Chapter 5 Outline
Microbial Growth and Nutrition
Introduction
5.1 Microbial Reproduction
 Most Prokaryotes Reproduce by Binary Fission
o The cell doubles in mass
o DNA replicates and the two strands separate
o Cytokinesis is an inward pinching of the cell membrane and cell wall
to separate the cell into two genetically identical cells
 Prokaryotes Reproduce Asexually
o The generation (or doubling) time is the interval of time between
successive binary fissions
o In pathogens, a shorter doubling time means a shorter incubation
period of disease
5.2 Microbial Growth
 A Bacterial Growth Curve Illustrates the Dynamics of Growth
o During the lag phase, no cell division occurs while bacteria adapt to
their new environment
o Exponential growth of the population occurs during the logarithmic
(log) phase
o Human disease symptoms usually develop during the log phase
o When reproductive and death rates equalize, the population enters the
stationary phase
o The accumulation of waste products and scarcity of resources causes
the population to enter the decline (exponential death) phase
o When reproductive and death rates equalize, the population enters the
stationary phase
o The accumulation of waste products and scarcity of resources causes
the population to enter the decline (exponential death) phase
 Endospores Are a Response to Nutrient Limitation
o Endospores are a highly resistant structure formed by species of
Bacillus and Clostridium when nutrient supplies are low
o A stressed cell undergoes asymmetrical cell division, creating a small
prespore and larger mother cell
o The prespore contains:
 Cytoplasm
 DNA
 dipicolinic acid, which stabilizes proteins and DNA
o The mother cell matures the prespore into an endospore, then
disintegrates, freeing the spore
o Endospores:
 are resistant to desiccation, heat
 undergo very few chemical reactions
o When environmental conditions are again favorable, protective layers
break down and the spore germinates into a vegetative cell
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Optimal Prokaryotic Growth Is Dependent on Several Physical Factors
o Temperature
 Each prokaryotic species has an optimal temperature for growth
and about a 30° range of acceptable temperatures
 Psychrophiles grow optimally below 15°C and make up the
largest portion of all prokaryotes on Earth
 Thermophiles multiply best around 60°C, living in compost
heaps and hot springs
 Hyperthermophiles are Archaea that grow optimally above 80°C,
found in seafloor hot-water vents
 Mesophiles thrive at the medium temperature range of 10° to
45°C, including pathogens that thrive in the human body
 Oxygen
• Many prokaryotes are obligate aerobes, which require oxygen to
grow
• Anaerobes do not or cannot use oxygen; aerotolerant species are
insensitive to oxygen, but obligate anaerobes are inhibited or killed
by oxygen
• Facultative prokaryotes grow either with oxygen or in reduced
oxygen environments
• Thioglycollate broth can be used to test an organism’s oxygen
sensitivity
• Carnophilic bacteria require an atmosphere low in oxygen and rich
in carbon dioxide
 pH
• The majority of species grow optimally at neutral (~7.0) pH
• Acidophiles are acid-tolerant prokaryotes
• For example, those used to turn milk into buttermilk, sour
cream, and yogurt
 Hydrostatic and Osmotic Pressure
• Barophiles can withstand incredibly high hydrostatic pressure
• For example, psychrophiles living at the bottom of the
ocean
• Halophiles are salt-tolerant prokaryotes
• They can maintain optimal osmotic pressure without
suffering from plasmolysis
5.3 Culture Media and Growth Measurements
 Culture Media Are of Two Basic Types
• A complex medium is a chemical unidentified medium such as a
nutrient broth or nutrient agar
• In a synthetic medium, the chemical composition of the medium is known
 Culture Media Can Be Devised to Select for or Differentiate between
Prokaryotic Species
• A selective medium contains ingredients to inhibit growth of certain
species and allow the growth of others
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A differential medium contains specific chemical to indicate species that
possess or lack a biochemical process
• Some “fastidious” organisms require an enriched medium containing
specific nutrients
• Many prokaryotes cannot be cultured in a laboratory
Population Measurements Are Made Using Pure Cultures
• A pure culture is a population consisting of only one species of
prokaryote
• The pour-plate isolation method allows separation of species through
dilution of a sample
The streak-plate isolation method spreads out individual cells to form discrete
colonies of species
Population Growth Can Be Measured in Several Ways
• Turbidity (cloudiness) of a sample can be measured with a
spectrophotometer
• One can perform a direct microscopic count
• Number of cells can be estimated through the most probable number test
• In the standard plate count procedure, colonies grow on a plate containing
a sample of diluted broth
• The number of colonies indicates the original number of viable
(living) cells in the broth