6.1 The Requirements for Growth

Microorganisms are classified into three primary groups on the basis of their preferred range of temperature: psychrophiles (cold-loving microbes), mesophiles (moderate-temperature–loving microbes), and thermophiles (heat-loving microbes).
The minimum growth temperature is the lowest temperature at which the species will grow.
- The maximum growth temperature is the highest temperature at which growth is possible.
The optimum growth temperature is the temperature at which the species grows best.
We will use the term psychrotrophs, which food microbiologists favor, for this group of spoilage microorganisms.
These organisms are called hyperthermophiles or, sometimes, extreme thermophiles.
Nonetheless, some bacteria, called acidophiles, are remarkably tolerant of acidity.
This osmotic loss of water causes plasmolysis, or shrinkage of the cell’s cytoplasm.
Some organisms, called extreme halophiles, have adapted so well to high salt concentrations that they actually require them for growth.
- In this case, they may be termed obligate halophiles.
More common are facultative halophiles, which do not require high salt concentrations but are able to grow at salt concentrations up to 2%, a concentration that inhibits the growth of many other organisms.
Microbes require very small amounts of other mineral elements, such as iron, copper, molybdenum, and zinc; these are referred to as trace elements.
Organisms that require oxygen to live are called obligate aerobes.
Anaerobes are bacteria that are unable to use molecular oxygen for energy-yielding reactions.
Aerotolerant anaerobes are fermentative and cannot use oxygen for growth, but they tolerate it fairly well.
Microaerophiles are aerobic and require oxygen.
Essential organic compounds an organism is unable to synthesize are known as organic growth factors; they must be directly obtained from the environment.

6.2 Biofilms

They more typically live in communities called biofilms, which are a thin, slimy layer encasing bacteria that adheres to a surface.
In bacterial cells, cell density alters gene expression in a process called quorum sensing.

6.3 Culture Media

A nutrient material prepared for the growth of microorganisms in a laboratory is called a culture medium.
Microbes that are introduced into a culture medium to initiate growth are called an inoculum.
The microbes that grow and multiply in or on a culture medium are referred to as a culture.
The medium must initially be sterile—that is, it must initially contain non living microorganisms—so that the culture will contain only the microbes (and their offspring) we add to the medium.
A chemically defined medium is one whose exact chemical composition is known.
Most heterotrophic bacteria and fungi, such as you would work with in an introductory lab course, are routinely grown on complex media made up of nutrients including extracts from yeasts, meat, or plants, or digests of proteins from these and other sources.
- Because anaerobes might be killed by exposure to oxygen, special media called reducing media must be used.
Microbes that grow better at high CO2 concentrations are called capnophiles.
Differential media make it easier to distinguish colonies of the desired organism from other colonies growing on the same plate.
Since bacteria present in small numbers can be missed, especially if other bacteria are present in much larger numbers, it is sometimes necessary to use an enrichment culture.

6.4 Obtaining Pure Cultures

A visible colony theoretically arises from a single spore or vegetative cell or from a group of the same microorganisms attached to one another in clumps or chains.
The isolation method most commonly used to get pure cultures is the streak plate method.

6.5 Preserving Bacterial Cultures

Deep-freezing is a process in which a pure culture of microbes is placed in a suspending liquid and quick-frozen at temperatures ranging from -50°C to -95°C.
During lyophilization (freeze-drying), a suspension of microbes is quickly frozen at temperatures ranging from -54°C to -72°C, and the water is removed by a high vacuum (sublimation).

6.7 The Growth of Bacterial Cultures

Bacteria normally reproduce by binary fission.
A few bacterial species reproduce by budding; they form a small initial outgrowth (a bud) that enlarges until its size approaches that of the parent cell, and then it separates.
The time required for a cell to divide (and its population to double) is called the generation time.
When a few bacteria are inoculated into an environment such as the large intestine or a liquid growth medium and the population is counted at intervals, it is possible to plot a bacterial growth curve that shows the growth of cells over time.
This period of little or no cell division is called the lag phase, and it can last for 1 hour or several days.
Eventually, the cells begin to divide and enter a period of growth, or logarithmic increase, called the log phase, or exponential growth phase.
Exponential growth stops because the bacteria approach the carrying capacity, the number of organisms that an environment can support.
The number of deaths eventually exceeds the number of new cells formed, and the population enters the death phase, or logarithmic decline phase.
The most frequently used method of measuring bacterial populations is the plate count.
- An important advantage of this method is that it measures the number of viable cells.
- One disadvantage may be that it takes some time, usually 24 hours or more, for visible colonies to form.
To reflect this reality, plate counts are often reported as colonyforming units (CFU).
To ensure that some colony counts will be within this range, the original inoculum is diluted several times in a process called serial dilution.
In the method known as the direct microscopic count, a measured volume of a bacterial suspension is placed within a defined area on a microscope slide.

