Prescott Home   Microbiology, 4/e               Prescott, Harley, Klein

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5 Microbial Nutrition



This chapter describes the basic nutritional requirements of microorganisms. Cells must have a supply of raw materials and energy in order to construct new cellular components. This chapter also describes the processes by which microorganisms acquire nutrients and provides information about the cultivation of microorganisms.



After reading this chapter you should be able to:

o list the ten elements that microorganisms require in large amounts (macronutrients/macroelements) and the six elements that they require in trace amounts (micronutrients)

o list the major nutritional categories and give the source of carbon, energy, and hydrogen/electrons for each of the categories

o compare the various processes (passive diffusion, facilitated diffusion, active transport, group translocation) by which cells can obtain nutrients from the environment

o describe the various types of culture media for microorganisms (synthetic, defined, selective, differential) and tell how each is normally used in the study of microorganisms

o describe the techniques used to obtain pure cultures (spread plate, streak plate, pour plate)



I. The Common Nutrient Requirements

A. Macroelements or macronutrients (C, O, H, N, S, P, K, Ca, Mg, Fe) are required by microorganisms in relatively large amounts

B. 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

C. Other elements may be needed by particular types of microorganisms

II. Requirements for Carbon, Hydrogen, and Oxygen-often satisfied together

A. Autotrophs use carbon dioxide as their sole or principal carbon source

B. Heterotrophs use reduced, preformed organic molecules (usually from other organisms) as carbon sources

C. Prototrophs are microorganisms requiring the same nutrients as most naturally occurring members of their species

D. Auxotrophs are mutated microorganisms that lack the ability to synthesize an essential nutrient and therefore must obtain it or a precursor from the surroundings

III. Nutritional Types of Microorganisms

A. Energy

1. Phototrophs use light as their energy source

2. Chemotrophs obtain energy from the oxidation of organic or inorganic compounds

B. Hydrogen/electrons

1. Lithotrophs use reduced inorganic compounds as their electron source

2. Organotrophs use reduced organic compounds as their electron source

C. Nutritional types of microorganisms-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. Photoorganotrophic heterotrophs

3. Chemolithotrophic autotrophs

4. Chemoorganotrophic heterotrophs

D. Mixotrophic organisms combine autotrophic and heterotrophic metabolic processes, relying on inorganic energy sources and organic carbon sources

E. Chemolithotrophs contribute greatly to the chemical transformation of elements that continually occur in the ecosystem

F. Some organisms show great metabolic flexibility and alter their metabolic patterns in response to environmental changes

IV. Requirements for Nitrogen, Phosphorus, and Sulfur-can be met by either organic or inorganic sources; some organisms have specific requirements for sources of these elements while others are more general

V. Growth Factors-organic compounds required by the cell because they are essential cell components (or precursors of these components) that the cell cannot synthesize

A. Amino acids-needed for protein synthesis

B. Purines and pyrimidines-needed for nucleic acid synthesis

C. Vitamins-function as enzyme cofactors

D. Knowledge of specific growth factor requirements makes possible quantitative growth-response assays

VI. Uptake of Nutrients by the Cell

A. 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 useful molecules (e.g., glycerol,H2O, O2, and CO2)

B. 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

C. 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

a. Saturable uptake rate

b. Requires an expenditure of metabolic energy

c. Can concentrate molecules inside the cell even when the concentration inside the cell is already higher than that outside the cell

2. ATP hydrolysis or protonmotive forces are the usual sources of metabolic energy

3. Types of active transport

a. Symport is the linked transport of two substances in the same direction

b. Antiport is the linked transport of two substances in opposite directions

D. Group translocation-a process in which molecules are modified as they are transported across the membrane

E. Iron uptake-the organism secretes siderophores that complex with the very insoluble ferric ion, which is then transported into the cell

VII. Culture Media

A. Synthetic (defined) media are media in which all components and their concentrations are known

B. 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

1. Peptones-protein hydrolysates prepared by partial proteolytic digestion of various protein sources

2. Extracts-aqueous extracts, usually of beef or yeast

C. Agar is a sulfated polymer used to solidify liquid media

D. 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 inhibits the growth of others

4. Differential media distinguish between different groups of bacteria on the basis of their biological characteristics

E. Some media can exhibit characteristics of more than one type (e.g., blood agar is enriched and differential, and distinguishes between hemolytic and nonhemolytic bacteria)

VIII. Isolation of pure cultures (a population of cells arising from a single cell)-can be accomplished from mixtures by a variety of procedures, including spread plates, streak plates, and pour plates

A. Colonies are macroscopically visible growths or clusters of microorganisms on solid media

B. Colony growth is most rapid at the colony's edge because oxygen and nutrients are more available; growth is slowest at the colony's center

C. Colony morphology helps microbiologists identify bacteria because individual species often form colonies of characteristic size and appearance



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