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Chapter 6
Microbial Nutrition and Growth
Nutrition is a process by which all living organisms obtain substances from their
environment to convert to metabolic uses. Nutrients are categorized by the amount
required (macronutrients or micronutrients), by chemical structure (organic or inorganic),
and by their importance to the organism’s survival (essential or nonessential).
Microorganisms are classified both by the chemical form of their nutrients and the
energy sources they utilize. Although the chemical form of nutrients varies widely, all
organisms require six elements—carbon, hydrogen, oxygen, nitrogen, phosphorus, and
sulfur—to survive, grow, and reproduce. Nutrients are transported into microorganisms
by two kinds of processes: active transport that expends energy and passive transport
that does not need energy input.
The environmental factors that control microbial growth are temperature; gases; pH;
osmotic, hydrostatic and atmospheric pressure; radiation; and other organisms in their
habitats. Environmental factors control microbial growth mainly by their influence on
microbial enzymes. Three cardinal temperatures for a microorganism describe its
temperature range and the temperature at which it grows best. These are the minimum
temperature, the maximum temperature, and the optimum temperature.
Microorganisms are classified by their temperature requirements as psychrophiles,
mesophiles, or thermophiles. Most eukaryotic microorganisms are aerobic, whereas
bacteria vary widely in their oxygen requirements from obligately aerobic to anaerobic.
Microorganisms live in association with other species that range from mutually
beneficial symbiosis to parasitism and antagonism. Biofilms are examples of complex
synergistic communities of microbes that behave differently than free-living
microorganisms.
The splitting of a parent bacterial cell to form a pair of similar size daughter cells is
known as binary fission. Microbial growth refers both to increase in cell size and
increase in number of cells in a population. The generation time is a measure of the
growth rate of a microbial population. It varies in length according to environmental
conditions. Microbial cultures in a nutrient-limited batch environment exhibit four distinct
stages of growth: the lag phase, the exponential growth (log) phase, the stationary
phase, and the death phase. Microbial cell populations show distinct phases of
growth in response to changing nutrient and waste conditions. Population growth can be
quantified by measuring colony numbers, the turbidity of a solution, and direct cell
counts.
6.1 Microbial Nutrition
1. List the essential nutrients of a bacterial cell.
2. Differentiate between macronutrients and micronutrients.
3. Construct four different terms that describe an organism’s sources of carbon and
energy.
4. Define saprobe and parasite.
5. Discuss diffusion and osmosis.
6. Identify the effects on a cell of isotonic, hypotonic, and hypertonic conditions.
7. Name two types of passive transport and three types of active transport.
6.2 Environmental Factors That Influence Microbes
8. Name five types of bacteria based on their temperature preferences.
9. Explain how different organisms deal with oxygen.
10. Name three physical factors besides temperature and oxygen requirements that
microbes must contend with.
11. List and describe the five types of associations microbes can have with their hosts.
12. Discuss characteristics of biofilms that differentiate them from planktonic bacteria.
6.3 The Study of Bacterial Growth
13. Describe the main way that bacteria divide.
14. Define doubling time and how it relates to exponential growth.
15. Compare and contrast the four phases of growth in a bacterial growth curve.
16. Identify three methods besides a growth curve to count bacteria.
Key Terms
Nutrition
Nutrients
Essential nutrient
Macronutrients
Micronutrients
Trace elements
Inorganic nutrient
Organic nutrient
Heterotroph
Autotroph
Phototroph
Chemotroph
Photoautotroph
Chemoautotrophs
Lithoautotrophs
Chemoheterotrophs
Saprobes
Parasites
Obligate
Facultative
Opportunistic
Pathogens
Passive transport
Active transport
Bulk transport
Diffusion
Osmosis
Permeable
Isotonic
Hypotonic
Hypertonic
Facilitated diffusion
Group translocation
Endocytosis
Phagocytosis
Pinocytosis
Optimum temperature
Psychrophile
Mesophile
Thermophile
Aerobe
Anaerobe
Microaerophile
Aerotolerant
Capnophiles
Acidophiles
Alkalinophiles
Halophiles
Barophiles
Symbiosis
Mutualism
Commensalism
Parasitism
Synergism
Antagonism
Normal microbial flora
Binary fission
Exponential
Lag phase
Exponential (log) phase
Stationary phase
Death phase
Direct (total) cell count
Viable nonculturable (VNC)
Extreme thermophile
Growth curve
Quorum sensing
Osmophiles
Hydrogen peroxide