Chapter 6: Microbial Growth

Optimum growth temperature

the temperature at which the species grows best

Nutrient broth

Complex medium in liquid form

Nutrient agar

Nutrient broth containing agar

Reducing media

Media contain ingredients such as sodium thioglycolate that chemically combine with dissolved oxygen and deplete the oxygen in the culture medium

Selective media

Media that suppress growth of unwanted bacteria and encourage the growth of the desired microbes

Differential media

Media that make it easier to distinguish colonies of the desired organism from other colonies growing on the same plate

Enrichment culture

Culture medium used for preliminary isolation that favors the growth of one particular microorganism but not others.

Streak plate method

A method of isolating a culture by spreading microorganisms over the surface of a solid culture medium

Deep-freezing

Process in which a pure culture of microbes is placed in a suspending liquid and quick-frozen at temperatures ranging from -50 degrees C to -95 degrees C

Lyophilization (freeze-drying)

A suspension of microbes is quickly frozen at temperatures ranging from -54 degrees C to -72 decrees C, and the water is removed by a high vacuum (sublimation)

Binary fission

Reproduction of a cell by division of a cell into two daughter cells

Budding

Bacteria form a small initial outgrowth (a bud) that enlarges until its size approaches that of the parent cell, then it separates

Generation time

The time required for a cell to divide (and its population to double)

Bacterial growth curve

A graph indicating the growth of a bacterial population over time

Lag phase

Period of little or no cell division

Log phase/exponential growth phase

When the cells begin to divide and enter a period of growth, or logarithmic increase

Stationary phase

Period of equilibrium in which the growth rate slows, the number of microbial deaths balances the number of new cells, and the population stabilizes

Death phase/logarithmic decline phase

When the number of deaths eventually exceeds the number of new cells formed

Plate count

A method of determining the number of bacteria in a sample by counting the number of colony-forming unites on a solid culture medium

Colony-forming units (CFU)

Plate counts that count the short segments of a chain of bacterial clump; don't assume each live bacterium grows and divides to produce a single colony like a plate count

Serial dilution

The process of diluting a sample several times

Pour plate method

A method of inoculating a solid nutrient medium by mixing bacteria in the melted medium and pouring the medium into a Petri dish to solidify

Filtration

The passage of a liquid or gas through a screenlike material; a 0.45- µm filter removes most bacteria

Direct microscopic count

Enumeration of cells by observation through a microscope

Psychrophiles

cold-loving microbes

Mesophiles

moderate-temperature-loving microbes

Thermophiles

heat-loving microbes

Minimum growth temperature

the lowest temperature at which the species will grow

Maximum growth temperature

the highest temperature at which growth is possible

Psychrotrophs

Organisms that are likely to be encountered in low-temperature food spoilage because they grow fairly well at refrigerator temperatures

Hyperthermophiles/extreme thermophiles

Microbes that have an optimum growth temperature of 80 degrees celsius or higher

Acidophiles

Bacteria that are remarkably tolerant of acidity

Plasmolysis

Shrinkage of the cell's cytoplasm

Extreme halophiles

Organisms that have adapted well to high salt concentrations

Obligate halophiles

Organisms that require high salt concentrations for growth

Facultative halophiles

Organisms that do not require high salt concentrations but are able to grow at salt concentrations up to 2%, a concentration that inhibits growth of many other organisms

Nitrogen fixation

When important bacteria use gaseous Nitrogen (N2) directly from the atmosphere

Trace elements

A chemical element (mineral) required in small amounts for growth, such as iron, copper, molybdenum, and zinc

Obligate aerobes

Organisms that require oxygen to live

Facultative anaerobes

Aerobic bacteria that have developed, or retained, the ability to continue growing in the absence of oxygen

Obligate anaerobes

Bacteria that are unable to use molecular oxygen for energy-yielding reactions

Singlet oxygen (1^O2-)

Normal molecular oxygen (O2) that has ben boosted into a higher-energy state and is extremely reactive

Superoxide radicals/superoxide anions (O2.-)

A toxic anion (O2-) with an unpaired electron

Superoxide dismutase (SOD)

An enzyme that destroys superoxide: O2- + O2- + 2H+ --> H2O2 + O2

Peroxide anion

An oxygen anion consisting of two atoms of oxygen; O2^2-

Catalase

An enzyme that breaks down hydrogen peroxide: 2 H2O2 --> 2 H2O + O2

Peroxidase

An enxyme that destroys hydrogen peroxide: H2O2 + 2 H+ --> 2 H2O

Ozone

O3.

Hydroxyl radical

A toxic form of oxygen (OH.) formed in cytoplasm by ionizing radiation and aerobic respiration

Aerotolerant anaerobes

An organism that does not use molecular oxygen (O2) but is not affected by its presence

Microaerophiles

Organisms that are aerobic (require oxygen) but grow only in oxygen concentrations lower than those in air (small amounts of oxygen).

Organic growth factors

Essential organic compounds an organism is unable to synthesize

Biofilm

Microbial community that usually forms as a slimy layer on a surface

Culture medium

A nutrient material prepared for the growth of microorganisms in a laboratory

Inoculum

Microbes that are introduced into a culture medium to initiate growth

Culture

The microbes that grow and multiply in or on a culture medium

Agar

A complex polysaccharide derived from a marine alga

Chemically defined medium

A medium whose exact chemical composition is known

Complex media

Culture medium in which the exact chemical composition is not known

Capnophiles

Microbes that grow better at high CO2 concentrations

Colony

A visible mass of microbial cells arising from one cell or from a group of the same microbes

Spread plate method

A plate count method in which inoculum is spread over the surface of a solid culture medium

Most probable number (MPN) method

The greater the number of bacteria in a sample, the more dilution is needed to reduce the density to the point at which no bacteria are left to grow in the tubes in a dilution series.

Turbidity

The cloudiness of a suspension

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