Selecting a Method to Control Microbial Growth - Online Tutor, Practice Problems & Exam Prep

Selecting an effective method for microbial growth control involves five key considerations: the types and numbers of microbes, the risk of infection, environmental factors, and the composition of the item being treated. Different microbes exhibit varying resistance levels, influencing the choice between disinfection and sterilization. The number of microbes affects the time required for effective treatment, while environmental conditions like temperature and pH can enhance or hinder effectiveness. Lastly, the material composition must be compatible with the chosen method to avoid damage. Understanding these factors is crucial for effective microbial control.

Selecting a Method to Control Microbial Growth

In this video, we're going to begin our lesson on selecting a method to control microbial growth. Selecting an effective method to control microbial growth is a challenging process. This is not only because there are many microbial growth control methods, but also because each method has a set of advantages and disadvantages. There isn't a perfect microbial growth control method that works in every scenario; each has its own pros and cons.

In order to select an appropriate microbial growth control method, there are some important considerations that should be made. Notice that we're listing these 5 important considerations in the table below. Our scientist is asking, "What method of microbial growth control should I be using?" To answer this question, you should consider the five points we have below. The first consideration is simply the types of microbes that are present. Different types of microbes have different characteristics, and knowing these can help dictate which methods are useful and which are not. Some microbial growth control methods may work for one type of microbe but not for others. We will talk more about this idea and each of these considerations in more detail as we move forward in our course.

The second consideration is the number of microbes. A large number of microbes may require a different control method than a smaller number. The third consideration is the overall risk of infection. If there is a low risk of infection, it may influence the choice of microbial growth control method, and a high risk of infection can also dictate the method used. Another important consideration is environmental factors, such as temperature and pH, which can impact the effectiveness of a microbial growth control method.

The final consideration when selecting a microbial growth control method is the composition of the item to be treated. Whether the item is made out of glass or plastic can make a difference. We will discuss each of these five considerations in more detail as we move forward in our course. This concludes our brief introduction to selecting a method to control microbial growth and the considerations that should be made before or while selecting a control method. I'll see you all in our next video to talk even more about each of these considerations.

Types of Microbes (Microbial Characteristics)

In this video, we're going to talk more about the first consideration to be made when selecting a microbial growth control method, and that is the types of microbes and the microbial characteristics that are present. And so once again, the types of microbes along with the microbial characteristics are going to be really important considerations when selecting a microbial growth control method. And so this is because some microbes are actually really highly resistant to particular microbial growth control treatments, meaning that they will not die by a specific control method. Whereas other microbes may be sensitive to the same exact treatment method, meaning that they will die in the presence of that treatment method. And so the type of microbe is going to be a very important consideration because it can help dictate whether a disinfection method can be used, or whether a more powerful sterilization procedure should be used. And so if we take a look at this image down below, on the left-hand side, we can get a better understanding of how different types of microbes can have different resistant levels to particular control methods. And so notice that on the far left-hand side over here, left-hand column, this is a column showing you different types of microbes. And notice that these different types of microbes are arranged based on their resistance to particular microbial control methods. So notice the ones that are towards the top with the whitish background are going to be less resistant. And that means that they are going to be more susceptible, meaning that they will die rather easily by a microbial growth control method. And as you start to go downwards towards the bottom, the microbes start to become more and more resistant. And of course, being resistant means that they are able to survive in the presence of that method. And so on the far right, what we're showing you is a column of the microbial control method that's going to be sufficient enough to basically control the particular microbe. And so notice that for enveloped viruses such as the SARS CoV 2 virus that causes COVID 19, HIV, and hepatitis B viruses, enveloped viruses are not very resistant. Okay? They're very susceptible. And so really low-level disinfection control methods are going to be suitable for dealing with enveloped viruses and also for dealing with most vegetative bacteria. Such as, for example, S. Aureus and N. Gonorrhea. Now notice that fungi and non-enveloped viruses, start to become more, a little bit more resistant. And so they require high-level disinfections methods in order to control these microbes. And you could see some examples of fungi such as ringworm and athlete's foot, and also non-enveloped viruses. Examples are poliovirus or polio and rhinovirus, which is responsible for the common cold. Then notice that mycobacterium and bacterial endospores are even more resistant, and so they require more powerful sterilization techniques. And so, if we were to use a low-level disinfection technique on one of these microbes here, mycobacterium or bacterial endospores, it would not be sufficient to control them. And so, only sterilization procedures are going to be sufficient for these microbes. And then, towards the very bottom what we have are prions. And recall that prions are going to be very difficult to control and so, it requires even more powerful control methods than sterilization such as for example, prion elimination techniques. And so those are the most resistant microbes, are the prions. And so basically, the big takeaway that you can get from this image is that the type of microbe really does matter. Because whether we're dealing with enveloped viruses or whether we're dealing with bacterial endospores, it really matters, because it will dictate what type of control method is going to be suitable. And, lower-level disinfection methods will not be suitable, to basically handle more resistant forms or more resistant types of microbes. Now over here on the far right what we're showing you is a graph that is showing you the time for sterilization. Basically how long it takes to kill a particular microbe. And so notice on the y axis it's showing the log of the number of viable cells. Basically showing you how many viable cells there are, how many living cells there are. The higher, the y value the more living cells. The lower the y value the less living cells the more dead cells there are. And so, on the x axis what we have is the time progressing from left to right. And so notice that we have 2 different curves on we have 2 different lines on this graph. We have this yellow line right here and then we have this orangish line right here. And notice that the yellow line represents vegetative bacterial cells whereas this orangish line right here represents endospores. And recall from our previous lesson videos and from this table over here, that endospores are fairly resistant and they require more stringent control methods. Whereas, vegetative bacteria are up here and they are less resistant, more susceptible. And so notice that the vegetative bacteria are able to be killed or, sterilized in a less amount of time, whereas the endospores, require a lot more time in order to be completely sterilized and completely killed off. And so, once again, this is just showing you how that, the time that's required for sterilization actually depends on the type of microbe that is present. And so, all of this is just to emphasize the fact that the types of microbes that are present are a really important consideration when selecting the microbial growth control method. And so this here concludes our brief lesson on this idea here, and we'll be able to learn more about the other considerations and apply these concepts here as we move forward. So I'll see you all in our next video.

