CHAPTER 27 PROKARYOTES AND THE ORIGINS OF

METABOLIC DIVERSITY

Bacillus anthracis spores in lung bronchiole

E. coli on surface of human skin

They’re (almost) everywhere!

• Prokaryotes were the earliest organisms on Earth and evolved alone for 1.5 billion years.

Vibrio cholerae - Gram-negative, facultatively anaerobic, causes Asiatic cholera.

Domain Archae (Archaebacteria)

• Lack peptidoglycan in c.w.

• More than one RNA poly-ase

• Histones assoc. with DNA.

• Introns in some genes

Halophilic bacterium

Current taxonomy recognizes two prokaryotic domains: domain Bacteria and domain Archaea.

Domain: Archae

Methanogens: form CH4 from H2 and CO2 or acetate. Requires anaerobic conditions (bottom of pond, landfill, sewage digester)

Halophiles: require extremes in salt (> 10%), contain pigment (bacteriorhodopsin) that can absorb light and create H+ gradient.

Thermoacidophiles: found in geothermal springs (high heat) and low pHs.

Alicyclobacillus spp. - an acidophilic, thermophilic, spore forming bacterium

Domain: Bacteria (Eubacteria)

• Contain peptidoglycan in c.w.

• One type of RNA poly-ase.

• Lack introns in genome.

• Sensitive to antibiotics (streptomycin and chlroamphinicol)

• Ex. Purple bacteria, free-living, enteric, mycoplasmas, actinomycetes (soil bacteria), cyanobacteria, spirochetes, chlamydias.

Structure and Morphology• Most prokaryotes are

unicellular; some colonial

• In nearly all prokaryotes, a cell wall maintains the shape of the cell, affords physical protection, and prevents the cell from bursting in a hypotonic environment.

• The most common shapes among prokaryotes are spheres (cocci), rods (bacilli), and helices.

Diplococcus

StreptococcusStaphylococcus

• Most bacterial cell walls contain peptidoglycan, a polymer of modified sugars cross-linked by short polypeptides.

• The walls of archaea lack peptidoglycan.

• Chemistry of cell wall allows for identification and classification. Gram staining.

Nearly all prokaryotes have a cell wall external to the plasma membrane

• Gram-positive bacteria have simpler cell walls, with large amounts of peptidoglycans and only one membrane. Stains purple.

• Gram-negative bacteria have more complex cell walls and less peptidoglycan. Have a double lipopolysaccharide membrane. Stains Pink.

• An outer membrane on the cell wall contains lipopolysaccharides, carbohydrates bonded to lipids.

• Among pathogenic bacteria, gram-negative species are generally more threatening than gram-positive species.

• The lipopolysaccharides on the walls are often toxic and the outer membrane protects the pathogens from the defenses of their hosts.

• Gram-negative bacteria are commonly more resistant than gram-positive species to antibiotics because the outer membrane impedes entry of antibiotics.

• Many prokaryotes secrete another sticky protective layer, the capsule, outside the cell wall.

• Composed of protein or polysaccharides.

• Capsules adhere the cells to their substratum.

• They may increase resistance to host defenses.

• They glue together the cells of those prokaryotes that live as colonies.

• i.e. R and S strains of Griffith experiment.

• Another way for prokaryotes to adhere to one another or to the substratum is by surface appendages called pili (or fimbriae).

• Pili/fimbriae can fasten pathogenic bacteria to the mucous membranes of its host.

• Formed from protein Pilin

• Sex Pili

Many prokaryotes are motile

• About half of all prokaryotes are capable of directional movement.

• The action of flagella, scattered over the entire surface or concentrated at one or both ends, is the most common method of movement.

• The flagella of prokaryotes differ in structure and function from those of eukaryotes.

• In a prokaryotic flagellum, chains of a globular protein wound in a tight spiral from a filament which is attached to another protein (the hook), and the basal apparatus.

• Rotation of the filament is driven by the diffusion of protons into the cell through the basal apparatus after the protons have been actively transported by proton pumps in the plasma membrane.

• A second motility mechanism is found in spirochetes, helical bacteria.

