Microbial Control: Physical and Chemical Methods

Principles of Microbial Control

Introduction to Microbial Control

Microbes are ubiquitous in the environment, including on surfaces, in water, and on human skin. While many are harmless or beneficial, some can cause disease or contamination. Controlling microbial growth is essential in medicine, food safety, and research laboratories. This chapter focuses on the physical and chemical methods used to reduce or eliminate microorganisms.

• Sterilization: Complete removal or destruction of all microorganisms, including endospores.

• Disinfection: Elimination of most pathogens (but not all microbes or endospores) from inanimate objects.

• Decontamination: Reduction of pathogens to safe levels, as determined by public health standards.

• Sanitization: Lowering microbial counts to safe public health levels, often by cleaning.

• Preservation: Inhibition of microbial growth to delay spoilage of perishable products.

Significance:

• Medical relevance: Ensures surgical instruments, hospital rooms, and patient care items are sterile or disinfected.

• Everyday relevance: Household cleaners, pasteurization, and handwashing protect health.

• Research relevance: Prevents contamination in experiments and ensures pure cultures.

Section 5.1 – Principles of Control

Definitions and Situational Considerations

This section introduces key definitions and the importance of context in choosing microbial control methods.

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

• Disinfection: Elimination of most pathogens (not endospores) from inanimate objects.

• Decontamination: Reduction of pathogens to safe levels.

• Sanitization: Reduction of microbial population to meet accepted health standards.

• Preservation: Inhibition of microbial growth to delay spoilage.

Situational Considerations:

• Daily life: Handwashing, disinfecting kitchen surfaces.

• Hospitals: Highest concern due to risk of infection.

• Laboratories: Prevent contamination of cultures and experiments.

• Food production: Avoid spoilage and foodborne illness.

Section 5.2 – Approaches to Control

Physical and Chemical Antimicrobial Procedures

Microbial control can be achieved through physical or chemical means. The choice depends on the type of microbe, the environment, and the intended use of the item being treated.

• Physical methods: Heat, filtration, irradiation, and high pressure.

• Chemical methods: Use of germicidal chemicals to disinfect or sterilize.

Highly Resistant Microbes

• Endospores: Highly resistant to heat, chemicals, and radiation.

• Mycobacterium: Waxy cell wall makes them resistant to many disinfectants.

• Pseudomonas: Can grow in some disinfectants; resistant to many chemicals.

• Naked viruses: Lack lipid envelope, making them more resistant to disinfectants.

• Prions: Infectious proteins; extremely resistant to conventional sterilization.

Example: Clostridium difficile endospores are difficult to eliminate from hospital environments.

Section 5.3 – Physical Methods

Heat, Filtration, Irradiation, and High-Pressure Methods

Physical methods are commonly used to control microbial growth by destroying or removing microorganisms.

• Moist Heat:

  • Boiling: Kills most microbes but is not reliable for sterilization (does not destroy endospores).

  • Pasteurization: Reduces microbial numbers without damaging food quality.

  • Autoclave: Uses pressurized steam at 121°C and 15 psi to destroy all microbes, including endospores.

• Dry Heat:

  • Incineration: Burns cell components; used for sterilizing inoculation loops.

  • Hot air ovens: Denature proteins and oxidize cell components; used for glassware.

• Filtration:

  • Liquid filtration: Removes microbes larger than 0.2 μm.

  • HEPA filters: Remove microbes from air in hospital rooms and biosafety cabinets.

• Radiation:

  • Ionizing radiation: Destroys DNA; used for sterilizing medical equipment and food.

  • Ultraviolet (UV) radiation: Damages DNA; used for disinfecting surfaces and air.

• High Pressure:

  • Used in the food industry to destroy microbes while preserving food quality.

Example: Autoclaving is more effective than boiling because it achieves higher temperatures and pressures, destroying endospores.

Section 5.4 – Chemical Methods

Germicidal Chemicals: Types, Actions, and Limitations

Chemical methods use various agents to disinfect or sterilize surfaces, instruments, and skin. The choice of chemical depends on its effectiveness, toxicity, and application.

Chemical	Mode of Action	Example Use	Limitations
Alcohols	Denature proteins, damage membranes	Hand sanitizers	Ineffective against endospores, some viruses
Aldehydes	Inactivate proteins, nucleic acids	Preserving specimens	Toxic
Biguanides	Disrupt membranes	Antiseptic (chlorhexidine)	Low toxicity
Ethylene oxide	Gas sterilant, destroys DNA	Medical devices	Toxic, explosive
Halogens	Oxidize proteins, cell components	Bleach, iodine	Corrosive, unstable
Metals	Bind proteins, inactivate enzymes	Silver nitrate	Toxic at high levels
Peroxygens	Oxidize proteins, denature enzymes	Hydrogen peroxide	Damaged by catalase
Phenolics	Destroy membranes, denature proteins	Lysol	Irritating, residues
Quats	Disrupt membranes	Disinfect surfaces	Not effective against endospores

Example: Alcohol-based hand sanitizers are effective for routine hand hygiene but do not kill endospores or some non-enveloped viruses.

Section 5.5 – Preserving Perishable Products

Methods to Prevent Spoilage

Preservation methods inhibit microbial growth to extend the shelf life of perishable products. These methods are essential in food safety and storage.

• Refrigeration: Slows microbial growth by lowering temperature.

• Freezing: Stops microbial growth but may not kill all microbes.

• Reducing water availability: Methods such as salting, sugaring, and drying inhibit microbial growth by creating a hypertonic environment.

Example: Salted fish does not spoil quickly because high salt concentrations draw water out of microbial cells, inhibiting their growth.

Chapter 5 Wrap-Up

Key Review Questions

• Why are some microbes (like endospores and prions) so difficult to control?

• How would you choose between physical and chemical methods for hospital cleaning protocols?

• Why is understanding the limitations of each control method as important as knowing what they kill?