The Control of Microbial Growth: Principles and Physical Methods

Chapter 7: The Control of Microbial Growth

Introduction

The control of microbial growth is essential in medical, industrial, and food settings to prevent infection, spoilage, and contamination. This chapter explores the terminology, principles, and physical methods used to control and eliminate microorganisms.

Key Terminology in Microbial Control

Definitions and Concepts

• Sepsis: Refers to microbial contamination, specifically the growth and spread of bacteria in blood and tissues, leading to a systemic inflammatory response.

• Asepsis: The absence of significant contamination. Aseptic techniques are crucial in surgery to prevent microbial contamination of wounds.

• Antisepsis: Removal of pathogens from living tissue, typically using chemical agents safe for skin or mucous membranes.

• Sterilization: The complete removal or destruction of all microbial life, including endospores.

• Commercial Sterilization: A process used especially in the food industry (e.g., canned foods) to kill Clostridium botulinum endospores and vegetative cells.

• Disinfection: Removal of pathogens from non-living surfaces using chemical or physical means.

• Degerming: Removal of microbes from a limited area, such as swabbing skin with alcohol before injection.

• Sanitization: Reducing microbial counts on eating utensils and food preparation areas to safe public health levels.

• Biocide/Germicide: Substance that kills microbes.

• Bacteriostasis: Inhibition of bacterial growth without killing the organisms.

The Rate of Microbial Death

Principles of Microbial Death

• Bacterial populations subjected to heat or antimicrobial chemicals usually die at a constant rate.

• When plotted logarithmically, the death curve appears as a straight line, indicating a constant percentage of the population is killed per unit time.

• The time required to kill a microbial population is proportional to the number of microbes present.

• Different microbial species and life cycle phases (e.g., endospores) have varying susceptibilities to control methods.

• Organic matter can interfere with the effectiveness of heat and chemical treatments.

• Longer exposure to lower heat can have the same effect as shorter exposure to higher heat.

Microbial Death Curves

• Logarithmic plotting of microbial death reveals a straight line, demonstrating that each minute, a fixed percentage of the population dies.

• One log decrease equals a 90% reduction in the population.

Factors Affecting Antimicrobial Effectiveness

• Number of microbes present

• Environmental factors (organic matter, temperature, biofilms)

• Time of exposure

• Microbial characteristics (e.g., resistance mechanisms, spore formation)

Actions of Microbial Control Agents

Mechanisms of Action

• Alteration of membrane permeability: Disruption of the plasma membrane, leading to leakage of cellular contents and cell death.

• Damage to proteins: Denaturation or breaking of hydrogen and covalent bonds in proteins, inactivating enzymes and structural proteins.

• Damage to nucleic acids: Interference with DNA and RNA replication and protein synthesis, preventing cell function and reproduction.

Physical Methods of Microbial Control

Heat

Heat is one of the most common and effective methods for controlling microbial growth. It works primarily by denaturing proteins and enzymes, leading to cell death.

• Moist heat (e.g., boiling, autoclaving) is more effective than dry heat for sterilization.

• Thermal death point (TDP): The lowest temperature at which all cells in a culture are killed in 10 minutes.

• Thermal death time (TDT): The minimal length of time required to kill all bacteria in a liquid culture at a given temperature.

• Decimal reduction time (DRT): The time (in minutes) required to kill 90% of a microbial population at a given temperature.

Moist Heat Methods

• Boiling: Kills most vegetative cells and viruses within 10 minutes but may not destroy all endospores.

• Autoclaving: Uses steam under pressure (typically 121°C at 15 psi for 15 minutes) to achieve sterilization. The steam must directly contact the material to be sterilized.

Pressure vs. Temperature of Steam

Increasing pressure raises the boiling point of water, allowing higher temperatures for sterilization. At higher altitudes, lower atmospheric pressure requires adjustments to achieve effective sterilization.

Pasteurization

Pasteurization reduces spoilage organisms and pathogens in foods and beverages. It does not sterilize but significantly decreases microbial load.

• Common methods include:

  • 63°C for 30 min (Low-temperature long-time, LTLT)

  • 72°C for 15 sec (High-temperature short-time, HTST)

  • 140°C for <1 sec (Ultra-high-temperature, UHT)

• Thermoduric (heat-resistant) organisms may survive but do not grow at storage temperatures.

Dry Heat Sterilization

Dry heat kills by oxidation and is used for materials that can withstand high temperatures without damage.

• Flaming: Used to sterilize inoculating loops.

• Incineration: Used to decontaminate disposable items (e.g., paper cups, dressings).

• Hot-air sterilization: Typically 170°C for 2 hours.

Equivalent treatments	Hot-air	Autoclave
	180°C for 90 minutes 170°C for 2 hours 160°C for 3 hours	121°C, 15 min

Low Temperature

Low temperatures inhibit microbial growth but do not necessarily kill microorganisms.

• Refrigeration: Slows microbial metabolism and growth; most pathogens do not reproduce at 0–7°C except Listeria.

• Deep freezing: Preserves microbes for long-term storage.

• Lyophilization: Freeze-drying for preservation of cultures.

• The effectiveness depends on the microorganism and the intensity of the application.

Filtration

Filtration is used to sterilize heat-sensitive materials by physically removing microbes from liquids or gases.

• Membrane filters with pore sizes of 0.22 µm or 0.45 µm are commonly used to remove bacteria; smaller pores (0.01 µm) can retain viruses and large proteins.

• HEPA filters are used to remove microbes from air in laboratory and clinical settings.

Desiccation

Desiccation (drying) prevents microbial growth by removing water, but many microbes can survive in a dormant state. Endospores are more resistant to desiccation than viruses.

Osmotic Pressure

High concentrations of salts and sugars create a hypertonic environment, causing plasmolysis (shrinkage of the cell membrane from the cell wall) and inhibiting microbial growth. This method is used in food preservation (e.g., curing meats, preserving fruits).

• Molds and yeasts are more tolerant of high osmotic pressures than bacteria.

Summary Table: Physical Methods of Microbial Control

Method	Mechanism	Application
Moist Heat (Autoclave)	Protein denaturation	Culture media, surgical instruments
Dry Heat	Oxidation	Glassware, metal instruments
Filtration	Physical removal	Heat-sensitive solutions, air
Low Temperature	Inhibits metabolism	Food preservation, culture storage
Desiccation	Prevents metabolism	Food preservation
Osmotic Pressure	Plasmolysis	Food preservation