Physical agents are non-chemical methods used to control microorganisms. These methods utilize environmental factors like temperature, pressure, radiation, and physical barriers to eliminate, inhibit, or remove microbes from various materials. They are widely applied in industrial settings for food preservation, in healthcare for sterilization, and in the laboratory for routine hygiene. These physical controls often work by disrupting the microbe’s cellular structure or interfering with its metabolic functions.
Manipulating Temperature to Inhibit Growth
Temperature manipulation is a primary physical method for controlling microbial populations. High heat achieves destruction through the denaturation of microbial proteins and enzymes. Moist heat, such as steam in an autoclave, penetrates cells efficiently, coagulating proteins. This requires less time and a lower temperature, typically 121°C for 15 minutes under pressure, to achieve sterilization.
Dry heat, used in a hot air oven or a flaming loop, relies on oxidation to destroy cellular components. Since water is absent, the process is less efficient, requiring higher temperatures and longer exposure times, such as 170°C for two hours, to achieve sterilization. Both moist and dry heat applications are used for sterilization, which removes all living organisms, including bacterial spores.
A milder thermal control method is pasteurization, a common preservation technique that reduces spoilage organisms and eliminates pathogens. This process does not achieve complete sterilization, as many heat-resistant bacterial spores survive the treatment. High-Temperature Short-Time (HTST) pasteurization heats liquid products like milk before rapid cooling.
Low temperatures are used for inhibiting microbial growth rather than killing organisms outright. Refrigeration slows the metabolic rate of most microorganisms, inhibiting their ability to reproduce and delaying spoilage.
Freezing halts microbial growth completely by turning available water into ice, making it inaccessible for cellular metabolism. Ice crystal formation can also cause physical damage to the microbial cell structure, leading to the death of some organisms.
Removing Water and Applying Pressure
Microorganisms require water to maintain metabolic function and grow. The physical removal of water, or desiccation, has been used for centuries as a method of food preservation. Drying prevents essential enzymatic and chemical reactions within the cell, effectively stopping growth.
Limiting water availability can also be achieved using high concentrations of solutes like salt or sugar, which controls microbes through osmotic pressure. When a microbial cell is placed in a hypertonic environment, water is drawn out of the cell. This process, known as plasmolysis, causes the cytoplasm to shrink and become dehydrated, inhibiting metabolic processes. This is the principle behind curing meats and making jams.
A modern, non-thermal method is High Pressure Processing (HPP), also known as pascalization. This technique subjects food products to high hydrostatic pressure. The immense pressure causes denaturation and damages the cell membranes of microorganisms. HPP is effective against vegetative cells and is valued in the food industry because it eliminates pathogens while preserving the product’s flavor and nutritional qualities.
Using Radiation to Damage Microbes
Radiation controls microbial growth by delivering energy waves that damage cellular components, particularly the genetic material. Non-ionizing radiation, primarily ultraviolet (UV) light, has low energy and poor penetrating ability, making it suitable only for surface and air disinfection. UV light works by forming covalent bonds between adjacent pyrimidine bases in a microbe’s DNA.
These resulting structures, called thymine dimers, create structural deformities in the DNA strand that inhibit DNA function. Because UV light is non-penetrating, it is often used in laboratory biosafety cabinets and for sanitizing surfaces and water.
In contrast, ionizing radiation, which includes gamma rays and X-rays, is highly energetic and deeply penetrating. This radiation controls microbes through two mechanisms: direct damage to the DNA molecule and indirect damage. The indirect effect involves the radiation interacting with water molecules in the cell, a process called radiolysis, which generates highly reactive hydroxyl free radicals. These free radicals then chemically attack and destroy DNA and proteins, leading to cellular death. Ionizing radiation is often referred to as “cold sterilization” and is used to sterilize disposable medical equipment and certain food products.
Physical Separation Through Filtration
Filtration is a method of microbial control that physically separates microbes from liquids or gases, rather than killing or inhibiting their growth. The process involves passing the fluid or air through a screen-like material containing pores small enough to trap particles and microorganisms. This technique is important for sterilizing heat-sensitive liquids, such as certain vaccines, antibiotics, and culture media.
For liquids, membrane filters are commonly used, with a pore size of 0.22 micrometers being standard to retain all bacteria. Air filtration often uses High-Efficiency Particulate Air (HEPA) filters, which are designed to remove at least 99.97% of airborne particles with a diameter of 0.3 micrometers. HEPA filters are widely used in clean rooms, surgical suites, and biological safety cabinets to maintain sterile air quality.