Which of the following questions are important to answer before attempting to control a microbial population?

• A. What type of microbe am I trying to control?
• B. What kind of environment is the microbe in?
• C. What is the size of the microbial population?
• D. All are important questions to answer.

Which microbe has the highest level of resistance to control methods that humans currently possess?

• A. Mycobacteria (bacteria that cause tuberculosis).
• B. Enveloped viruses (Covid-19 virus, HIV, etc.).
• C. Prions.
• D. Bacterial endospores.

Number of Microbes

In this video, we're going to talk about the second consideration when selecting a method for microbial growth control, which is the number of microbes that are present. When a microbial population is treated with either a physical or a chemical process, usually the microbes within that population die at a constant rate, rather than all at once. This means that the larger the initial population of microbes is, the longer it will take for the entire population to be destroyed. Washing and scrubbing can help to remove microbes and biofilms, ultimately reducing the time to sterilize or disinfect an item if you're able to remove some of the microbes present just by physical disruption.

Below, we are showing you these two plots, this one and this one over here. These are known as microbial death curves, which are plots of the death of a microbial population over time due to either a physical or a chemical microbial control treatment method. On these plots, you will find a value known as the decimal reduction time or just the D-value for short. The decimal reduction time or D-value refers to the specific amount of time required to kill 90% of a microbial population under set conditions. It's important to note that the greater the D-value is, the more resistant the microbial population is to the specific treatment method.

Take a look at our image below; we are showing you a microbial death curve along with the D-value. Notice on the y-axis we have the number of living microbial cells, and on the x-axis, we have the amount of time in minutes. At time 0, we're starting with a population of 10,000 microbes. When applying this particular microbial growth control method, the population starts to decrease over time. When 90% of the population is gone, going from 10,000 cells down to 1,000 cells, this is a 90% drop-off. This is on a logarithmic scale y-axis. This population after one D-value shows that the D-val

The decimal reduction time refers to the amount of time it takes to which of the following?

• A. Reduce a microbial population by 10%.
• B. Reduce a microbial population by 0.1%.
• C. Reduce a microbial population by 90%.
• D. Completely eliminate a microbial population.

Which microbial control method is most effective for killing a population of Bacteria X?

• A. A microbial control method with a D-value of 1 hour.
• B. A microbial control method with a D-value of 10 minutes.
• C. A microbial control method with a D-value of 30 minutes.
• D. A microbial control method with a D-value of 2 hours.

