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Bacterial Evolution & Classification 

  • Most numerous organisms on earth
  • Earliest life forms (fossils date 2.5 billion years old)
  • Microscopic prokaryotes (no nucleus nor membrane-bound organelles)
  • Contain ribosomes
  • Infoldings of the cell membrane carry on photosynthesis & respiration
  • Surrounded by protective cell wall containing peptidoglycan (protein-carbohydrate)
  • Many are surrounded by a sticky, protective coating of sugars called the capsule or glycocalyx (can attach to other bacteria or host)
  • Have only one circular chromosome
  • Have small rings of DNA called plasmids
  • May have short, hairlike projections called pili on cell wall to attach to host or another bacteria when transferring genetic material
  • Most are unicellular

  • Found in most habitats
  • Most bacteria grow best at a pH of 6.5 to 7.0
  • Main decomposers of dead organisms so recycle nutrients
  • Some bacteria breakdown chemical & oil spills
  • Some cause disease 
  • Move by flagella, gliding over slime they secrete ( e.g. Myxobacteria)
  • Some can form protective endospores around the DNA when conditions become unfavorable; may stay inactive several years & then re-activate when conditions favorable
  • Classified by their structure, motility (ability to move), molecular composition, & reaction to stains (Gram stain)
  • Grouped into 2 kingdoms — Eubacteria (true bacteria) & Archaebacteria (ancient bacteria)
  • Once grouped together in the kingdom Monera


Cell Wallprotects the cell and gives shape
Outer Membraneprotects the cell against some antibiotics (only present in Gram negative cells)
Cell Membraneregulates movement of materials into and out of the cell; contains enzymes important to cellular respiration
Cytoplasmcontains DNA, ribosomes, and organic compounds required to carry out life processes
Chromosomecarries genetic information inherited from past generations
Plasmidcontains some genes obtain through genetic recombination
Capsule, and slime layerprotects the cell and assist in attaching the cell to other surfaces
Endosporeprotects the cell against harsh environmental conditions, such as heat or drought
Pilus (Pili)assist the cell in attaching to other surfaces, which is important for genetic recombination
Flagellummoves the cell


Kingdom Archaebacteria

  •   Found in harsh environments (undersea volcanic vents, acidic hot springs, salty water)
  • Cell walls without peptidoglycan
  • Subdivided into 3 groups based on their habitat — methanogens, thermoacidophiles, & extreme halophiles


  • Live in anaerobic environments (no oxygen)
  • Obtain energy by changing H2 and CO2 gas into methane gas
  • Found in swamps, marshes, sewage treatment plants, digestive tracts of animals
  • Break down cellulose for herbivores (cows)
  • Produce marsh gas or intestinal gas (methane)

Extreme Halophiles

  •   Live in very salty water
  •   Found in the Dead Sea, Great Salt Lake, etc.
  • Use salt to help generate ATP (energy)

Thermoacidophiles (Thermophiles)

  • Live in extremely hot  (1100C) and acidic (pH 2) water
  • Found in hot springs in Yellowstone National Park, in volcanic vents on land, & in cracks on the ocean floor that leak scalding acidic water

Kingdom Eubacteria (true bacteria)

  • Most bacteria in this kingdom
  • Come in 3 basic shapes — cocci (spheres), bacilli (rod shaped), spirilla (corkscrew shape)

  • Bacteria can occur in pairs ( diplo– bacilli or cocci)
  •   Bacteria occurring in chains are called strepto- bacilli or cocci
  • Bacteria in grapelike clusters are called staphylococci
  • Most are heterotrophic (can’t make their own food)
  • Can be aerobic (require oxygen) or anaerobic (don’t need oxygen)
  • Subdivided into 4 phyla — Cyanobacteria (blue-green bacteria), Spirochetes, Gram-positive, & Proteobacteria
  • Can be identified by Gram staining (gram positive or gram negative)  

Gram Staining

  • Developed in 1884 by Danish microbiologist, Hans Gram
  •   Bacteria are stained purple with Crystal Violet & iodine; rinsed with alcohol to decolorize; then restained with Safranin (red dye)

  • Bacterial cell walls either stain purple or reddish-pink

Gram-positive bacteria (Gram +)

  • Thick layer of peptidoglycan (protein-sugar) complex in cell walls & single layer of lipids
  • Stain purple

  • Lactobacilli are used to make yogurt, buttermilk ….
  • Actinomycetes make antibiotics like tetracycline & streptomycin
  • Disease-causing gram + bacteria produce poisons called toxins
  • Clostridium causes tetanus or lockjaw
  • Streptococcus cause infections such as “strep” throat

  • Staphylococci cause “staph” infections

  • Also cause toxic shock, bacterial pneumonia, botulism (food poisoning), & scarlet fever
  • Can be treated with penicillin (antibiotics) & sulfa drugs

Gram-negative bacteria (Gram -)

  • Cell walls have a thin layer of peptidoglycan & an extra layer of lipids on the outside
  • Stain pink or reddish 

