Microbial Control in the Environment: Principles, Methods, and Evaluation

Microbial Control in the Environment

Introduction

Microbial control is essential in preventing infectious diseases, ensuring food safety, and maintaining sterile environments in healthcare and laboratory settings. This study guide covers key terminology, methods, and evaluation techniques for controlling microbial growth in the environment.

Learning Objectives

• Learn the definitions and terminology used when controlling microbial growth in the environment.

• Explain and apply how death rate is used to calculate how long it takes to kill microbes.

• Discuss how different physical and chemical methods are used to control microbes.

Why Control Microbial Agents?

Importance of Microbial Control

• Infectious diseases such as tuberculosis, malaria, and HIV/AIDS are leading causes of death worldwide.

• Controlling microbial agents reduces transmission, prevents outbreaks, and protects vulnerable populations.

• Applications include healthcare, food industry, and public health.

Terminology of Microbial Control

Antisepsis

• Antisepsis: The use of chemical agents to inhibit or destroy microorganisms on living tissue, most commonly the skin.

• The chemical used is called an antiseptic (e.g., iodine, ethanol).

• Example: Cleaning a wound with iodine.

Aseptic Technique

• Aseptic technique: Working in an environment or performing procedures free of pathogenic contaminants.

• Examples: Surgical preparation, flaming lab equipment.

• Essential for laboratory and medical procedures to prevent infection.

Semmelweis and the History of Aseptic Technique

• Ignaz Semmelweis (1818-1863) discovered that handwashing in lime water reduced maternal mortality by preventing infections caused by cadaver particles.

• Pioneered the development of aseptic techniques in medicine.

• Graphical data shows a significant drop in infection rates after implementing handwashing protocols.

-cide/-cidal vs. -static/-stasis

• -cide/-cidal: Indicates microbe destruction (e.g., fungicidal, bactericidal, virucide).

• Examples: Ethylene oxide, propylene oxide, aldehydes.

• -static/-stasis: Indicates inhibition but not complete destruction (e.g., fungistatic, bacteriostatic, virustatic).

• Examples: Refrigeration, freezing.

Disinfection

• Disinfection: Destruction of most microorganisms on nonliving surfaces using agents such as phenolics, alcohols, aldehydes, and soaps.

• More harsh than antisepsis since living cells are not a concern.

De-germing

• De-germing: Removal of microbes by mechanical means (e.g., swabbing with alcohol, washing hands).

• Physical scrubbing under water is a major factor in de-germing.

Sterilization

• Sterilization: Removal or destruction of all microbes from an area, including viruses, bacteria, and eukaryotes (but not prions).

• Practical sterilization often means killing all bacteria, commonly achieved with autoclaves (pressure and heat).

• Autoclave: High pressure saturated steam at 121°C (249°F) for 15-20 minutes.

Pasteurization

• Pasteurization: Uses heat to kill pathogens and reduce food spoilage, especially in milk and beverages.

• Invented by Louis Pasteur; increases shelf life and safety of food products.

• Dairy pasteurization involves heating milk to a specific temperature for a set time to kill harmful bacteria.

Mechanisms of Antimicrobial Action

Categories of Damage

Damage to Cell Walls and Membranes	Damage to Proteins and Nucleic Acids
Cell wall maintains integrity of cell shape. Damage causes cells to burst due to osmotic effects. Cytoplasmic membrane controls passage of chemicals; damage disrupts this function.	Protein function depends on 3-D shape; denatured by extreme heat or chemicals. Chemicals, radiation, and heat can alter/destroy nucleic acids. Can produce fatal mutations or halt protein synthesis.

Death Rate of Microbes

Microbial Death Rate

• Antimicrobials kill a constant percentage of the population per unit time, not all at once.

• Death rate can be modeled mathematically:

$N_t = N_0 \times (1 - k)^t$

• Where $N_t$ is the number of microbes at time $t$, $N_0$ is the initial number, and $k$ is the fraction killed per time interval.

• Example: If 50% are killed every 5 minutes, starting with 256 microbes, it takes 45 minutes to kill all.

Factors in Choosing Microbial Control Methods

Considerations

• Inexpensive, fast-acting, stable during storage.

• Capable of controlling microbial growth while being harmless to humans, animals, and objects.

• The site to be treated is a major factor (e.g., cannot use harsh chemicals on living tissue).

• Patient health and site of medical procedure are important considerations.

Relative Susceptibility of Microorganisms

Classification of Germicides

Level	Effectiveness	Examples of Targets
High	Kill all pathogens including endospores	Prions, bacterial endospores
Intermediate	Kill fungal spores, protozoan cysts, viruses, pathogenic bacteria	Mycobacteria, cysts of protozoa, active-stage protozoa, most Gram-negative bacteria, fungi
Low	Kill vegetative bacteria, fungi, protozoa, some viruses	Most Gram-positive bacteria, enveloped viruses

Methods for Evaluating Disinfectants and Antiseptics

Phenol Coefficient

• Measures efficacy of disinfectants/antiseptics by comparing to phenol.

• Phenol coefficient > 1.0 indicates greater effectiveness than phenol.

• Historically important, now replaced by newer methods.

Dilution Test (Current US Standard)

• Metal cylinders are dipped into bacterial cultures, then immersed in disinfectant dilutions.

• Cylinders are washed and placed in growth medium; the most effective agents prevent growth at highest dilution.

• New standard procedures are being developed.

Summary and Take Home Messages

• The choice of microbial control method depends on the type of surface and the microbes to be eliminated.

• Antisepsis is used for living tissue; disinfection for nonliving surfaces.

• The dilution test is the current standard for evaluating disinfectants in the US.

• New standards are under development to improve microbial control practices.

Additional info: Dairy pasteurization typically heats milk to 72°C for 15 seconds (high-temperature, short-time method) to kill pathogens without affecting taste.