Control of Microbial Growth: Principles and Methods

Control of Microbial Growth

Introduction

The control of microbial growth is essential in medical, industrial, and everyday settings to prevent infection, spoilage, and contamination. This chapter explores the terminology, principles, and methods used to control microbial populations.

Terminology of Microbial Control

Key Definitions

• Sepsis: Refers to bacterial contamination.

• Asepsis: The absence of significant contamination. Aseptic techniques prevent microbial contamination of wounds.

• Sterilization: Removing and destroying all microbial life.

• Commercial Sterilization: Killing Clostridium botulinum endospores from canned goods.

• Disinfection: Destroying harmful microorganisms on inanimate objects.

• Antisepsis: Destroying harmful microorganisms from living tissue.

• Degerming: Mechanical removal of microbes from a limited area.

• Sanitization: Lowering microbial counts on eating utensils to safe levels.

• Biocide (germicide): Treatments that kill microbes.

• Bacteriostasis: Inhibiting, not killing, microbes.

The Rate of Microbial Death

Factors Affecting Effectiveness of Treatment

• Number of microbes: Larger populations take longer to eliminate.

• Environment: Presence of organic matter, temperature, and biofilms can protect microbes.

• Time of exposure: Longer exposure increases effectiveness.

• Microbial characteristics: Species and life stage (e.g., endospores) affect susceptibility.

Understanding the Microbial Death Curve

• Microbial death often follows a logarithmic decline.

• If the rate of killing is constant, larger populations require more time to achieve sterility.

Example: Killing all cells in a population of 106 takes longer than in a population of 103 at the same rate of killing.

Actions of Microbial Control Agents

• Alteration of membrane permeability: Causes cell contents to leak, interfering with cell growth.

• Damage to proteins (enzymes): Heat and chemicals can denature proteins, causing loss of function.

• Damage to nucleic acids: Heat, chemicals, and radiation can damage DNA and RNA, preventing replication and normal function.

Physical Methods of Microbial Control

• Heat

• Filtration

• Low Temperature

• High Pressure

• Desiccation

• Osmotic Pressure

• Radiation

Moist Heat Sterilization

• Denatures proteins

• Uses steam and pressure (e.g., autoclave)

• Kills endospores

• Effectiveness depends on volume and sterilization time

• Test strips are used to indicate sterility

Pasteurization

• Reduces spoilage organisms and pathogens

• High-temperature short-time (HTST): 72°C for 15 seconds

• Thermoduric organisms may survive

Filtration

• Passage of substance through a screen-like material

• Used for heat-sensitive materials

• HEPA filters remove microbes >0.3 μm

• Membrane filters remove microbes >0.22 μm; some can filter viruses and large proteins

Low Temperature & Desiccation

• Low temperature: Bacteriostatic effect (slows metabolism)

  • Refrigeration

  • Deep-freezing

  • Lyophilization (freeze drying)

• Desiccation: Absence of water prevents metabolism; bacteria remain alive but cannot grow; endospores can survive for years

Principles of Effective Disinfection

• Concentration of disinfectant

• Presence of organic matter

• pH

• Time of exposure

The Disk-Diffusion Method

• Evaluates efficacy of chemical agents

• Filter paper disks are soaked in a chemical and placed on a culture

• Zone of inhibition around disks indicates effectiveness

Chemical Methods of Microbial Control

Alcohols

• Denature proteins and dissolve lipids

• No effect on endospores and nonenveloped viruses

• Ethanol and isopropanol require water for effectiveness

Heavy Metals (Oligodynamic Action)

• Denature proteins

• Examples: Silver (Ag), Mercury (Hg), Copper (Cu), Zinc (Zn)

Surface-Active Agents

Agent	Action
Soap	Degerming; emulsification
Acid-anionic sanitizers	Anions react with plasma membrane
Quaternary ammonium compounds (quats)	Cations are bactericidal, denature proteins, disrupt plasma membrane; broad spectrum but ineffective against endospores and mycobacteria

Effectiveness of Chemical Antimicrobials

Chemical Agent	Effect against Endospores	Effect against Mycobacteria
Glutaraldehyde	Fair	Good
Chlorines	Fair	Fair
Alcohols	Poor	Good
Iodine	Poor	Good
Phenolics	Poor	Good
Chlorhexidine	None	Fair
Bisphenols	None	None
Quats	None	None
Silver	None	None

Additional info: The effectiveness of chemical agents varies depending on the type of microorganism and the presence of protective structures such as endospores and mycobacterial cell walls.