a parasite. This is the case with many disease causing parasites.
A heterotroph that feeds on dead or decaying matter is also called a saprophyte. By decomposing matter, saprophytic bacteria release carbon and other elements for use by other organisms. Saprophytes are therefore essential to the recycling of nutrients in every ecosystem.
demonstrate sterile technique while preparing a streak plateLAB: Culturing Bacteria pg. 305 Modern Biology
evaluate the effectiveness of various antibiotics, disinfectants, and antiseptics on bacteria culturesThe prokaryotic ribosome is slightly smaller than the eukaryotic and differs in composition enough that selective antibiotics like tetracycline and chloramphenical, bind to the ribosomes of prokaryotes and block protein synthesis, while not inhibiting eukaryotic ribosomes.
Many antibiotics, including penicillins, inhibit the the synthesis of cross-links in peptidoglycan and prevent the formation of a functional wall, particularly in gram-positive bacteria. These drugs are like selective bullets that cripple many species of infectious bacteria without adversely affecting humans and other eukaryotes, which do not make peptidoglycan.
More than half of our antibiotics (including streptomycin, neomycin, erythromycin, auromycin, tetracycline, and others) come from soil bacteria. In the wild these compounds prevent encroachment by other microbes.
COMPLETE: review question 16 on page 223. The answer can be found by reading HUMAN DISEASES beginning on page 221.
explain processes by which bacteria adapt to become resistant to antibioticsREAD antibiotics and bacterial resistance on page 222 and top of 223 and answer review questions 12 and 13 on page 223.
give examples of the beneficial roles of bacteriaHumans have learned many ways of exploiting the diverse metabolic capabilities of prokaryotes, both for scientific research and for practical purposes.
- Much of what we know about metabolism and molecular biology has been learned in laboratories using bacteria as relatively simple model systems. [In fact the E. coli is the best understood of all organisms.
- Because of the relatively simple organization of prokaryotic genomes and the ability of cells to take up foreign DNA from the surrounding solution, bacteria were the first choice for recombinant DNA research and genetic engineering.
- Methanogens are important decomposers employed for sewage treatment. Some farmers have experimented with use of these microbes to convert garbage and dung to methane a valuable fuel.
- Some soil species of pseudomonads decompose pesticides and other synthetic compounds.
- The chemical industry grows immense cultures of bacteria that produce acetone, butanol, and several other products.
- Pharmaceutical companies culture bacteria that make vitamins and antibiotics.
- The food industry uses bacteria to convert milk to yogurt and various kinds of cheese.