Physical Methods of Microbial Control

Heat Sterilization

Heat is the oldest and most widely used method for sterilization in microbiology. It works by denaturing proteins, interfering with the integrity of cytoplasmic membranes and cell walls, and disrupting the structure and function of nucleic acids.

• Thermal Death Point: The lowest temperature that kills all cells in a broth in 10 minutes.

• Thermal Death Time: The time required to sterilize a volume of liquid at a set temperature.

• Autoclaving: Uses pressurized steam to achieve temperatures above boiling, typically 121°C at 15 psi for 15 minutes, effectively destroying most microbes.

Moist Heat

• More effective than dry heat due to water's superior heat conductivity.

• Boiling: Kills vegetative cells of bacteria, fungi, protozoan trophozoites, and most viruses within 10 minutes.

• Not effective against bacterial endospores, protozoan cysts, and some viruses (e.g., hepatitis viruses).

Dry Heat

• Used for materials that cannot be sterilized by boiling or steam (e.g., powders, oils).

• Requires higher temperatures and longer exposure times than moist heat.

• Incineration: Ultimate means of sterilization for contaminated dressings, bags, and paper cups.

Pasteurization

Pasteurization uses heat to kill pathogens and reduce food spoilage, especially in milk and beverages.

Refrigeration and Freezing

Lowering temperature slows microbial metabolism, growth, and reproduction.

• Slow freezing is more effective than quick freezing.

• Many bacteria, endospores, and viruses can survive subfreezing temperatures for years.

• Freezing does not kill all microorganisms (e.g., ancient viruses found in permafrost).

Desiccation and Lyophilization

Desiccation (drying) inhibits microbial growth by removing water, essential for metabolism.

• Used for food preservation (fruits, beans, grains, nuts).

• Kills most pathogenic bacteria, but molds can grow on dried food.

• Lyophilization: Combines drying and freezing for long-term preservation of microbial cultures.

Filtration

Filtration removes microbes by passing liquids or gases through a filter with pores small enough to trap microorganisms.

• Filters made from various materials (e.g., cellulose, glass fiber).

• Used in labs (cotton plugs) and by medical personnel (masks).

HEPA Filters

• High Efficiency Particulate Air (HEPA) filters: Remove microbes from air in research and hospital settings.

• Used for highly contagious or susceptible patients (e.g., tuberculosis, burn patients).

Osmotic Pressure

High concentrations of salt or sugar create hypertonic environments that inhibit microbial growth by drawing water out of cells.

• Used in food preservation (e.g., salted meats, jams, pickles).

• Fungi are more resistant than bacteria to hypertonic conditions.

Radiation

Ionizing Radiation

• Wavelengths shorter than 1 nm (e.g., gamma rays, electron beams).

• Ejects electrons, creates ions, disrupts hydrogen bonding, oxidizes bonds, and denatures molecules (e.g., DNA).

• Gamma rays penetrate well but require long exposure; electron beams are effective but do not penetrate deeply.

Nonionizing Radiation

• Wavelengths greater than 1 nm (e.g., UV light, visible light, radio waves).

• UV light at 260 nm causes thymine dimers, inhibiting DNA replication and causing structural damage.

Chemical Methods of Microbial Control

Overview

Chemical agents affect microbial cell walls, membranes, proteins, or DNA. Their effectiveness varies with environmental conditions and is often greater against enveloped viruses and vegetative cells.

Phenols

• Intermediate to low-level disinfectants.

• Denature proteins and disrupt cell membranes.

• Remain active in presence of organic matter.

• Used in healthcare settings, labs, and homes.

• Disagreeable odor and possible side effects.

Alcohols

• Intermediate-level disinfectants (e.g., isopropanol, ethanol).

• Denature proteins and disrupt membranes.

• More effective than soap for hand hygiene.

• 70% ethanol swab kills most bacteria before injection.

Halogens

• Include iodine, chlorine, bromine, fluorine.

• Intermediate-level antimicrobial chemicals.

• Damage enzymes by oxidation or denaturation.

• Used in water treatment, disinfectants, and antiseptics.

Oxidizing Agents

• Examples: peroxides, ozone, peracetic acid.

• High-level disinfectants and antiseptics.

• Kill by oxidation of microbial enzymes.

• Hydrogen peroxide disinfects surfaces but is less effective on open wounds due to catalase in human cells.

• Ozone used for water treatment.

Equation:

Surfactants

• Soaps: hydrophilic and hydrophobic ends break up oils, good for degerming but not antimicrobial.

• Detergents: positively charged, more soluble in water, disrupt cell membranes.

• Quats (quaternary ammonium compounds): low-level disinfectants, colorless, odorless, harmless to humans.

Heavy Metals

• Denature proteins, low-level bacteriostatic and fungistatic agents.

• Examples: silver nitrate (prevents blindness from N. gonorrhoeae), thimerosal (preserves vaccines), copper compounds (control algae).

• Can be toxic to humans.

Aldehydes

• Contain terminal -CHO groups, cross-link functional groups to denature proteins and inactivate nucleic acids.

• Glutaraldehyde disinfects and sterilizes; formalin used in embalming and room/instrument disinfection.

• Hospitals use 2% glutaraldehyde for equipment disinfection.

Gaseous Agents

• Examples: ethylene oxide, propylene oxide, beta-propiolactone.

• Used in closed chambers for sterilizing items that cannot be exposed to heat or moisture.

• Denature proteins and DNA by cross-linking functional groups.

• Used in hospitals and dental offices; can be hazardous and explosive.

Enzymes

• Proteins produced by organisms to act against microbes.

• Lysozyme in human tears digests peptidoglycan in bacterial cell walls.

• Lysozyme used in cheese production to reduce bacteria.

• Prionzyme can remove prions from medical instruments.

Biosafety Levels

Classification of Laboratory Safety

Biosafety levels (BSL) are government classifications for laboratory safety procedures when handling pathogens.

Summary

• Various physical and chemical methods are used to control microbial growth.

• The choice of method depends on the material, targeted pathogen, and desired level of sterility.