Controlling Microbial Growth in the Environment: Principles and Methods

Basic Principles of Microbial Control

Terminology of Microbial Control

Understanding the terminology of microbial control is essential for effective communication and application in microbiology. The following table summarizes key terms, their definitions, examples, and comments on their use:

Term	Definition	Examples	Comments
Antisepsis	Reduction in the number of microorganisms and viruses, particularly potential pathogens, on living tissue	Iodine; alcohol	Antiseptics are frequently disinfectants whose strength has been reduced to make them safe for living tissues.
Aseptic	Refers to an environment or procedure free of pathogenic contaminants	Preparation of surgical field; hand washing; flame sterilization of laboratory equipment	Scientists, laboratory technicians, and health care workers routinely follow aseptic techniques.
-cide/-cidal	Suffixes indicating destruction of a type of microbe	Bactericide; fungicide; germicide; virucide	Germicides include ethylene oxide, propylene oxide, and aldehydes.
Degerming	Removal of microbes by mechanical means	Handwashing; alcohol swabbing at site of injection	Chemicals play a secondary role to the mechanical removal of microbes.
Disinfection	Destruction of most microorganisms and viruses on nonliving tissue	Phenolics; alcohols; aldehydes; soaps	Disinfectants are used only on inanimate objects.
Pasteurization	Use of heat to destroy pathogens and reduce the number of spoilage microorganisms in foods and beverages	Pasteurized milk and fruit juices	Heat treatment is brief to minimize alteration of taste and nutrients; microbes still remain.
Sanitization	Removal of pathogens from objects to meet public health standards	Washing tableware in scalding water in restaurants	Standards vary among governmental jurisdictions.
-stasis/-static	Suffixes indicating inhibition, but not complete destruction, of a type of microbe	Bacteriostatic; fungistatic; virustatic	Germistatic agents include some chemicals, refrigeration, and freezing.
Sterilization	Destruction of all microorganisms and viruses in or on an object	Preparation of microbiological culture media and canned food	Typically achieved by steam under pressure, incineration, or ethylene oxide gas.

Action of Antimicrobial Agents

Antimicrobial agents control microbial growth by targeting essential cellular structures and functions:

• Alteration of cell walls and membranes: Damaging the cell wall can cause cells to burst due to osmotic pressure. Disruption of the cytoplasmic membrane leads to leakage of cellular contents.

• Damage to proteins and nucleic acids: Denaturation of proteins (by heat or chemicals) disrupts their function. Damage to nucleic acids (by chemicals, radiation, or heat) can result in fatal mutations or halt protein synthesis.

• Relative resistance: Nonenveloped viruses are generally more tolerant of harsh conditions than enveloped viruses.

The Selection of Microbial Control Methods

Ideal Characteristics of Antimicrobial Agents

• Inexpensive

• Fast-acting

• Stable during storage

• Harmless to humans, animals, and objects

Factors Affecting Efficacy

• Site to be treated: The method must be appropriate for the material or tissue being treated (e.g., harsh chemicals cannot be used on living tissues).

• Relative susceptibility of microorganisms: Microbes vary in their resistance to antimicrobial agents. Germicides are classified as high, intermediate, or low effectiveness.

• Environmental conditions: Temperature, pH, and presence of organic matter can affect efficacy.

Methods for Evaluating Disinfectants and Antiseptics

• Phenol coefficient: Compares an agent’s effectiveness to phenol. A value greater than 1.0 indicates higher efficacy than phenol.

• Use-dilution test: Metal cylinders are contaminated with bacteria, exposed to disinfectant, and then incubated to assess microbial survival. The most effective agents prevent growth at the highest dilution.

• Kelsey-Sykes capacity test: Bacterial suspensions are exposed to the chemical, and samples are incubated to determine the minimum effective time.

• In-use test: Swabs are taken before and after disinfectant application to determine real-world effectiveness.

Physical Methods of Microbial Control

Heat-Related Methods

Heat is a widely used method for controlling microbial growth, acting by denaturing proteins, disrupting membranes, and damaging nucleic acids.