  • Lipid layer prevents the purple stain & antibiotics from entering (antibiotic resistant)
  • Some are photosynthetic but make sulfur, not oxygen
  • Rhizobacteria grow in root nodules of legumes (soybeans, peanuts…) & fix nitrogen form the air for plants
  • Rickettsiae are parasitic bacteria carried by ticks that cause Rocky Mountain spotted fever
  • Spirochetes can cause syphilis & Lyme disease

Phylum Cyanobacteria

  • Gram negative
  •   Carry on photosynthesis & make oxygen
  • Called blue-green bacteria
  • Contain pigments called phycocyanin (red & blue) & chlorophyll a (green)
  •    May be red, yellow, green, brown, black, or blue-green
  • Some grow in chains (e.g. Oscillatoria)  & have specialized cells called heterocysts that fix nitrogen


  •  First bacteria to re-enter devastated areas
  • Anabaena that live on nitrates & phosphates in water can overpopulate & cause “population blooms” or eutrophication
  •   After eutrophication, the cyanobacteria die, decompose, & use up all the oxygen for fish

Phylum Spirochetes

  •   Gram positive
  • Have flagella at each end so move in a corkscrew motion
  •   Some are aerobic (require oxygen); others are anaerobic
  • May be free-living, parasitic, or live symbiotically with another organism  

Phylum Gram Positive bacteria

  • Most are Gram +, but some are Gram –
  • Lactobacilli grow in milk & make lactic acid (forms yogurt, cottage cheese, buttermilk) & also found on teeth & cause tooth decay
  • Actinomycetes grow in the soil & make antibiotics
  • Gram + members are found in the oral & intestinal cavities & slow the growth of disease-causing bacteria

Phylum Proteobacteria

  • Largest & most diverse bacterial group
  • Subdivided into Enteric bacteria, Chemoautotrophic bacteria, & Nitrogen-fixing bacteria  

Enteric bacteria

  • Gram negative heterotrophs
  • Can live in aerobic & anaerobic environments
  • Includes E. coli that lives in the intestinal tract making vitamin K & helping break down food
  • Salmonella causes food poisoning


  • Gram negative bacteria that obtain energy from minerals  
  • Iron-oxidizing bacteria found in freshwater ponds use iron salts for energy

Nitrogen-Fixing bacteria

  • Rhizobium are Gram negative & live in legume root nodules

  • 80% of atmosphere is N2, but plants can’t use nitrogen gas
  • Nitrogen-fixing bacteria change N2 into usable ammonia (NH3)
  • Important part of the Earth’s nitrogen cycle

Methods of Nutrition

  •  Saprobes feed on dead organic matter
  •  Parasites feed on a host cell
  •  Photoautotrophs use sunlight for energy, but get carbon from organic compounds (not CO2) to make their own food  
  • Chemoautotrophs obtain food by oxidizing inorganic substances like sulfur, instead of using sunlight

Methods of Respiration

  •   Obligate aerobic bacteria can’t live without oxygen; (tuberculosis bacteria)
  •  Obligate anaerobes die if oxygen is present; (tetanus bacteria that causes lockjaw)
  • Facultative anaerobes do not need oxygen, but don’t die if oxygen is present; (E. coli)
  • Anaerobes carry on fermentation, while aerobes carry on cellular respiration 

Bacterial Reproduction & Genetic Recombination

  • Most bacteria reproduce asexually by binary fission (chromosome replicates & then the cell divides)  
  •   Bacteria replicate (double in number) every 20 minutes under ideal conditions  
  • Bacteria contain much less DNA than eukaryotes
  • Bacterial plasmids are used in genetic engineering to carry new genes into other organisms  
  • Bacteria recombine genetic material in 3 ways — transformation, conjugation, & transduction


  • Sexual reproductive method
  • Two bacteria form a conjugation bridge or tube between them

  •   Pili hold the bacteria together
  •   DNA is transferred from one bacteria to the other       


  • Bacteria pick up pieces of DNA from other dead bacterial cells
  • New bacterium is genetically different from original


  • A bacteriophages (virus) carries a piece of DNA from one bacteria to another

  • Human insulin is produced in the lab by this method

Pathogenic bacteria

  •   Known as germs or pathogens
  • Cause disease
  • Can produce poisonous toxins
  • Endotoxins are made of lipids & carbohydrates by Gram – bacteria & released after the bacteria die (cause high fever, circulatory vessel damage…)
  • E. coli  produce endotoxins
  • Exotoxins are made of protein by Gram + bacteria 
  • Clostridium tetani produce exotoxins
  • Antibiotics interfere with cellular functions (Penicillin interferes with synthesis of the cell wall; tetracycline interferes with protein synthesis)
  • Some antibiotics are made by bacteria or fungi
  • Broad-spectrum antibiotics affect a wide variety of organisms
  • Bacteria can mutate and become antibiotic resistant (often results from overuse of antibiotics)