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Physical Methods for Microbial Control: Types & Effectiveness

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

In this lesson we'll be learning about physical methods for microbial control using heat. We'll cover incineration, dry heat, and boiling water. For each type we'll look at the effectiveness, uses, and drawbacks.

What Is Microbial Control?

On all of the surfaces in our homes, and even our own skin, lives an entirely microscopic world. Microbes, or microscopic organisms, are versatile living things that colonize every surface of the planet. Many of these microbes are our friends, breaking down waste, helping us digest our food, and keeping us safe from disease-causing organisms.

Other microbes however, are pathogenic, meaning they cause disease. These microbes try to take over our body and steal resources from our own cells, which makes us sick. Thus, although not all microbes are bad, microbial control is definitely necessary for human health.

Microbial control is the process of destroying unwanted microbes. Microbial control has improved throughout human history. Today, many countries have clean drinking water, sterilized food and medical equipment. But with something so small and numerous as microbes, how can we actually ever get rid of them?

Humans use many methods to destroy microbes, but today we'll be focusing on types of physical methods of microbial control, which can involve using heat, radiation, filtration, low temperature or changes in desiccation to destroy microbes. Physical methods are in contrast to chemical methods, which use chemical reactions for disinfection. In this lesson, we'll be focusing on physical methods using heat.

Types of Physical Microbial Control

1. Incineration

Have you ever had gym clothes so smelly you think you should just burn them? Although it probably isn't recommended to burn your clothing instead of giving it a good wash, it certainly does eliminate the smelly microbes coating the surface. Incineration, or burning materials to ashes, is one of the most effective measures to control microbial growth.

Although highly effective, incineration isn't a method you can use with just anything. It's usually reserved as a method to dispose of contaminated waste. After all, if you incinerate important materials they would be microbe free, but they would also be ashes at the end, and you would be unable to use them.

Municipal waste being incinerated for disposal
incineration

Incineration is used mostly to dispose of biological waste, such as medical waste or animal carcasses. It can also be used to dispose of disposable items that may be contaminated, such as dressings or bandages.

2. Dry Heat

Sometimes, you may want to sterilize something, but not completely burn it to ashes. In this case, using dry heat can be helpful. Dry heat is the process of applying dry, heated air to objects to sterilize them. The benefits of this method are that it is incredibly effective for not only killing bacteria but also destroying viruses and vegetative spores that might persist in other control methods.

Dry heat also has the benefit of sterilizing objects that shouldn't be exposed to moist heat. For example, powders, oils or other substances that can't be exposed to water could be sterilized using this technique as long as they are heat stable. However, dry heat is most commonly used to sterilize glassware, instruments, needles and syringes.

Dry heat works by oxidizing molecules inside the microbe. During oxidation, proteins and other biological molecules lose electrons. This changes their three-dimensional structure in a process called denaturation. If these molecules lose their structure, they are no longer able to do their function. They stop working and thus the microbe dies.

However, dry air conducts heat much slower than moist air. Therefore, one drawback to this method is that extremely high temperatures must be used over a long period of time. Objects to be sterilized with this method must be exposed to temperatures above 338 degrees Fahrenheit for at least 2 hours.

3. Boiling Water

If you've ever been hiking on a multi-day trip, you know one of the most important tools is the ability to purify your water. Although there are tablets and filters available, one of the oldest methods for keeping clean water on the trail is to simply boil it. Boiling requires heating water to 212 degrees Fahrenheit for at least ten minutes.

Boiling water is one method of microbial control
boiling water

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