CHAPTER
6
Microbial Nutrition and Growth
Chapter Outline
Growth Requirements (pp. 161–169)
Nutrients: Chemical and Energy Requirements
Culturing Microorganisms (pp. 169–177)
Clinical Sampling
Growth of Microbial Populations (pp. 177–186)
Generation Time
Mathematical Considerations in Population Growth
Chapter Summary
Growth Requirements (pp. 161–169)
Microbiologists use the term growth to indicate an increase in a population of microbes rather
than an increase in size. Microbial growth depends on the metabolism of nutrients, and results in
the formation of a discrete colony, an aggregation of cells arising from a single parent cell, or a
Nutrients: Chemical and Energy Requirements
All cells require three things to conduct metabolism: a carbon source, a source of energy, and a
source of electrons or hydrogen atoms.
Chapter 6 Microbial Nutrition and Growth
Sources of Carbon, Energy, and Electrons
Organisms can be categorized into one of two groups based on their source of carbon:
autotrophs utilize carbon dioxide as their source of carbon, while heterotrophs catabolize
2. Chemoautotrophs use carbon dioxide as a carbon source but obtain energy from redox
reactions involving organic or inorganic molecules.
4. Chemoheterotrophs use organic compounds for both energy and carbon.
Oxygen Requirements
Obligate aerobes require oxygen as the final electron acceptor of the electron transport chain,
whereas obligate anaerobes cannot tolerate oxygen and use an electron acceptor other than
oxygen. Toxic forms of oxygen are highly reactive and cause a chain of vigorous oxidation.
Four forms of oxygen are toxic:
1. Singlet oxygen (1O2) is molecular oxygen with electrons that have been boosted to a higher
2. Superoxide radicals (O2–) are formed during the incomplete reduction of oxygen during
3. Peroxide anion (O22–) is a component of hydrogen peroxide, which is formed during
4. Hydroxyl radicals (OH) result from ionizing radiation and from the incomplete reduction
of hydrogen peroxide. Hydroxyl radicals are the most reactive of the four toxic forms of
oxygen, but because hydrogen peroxide does not accumulate in aerobic cells, the threat of
hydroxyl radicals is virtually eliminated in aerobic cells.
Nitrogen Requirements
Nitrogen is a growth-limiting nutrient for many microorganisms, which acquire it from organic
and inorganic nutrients. Photosynthetic organisms can reduce nitrate (NO3–) to ammonium
Instructor’s Manual for Microbiology with Diseases by Body System, 5e
Other Chemical Requirements
Carbon, hydrogen, oxygen, and nitrogen account for more than 95% of the dry weight of cells.
Other important elements are phosphorus, sulfur, calcium, manganese, magnesium, copper, and
Physical Requirements
In addition to chemical nutrients, organisms have physical requirements for growth, including
specific conditions of temperature, pH, osmolarity, and pressure.
Temperature
Because both proteins and lipids are temperature sensitive, different temperatures have different
effects on the survival and growth rates of microbes. Though microbes survive within the limits
imposed by a minimum growth temperature and a maximum growth temperature, an organism’s
metabolic activities produce the highest growth rate at the optimum growth temperature.
Some microbes survive, but do not grow, at temperatures below their minimum growth
temperature.
Microbes are described in terms of their temperature requirements as (from coldest to
warmest):
Organisms are sensitive to changes in acidity because hydrogen ions and hydroxyl ions interfere
with hydrogen bonding within the molecules of proteins and nucleic acids; as a result,
organisms have ranges of acidity that they prefer and can tolerate. Most bacteria and protozoa
Chapter 6 Microbial Nutrition and Growth
Physical Effects of Water
Osmotic Pressure. Microorganisms require water to dissolve enzymes and nutrients and to act
as a reactant in many metabolic reactions. Osmosis is the diffusion of water across a membrane.
Osmotic pressure restricts cells to certain environments. Whereas the cell walls of some
microbes protect them from osmotic shock, osmosis can cause other cells to die from either
Associations and Biofilms
In nature, most organisms live in associations with other organisms. Relationships in which one
organism harms or even kills another are considered antagonistic. In synergistic relationships,
members of an association cooperate such that each receives benefits that exceed those that
would result if each lived separately. In symbiotic relationships, organisms live in close
nutritional or physical contact, becoming interdependent.
