(1) Chapter 6

Microbial Growth

What does microbial growth mean?

An increase in the number of cells, not an increase in cell size.

How do bacteria usually reproduce?

By binary fission.

What is binary fission?

A form of asexual reproduction where one cell divides into two genetically identical cells.

What is generation time?

The time required for one bacterial cell to divide into two.

What determines how fast bacteria grow?

Environmental conditions such as temperature, pH, nutrients, oxygen, and osmotic pressure.

What is an optimum growth condition?

The condition under which a microorganism grows best.

What are the three major physical requirements for microbial growth?

Temperature, pH, and osmotic pressure.

What are the major chemical requirements for microbial growth?

Carbon, nitrogen, sulfur, phosphorus, trace elements, oxygen, and organic growth factors.

Why is carbon important for microbial growth?

It forms the backbone of all organic molecules.

Why is nitrogen important?

It is needed to make proteins and nucleic acids.

Why is sulfur needed?

It is required to make certain amino acids and vitamins.

Why is phosphorus needed?

It is required for ATP, nucleic acids, and phospholipids.

What are trace elements?

Small amounts of minerals needed as enzyme cofactors.

Give examples of trace elements.

Iron, zinc, manganese, cobalt, and molybdenum.

What are organic growth factors?

Organic compounds that organisms cannot synthesize and must obtain from the environment.

Examples of organic growth factors include?

Vitamins, amino acids, purines, and pyrimidines.

What is the optimum temperature for most human pathogens?

About 37°C.

What are psychrophiles?

Cold-loving microorganisms that grow best around 15°C or lower.

Where are psychrophiles commonly found?

Polar regions, glaciers, and deep oceans.

Can psychrophiles grow below freezing?

Yes, although growth is slow.

What are psychrotrophs?

Microorganisms that grow in refrigerators but grow best at room temperature.

Which microorganisms commonly spoil refrigerated food?

Psychrotrophs.

What temperature range do psychrotrophs tolerate?

About 0°C to 30°C, with optimum around 20–30°C.

What are mesophiles?

Microorganisms that grow best between about 25°C and 40°C.

Which temperature group includes most human pathogens?

Mesophiles.

What are thermophiles?

Heat-loving microorganisms.

What is the optimum temperature for thermophiles?

About 50–60°C.

Where are thermophiles commonly found?

Hot springs and compost piles.

What are hyperthermophiles?

Microorganisms that grow best above 80°C.

Where are hyperthermophiles found?

Hydrothermal vents and geysers.

Which domain contains many hyperthermophiles?

Archaea.

Why doesn't refrigeration kill bacteria?

It is bacteriostatic—it slows or stops growth rather than killing cells.

What is the food danger zone?

Approximately 5°C–60°C (41°F–140°F), where bacteria grow rapidly.

Why should hot food be refrigerated quickly?

To reduce the time bacteria spend multiplying in the danger zone.

Why should leftovers be stored in shallow containers?

They cool more quickly, reducing bacterial growth.

What happens if food cools too slowly?

Bacteria have more time to multiply and produce toxins.

What is the typical optimum pH for bacteria?

6.5–7.5.

What is the typical optimum pH for fungi?

About 5–6.

Which generally tolerate more acidic environments: bacteria or fungi?

Fungi.

What are acidophiles?

Microorganisms that grow best in acidic environments.

What are alkaliphiles?

Microorganisms that grow best in alkaline environments.

Why does pH affect microbial growth?

It influences enzyme activity and protein stability.