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Microbiology, Fifth Edition
Microbiology, 5/e
Lansing M Prescott, Augustana College
Donald A Klein, Colorado State University
John P Harley, Eastern Kentucky University
Microbial Nutrition

Study Outline

  1. The Common Nutrient Requirements
    1. Macroelements, also known as macronutrients (C, O, H, N, S, P, K, Ca, Mg, Fe), are required by microorganisms in relatively large amounts
    2. Trace elements or micronutrients (Mn, Zn, Co, Mo, Ni, Cu) are required in trace amounts by most cells and are often adequately supplied in the water used to prepare the media or in the regular media components
    3. Other elements may be needed by particular types of microorganisms
  2. Requirements for Carbon, Hydrogen, and Oxygen
    1. Autotrophs use carbon dioxide as their sole or principal carbon source
    2. Heterotrophs use reduced, preformed organic molecules (usually from other organisms) as carbon sources
  3. Nutritional Types of Microorganisms
    1. All organisms need a source of energy and electrons
      1. Energy
        1. Phototrophs use light as their energy source
        2. Chemotrophs obtain energy from the oxidation of organic or inorganic compounds
      2. Electrons
        1. Lithotrophs use reduced inorganic compounds as their electron source
        2. Organotrophs use reduced organic compounds as their electron source
  4. Most microorganisms can be categorized as belonging to one of four major nutritional types depending on their sources of carbon, energy, and electrons:
    1. Photolithotrophic autotrophs
    2. Chemoorganotrophic heterotrophs
    3. Photoorganotrophic heterotrophs
    4. Chemolithotrophic autotrophs
  5. Some organisms show great metabolic flexibility and alter their metabolic patterns in response to environmental changes; mixotrophic organisms combine autotrophic and heterotrophic metabolic processes, relying on inorganic energy sources and organic carbon sources
  6. Requirements for Nitrogen, Phosphorus, and Sulfur
    1. Nitrogen is needed for the synthesis of amino acids, purines, pyrimidines and other molecules; depending on the organism, nitrogen can be supplied by organic molecules, by assimilatory nitrate reduction, or by nitrogen fixation
    2. Phosphorus is present in nucleic acids, phospholipids, nucleotides and other molecules; most microorganisms use inorganic phosphate to meet their phosphorus needs
    3. Sulfur is needed for the synthesis of certain amino acids and other molecules; most microorganism meet their sulfur needs by assimilatory sulfate reduction
  7. Growth Factors
    1. Growth factors are organic compounds required by the cell because they are essential cell components (or precursors of these components) that the cell cannot synthesize; there are three major classes:
      1. Amino acids-needed for protein synthesis
      2. Purines and pyrimidines-needed for nucleic acid synthesis
      3. Vitamins-function as enzyme cofactors
    2. Knowledge of specific growth factor requirements makes possible quantitative growth-response assays
  8. Uptake of Nutrients by the Cell
    1. Passive diffusion-a phenomenon in which molecules move from an area of high concentration to an area of low concentration because of random thermal agitation
      1. Requires a large concentration gradient for significant levels of uptake
      2. Limited to only a few small molecules (e.g., glycerol, H2O, O2, and CO2)
    2. Facilitated diffusion-a process that involves a carrier molecule (permease) to increase the rate of diffusion; net effect is limited to movement from an area of higher concentration to an area of lower concentration
      1. Requires a smaller concentration gradient than passive diffusion
      2. The rate plateaus when the carrier becomes saturated (i.e., when it is binding and transporting molecules as rapidly as possible)
      3. Generally more important in eucaryotes rather than procaryotes
    3. Active transport-a process in which metabolic energy is used to move molecules to the cell interior where the solute concentration is already higher (i.e., it runs against the concentration gradient)
      1. Characteristics of active transport
        1. Saturable uptake rate
        2. Requires an expenditure of metabolic energy
        3. Can concentrate molecules inside the cell even when the concentration inside the cell is already higher than that outside the cell
      2. ATP-binding cassette transporters (ABC transporters) use ATP to drive transport against a concentration gradient; they are observed in bacteria, archaea and eucaryotes
      3. Proton motive forces can also be used to power active transport
      4. Types of active transport
        1. Symport is the linked transport of two substances in the same direction
        2. Antiport is the linked transport of two substances in opposite directions
    4. Group translocation-a process in which molecules are modified as they are transported across the membrane
    5. Iron uptake-the organism secretes siderophores that complex with the very insoluble ferric ion, which is then transported into the cell
  9. Culture Media
    1. A culture medium is a solid or liquid preparation used to grow, transport, and store microorganisms
    2. Synthetic (defined) media are media in which all components and their concentrations are known
    3. Complex media are media that contain some ingredients of unknown composition and/or concentration; this type supplies amino acids, vitamins, growth factors, and other nutrients
    4. Types of Media
      1. General purpose media will support the growth of many microorganisms
      2. Enriched media are supplemented by blood or other special nutrients to encourage the growth of fastidious heterotrophs
      3. Selective media favor the growth of particular microorganisms and inhibit the growth of others
      4. Differential media distinguish between different groups of bacteria on the basis of their biological characteristics
  10. Isolation of Pure Cultures
    1. A pure culture is a population of cells arising from a single cell
    2. The spread plate and streak plate methods separate cells on an agar surface such that each cell grows into a completely isolated colony (a macroscopically visible growth or cluster of microorganisms on a solid medium)
    3. The pour plate method involves diluting a sample to decrease the number of microorganisms, mixing the dilution with agar, and then pouring the mixture into a petri dish
    4. Colony morphology helps microbiologists identify bacteria because individual species often form colonies of characteristic size and appearance








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