Overall Risk of Infection

In this video, we're going to talk about the 3rd consideration when selecting a microbial growth control method, and that is the overall risk of infection that the item being treated presents. And so, once again, the risk of infection that a particular item presents is going to dictate the method of controlling microbial growth that's going to be used. Now, medical instruments and tools are actually categorized into 3 groups based on the risk of infecting a patient. And so, we've got those 3 groups numbered down below, and we've also got images of these 3 groups down below in the image. Now the first group is actually going to be the non-critical instruments. And as their name implies, these are going to be medical instruments and tools that present a really low risk of infection, a really low risk of infecting the patient. And that's usually because these items only make contact with the skin on the outside of a patient. And so, these items only need to be treated with low-level disinfectants because they present a low risk of infection. And so, if we take a look at our image down below at the overall risk of infection in these three groups, over here on the far left, we have the non-critical instruments, which once again present a really low risk of infection. And they're going to usually make contact only with the skin of the patient, and so, they do not penetrate into the tissues or anything like that. And so, the items such as stethoscopes, which measure heart rate, and crutches which can help you walk around if you have an injured foot, and even items like blood pressure cuffs are all examples of non-critical medical instruments that present a low risk of infection. And so, only low-level disinfection is required for these low-risk non-critical instruments. Now the next group that we have are these semi-critical instruments. And semi is a root that means partially critical. And so, as their name implies, these instruments are going to present a medium risk of infection, or a medium risk of infecting the patient. And so, these require higher-level disinfection methods. And so, they're going to be more powerful because the instrument has a greater risk of infecting the patient. And so if we take a look at our image down below, notice the semi-critical instruments are right here in the middle, and these once again present a medium risk of infection to the patient. And so this could include examples of instruments that are going to be semi-critical like endoscopes, which are basically these scopes that can go into the gastrointestinal tract of a human. So they're going into the body, but they're not actually penetrating into any tissues. And so, they do present a medium risk of infection. And then, of course, last but not least, what we have are the critical instruments. And critical instruments, as their name implies, they present a really high risk of infection or a really high risk of infecting the patient. And so, therefore, these critical instruments must be sterile in order, for them to be used appropriately in a medical setting. And so if we take a look at our image down below at the critical instruments, once again, these present a really high risk of infection. And so, these are going to be such as surgical instruments that are going to penetrate into the tissue of the patient. And so, they need to be sterile, which means that no living microorganisms are going to be present on them. And so, this here concludes our lesson on how the overall risk of infection is an important consideration when selecting the microbial growth control method. And once again, depending on if it's a low risk of infection, a medium risk of infection, or a high risk of infection, that might dictate whether low-level disinfection methods, high-level disinfection methods, or sterilization methods are required for that particular item that's going to be treated. And so, we'll be able to get some practice applying these concepts here as we move forward. But for now, I'll see you in our next video.

Environmental Factors

In this video, we're going to talk about the 4th consideration when selecting a microbial growth control method, and that is environmental factors. Environmental factors such as the temperature, for example, and the pH can influence the effectiveness of a microbial growth control method. For example, if the temperature or pH ranges are not correct within the appropriate range, then the microbial growth control method may be ineffective, meaning it may not work as intended. On top of environmental factors such as temperature and pH, other factors such as substances including grease, dirt, and body fluids can interfere with some microbial growth control methods. Therefore, these items should be cleaned out appropriately prior to implementing any disinfection or sterilization control methods.

If we take a look at our image below, we can get a better understanding of how environmental factors affect microbial growth control methods. We're showing you a microbial death curve, with the use of sodium hypochlorite as a chemical microbial growth control method. Notice on the y-axis we have the number of living microbial cells, and on the x-axis we have the amount of time that these cells have been exposed to the microbial growth control method. Notice that we have two lines here; one in orange and one in blue. The orange line represents a hot temperature of 55 degrees Celsius, and the blue line represents a colder temperature of 50 degrees Celsius. Notice that there is only a 5 degrees Celsius difference, which you might think might not make a significant difference in the control method. However, in this particular example, it does make a significant difference.

For the hotter temperatures, the d value is much smaller. The d value, the decimal reduction time, is the amount of time it takes to kill 90% of the population. The d value here for the orange curve is 10 minutes to kill 90% of the population, starting from an initial population of 10,000 and going down to 1,000. The hotter temperature is more effective than the colder temperature because the d value for the colder temperature is double and shows that these microbes are more resistant to colder temperatures. It takes a lot longer to kill all of the microbes when the temperature is colder and much less time when it is hotter.

