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Microbial growth

Increase in the number of cells, NOT an increase in individual cell size.

Exponential growth

Cells repeatedly divide so the population doubles: 1 → 2 → 4 → 8 → 16…

Generation time

Time required for microbial cells to double in number.

Three factors regulating microbial growth

Nutrient availability, environmental conditions, and generation time.

Environmental conditions affecting microbial growth

Temperature, pH, and osmotic pressure.

CFU (colony-forming unit)

A viable bacterium or viable unit capable of growing into a visible colony.

Bacterial colony

A visible mass of millions of clonal bacteria that originated from a viable bacterial cell or unit.

Vibrio natriegens

A very rapidly growing bacterium that can double in less than 10 minutes.

Main chemical requirement for bacterial growth

Water.

CHONPS

Carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur.

Trace elements

Elements bacteria require in very small amounts for growth.

Organic growth requirements

Can include glucose, vitamins/coenzymes, amino acids, purines, and pyrimidines.

Psychrophile

A microorganism that grows best at cold temperatures.

Mesophile

A microorganism that grows best at moderate temperatures; most bacteria are mesophiles.

Thermophile

A microorganism that grows best at high temperatures.

Hyperthermophile

A microorganism that grows best at extremely high temperatures.

Cardinal temperatures

The minimum, optimum, and maximum temperatures for microbial growth.

Minimum growth temperature

The lowest temperature at which an organism can still grow; growth is slow.

Optimum growth temperature

The temperature at which an organism grows fastest.

Maximum growth temperature

The highest temperature at which an organism can still grow.

Effect of temperatures above the maximum

Proteins and enzymes can denature, causing growth to stop and potentially killing the cell.

Effect of refrigerator temperature (~5°C)

Slows or stops the growth of most microbes; it does not necessarily kill them.

Obligate aerobe

Requires oxygen to grow.

Facultative anaerobe

Can grow with or without oxygen.

Obligate anaerobe

Cannot tolerate oxygen and may die when exposed to it.

Reactive oxygen species (ROS)

Harmful molecules produced during oxygen metabolism, including superoxide and hydrogen peroxide.

Why oxygen harms obligate anaerobes

They lack adequate defenses against ROS, causing cellular damage and death.

Preferred pH of most bacteria

Approximately pH 6.5–7.5, near neutral.

Acidophile

A microorganism adapted to grow in low-pH, acidic environments.

Why pH below 4 preserves food

The acidic environment inhibits the growth of most bacteria.

Lag phase

Cells adjust to the new environment and prepare for growth; little or no population increase occurs.

Log phase

Cells divide at their maximum rate, producing exponential population growth.

Stationary phase

Cell division rate equals cell death rate, so population size stays approximately constant.

Death phase

Cell death rate exceeds cell division rate, causing the living population to decrease.

Growth curve order

Lag → Log → Stationary → Death.

Why stationary phase occurs

Nutrients become limited and waste products accumulate.

Best phase to study actively growing bacteria

Log phase.

Why log-phase cells are sensitive to antibiotics

They are actively replicating DNA, synthesizing cell walls, and dividing.

Turbidity

Cloudiness in liquid culture caused by bacterial growth.

Optical density (OD)

A measurement of how much light is scattered by cells in a bacterial culture.

How is OD measured?

Using a spectrophotometer, typically with 600 nm light.

Higher OD

A higher OD indicates more total bacterial cells or cell material.

Does OD measure viability?

No. OD cannot determine whether bacterial cells are alive or dead.

Why can dead bacteria increase OD?

Dead intact cells can still scatter light.

Why shake an aerobic bacterial culture?

To mix oxygen into the culture and keep cells suspended.

Main limitation of OD

OD measures total cells/cell material, not the number of viable cells.

Optical density (OD)

Measures total bacterial cells/cell material, including intact dead cells.

Higher OD

Indicates more total bacterial cells or cell material.

Can OD measure viability?

No. Dead intact cells can still contribute to OD.

Colony-forming unit (CFU)

Represents a viable bacterium/unit capable of growing into a visible colony.

CFU assay

Plate a culture sample on agar, incubate it, then count the colonies.

Main difference between OD and CFU

OD measures total cells/cell material; CFU measures viable cells.

Binary fission

Bacterial cell division that produces two daughter cells.

Septum

The division site that separates a bacterial cell into two daughter cells.

FtsZ

Forms the Z-ring at the middle of the cell for division.

FtsA and ZipA

Help anchor and recruit the bacterial division machinery.

FtsK

Helps separate chromosomes into the daughter cells.

Fts mutant at 42°C

Cell grows and elongates but cannot divide, producing long filaments.

Min system

Prevents division at the poles so the Z-ring forms near the middle.

MinC

Directly inhibits FtsZ polymerization.

Defective Min system

Causes division near the poles, producing minicells.

MinC

Directly inhibits FtsZ polymerization and prevents Z-ring formation.

MinD

Recruits MinC to the membrane near the cell poles.

MinE

Drives movement of MinC and MinD away from a pole, causing their oscillation.

Purpose of the Min system

Prevents FtsZ from forming at the poles so the Z-ring forms near the middle.