Bacteria are prokaryotic and unicellular (although some join together to form colonies or link up in chains). When people think of bacteria, they immediately think of diseases – but disease-causing bacteria only make up a small fraction of all bacteria. Bacteria play an important role in ecosystems (decomposers, nitrogen fixation), and in edible products (cheese, yogurt, vinegar, butter). There are also many helpful bacteria in the human digestive tract.

Bacterial Structure

  • unicellular, prokaryotic cells
  • No membrane-bound organelles
    • Nucleus
    • Vacuole
    • Mitochondria
    • Chloroplasts
  • What they still have:
    • Ribosomes
    • Cytoplasm
    • DNA
    • Cell membrane
  • They also have structures that eukaryotes lack:
    • Cell Wall (diff. from plant cell wall, only in some bacteria, give rigidity, added protection)
    • Capsule (Sometimes present, added protection against environment, antibodies, WBCs)
    • Pili (Protrusions on cell wall that allow them to attach to surfaces and exchange DNA with other bacteria)
    • Flagellum – makes bacteria motile

Classifying Bacteria

A species is defined as ‘a group of organisms that can only breed with one another’. This def’n is hard to apply to bacteria because they reproduce asexually, but can potentially swap genes with other bacteria. There are many different ways to classify bacterial species:


  • The most common way to classify bacteria
  • Coccus
  • Bacillus
  • Spirillium
  • Helix

  • Incorporated into this classification system is a series of prefixes that indicate the number and/or arrangement of the bacteria
  • Mono
  • Diplo
  • Strepto
  • Staphylo
  • These prefixes do not apply to spirillium or helix bacteria as these only exist as singles

Cell Wall Structure

  • In 1884, Danish physicist Hans Gram used a stain on bacteria that allowed the differences in cell wall structure and thickness to be seen
  • The stain, now called a Gram Stain, highlights basic differences in the arrangements of amino acid and sugar molecules in bacterial cell walls.
  • There are two types of cells based on cell wall structure: Gram-Positive and Gram-Negative bacteria
  • Gram-Positive bacteria have a thick protein layer on their cell wall and stain purple
  • Gram-Negative bacteria have a thin protein layer on their cell wall and stain pink

Carbon and Energy Sources

  • Bacteria use widely different sources of energy and materials that other organisms cannot
  • Bacteria can be classified into four groups based on sources of nutrients and energy: photoautotrophs, photoheterotrophs, chemoautotrophs, chemoheterotrophs
  • See Fig 12.8, page 421 for an explanation of these bacteria


  • With regard to oxygen requirements, four types can be distinguished:

Obligate Aerobes

–         Bacteria that require oxygen for their metabolic processes

–         Example: Mycobacterium tuberculosis (tuberculosis)

Obligate Anaerobes

–         Bacteria that cannot be in the presence of oxygen (they will die)

–         Example: Clostridium tetani (tetanus)

Facultative Anaerobes

–         Bacteria that can metabolize either with or without oxygen

–         Example: Escherichia coli (E. coli)


–         Bacteria requiring a low dose of atmospheric oxygen (approx 5%) and an increased carbon dioxide level.

–         Example: Helicobacter pylori (ulcers)

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