Controlling Microbial Growth: Methods and Principles 7

Controlling Microbial Growth CH 7

The Terminology of Microbial Control

Understanding the terminology used in microbial control is essential for effective application in laboratory and clinical settings.

• Sepsis: Refers to microbial contamination, especially in tissues or blood.

• Asepsis: The absence of significant contamination; practices that prevent microbial contamination of wounds.

• Antiseptic: Agents used on living tissues to reduce or eliminate pathogens.

• Disinfection: Removing or killing most microorganisms (except some spores) from inanimate objects.

• Sanitization: Lowering microbial counts to safe public health levels.

• Degerming: Mechanically removing microbes from a limited area (e.g., skin before injection).

• Decontamination: Reducing pathogens to safe levels.

• Sterilization: The complete removal or destruction of all forms of microbial life, including endospores.

• Bacteriostasis: Inhibiting, not killing, microbes.

Measuring Effectiveness of Antimicrobial Treatments

Several parameters are used to evaluate the effectiveness of microbial control methods.

• Thermal Death Point (TDP): The lowest temperature at which all cells in a culture are killed in 10 minutes.

• Thermal Death Time (TDT): The minimum time required to kill all microbes in a sample at a specific temperature.

• Decimal Reduction Time (D-Value, DRT): The time (in minutes) required to kill 90% of a population at a given temperature.

Factors Affecting Microbial Death

The rate at which microbes are killed depends on several factors:

• Number of microbes present

• Environmental influences (e.g., organic matter, temperature, biofilms)

• Time of exposure

• Microbial characteristics (e.g., spore-formers, cell wall structure)

Physical Methods of Microbial Control

Dry Heat Sterilization

• Flaming: Direct exposure to flame (e.g., inoculating loops).

• Incineration: Burning to ashes (e.g., disposal of contaminated materials).

• Hot Air Oven: Sterilization at 170°C for 2 hours.

Moist Heat Sterilization

• Boiling: Kills most vegetative cells but not all endospores.

• Autoclaving: Steam under pressure (121°C, 15 psi, 15 min) is effective for sterilization.

Filtration

• Removes microbes from solutions by passing through filters with pore sizes as small as 0.22 μm.

• Used for heat-sensitive liquids (e.g., antibiotics, vaccines).

Radiation

• Ionizing Radiation: (X-rays, gamma rays, electron beams) damages DNA by causing lethal mutations.

• Non-ionizing Radiation: (UV light) causes thymine dimers in DNA, inhibiting replication.

• Microwaves: Kill by heat; not especially antimicrobial.

Principles of Effective Disinfection

Several factors influence the effectiveness of disinfectants:

• Concentration of disinfectant

• Organic matter present

• pH

• Time of exposure

Chemical Methods of Microbial Control

Use-Dilution Test

This test evaluates the effectiveness of disinfectants against bacteria dried onto metal rings.

• Rings are dipped in test bacteria, dried, then placed in disinfectant for 10 minutes at 20°C.

• Rings are transferred to culture media to determine if bacteria survived treatment.

Disk-Diffusion Method

• Filter paper disks are soaked in disinfectant and placed on agar plates inoculated with bacteria.

• Zones of inhibition indicate effectiveness.

Types of Disinfectants and Antiseptics

• Phenolics: Disrupt plasma membranes and denature proteins. Effective against organic matter. Example: Lysol.

• Bisphenols: Disrupt plasma membranes. Example: Triclosan.

• Biguanides: Disrupt plasma membranes. Example: Chlorhexidine.

• Halogens: Oxidizing agents that damage proteins and membranes. Examples: Iodine, chlorine.

• Alcohols: Denature proteins and dissolve lipids. Examples: Ethanol, isopropanol.

• Heavy Metals: Denature proteins. Examples: Silver nitrate, copper sulfate.

• Surface-Active Agents (Surfactants): Soaps and detergents that mechanically remove microbes.

• Quaternary Ammonium Compounds (Quats): Disrupt membranes. Example: Benzalkonium chloride.

• Aldehydes: Inactivate proteins by cross-linking functional groups. Examples: Glutaraldehyde, formaldehyde.

• Gaseous Sterilants: Used for heat-sensitive materials. Example: Ethylene oxide.

• Peroxygens: Oxidizing agents used on contaminated surfaces. Examples: Hydrogen peroxide, peracetic acid.

Comparison Table: Physical and Chemical Methods of Microbial Control

Method	Mechanism	Example/Application
Autoclaving	Denatures proteins via moist heat under pressure	Sterilizing media, instruments
Filtration	Physical removal of microbes	Heat-sensitive solutions
UV Radiation	Damages DNA (thymine dimers)	Surface sterilization
Alcohols	Denature proteins, dissolve lipids	Skin antisepsis
Halogens	Oxidation of cellular components	Water disinfection, wound cleaning
Aldehydes	Cross-linking of proteins	Medical equipment sterilization

Additional info:

• Some methods (e.g., boiling) are not reliable for sterilization due to endospore resistance.

• Organic matter can inhibit the action of many disinfectants.

• Proper selection of method depends on the nature of the item, type of microbe, and intended use.