Culturing Microorganisms (pp. 169–177
Microbiologists culture (cultivate) microorganisms by transferring an inoculum (plural,
inocula)—a sample—from a clinical or environmental specimen into a medium (plural, media),
Clinical Sampling
A clinical specimen is a sample of human material, such as feces, saliva, cerebrospinal fluid, or
blood that is examined and tested for the presence of microorganisms. Clinical specimens must
Instructor’s Manual for Microbiology with Diseases by Body System, 5e
Obtaining Pure Cultures
Suspected pathogens must be isolated from the normal microbiota in culture. Scientists use
several techniques to isolate organisms in pure cultures (axenic cultures) composed of cells
Culture Media
Microorganisms vary in their nutritional requirements. A variety of media are available for
microbiological cultures. A common example is nutrient broth, which contains digested
proteins. Agar, a complex polysaccharide, is a useful compound because it is difficult for
microbes to digest, solidifies at temperatures below 40°C, and does not melt below 100°C. Still–
warm liquid agar media can be poured into Petri dishes, which are called Petri plates once the
agar solidifies. When warm agar media are poured into test tubes that are then placed at an angle
and left to cool until the agar solidifies, the result is slant tubes or slants.
Various culture media are used to culture microbes. The general types of culture media
include the following:
A medium for which the precise chemical composition is known is called a defined medium
(or synthetic medium).
Chapter 6 Microbial Nutrition and Growth
among the different kinds of bacteria growing on the medium. For example, Streptococcus
pneumoniae (alpha-hemolysis: partially digested red blood cells), Streptococcus pyogenes
(beta-hemolysis: completely digested red blood cells), and Enterococcus faecalis (gamma-
hemolysis: no lysis) can be distinguished based on differences in the bacteria’s utilization of
Special Culture Techniques
Special culture techniques required for the growth of some microbes include the following:
Animal and cell cultures allow for the growth of microorganisms for which artificial media
are inadequate. For example, viruses and obligate intracellular parasites must reproduce
inside cells. Mammals, bird eggs, and cultures of living cells are used.
Low-oxygen cultures favor the growth of microorganisms that thrive in environments
Preserving Cultures
Slowing the metabolism by refrigeration is often the best technique for storing bacterial cultures
for short periods of time. Deep-freezing and lyophilization are used for long-term storage of
Growth of Microbial Populations (pp. 177–186)
Most unicellular microorganisms reproduce by binary fission, a process in which a cell grows to
twice its normal size and then divides in half to produce two daughter cells of equal size.
Generation Time
The time required for a bacterial cell to grow and divide is its generation time. Viewed another
Instructor’s Manual for Microbiology with Diseases by Body System, 5e
Mathematical Considerations in Population Growth
With binary fission, any given cell divides to form two cells; then each of these new cells
divides into two, for a total of four cells, and then four becomes eight, and so on. This type of
Phases of Microbial Population Growth
A graph that plots the number of bacteria growing in a population over time is called a growth
curve. When microbial growth is plotted on a semilogarithmic scale (which uses a logarithmic
scale for the y-axis) the plot of the population’s growth results in a straight line. When bacteria
are grown in a broth, the typical microbial growth curve has four distinct phases:
Continuous Culture in a Chemostat
A chemostat is an open system device used to maintain a microbial culture in a particular phase
of microbial population growth by supplying fresh medium while removing spent medium,
Measuring Microbial Reproduction
Because of each cell’s small size and incredible rate of reproduction, it is not possible to count
every single one in a population. Microbiologists estimate population size by counting the
number of cells in small, representative samples, and then multiplying to estimate the number of
cells in the entire specimen. Microbiologists use either direct or indirect methods to estimate the
number of cells.
Direct methods not requiring incubation include:
In microscopic counts, a sample is placed on a cell counter (Petroff-Hausser counting
chamber), a glass slide with an etched grid, and viewed through a microscope. It is designed
cytometry is a variation using fluorescent dyes or antibodies to detect cells.
Chapter 6 Microbial Nutrition and Growth
When population densities are high, microbiologists use serial dilution and viable plate
The most probable number (MPN) method is a statistical estimating technique based on
the fact that the more bacteria are in a sample, the more dilutions are required to reduce their
number to zero. Each dilution is used to inoculate a set of tubes. After incubation, the number
of tubes showing growth at each dilution is determined and compared to an MPN table to
estimate the number of cells in the sample. The MPN method is useful when counting