Macroelements, also known as macronutrients (C, O, H, N, S, P, K, Ca, Mg,
Fe), are required by microorganisms in relatively large amounts
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
Other elements may be needed by particular types of microorganisms
Requirements for Carbon, Hydrogen, and Oxygen
Autotrophs use carbon dioxide as their sole or principal carbon source
Heterotrophs use reduced, preformed organic molecules (usually from other organisms)
as carbon sources
Nutritional Types of Microorganisms
All organisms need a source of energy and electrons
Energy
Phototrophs use light as their energy source
Chemotrophs obtain energy from the oxidation of organic or inorganic compounds
Electrons
Lithotrophs use reduced inorganic compounds as their electron source
Organotrophs use reduced organic compounds as their electron source
Most microorganisms can be categorized as belonging
to one of four major nutritional types depending on their sources of carbon,
energy, and electrons:
Photolithotrophic autotrophs
Chemoorganotrophic heterotrophs
Photoorganotrophic heterotrophs
Chemolithotrophic autotrophs
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
Requirements for Nitrogen, Phosphorus, and Sulfur
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
Phosphorus is present in nucleic acids, phospholipids, nucleotides and other
molecules; most microorganisms use inorganic phosphate to meet their phosphorus
needs
Sulfur is needed for the synthesis of certain amino acids and other
molecules; most microorganism meet their sulfur needs by assimilatory sulfate
reduction
Growth Factors
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:
Amino acids-needed for
protein synthesis
Purines and pyrimidines-needed for nucleic acid synthesis
Vitamins-function as enzyme cofactors
Knowledge of specific growth factor
requirements makes possible quantitative growth-response assays
Uptake of Nutrients by the Cell
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
Requires a large concentration gradient for significant levels of uptake
Limited to only a few small molecules (e.g., glycerol, H2O, O2, and
CO2)
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
Requires a smaller concentration gradient than passive diffusion
The rate plateaus when the carrier becomes saturated (i.e., when it
is binding and transporting molecules as rapidly as possible)
Generally more important in eucaryotes rather than procaryotes
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)
Characteristics of active transport
Saturable uptake rate
Requires an expenditure of metabolic energy
Can concentrate molecules inside the cell even when the concentration
inside the cell is already higher than that outside the cell
ATP-binding cassette transporters (ABC transporters) use ATP to drive
transport against a concentration gradient; they are observed in bacteria,
archaea and eucaryotes
Proton motive forces can also be used to power active transport
Types of active transport
Symport is the linked transport of two substances in the same
direction
Antiport is the linked transport of two substances in opposite
directions
Group translocation-a process in which molecules are modified as they
are transported across the membrane
Iron uptake-the organism secretes siderophores that complex with the
very insoluble ferric ion, which is then transported into the cell
Culture Media
A culture medium is a solid or liquid preparation used to grow, transport,
and store microorganisms
Synthetic (defined) media are media in which all components and their
concentrations are known
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
Types of Media
General purpose media will support the growth of many microorganisms
Enriched media are supplemented by blood or other special nutrients
to encourage the growth of fastidious heterotrophs
Selective media favor the growth of particular microorganisms and inhibit
the growth of others
Differential media distinguish between different groups of bacteria
on the basis of their biological characteristics
Isolation of Pure Cultures
A pure culture is a population of cells arising from a single cell
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)
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
Colony morphology helps microbiologists identify bacteria because individual
species often form colonies of characteristic size and appearance
