Review of Physical Methods to Control Microbial Growth - Online Tutor, Practice Problems & Exam Prep

Controlling microbial growth involves various physical methods. Dry heat, such as incineration and hot air ovens, eliminates microbes without moisture. Moist heat techniques, including boiling and autoclaving, utilize steam. Low temperatures, like refrigeration and freezing, inhibit growth. Desiccation removes moisture, while lyophilization combines freezing and drying. Filtration uses small pores to separate microbes from liquids or air, and irradiation employs ionizing or non-ionizing radiation to kill microbes. High pressure processing applies extreme pressure to destroy microbial cells, ensuring effective sterilization and preservation.

Review of Physical Methods to Control Microbial Growth

In this video, we're going to review some of the information that we've already covered in our previous lesson videos on the physical methods of controlling microbial growth. We're going to do that by completing the interactive blanks that we have in the table below. Notice that this table focuses on the physical methods of controlling microbial growth. We have the specific control method in the left-hand column, and a description of the control method in the right-hand column. The very first method that we have here is dry heat, which recall uses heat that has absolutely no moisture or liquid content. This includes techniques such as incineration, burning with a direct flame, or using a hot air oven.

Next, we have moist heat. As its name implies, moist heat is heat that actually has moisture or liquid content. This includes techniques such as boiling, using pasteurization, and pressurized steam using an autoclave. Following that, we have low temperatures. Rather than applying heat, using low temperatures will inhibit the growth of microbes, helping to preserve foods. We can implement this using refrigeration or freezing.

Next, we have desiccation, the process of drying out. You can think of a cactus, found in deserts and really dry areas, to help remind you of desiccation and the fact that this is the process of removing moisture from cells to kill microbes. Following desiccation, we have lyophilization, the process of freeze-drying. It's almost like combining low temperatures and desiccation, including some of your favorites like ice cream.

Next, we have filtration. Filtration is the process of using filters with very small pores, pores that are so small that they make microbes appear large, and the microbes are too large to fit through the pores. It filters out the microbes and allows the liquid or the air to pass through and become sterilized. Recall that the HEPA filter, the high-efficiency particulate air filter, is a specific type of filter for filtering air.

Next, we have irradiation, the process of exposing an object to radiation to kill microbes. We talked about two different types of radiation: ionizing radiation, which has high penetrative power and sufficient energy to ionize molecules creating reactive oxygen species, and non-ionizing radiation, which has less penetrative power and less energy and must be used directly on the microbes to be effective.

Last but not least, we have high-pressure processing. As its name implies, it is a process that uses extremely high pressures to destroy and kill microbes. This concludes our brief review of the physical methods of controlling microbial growth, and we'll be able to get a little bit more practice moving forward. Then we'll discuss the chemical methods of controlling microbial growth. I'll see you all in our next video.

Which of the following microbial control methods does not actually kill microbes or inhibit their growth but instead removes them physically from samples?

• A. Filtration.
• B. Desiccation.
• C. Lyophilization.
• D. Non-Ionizing Radiation.

Which method of physical microbial growth control requires extremely high temperatures and long periods of time to kill microbes?

• A. Moist heat control techniques.
• B. Pasteurization.
• C. Dry heat control techniques.
• D. Lyophilization.

Here’s what students ask on this topic:

What are the differences between dry heat and moist heat methods in controlling microbial growth?

Dry heat methods, such as incineration and hot air ovens, eliminate microbes by using heat without moisture. This method is effective for materials that can withstand high temperatures and is often used for sterilizing glassware and metal instruments. Moist heat methods, including boiling, pasteurization, and autoclaving, utilize steam and are more effective at lower temperatures compared to dry heat. Moist heat denatures proteins and disrupts cell membranes, making it suitable for sterilizing liquids, culture media, and surgical instruments. Autoclaving, which uses pressurized steam, is particularly effective for achieving complete sterilization.

How does refrigeration inhibit microbial growth?

Refrigeration inhibits microbial growth by slowing down the metabolic processes of microbes. At low temperatures, typically around 4°C (39°F), the enzymatic activities and cellular functions of microbes are significantly reduced, which prevents them from reproducing and spreading. This method is commonly used to preserve perishable foods, pharmaceuticals, and biological samples. While refrigeration does not kill microbes, it effectively prolongs the shelf life of products by keeping microbial activity at a minimum.

What is the role of HEPA filters in microbial control?

HEPA (High-Efficiency Particulate Air) filters play a crucial role in microbial control by trapping airborne particles, including microbes, with a high efficiency. These filters have very small pores, typically around 0.3 micrometers, which are capable of capturing particles as small as bacteria, viruses, and fungal spores. HEPA filters are commonly used in cleanrooms, hospitals, laboratories, and air purifiers to maintain sterile environments and prevent the spread of airborne infections.

How does ionizing radiation differ from non-ionizing radiation in microbial control?

Ionizing radiation, such as gamma rays and X-rays, has high penetrative power and sufficient energy to ionize molecules, creating reactive oxygen species that damage microbial DNA and cellular structures. This method is effective for sterilizing medical equipment, pharmaceuticals, and food products. Non-ionizing radiation, such as ultraviolet (UV) light, has less penetrative power and energy. It must be used directly on the surface of the microbes to be effective, as it causes thymine dimers in DNA, leading to mutations and cell death. UV light is commonly used for disinfecting surfaces, air, and water.

What is lyophilization and how is it used in microbial control?

Lyophilization, also known as freeze-drying, is a process that combines freezing and drying to preserve microbial cultures, pharmaceuticals, and food products. In this method, the material is first frozen, and then the surrounding pressure is reduced to allow the frozen water to sublimate directly from solid to gas. This process removes moisture, which inhibits microbial growth and preserves the material for long-term storage. Lyophilization is particularly useful for preserving the viability and stability of microbial cultures and sensitive biological materials.