The Control of Microbial Growth

Introduction

The control of microbial growth is essential in medical, industrial, and everyday settings to prevent infection, spoilage, and contamination. This chapter outlines the terminology, mechanisms, and methods used to control microbial populations, both physically and chemically.

Terminology of Microbial Control

Key Definitions

• Sepsis: Refers to bacterial contamination.

• Asepsis: The absence of significant contamination; aseptic techniques are used to prevent microbial contamination, especially in surgery.

• Sterilization: Removal or destruction of all microbial life, including endospores.

• Commercial Sterilization: Process used in the food industry to destroy Clostridium botulinum endospores in canned goods.

• Disinfection: Destruction of harmful microorganisms, typically on inanimate objects.

• Antisepsis: Destruction of harmful microorganisms on living tissue.

• Degerming: Mechanical removal of microbes from a limited area (e.g., skin before injection).

• Sanitization: Lowering microbial counts on eating utensils to safe public health levels.

• Biocide (Germicide): Agents that kill microbes.

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

Example: Using alcohol swabs before injections is an example of degerming and antisepsis.

The Rate of Microbial Death

Patterns and Factors Affecting Microbial Death

• Microbial death occurs at a constant rate when exposed to microbial control agents.

• Effectiveness of treatment depends on:

  • Number of microbes present

  • Environmental factors (e.g., presence of organic matter, temperature, biofilms)

  • Time of exposure to the agent

  • Microbial characteristics (e.g., endospore formation, cell wall structure)

Example: A solution with a higher number of bacteria takes longer to sterilize than one with fewer bacteria.

Actions of Microbial Control Agents

Effects on Cellular Structures

• Alteration of membrane permeability: Damages the plasma membrane, causing leakage of cellular contents.

• Damage to proteins (enzymes): Denaturation or inactivation of enzymes disrupts cell metabolism.

• Damage to nucleic acids: Prevents replication and protein synthesis, leading to cell death.

Example: Chemical agents that disrupt plasma membranes can also affect human cells, but selectivity and concentration are important factors.

Physical Methods of Microbial Control

Heat

• Heat denatures enzymes, leading to loss of function and cell death.

• Thermal Death Point (TDP): Lowest temperature at which all cells in a liquid culture are killed in 10 minutes.

• Thermal Death Time (TDT): Minimum time required to kill all bacteria in a liquid culture at a given temperature.

• Decimal Reduction Time (DRT): Time (in minutes) to kill 90% of a population at a given temperature. Equation: (where D = DRT, t = time, N0 = initial population, N = final population)

Moist Heat Sterilization

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

• Autoclaving: Steam under pressure (121°C at 15 psi for 15 min) kills all organisms and endospores. Steam must contact all surfaces.

• Pasteurization: Reduces spoilage organisms and pathogens. Equivalent treatments include:

  • 63°C for 30 min

  • High-Temperature Short-Time (HTST): 72°C for 15 sec

  • Ultra-High-Temperature (UHT): 140°C for 4 sec

  Note: Thermoduric organisms may survive pasteurization.

Dry Heat Sterilization

• Kills by oxidation (e.g., flaming, incineration, hot-air sterilization).

Filtration

• Physical removal of microbes by passage through a filter.

• Used for heat-sensitive materials.

• High-Efficiency Particulate Air (HEPA) filters remove microbes >0.3 µm.

• Membrane filters remove microbes >0.22 µm.

Other Physical Methods

• Low Temperature: Bacteriostatic effect (refrigeration, deep-freezing, lyophilization).

• High Pressure: Denatures proteins.

• Desiccation: Absence of water prevents metabolism.

• Osmotic Pressure: High concentrations of salts and sugars create a hypertonic environment, causing plasmolysis.

Radiation

• Ionizing Radiation: (X-rays, gamma rays, electron beams) Ionizes water to create reactive hydroxyl radicals, damaging DNA and causing lethal mutations.

• Nonionizing Radiation: (UV, 260 nm) Damages DNA by creating thymine dimers.

• Microwaves: Kill by heat, not especially antimicrobial.

Chemical Methods of Microbial Control

Principles of Effective Disinfection

• Effectiveness depends on:

  • Concentration of disinfectant

  • Presence of organic matter

  • pH

  • Time of exposure

Testing Disinfectant Efficacy

• Use-Dilution Test: Metal cylinders with dried bacteria are exposed to disinfectant, then transferred to culture media to check for survival.

• Disk-Diffusion Method: Filter paper disks soaked in chemical agents are placed on a microbial culture; zones of inhibition indicate effectiveness.

Types of Chemical Agents

Additional Chemical Agents and Applications

• Heavy Metals: Silver nitrate (prevents ophthalmia neonatorum), mercuric chloride (prevents mildew), copper sulfate (algicide), zinc chloride (mouthwash).

• Chemical Food Preservatives: Sulfur dioxide (wine), organic acids (sorbic, benzoic, calcium propionate), nitrites/nitrates (prevent endospore germination in meats).

• Antibiotics: Bacteriocins (e.g., nisin, natamycin) prevent spoilage in cheese.

• Plasma: Electrically excited gas with free radicals; used for sterilizing tubular instruments.

• Supercritical Fluids: CO2 with properties of both gas and liquid; used for sterilizing medical implants.

• Peroxygens: Ozone (O3), hydrogen peroxide (H2O2), peracetic acid; used as oxidizing agents for sterilization.

Microbial Characteristics and Microbial Control

Microbial Resistance

• Endospores: Highly resistant to most control methods.

• Gram-negative bacteria: More resistant to chemical biocides than gram-positive bacteria due to their outer membrane.

• Other factors: Mycobacteria, cysts, and prions also show high resistance.

Example: Pseudomonas species and Mycobacterium tuberculosis are notably resistant to many disinfectants.

Summary Table: Physical and Chemical Methods of Microbial Control

Additional info: The effectiveness of any microbial control method depends on the type of microbe, the environment, and the method's mechanism of action. For example, prions are resistant to standard autoclaving, and organic matter can inhibit the action of many disinfectants.