Control of Microbial Growth: Methods, Agents, and Laboratory Safety

Control of Microbial Growth

Biosafety Levels (BSL)

Laboratories are classified into four biosafety levels based on the organisms handled and the risk they pose to humans and the environment.

• BSL-1: Microbes not known to cause disease in healthy hosts; minimal risk. Standard microbiological practices are sufficient.

• BSL-2: (e.g., Normandale lab) Indigenous microbes associated with diseases of varying severity; moderate risk. Requires additional precautions such as limited access and use of personal protective equipment (PPE).

• BSL-3: Indigenous or exotic microbes causing serious or potentially lethal diseases via respiratory transmission. Requires controlled access, specialized ventilation, and PPE.

• BSL-4: Dangerous and exotic microbes with high risk of aerosol-transmitted infections, often fatal without treatment or vaccines. Requires maximum containment and specialized facilities.

Example: Most teaching labs, including Normandale, operate at BSL-2.

Definitions of Key Terms in Microbial Control

• Sterilization: Complete elimination of all forms of microbial life, including vegetative cells, endospores, and viruses, from an inanimate object. Methods include autoclaving, chemicals, and radiation.

• Aseptic Technique: Procedures used to prevent contamination of sterile environments or cultures by unwanted microbes.

• Disinfection: Reduction or destruction of microbial load on inanimate objects using heat or antimicrobial chemicals (e.g., bleach, phenols).

• Antisepsis: Reduction of microbial load on living tissue using antimicrobial chemicals (e.g., isopropyl alcohol, iodine).

• Degerming: Removal of microbes from living tissue by mechanical means (e.g., handwashing with soap or alcohol swabs).

• Sanitizing: Reduction of microbial load on inanimate objects to safe public health levels (e.g., commercial dishwashing).

Cidal vs. Static Agents and Prefixes

• Cidal Agents: Cause irreversible cell death. Microbes do not recover after removal of the agent (e.g., bactericidal, virucidal, fungicidal).

• Static Agents: Inhibit growth and reproduction without killing. Microbes may resume growth if the agent is removed (e.g., bacteriostatic, fungistatic).

• Prefixes:

  • Bacteri-: Refers to bacteria (e.g., bactericide kills bacteria).

  • Viri-: Refers to viruses (e.g., virucidal inactivates viruses).

  • Fungi-: Refers to fungi (e.g., fungistatic inhibits fungal growth).

Microbial Death Curve

A microbial death curve is a graphical representation showing the rate at which a microbial population is killed when exposed to a control agent (e.g., heat, chemicals). The death rate is typically constant over time, meaning the same percentage of remaining microbes is killed in each time interval.

• D-value: The time required to kill 90% of the microbial population under specific conditions.

Application: Used to determine the time needed for sterilization processes.

Methods of Microbial Control

Physical Methods

• Heat

  • Dry Heat:

    • Incineration: Sterilizes by burning; destroys all microbes, including endospores and prions. Used for inoculating loops.

    • Oven: Sterilizes by denaturing proteins and dehydrating cells; effective against vegetative cells, fungi, viruses, and endospores.

  • Moist Heat:

    • Autoclave: Sterilizes by denaturing proteins and altering membranes; kills all vegetative cells and most endospores (extended time for prions).

    • Boiling: Disinfects by denaturing proteins; kills most vegetative cells (not endospores).

    • Pasteurization: Disinfects by denaturing proteins; kills some pathogens (e.g., Salmonella, E. coli), prevents food spoilage.

    • Ultra-High Temperature (UHT): Sterilizes; kills all vegetative cells and endospores, used for shelf-stable foods.

  • Lack of Heat:

    • Refrigeration (0–7°C): Inhibits metabolism, slows cell division.

    • Freezing (< -2°C): Stops metabolism, may kill some microbes.

• Pressure (Pascalization): Disinfects by denaturing proteins and causing cell lysis; used in food preservation.

• Desiccation and Lyophilization:

  • Desiccation: Drying inhibits metabolism; used for dried fruits and meats.

  • Lyophilization: Rapid freezing under vacuum; preserves food and lab cultures.

• Radiation:

  • Ionizing Radiation: Sterilizes by introducing double-strand breaks in DNA (e.g., X-rays, gamma rays); used for spices, heat-sensitive items.

  • Non-ionizing Radiation: Disinfects using UV light; causes thymine dimers in DNA, effective on surfaces.

• Filtration:

  • HEPA Filters: Disinfect air by trapping microbes; used in operating rooms and safety cabinets.

  • Membrane Filters: Sterilize liquids by removing bacteria, fungi, and endospores (some viruses may pass).

Chemical Methods

• Phenolics: Disinfectants and antiseptics that disrupt cell membranes (e.g., triclosan).

• Heavy Metals: Topical antiseptics that bind proteins and inhibit enzymes (e.g., mercury, silver).

• Halogens: Antiseptics and disinfectants that oxidize and destabilize cellular macromolecules (e.g., iodine, chlorine).

• Alcohols: Antiseptics and disinfectants that denature proteins and disrupt membranes (e.g., ethanol, isopropanol).

• Surfactants: Degerming agents and disinfectants that lower surface tension and disrupt membranes (e.g., quaternary ammonium compounds).

• Bisbiguanides: Antiseptics that disturb cell membranes; used for surgical hand scrubs (e.g., chlorhexidine).

• Alkylating Agents:

  • Ethylene Oxide: Sterilizing gas that penetrates plastics and fabrics; advantages include sterilizing heat-sensitive items; disadvantages include being explosive and carcinogenic.

• Peroxygens: Antiseptics and disinfectants that produce free radicals (e.g., hydrogen peroxide, peracetic acid).

• Supercritical Fluids: Sterilize by penetrating cells and forming carbonic acid, lowering intracellular pH (e.g., carbon dioxide); used for delicate tissues, food, and medical devices.

• Food Preservation Methods: Decrease pH and inhibit enzymes (e.g., sorbic acid, nitrates).

Levels of Germicides

Level	Effectiveness	Targets
High-level	Kills all microbes, including endospores (sterilization with sufficient contact time)	Vegetative cells, fungi, viruses, endospores
Intermediate-level	Kills most microbes, not endospores	Vegetative cells, most viruses
Low-level	Kills only some microbes	Certain vegetative bacteria, fungi, enveloped viruses

Disk-Diffusion Assay

This assay tests the effectiveness of chemicals against bacteria. Paper disks soaked in chemicals are placed on a bacterial lawn. The chemical diffuses, and a clear zone of inhibition forms if bacteria are killed or growth is prevented. The size of the zone indicates the chemical's effectiveness.

In-Use Test

A positive in-use test means microbes are growing in the disinfectant solution, indicating contamination and loss of effectiveness.

Additional info: The D-value is mathematically expressed as the time required to reduce the microbial population by one log (90%) under specific conditions: $D = \frac{t}{\log_{10}(N_0/N)}$, where $N_0$ is the initial population and $N$ is the final population.