• Thermal death point (TDP): The lowest temperature that kills all cells in a broth in 10 minutes.

• Thermal death time (TDT): The time required to sterilize a volume of liquid at a set temperature.

Decimal Reduction Time (D-value)

The D-value is the time required to kill 90% of the microorganisms present. It is a key parameter in sterilization processes.

Moist Heat Methods

• Boiling: Kills most vegetative cells and viruses, but not endospores or some viruses. Boiling time is critical and varies with elevation.

• Autoclaving: Uses pressurized steam (121°C, 15 psi, 15 min) to achieve sterilization. Pressure increases the boiling point of water, allowing higher temperatures for effective sterilization.

• Pasteurization: Reduces pathogens in foods and beverages without sterilization. Methods include batch, flash, and ultrahigh-temperature pasteurization.

• Ultrahigh-temperature sterilization: 140°C for 1 second, followed by rapid cooling. Allows storage at room temperature.

Dry Heat Methods

• Used for materials that cannot be sterilized with moist heat.

• Requires higher temperatures and longer times.

• Incineration is the ultimate means of sterilization.

Other Physical Methods

• Refrigeration and Freezing: Inhibit microbial metabolism and growth. Psychrophilic microbes can still grow at low temperatures. Slow freezing is more effective than quick freezing.

• Desiccation and Lyophilization: Drying removes water, inhibiting growth. Lyophilization (freeze-drying) is used for long-term preservation.

• Filtration: Physically removes microbes from air and liquids using membrane filters. High-efficiency particulate air (HEPA) filters are used in biological safety cabinets.

• Osmotic Pressure: High concentrations of salt or sugar inhibit microbial growth by causing water loss from cells. Fungi are more tolerant than bacteria.

• Radiation: Ionizing radiation (e.g., gamma rays) creates ions that damage DNA and proteins. Nonionizing radiation (e.g., UV light) causes DNA mutations and is used for surface disinfection.

Biosafety Levels

Laboratories are classified into four biosafety levels (BSL-1 to BSL-4) based on the risk associated with the pathogens handled:

• BSL-1: Non-pathogenic microbes

• BSL-2: Moderately hazardous agents

• BSL-3: Pathogens handled in safety cabinets

• BSL-4: Dangerous and exotic pathogens (e.g., Ebola virus)

Chemical Methods of Microbial Control

Overview

Chemical agents control microbial growth by affecting cell walls, membranes, proteins, or DNA. Their effectiveness varies with environmental conditions and the type of microbe.

Major Classes of Chemical Agents

• Phenol and Phenolics: Intermediate- to low-level disinfectants that denature proteins and disrupt membranes. Effective in the presence of organic matter and remain active for prolonged periods.

• Alcohols: Intermediate-level disinfectants that denature proteins and disrupt membranes. Commonly used for skin antisepsis (e.g., 70% ethanol).

• Halogens: Intermediate-level agents that damage enzymes via oxidation or denaturation. Examples include iodine, chlorine, and bromine.

• Oxidizing Agents: High-level disinfectants (e.g., hydrogen peroxide, ozone, peracetic acid) that kill by oxidizing microbial enzymes.

• Surfactants: Surface-active agents (e.g., soaps and detergents) that reduce surface tension. Quaternary ammonium compounds (quats) are low-level disinfectants.

• Heavy Metals: Ions such as silver, mercury, and copper denature proteins. Used as low-level bacteriostatic and fungistatic agents.

• Aldehydes: Compounds like glutaraldehyde and formalin cross-link functional groups to denature proteins and inactivate nucleic acids.

• Gaseous Agents: Sterilize in closed chambers by denaturing proteins and DNA. Used for heat-sensitive materials but can be hazardous.

• Enzymes: Antimicrobial enzymes (e.g., lysozyme) digest cell walls. Prionzyme can remove prions from medical instruments.

• Antimicrobials: Antibiotics and synthetic chemicals are used for disease treatment and sometimes for environmental control.

Development of Resistant Microbes

Overuse of antiseptics and disinfectants can promote the development of resistant microbial strains. There is little evidence that such products significantly improve human or animal health.