Control of Microbial Growth: Principles and Methods

Chapter 7 – Control of Microbial Growth

Definitions of Key Terms in Microbial Control

Understanding the terminology related to microbial control is essential for interpreting laboratory protocols and public health guidelines. The following definitions clarify the scope and application of various control methods:

• Sterilization: The complete destruction of all forms of microbial life, including endospores. Heating is the most common method.

• Disinfection: The destruction of vegetative (non-endospore-forming) pathogens, typically using chemicals, UV radiation, boiling water, or steam.

• Antisepsis: Application of a disinfectant (antiseptic) to living tissue to reduce the risk of infection.

• Degerming: Mechanical removal of most microbes from a limited area (e.g., using an alcohol swab before injection).

• Sanitization: Treatment of publicly used articles to lower microbial counts to safe public health levels, minimizing disease transmission (e.g., high-temperature washing or chemical disinfectants).

• Germicide: An agent that kills microorganisms (except endospores).

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

• Asepsis: The absence of significant contamination by pathogens.

Patterns of Microbial Death

Microbial death due to control agents follows predictable patterns, influenced by several factors:

• Microbial death occurs at a constant rate (straight line on a logarithmic scale).

• Factors affecting effectiveness:

  • Number of microbes: Higher numbers require longer treatment times.

  • Microbial characteristics: Some microbes are more resistant than others.

  • Environmental influences: Organic matter can inhibit antimicrobial action.

  • Time of exposure: Longer exposure increases effectiveness.

Example: Killing 90% of a bacterial population in one minute means that each additional minute kills 90% of the remaining population.

Effects of Microbial Control Agents on Cellular Structures

Control agents target essential cellular components:

• Plasma membrane: Disruption increases permeability, causing leakage of cell contents (e.g., quaternary ammonium compounds).

• Proteins: Denaturation destroys enzymes and structural proteins, leading to cell death.

• Nucleic acids: Damage prevents replication and enzyme synthesis, which is lethal to the cell.

Physical Methods of Microbial Control

Physical methods are widely used to control microbial growth, each with specific mechanisms and applications:

Heat

• Moist heat: Kills by coagulating proteins (breaking hydrogen bonds). Methods include boiling, autoclaving, and pasteurization.

  • Boiling: 100°C for 10 minutes kills most pathogens, but not all endospores or some viruses.

  • Autoclaving: Steam under pressure (15 psi, 121°C) for 15 minutes sterilizes all organisms and endospores.

  • Pasteurization: Mild heating to kill spoilage organisms without damaging product quality.

• Dry heat: Kills by oxidation (e.g., direct flaming, hot-air sterilization at 170°C for 2 hours).

Filtration

• Removes microbes from liquids or gases by passing them through filters with small pores.

• Used for heat-sensitive materials (e.g., culture media, vaccines).

• HEPA filters remove particles >0.3 μm; some filters retain viruses (pores as small as 0.01 μm).

Low Temperatures

• Refrigeration (0–7°C) slows microbial growth; most pathogens do not grow at these temperatures.

• Slow freezing forms ice crystals that disrupt cell structure.

Desiccation

• Absence of water prevents growth and reproduction, but microbes may remain viable for years.

Osmotic Pressure

• High concentrations of salt or sugar cause plasmolysis, inhibiting growth (used in food preservation).

• Molds and yeasts are more tolerant than bacteria to high osmotic pressure.

Radiation

• Ionizing radiation (gamma rays, X-rays): Short wavelength, high energy; produces hydroxyl radicals that damage DNA, causing mutations and cell death.

• Nonionizing radiation (UV light): Longer wavelength; causes thymine dimers in DNA, inhibiting replication. Most effective at ~260 nm. Limited penetration.

Microbial Growth and Environmental Factors

The effectiveness of control methods depends on microbial type and environmental conditions:

• Gram-positive bacteria are generally more susceptible to disinfectants than gram-negative bacteria, which have protective outer membranes.

• Pseudomonads and mycobacteria are especially resistant due to unique structural features.

• Endospores, protozoan cysts, and some viruses are highly resistant to chemical agents.

• Organic matter (e.g., vomit, feces) can inhibit disinfectant action.

• Disinfectants are often more effective at higher temperatures.

Factors Related to Effective Disinfection

Several factors influence the success of disinfection:

• Concentration: Use as specified by manufacturer.

• Nature of material: Presence of organic matter and pH can affect efficacy.

• Contact: Surfaces may need cleaning before disinfection; adequate contact time is essential.

• Temperature: Higher temperatures generally increase effectiveness.

Disinfectants can be evaluated using use-dilution tests and filter paper tests.

Chemical Methods of Microbial Control

Chemical agents are used to control microbial growth on living tissue and inanimate objects. Their effectiveness varies with the agent and the target organism.

Chemical Agent	Mechanism of Action	Applications/Notes
Phenol & Phenolics	Disrupt plasma membranes, denature proteins	Active in presence of organic matter; used in Lysol, hexachlorophene
Biguanides (e.g., Chlorhexidine)	Disrupt plasma membranes	Used for skin/mucous membranes; not sporicidal
Halogens (Iodine, Chlorine)	Oxidize cellular components, disrupt protein function	Iodine (tincture, iodophor), chlorine (water disinfection, bleach)
Alcohols (Ethanol, Isopropanol)	Denature proteins, disrupt membranes	Effective at 70%; not sporicidal; used for skin, surfaces
Heavy Metals (Silver, Mercury, Copper)	Denature proteins	Silver nitrate, silver dressings; limited use due to toxicity
Surface-Active Agents (Soaps, Detergents)	Mechanical removal, decrease surface tension	Soaps for degerming; some contain antibacterial agents
Quaternary Ammonium Compounds (Quats)	Disrupt membranes	Effective against gram-positive bacteria, enveloped viruses; not sporicidal
Chemical Food Preservatives	Inhibit microbial metabolism	Sodium benzoate (molds), sodium nitrate (botulism prevention)
Aldehydes (Formaldehyde, Glutaraldehyde)	Cross-link functional groups in proteins	Used for instrument disinfection; glutaraldehyde is less irritating
Gaseous Chemosterilizers (Ethylene oxide)	Denature proteins	Used for heat-sensitive medical supplies; very penetrating
Peroxygens (Hydrogen peroxide, Peracetic acid)	Oxidize cellular components	Effective on inanimate objects; peracetic acid is sporicidal

Additional info:

• Use-dilution test: Standard method for evaluating disinfectant effectiveness by exposing metal cylinders to bacteria, then to the disinfectant, and finally to growth media to check for survivors.

• Filter paper test: Disks soaked in disinfectant are placed on agar inoculated with bacteria; zones of inhibition indicate effectiveness.