Controlling Microbial Growth in the Environment

Controlling Microbial Growth in the Environment

Introduction to Microbial Control

Microbial control is essential in healthcare, industry, and daily life to prevent infection, spoilage, and contamination. This chapter explores the principles, methods, and effectiveness of physical and chemical agents used to control microbial growth.

Terminology of Microbial Control

Understanding the terminology is crucial for distinguishing between different levels and methods of microbial control.

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 healthcare workers routinely follow standard 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	Hand washing; alcohol swabbing at site of injection	Scrubbing is the most important part of this action; 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	The term is used primarily in relation to pathogens. 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-tolerant microbes survive pasteurization.
Sanitization	Removal of pathogens from objects to meet public health standards	Washing tableware in scalding water in restaurants	Standards of sanitization 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.

Basic Principles of Microbial Control

• Action of Antimicrobial Agents: Agents may damage cell walls, membranes, proteins, or nucleic acids, leading to cell death or inhibition of growth.

• Cell Wall and Membrane: Damage can cause cells to burst due to osmotic effects or leak cellular contents.

• Proteins and Nucleic Acids: Denaturation or destruction halts essential cellular functions and can produce fatal mutations.

• Nonenveloped Viruses: These are generally more resistant to harsh conditions than enveloped viruses.

Microbial Death Rate

Microbial death rate refers to the constant percentage of a microbial population killed per unit time when exposed to a particular agent or condition.

Selection of Microbial Control Methods

• Ideal Agents: Should be inexpensive, fast-acting, stable during storage, and harmless to humans, animals, and objects.

• Factors Affecting Efficacy: Site to be treated, susceptibility of microorganisms, and environmental conditions (e.g., temperature, pH, organic matter).

Relative Susceptibility of Microorganisms

Microorganisms vary in their resistance to antimicrobial agents. Prions are the most resistant, while enveloped viruses are the most susceptible.

Effect of Temperature on Efficacy

Higher temperatures generally increase the efficacy of antimicrobial chemicals by accelerating chemical reactions and denaturation processes.

Biosafety Levels

• BSL-1: Handling non-pathogenic microbes.

• BSL-2: Handling moderately hazardous agents.

• BSL-3: Handling microbes in safety cabinets.

• BSL-4: Handling highly dangerous or exotic microbes (e.g., Ebola virus).

Physical Methods of Microbial Control

Heat-Related Methods

Heat is one of the most common physical methods for controlling microbial growth. It denatures proteins, disrupts membranes, and destroys nucleic acids.

• Thermal Death Point (TDP): Lowest temperature that kills all cells in a broth in 10 minutes.

• Thermal Death Time (TDT): Time to sterilize a volume of liquid at a set temperature.

• Decimal Reduction Time (D): Time required to kill 90% of the microorganisms at a specific temperature.

Moist Heat Methods

• Boiling: Kills most vegetative cells, 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.

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

• Ultrahigh-Temperature Sterilization: 140°C for 1–3 seconds, then rapid cooling; allows storage at room temperature.

Dry Heat

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

• Requires higher temperatures and longer times (e.g., incineration).

Refrigeration and Freezing

Low temperatures decrease microbial metabolism, growth, and reproduction. Slow freezing is more effective than quick freezing due to the formation of ice crystals that damage cell structures.

Desiccation and Lyophilization

• Desiccation: Drying inhibits microbial growth by removing water.

• Lyophilization: Freeze-drying used for long-term preservation; prevents ice crystal formation.

Filtration

Filtration physically removes microbes from air and liquids using filters with specific pore sizes. HEPA filters are used in biological safety cabinets.

Pore Size (µm)	Smallest Microbes That Are Trapped
5	Multicellular algae, animals, and fungi
3	Yeasts and larger unicellular algae
1.2	Protozoa and small unicellular algae
0.45	Largest bacteria
0.22	Largest viruses and most bacteria
0.025	Larger viruses and pliable bacteria (mycoplasmas, rickettsias, chlamydias, and some spirochetes)
0.01	Smallest viruses

Osmotic Pressure

High concentrations of salt or sugar create hypertonic environments, causing cells to lose water and inhibiting microbial growth. Fungi are more resistant to osmotic pressure than bacteria.

Radiation

• Ionizing Radiation: (e.g., electron beams, gamma rays, X-rays) creates ions that disrupt cellular molecules, effective for sterilization of medical and laboratory equipment and food preservation.

• Nonionizing Radiation: (e.g., UV light) causes DNA damage (pyrimidine dimers), suitable for disinfecting air, transparent fluids, and surfaces.

Summary Table: Physical Methods of Microbial Control

Chemical Methods of Microbial Control

Overview

Chemical agents target cell walls, membranes, proteins, or DNA. Their effectiveness depends on environmental conditions and the type of microbe.

Major Classes of Chemical Agents

• Phenol and Phenolics: Denature proteins and disrupt membranes; effective in presence of organic matter; used in healthcare and household products.

• Alcohols: Intermediate-level disinfectants; denature proteins and disrupt membranes; used for skin antisepsis.

• Halogens: Intermediate-level agents (e.g., iodine, chlorine, bromine); denature enzymes; used in water treatment and antiseptics.

• Oxidizing Agents: High-level disinfectants (e.g., peroxides, ozone, peracetic acid); kill by oxidation of enzymes; used for surfaces and equipment.

• Surfactants: Soaps and detergents; reduce surface tension; quaternary ammonium compounds (quats) disrupt membranes.

• Heavy Metals: Denature proteins; low-level agents (e.g., silver nitrate, thimerosal, copper).

• Aldehydes: Cross-link functional groups in proteins and nucleic acids; high-level disinfectants (e.g., glutaraldehyde, formalin).

• Gaseous Agents: Sterilize in closed chambers; denature proteins and DNA; used for heat-sensitive materials.

• Enzymes: Antimicrobial enzymes (e.g., lysozyme) digest cell walls; used in food and medical applications.

• Antimicrobials: Antibiotics and synthetic agents; primarily for disease treatment, sometimes for environmental control.

Summary Table: Chemical Methods of Microbial Control

Evaluating Disinfectants and Antiseptics

• Phenol Coefficient: Compares effectiveness to phenol; values >1 indicate greater efficacy.

• Use-Dilution Test: Standard test in the U.S.; measures effectiveness at different concentrations.

• Kelsey-Sykes Capacity Test: Used in the EU; measures minimum effective time.

• In-Use Test: Monitors effectiveness in actual conditions by sampling before and after application.

Development of Resistant Microbes

Overuse of antiseptics and disinfectants can promote the development of resistant microbial strains. There is little evidence that routine use of these products improves human or animal health.