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Chapter 6: Microbial Growth

6.1 The Requirements for Growth

Microorganisms are classified into three primary groups on the basis of their preferred range of temperature: psychrophiles (cold-loving microbes), mesophiles (moderate-temperature–loving microbes), and thermophiles (heat-loving microbes).
The minimum growth temperature is the lowest temperature at which the species will grow.
- The maximum growth temperature is the highest temperature at which growth is possible.
The optimum growth temperature is the temperature at which the species grows best.
We will use the term psychrotrophs, which food microbiologists favor, for this group of spoilage microorganisms.
These organisms are called hyperthermophiles or, sometimes, extreme thermophiles.
Nonetheless, some bacteria, called acidophiles, are remarkably tolerant of acidity.
This osmotic loss of water causes plasmolysis, or shrinkage of the cell’s cytoplasm.
Some organisms, called extreme halophiles, have adapted so well to high salt concentrations that they actually require them for growth.
- In this case, they may be termed obligate halophiles.
More common are facultative halophiles, which do not require high salt concentrations but are able to grow at salt concentrations up to 2%, a concentration that inhibits the growth of many other organisms.
Microbes require very small amounts of other mineral elements, such as iron, copper, molybdenum, and zinc; these are referred to as trace elements.
Organisms that require oxygen to live are called obligate aerobes.
Anaerobes are bacteria that are unable to use molecular oxygen for energy-yielding reactions.
Aerotolerant anaerobes are fermentative and cannot use oxygen for growth, but they tolerate it fairly well.
Microaerophiles are aerobic and require oxygen.
Essential organic compounds an organism is unable to synthesize are known as organic growth factors; they must be directly obtained from the environment.

6.2 Biofilms

They more typically live in communities called biofilms, which are a thin, slimy layer encasing bacteria that adheres to a surface.
In bacterial cells, cell density alters gene expression in a process called quorum sensing.

6.3 Culture Media

A nutrient material prepared for the growth of microorganisms in a laboratory is called a culture medium.
Microbes that are introduced into a culture medium to initiate growth are called an inoculum.
The microbes that grow and multiply in or on a culture medium are referred to as a culture.
The medium must initially be sterile—that is, it must initially contain non living microorganisms—so that the culture will contain only the microbes (and their offspring) we add to the medium.
A chemically defined medium is one whose exact chemical composition is known.
Most heterotrophic bacteria and fungi, such as you would work with in an introductory lab course, are routinely grown on complex media made up of nutrients including extracts from yeasts, meat, or plants, or digests of proteins from these and other sources.
- Because anaerobes might be killed by exposure to oxygen, special media called reducing media must be used.
Microbes that grow better at high CO2 concentrations are called capnophiles.
Differential media make it easier to distinguish colonies of the desired organism from other colonies growing on the same plate.
Since bacteria present in small numbers can be missed, especially if other bacteria are present in much larger numbers, it is sometimes necessary to use an enrichment culture.

6.4 Obtaining Pure Cultures

A visible colony theoretically arises from a single spore or vegetative cell or from a group of the same microorganisms attached to one another in clumps or chains.
The isolation method most commonly used to get pure cultures is the streak plate method.

6.5 Preserving Bacterial Cultures

Deep-freezing is a process in which a pure culture of microbes is placed in a suspending liquid and quick-frozen at temperatures ranging from -50°C to -95°C.
During lyophilization (freeze-drying), a suspension of microbes is quickly frozen at temperatures ranging from -54°C to -72°C, and the water is removed by a high vacuum (sublimation).

6.7 The Growth of Bacterial Cultures

Bacteria normally reproduce by binary fission.
A few bacterial species reproduce by budding; they form a small initial outgrowth (a bud) that enlarges until its size approaches that of the parent cell, and then it separates.
The time required for a cell to divide (and its population to double) is called the generation time.
When a few bacteria are inoculated into an environment such as the large intestine or a liquid growth medium and the population is counted at intervals, it is possible to plot a bacterial growth curve that shows the growth of cells over time.
This period of little or no cell division is called the lag phase, and it can last for 1 hour or several days.
Eventually, the cells begin to divide and enter a period of growth, or logarithmic increase, called the log phase, or exponential growth phase.
Exponential growth stops because the bacteria approach the carrying capacity, the number of organisms that an environment can support.
The number of deaths eventually exceeds the number of new cells formed, and the population enters the death phase, or logarithmic decline phase.
The most frequently used method of measuring bacterial populations is the plate count.
- An important advantage of this method is that it measures the number of viable cells.
- One disadvantage may be that it takes some time, usually 24 hours or more, for visible colonies to form.
To reflect this reality, plate counts are often reported as colonyforming units (CFU).
To ensure that some colony counts will be within this range, the original inoculum is diluted several times in a process called serial dilution.
In the method known as the direct microscopic count, a measured volume of a bacterial suspension is placed within a defined area on a microscope slide.