Chapter 6

Classify microbes into five groups on the basis of preferred temperature range.
• Identify how and why the pH of culture media is controlled.
• Explain the importance of osmotic pressure to microbial growth.
• Name a use for each of the four elements (carbon, nitrogen, sulfur, and phosphorus)
needed in large amounts for microbial growth.
• Explain how microbes are classified on the basis of oxygen requirements.
The Requirements for Growth
 The growth of a population is an increase in the number of cells.
 The requirements for microbial growth are both physical and chemical.
Physical Requirements
 On the basis of preferred temperature ranges, microbes are classified as psychrophiles
(cold-loving), mesophiles (moderate-temperature–loving), and thermophiles (heat-
loving).
 The minimum growth temperature is the lowest temperature at which a species will
grow, the optimum growth temperature is the temperature at which it grows best, and
the maximum growth temperature is the highest temperature at which growth is
possible.
 Most bacteria grow best at a pH value between 6.5 and 7.5.
 In a hypertonic solution, most microbes undergo plasmolysis; halophiles can tolerate
high salt concentrations.
Chemical Requirements
 All organisms require a carbon source; chemoheterotrophs use an organic molecule,
and autotrophs typically use carbon dioxide.
 Nitrogen is needed for protein and nucleic acid synthesis.
 On the basis of oxygen requirements, organisms are classified as obligate aerobes,
facultative anaerobes, obligate anaerobes, aerotolerant anaerobes, and
microaerophiles.
 Aerobes, facultative anaerobes, and aerotolerant anaerobes must have the enzymes
superoxide dismutase and either catalase or peroxidase
 Other chemicals required for microbial growth include sulfur, phosphorus, trace
elements, and, for some microorganisms, organic growth factors.
Biofilms
• Describe the formation of biofilms and their potential for causing infection.

 Microbes adhere to surfaces and accumulate as biofilms on solid surfaces in contact
with water.
 Most bacteria live in biofilms.
 Microbes in biofilms are more resistant to antibiotics than are free-swimming microbes.
Culture Media
• Distinguish chemically defined and complex media.
• Justify the use of each of the following: anaerobic techniques, living host cells,
candle jars, selective and differential media, enrichment medium.
• Differentiate biosafety levels 1, 2, 3, and 4.
 A culture medium is any material prepared for the growth of bacteria in a laboratory.
 Microbes that grow and multiply in or on a culture medium are known as a culture.
 Agar is a common solidifying agent for a culture medium.
Chemically Defined Media
 A chemically defined medium is one in which the exact chemical composition is known.
Complex Media
 A complex medium is one in which the exact chemical composition varies slightly from
batch to batch.
Anaerobic Growth Media and Methods
 Reducing media chemically remove molecular oxygen (O2) that might interfere with the
growth of anaerobes.
 Petri plates can be incubated in an anaerobic jar, anaerobic chamber, or OxyPlate™.
Special Culture Techniques
 Some parasitic and fastidious bacteria must be cultured in living animals or in cell
cultures.
 CO2 incubators or candle jars are used to grow bacteria that require an increased
CO2 concentration.
 Procedures and equipment to minimize exposure to pathogenic microorganisms are
designated as biosafety levels 1 through 4.
Selective and Differential Media
 By inhibiting unwanted organisms with salts, dyes, or other chemicals, selective media
allow growth of only the desired microbes.
 Differential media are used to distinguish different organisms.
Enrichment Culture
 An enrichment culture is used to encourage the growth of a particular microorganism in
a mixed culture.

Obtaining Pure Cultures
• Define colony.
• Describe how pure cultures can be isolated by using the streak plate method.
 A colony is a visible mass of microbial cells that theoretically arose from one cell.
 Pure cultures are usually obtained by the streak plate method.
Preserving Bacterial Cultures
• Explain how microorganisms are preserved by deep-freezing and lyophilization
(freeze-drying).
 Microbes can be preserved for long periods of time by deep-freezing or
lyophilization (freeze-drying).
The Growth of Bacterial Cultures
• Define bacterial growth, including binary fission.
Compare the phases of microbial growth, and describe their relation to generation
time.
• Explain at least one direct methods of measuring cell growth.
• Differentiate direct and indirect methods of measuring cell growth.
• Explain at least one indirect methods of measuring cell growth.
Bacterial Division
 The normal reproductive method of bacteria is binary fission, in which a single cell
divides into two identical cells.
 Some bacteria reproduce by budding, aerial spore formation, or fragmentation.
Generation Time
 The time required for a cell to divide or a population to double is known as the
generation time.
Logarithmic Representation of Bacterial Populations
 Bacterial division occurs according to a logarithmic progression (two cells, four cells,
eight cells, and so on).
Phases of Growth
 During the lag phase, there is little or no change in the number of cells, but metabolic
activity is high.
 During the log phase, the bacteria multiply at the fastest rate possible under the
conditions provided.
 During the stationary phase, there is an equilibrium between cell division and death.
 During the death phase, the number of deaths exceeds the number of new cells formed.

Direct Measurement of Microbial Growth
 A heterotrophic plate count reflects the number of viable microbes and assumes that
each bacterium grows into a single colony; plate counts are reported as number of
colony-forming units (CFU).
 A plate count may be done by either the pour plate method or the spread plate method.
Estimating Bacterial Numbers by Indirect Methods
 A spectrophotometer is used to determine turbidity by measuring the amount of light
that passes through a suspension of cells