Microbial Control Methods Overview

Microbial Growth Control

• Microbial control methods can be physical or chemical
• Physical methods include heat, filtration, and radiation
• Chemical methods include disinfectants, germicides, and antibiotics
• When bacteria are treated with a killing agent, they usually die at a constant rate. This is called the death rate.

Microbial Control Methods: Terminology

• Sanitization: A mechanical/physical cleaning technique that removes microbes.
• Disinfection: A process that destroys vegetative microbes, but not endospores, usually on inanimate objects.
• Sterilization: A process that destroys all viable microbes, including viruses and endospores.
• Antisepsis: Chemicals applied to body surfaces to destroy or inhibit vegetative pathogens.
• Chemotherapy: Chemicals used to kill or inhibit microorganisms within tissues.

Factors Affecting Effectiveness of Antimicrobial Agents

• Population Size: Larger populations require more time for killing.
• Species/Life Cycle: Different microbes have varying sensitivities to antimicrobial agents.
• Population Composition: Microbes differ distinctly in how they respond to antimicrobial treatments.
• Organic Matter: Presence of organic matter influences antimicrobial effectiveness.
• Environmental Factors (pH and Temperature): Antimicrobial effectiveness changes with environmental conditions.
• Mode of Action: Different antimicrobial methods have varied effects.
• Concentration/Dosage: Higher concentrations typically act faster.
• Duration of Exposure: Extended exposure usually results in more deaths.

Antimicrobial Agents: Action Mechanisms

• Alteration of Cell Walls/Cytoplasmic Membranes: Disrupts the structure and function of the cell.
• Interference with Proteins/Nucleic Acids: Interferes with protein and nucleic acid function.
• Damage to Proteins and Nucleic Acids: Chemicals, radiation, and heat affect 3-D protein shape and nucleic acids, in turn resulting in mutations and cell death.
• Denaturation: Extreme heat, and certain chemicals can alter protein shape and function by breaking its three-dimensional structure.

Relative Susceptibility of Microorganisms

• Enveloped viruses, gram-positive bacteria and non-enveloped viruses, fungi generally are more susceptible to microbial treatment.
• Gram-negative bacteria, active-stage protozoa, protozoa cysts, mycobacteria, bacterial endospores generally are more resistant.

Gram Positive vs Gram Negative Bacteria

• Gram-positive bacterial cell walls have a thick peptidoglycan layer, less complex and thus more penetrable to antimicrobial agents.
• Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane containing lipopolysaccharides. This outer layer significantly reduces the penetration of many antimicrobial agents, thus higher resistance.

Strength of Germicides

• High-level: Kills all pathogens, including bacterial endospores (some aldehydes).
• Intermediate-level: Kills fungal spores, protozoan cysts, viruses, and pathogenic bacteria (some chlorine).
• Low-level: Kills vegetative bacteria, fungi, and protozoa (some alcohols, detergents/soaps).

Methods of Control: Physical vs Chemical

• Antimicrobial agents include physical factors (e.g., heat) and chemical factors (e.g., disinfectants, antibiotics).
• Bacterial populations treated with killing agents usually die at a constant rate.

Death Rate

• Bacterial populations die at a constant rate when exposed to killing agents, whether physical or chemical.
• The graph of bacterial population vs exposure time has a straight downwards slope.