Physical Methods Used to Control Microbial Growth

Heat

Moist Heat OR Boiling or Flowing Steam

Mechanism of Action: Protein Denaturation

Kills vegetative bacterial and fungal pathogens and many viruses within 10 min; less effective on endospores.

Autoclaving

Mechanism of Action: Protein Denaturation

Very effective method of sterilization; at about 15 psi of pressure (121°C), all vegetative cells and their endospores are killed in about 15 min.

Pasteurization

Mechanism of Action: Protein Denaturation

Heat treatment for milk (72°C for about 15 sec) that kills all pathogens and most nonpathogens.

Dry Heat

Direct Flaming

Mechanism of Action: Burning contaminants to ashes

Very effective method of sterilization. Used for inoculating loops.

Incineration

Mechanism of Action: Burning to ashes

Very effective method of sterilization. Used for disposal of contaminated dressings, animal carcasses, and paper.

Hot Air Sterilization

Mechanism of Action: Oxidation

Very effective method of sterilization but requires temperature of 170°C for about 2 hr. Used for empty glassware.

Filtration

Mechanism of Action: Separation of bacteria from suspending liquid

Removes microbes by passage of a liquid or gas through a screen-like material; most filters in use consist of cellulose acetate or nitrocellulose. Useful for sterilizing liquids (e.g., enzymes, vaccines) that are destroyed by heat.

Refrigeration

Mechanism of Action: Decreased chemical reactions and possible changes in proteins

Has a bacteriostatic effect.

Deep-freezing

Mechanism of Action: Decreased chemical reactions and possible changes in proteins

An effective method for preserving microbial cultures, food, and drugs.

Lyophilization

Mechanism of Action: Decreased chemical reactions and possible changes in proteins

Most effective method for long-term preservation of microbial cultures, food, and drugs.

High Pressure

Mechanism of Action: Alteration of molecular structure of proteins and carbohydrates

Preserves colors, flavors, nutrient values of fruit juices.

Desiccation

Mechanism of Action: Disruption of metabolism

Involves removing water from microbes; primarily bacteriostatic.

Osmotic Pressure

Mechanism of Action: Plasmolysis

Results in loss of water from microbial cells.

Ionizing Radiation

Mechanism of Action: Destruction of DNA

Used for sterilizing pharmaceuticals and medical and dental supplies.

Nonionizing Radiation

Mechanism of Action: Damage to DNA

Radiation is not very penetrating.