Controlling Microbial Growth in the Environment: Principles and Methods

Controlling Microbial Growth in the Environment

Introduction

This chapter covers the fundamental principles and methods used to control the growth of microorganisms in various environments. Understanding these concepts is essential for preventing infection, ensuring food safety, and maintaining sterile conditions in laboratory and medical settings.

Terminology of Microbial Control

Key Definitions and Concepts

• Antisepsis: Reduction of the number of microorganisms and viruses, particularly potential pathogens, on living tissue. Example: Use of iodine or alcohol to prepare skin for an injection.

• Aseptic: Refers to an environment or procedure free of pathogenic contaminants. Example: Preparation of surgical field, flame sterilization of laboratory equipment.

• Disinfection: Use of physical or chemical agents to destroy or inhibit microorganisms, especially on inanimate objects. Example: Use of phenolics, alcohols, aldehydes, or soaps on equipment or surfaces.

• Degerming: Removal of microbes by mechanical means. Example: Hand washing, alcohol swabbing at site of injection.

• Sanitization: Removal of pathogens from objects to meet public health standards. Example: Washing tableware in scalding water in restaurants.

• Pasteurization: Use of heat to destroy pathogens and reduce the number of spoilage microorganisms in foods and beverages. Example: Pasteurized milk and fruit juices.

• -stasis/-static: Suffixes indicating inhibition but not complete destruction of a type of microbe. Example: Bacteriostatic, fungistatic, virustatic.

• -cide/-cidal: Suffixes indicating destruction of a type of microbe. Example: Bactericide, fungicide, virucide.

• Sterilization: Destruction of all microorganisms and viruses in or on an object. Example: Preparation of microbiological culture media and canned food.

Term	Definition	Example	Comments
Antisepsis	Reduction of microorganisms/viruses on living tissue	Iodine, alcohol for skin prep	Antiseptics often less concentrated than disinfectants
Aseptic	Free of pathogenic contaminants	Surgical prep, flame sterilization	Used in lab and clinical settings
Disinfection	Destruction of most microorganisms/viruses on nonliving tissue	Phenolics, alcohols, aldehydes, soaps	Primarily for pathogens
Degerming	Removal of microbes by mechanical means	Hand washing, alcohol swabbing	Often secondary to mechanical removal
Sanitization	Removal of pathogens to meet public health standards	Washing tableware in restaurants	Standards set by authorities
Pasteruization	Use of heat to destroy pathogens/reduce spoilage	Milk, fruit juices	Heat treatment preserves taste/nutrients
Sterilization	Destruction of all microorganisms/viruses	Autoclaving, incineration	Typically achieved by steam, pressure, or gas

Basic Principles of Microbial Control

Microbial Death Rates

• Microbial death is defined as the permanent loss of reproductive ability under ideal environmental conditions.

• The microbial death rate is often constant for a microorganism under a particular set of conditions.

• Death rate is typically expressed as a logarithmic decline in the number of viable organisms over time.

Equation:

$N_t = N_0 imes 10^{-kt}$ Where $N_t$ = number of surviving microbes at time $t$, $N_0$ = initial number, $k$ = death rate constant, $t$ = time.

Example: If 90% of a microbial population dies every minute, the death rate is constant and can be plotted as a straight line on a logarithmic scale.

Actions of Antimicrobial Agents

• Alteration of cell walls and membranes:

  • Cell wall maintains cell integrity; damage causes cells to burst due to osmotic effects.

  • Cytoplasmic membrane controls passage of chemicals; damage leads to leakage of cellular contents.

  • Nonenveloped viruses are more tolerant of harsh conditions than enveloped viruses.

• Damage to proteins and nucleic acids:

  • Protein function depends on 3-D shape; extreme heat or chemicals can denature proteins.

  • Chemicals, radiation, and heat can alter or destroy nucleic acids, producing fatal mutations and halting protein synthesis.

Selection of Microbial Control Methods

Factors in Selecting Control Agents

• Agents should be inexpensive, fast-acting, stable during storage, and capable of controlling microbial growth without harming humans, animals, or objects.

• The method of microbial control depends on the site of medical procedure and the nature of the item to be treated.

• Harsh chemicals and extreme heat cannot be used on humans, animals, or fragile objects.

Relative Susceptibility of Microorganisms

Microorganisms vary in their resistance to antimicrobial agents. The following is a general ranking from most resistant to most susceptible:

Most Resistant	Most Susceptible
Prions	Enveloped viruses
Bacterial endospores	Most Gram-positive bacteria
Cysts of Cryptosporidium	Large nonenveloped viruses
Mycobacteria	Vegetative fungi
Cysts of other protozoa	Fungal spores
Small nonenveloped viruses	Active-stage protozoa (trophozoites)

Germicide Classification

• High-level germicides: Kill all pathogens, including endospores.

• Intermediate-level germicides: Kill fungal spores, protozoan cysts, viruses, and pathogenic bacteria.

• Low-level germicides: Kill vegetative bacteria, fungi, protozoa, and some viruses.

Environmental Conditions Affecting Efficacy

• Temperature and pH: Higher temperatures and optimal pH can increase the effectiveness of antimicrobial agents by increasing the rate of chemical reactions and microbial death rates.

• Organic materials: Presence of organic matter (e.g., blood, feces) can interfere with the penetration and effectiveness of heat, chemicals, and some forms of radiation, and may inactivate chemical disinfectants.

Example: Disinfectants may be less effective on surfaces contaminated with organic material, requiring pre-cleaning before application.