Physical and Chemical Controls of Microorganisms

Overview and Learning Objectives

This chapter explores the principles and practices used to control microbial growth in the environment and on surfaces. Students will learn about aseptic procedures, terminology, mechanisms of action (MOAs), microbial death, and both physical and chemical methods of microbial control.

• Aseptic procedures: Techniques to prevent contamination by microorganisms.

• Microbial control: Methods to reduce or eliminate microorganisms to prevent infection and spoilage.

Terminology of Microbial Control

Key Terms and Definitions

Understanding the terminology is essential for discussing microbial control methods. The following table summarizes important terms:

Primary Targets and Resistance of Microbial Control

Microbial Resistance Hierarchy

Microbial control methods target organisms that cause infection or spoilage. The effectiveness of these methods depends on the resistance of the microorganism.

• Most resistant: Prions, bacterial endospores, mycobacteria

• Least resistant: Enveloped viruses, most Gram-positive bacteria

Factors Affecting Microbial Control

Environmental and Microbial Factors

The effectiveness of antimicrobial treatments depends on several factors:

• Number of microbes: Higher populations require longer treatment.

• Environment: Presence of organic matter, temperature, concentration, and biofilms can affect efficacy.

• Time of exposure: Longer exposure increases effectiveness.

• Microbial characteristics: Cell wall structure, resistance mechanisms, and presence of protective layers (e.g., glycocalyx).

Modes of Action of Antimicrobial Agents

How Agents Affect Microbial Cells

Antimicrobial agents work by targeting essential structures and functions in microbial cells:

• Cell wall: Blocking synthesis, digesting, or breaking down the wall leads to cell lysis.

• Cell membrane: Disruption causes loss of integrity and cell death.

• Protein and nucleic acid synthesis: Binding to ribosomes or DNA prevents transcription and translation; mutagenic agents cause mutations.

• Protein denaturation: Changes in shape or blocking active sites render enzymes inactive.

Physical Methods of Microbial Control

Heat Methods

Heat is a widely used method for microbial control, including both dry and moist heat techniques.

• Dry heat: Kills by oxidation (flaming, incineration, hot-air sterilization).

• Moist heat: Includes pasteurization, boiling, and steam under pressure (autoclaving).

• Autoclave: Most efficient pressure-temperature combination for sterilization is 15 psi yielding 121°C.

Pasteurization

Pasteurization is used to disinfect beverages by applying heat to kill pathogens and spoilage organisms while preserving flavor and nutritional value.

• Flash method: 71.6°C for 15 seconds

• Batch method: 63–66°C for 30 minutes

• Ultra-high-temperature: 140°C for less than 1 second

• Limitations: Does not kill prions, endospores, or thermoduric microbes

Radiation Methods

Radiation is used to kill microbes by damaging their DNA or cellular components.

• Ionizing radiation: Gamma rays, X-rays, electron beams; cause DNA mutations and are used for sterilizing pharmaceuticals and medical supplies.

• Nonionizing radiation (UV): Causes thymine dimers in DNA, used for disinfecting surfaces and air.

• Microwave radiation: Kills by heating water molecules; less effective for dry materials and endospores.

Filtration Methods

Filtration physically removes microbes from air and liquids by passing them through filters with defined pore sizes.

• HEPA filters: Remove microbes larger than 0.3 µm from air.

• Membrane filtration: Removes microbes larger than 0.22 µm from liquids.

• Applications: Used for heat-sensitive liquids, water purification, and air decontamination.

Other Physical Methods

Additional physical methods include:

• Low temperature: Inhibits microbial growth (refrigeration, deep-freezing, lyophilization).

• High pressure: Denatures proteins.

• Desiccation: Prevents metabolism by removing water.

• Osmotic pressure: Causes plasmolysis by drawing water out of cells.

Chemical Methods of Microbial Control

Choosing a Microbicidal Chemical

Effective chemicals for microbial control should act rapidly, be stable, have broad-spectrum activity, penetrate surfaces, resist inactivation, and be affordable and safe.

Types of Chemical Agents

The following table summarizes common chemical methods:

Phenols and Phenolics

Phenol (carbolic acid) and its derivatives are used as disinfectants and antiseptics. They denature proteins and disrupt cell membranes, but are less commonly used today due to toxicity and odor.

• Examples: Cresols (Lysol), biphenols (pHisoHex), O-phenylphenol, pine oil

• Advantages: Stable, persistent, active in presence of organic compounds

Alcohols

Alcohols are effective disinfectants and antiseptics, acting by denaturing proteins and disrupting membranes. Ethanol and isopropanol are commonly used.

• Optimum concentration: 70% for maximum effectiveness

• Applications: Skin antisepsis, instrument disinfection

Halogens

Halogens such as iodine and chlorine are widely used for disinfection and antisepsis.

• Iodine: Used in tinctures and iodophors; denatures proteins by breaking disulfide bonds.

• Chlorine: Forms hypochlorous acid in water; used for water, pool, and sewage disinfection.

Biguanides

Chlorhexidine is a biguanide used for skin antisepsis and surgical scrubs. It targets cell membranes and protein structure, is bactericidal for both Gram-positive and Gram-negative bacteria, and is inactive against spores.

Peroxygens (Oxidizing Agents)

Peroxygens such as ozone and hydrogen peroxide oxidize cellular components and disrupt membranes and proteins.

• Ozone: Used for water disinfection, more effective than chlorine but less stable.

• Hydrogen peroxide: Used as antiseptic and disinfectant; sporicidal at high temperatures.

• Benzoyl peroxide: Used in acne medications.

Surfactants (Detergents)

Surfactants reduce surface tension and aid in the removal of microbes. Cationic detergents are more effective than anionic detergents.

• Soaps: Weak microbicides, enhanced when combined with other agents.

Heavy Metals

Heavy metals such as silver, mercury, and copper denature proteins and are effective in small amounts (oligodynamic action).

• Silver nitrate: Used to prevent eye infections in infants.

• Mercury compounds: Used for skin wound disinfection.

• Copper sulfate: Used to kill algae in pools and tanks.

Aldehydes

Aldehydes such as formaldehyde and glutaraldehyde are highly effective disinfectants that inactivate proteins by forming covalent crosslinks.

• Formalin: Used for specimen preservation and vaccine inactivation.

• Glutaraldehyde: Used for disinfecting hospital instruments; sporicidal at high concentrations.

Gaseous Sterilants

Gaseous agents such as ethylene oxide denature proteins and are used for sterilizing heat-sensitive materials.

Enzymes

Enzymes such as lysozyme and prionzyme act against microorganisms by digesting cell walls or removing prions from medical instruments.

Dyes as Antimicrobial Agents

Certain dyes are used in chemotherapy and wound treatment, but have limited applications due to staining and narrow spectrum.

Acids and Alkalis

Extreme pH can destroy or inhibit microbial cells, but their use is limited due to corrosive and hazardous properties.

Evaluating Disinfectants

Disk-Diffusion Method

The disk-diffusion method is used to evaluate the effectiveness of disinfectants against specific microbial species. The zone of inhibition indicates the degree of microbial control.

Summary and Take Home Points

• Know the terminology associated with microbial control.

• Understand the physical methods of control discussed.

• Know the different targets, pros, and cons of the chemical agent groups discussed.