• Two or more helical filaments under (internalized) the cell wall are attached to a basal motor attached to the cell.

• When the filaments rotate, the cell moves like a corkscrew. Useful in thick fluids.

• A third mechanism occurs in cells that secrete a jet of slimy threads that anchors the cells to the substratum.

• The cell glides along at the growing end of threads.

• In a relatively uniform environment, a flagellated cell may wander randomly.

• In a heterogenous environment, many prokaryotes are capable of taxis, movement toward or away from a stimulus.

• With chemotaxis, binding between receptor cells on the surface and specific substances results in movement toward the source (positive chemotaxis) or away (negative chemotaxis).

• Other prokaryotes can detect the presence of light (phototaxis) or magnetic fields (magnetotaxis).

Growth, reproduction and genetic exchange• Binary fission; Conjugation;

Transduction; Transformation

• Endospore formation. Resting structures that withstand high heat, radiation, desiccation, toxins. Allow survival for hundreds of years. Form during unfavorable conditions. Low in water content (15% vs. 90% of normal cell). Tough wall forms that is one of the toughest biological structures.

Clostridium botulinum - Causes botulism.

Endospore

Metabolic Diversity

• Nutritional modes depend on how an organism obtains energy and a carbon source from the environment to build the organic molecules of cells.

• Photoautotrophs are photosynthetic organisms that harness light energy to drive the synthesis of organic compounds from carbon dioxide.

• Among the photoautotrophic prokaryotes are the cyanobacteria.

• Among the photosynthetic eukaryotes are plants and algae.

Cyanobacteria

• Chemoautotrophs need only CO2 as a carbon source, but they obtain energy by oxidizing inorganic substances, rather than light.

• These substances include hydrogen sulfide (H2S), ammonia (NH3), and ferrous ions (Fe2+) among others. Use these to form H+ gradient then ATP.

• This nutritional mode is unique to prokaryotes.

• Symbionts with tubeworms at hydrothermal vents

• Photoheterotrophs use light to generate ATP but obtain their carbon in organic form.

• This mode is restricted to prokaryotes.

• Chemoheterotrophs must consume organic molecules for both energy and carbon.

• This nutritional mode is found widely in prokaryotes, protists, fungi, animals, and even some parasitic plants.

The majority of known prokaryotes are chemoheterotrophs. • These include saprobes,

decomposers that absorb nutrients from dead organisms, and parasites, which absorb nutrients from the body fluids of living hosts.

• Some of these organisms (such as Lactobacillus) have very exacting nutritional requirements, while others (E. coli) are less specific in their requirements.

Lactobacillus sp.

• Prokaryotes are responsible for the key steps in the cycling of nitrogen through ecosystems.

• Some chemoautotrophic bacteria convert ammonium (NH4

+) to nitrite (NO2

-).

• Others “denitrify” nitrite or nitrate (NO3

-) to N2, returning N2 gas to the atmosphere.

• A diverse group of prokaryotes, including cyanobacteria, can use atmospheric N2 directly.

• During nitrogen fixation, they convert N2 to NH4

+, making atmospheric nitrogen available to other organisms for incorporation into organic molecules.

Nitrosomonas spp

• Nitrogen fixing cyanobacteria are the most self-sufficient of all organisms.

• They require only light energy, CO2, N2, water and some minerals to grow.

• The presence of oxygen has a positive impact on the growth of some prokaryotes and a negative impact on the growth of others.

• Obligate aerobes require O2 for cellular respiration.

• Facultative anerobes will use O2 if present but can also grow by fermentation in an anaerobic environment.

• Obligate anaerobes are poisoned by O2 and use either fermentation or anaerobic respiration.

• In anaerobic respiration, inorganic molecules other than O2 accept electrons from electron transport chains.

Yersinia pestis

•Gram-negative, facultatively anaerobic, rod.

•Transmitted by the rat flea to humans, it caused the bubonic plague.

Helicobacter pylori

•Gram-negative, tiny flagella at the end of the cell.

•main cause of chronic superficial gastritis, associated with both gastric and duodenal ulcers

•lives in the interface between the surface of gastric epithelial cells (the lining of the stomach).