This emphasizes the fact that household bleach is more effective at killing microbes at hot temperatures, specifically temperatures of 55 degrees Celsius. This shows that even small temperature changes, like 5 degrees Celsius, can have a major impact on the microbial growth control method. Environmental factors are an important consideration when selecting a microbial growth control method. This concludes our lesson on this idea, and we'll be able to get some more practice and talk about the 5th and final consideration of selecting a microbial growth control method in our next video. See you all there.

Match the type of instrument with its correct level of microbial infection risk.

• A. Critical Instruments:High risk.
• B. Non-Critical Instruments:Medium risk.
• C. Semi-Critical Instruments:Low risk.
• D. All are matched correctly.

What can affect the effectiveness of a microbial growth control method?

• A. The temperature of the environment the control method is being used in.
• B. The pH of the environment the control method is being used in.
• C. The presence of grease, dirt, or body fluids on the surface contaminated with microbes.
• D. The type of microbe you are trying to control.
• E. All of the above.

Composition of Item to Be Treated

In this video, we're going to talk about the 5th consideration when selecting a microbial growth control method, and that is the composition of the item to be treated. Some physical and chemical processes that control microbial growth are actually inappropriate for certain types of material. Physical methods like heat and irradiation can actually cause damage to some types of plastics. This is a consideration that needs to be made. Also, liquid chemical disinfectants can also cause damage to certain types of materials such as electrical equipment, for example. All that we're trying to say here is that the composition of the item to be treated has to be compatible with the microbial growth control method. Depending on the material or the composition of the item, that will dictate what microbial growth control method can be used. If we take a look at the left-hand side over here, notice that we have this plate or this dish here that's made of plastic, and this plastic plate actually melted from having excessive heat. Although the heat may have killed the microbes, it also damaged the plastic and now this dish is no longer usable. That would not be the best microbial control method for this type of plastic material. Over here on the right what we're showing you is some electrical equipment. Of course, liquids are going to damage most electronic equipment, most electrical equipment. Liquids are not going to be compatible with electrical equipment. Perhaps another type of method such as a gas could be used as a replacement. This here concludes our brief lesson on this 5th consideration, the composition of the item to be treated when selecting a microbial growth control method. We'll be able to get some practice applying these concepts as we move forward. So I'll see you all in our next video.

Why must the composition of the item being treated be compatible with the microbial growth control method?

• A. If the item and the control method are not compatible then the microbes will die faster.
• B. If the item and the control method are not compatible then the item can be damaged by the control method.
• C. If the item and the control method are not compatible then the control method will be ineffective.
• D. None of the above.

Here’s what students ask on this topic:

What are the key considerations when selecting a method to control microbial growth?

When selecting a method to control microbial growth, five key considerations must be taken into account: the types of microbes, the number of microbes, the risk of infection, environmental factors, and the composition of the item being treated. Different microbes exhibit varying resistance levels, influencing whether disinfection or sterilization is appropriate. The number of microbes affects the time required for effective treatment. Environmental conditions like temperature and pH can enhance or hinder the effectiveness of the control method. Lastly, the material composition must be compatible with the chosen method to avoid damage. Understanding these factors is crucial for effective microbial control.

How do the types of microbes affect the choice of microbial growth control methods?

The types of microbes significantly affect the choice of microbial growth control methods because different microbes have varying resistance levels. For example, enveloped viruses like SARS-CoV-2 are less resistant and can be controlled with low-level disinfection methods. In contrast, bacterial endospores and prions are highly resistant and require more stringent sterilization techniques. Understanding the specific type of microbe helps determine whether a disinfection method will suffice or if a more powerful sterilization procedure is necessary. This ensures that the chosen method is effective in eliminating the targeted microbes.

Why is the number of microbes an important consideration in microbial growth control?

The number of microbes is an important consideration because it affects the time required for effective microbial control. Microbes typically die at a constant rate when exposed to a control method, meaning larger populations take longer to eliminate. For instance, if a microbial population starts at 10,000 cells, it will take more time to reduce this population to zero compared to a smaller initial population. Washing and scrubbing can help reduce the number of microbes, thereby decreasing the time needed for disinfection or sterilization. Understanding the initial microbial load helps in selecting the appropriate control method and duration.

How do environmental factors influence the effectiveness of microbial growth control methods?

Environmental factors such as temperature and pH can significantly influence the effectiveness of microbial growth control methods. For example, higher temperatures can enhance the effectiveness of chemical disinfectants like sodium hypochlorite, reducing the time required to kill microbes. Conversely, lower temperatures may make the control method less effective. Additionally, substances like grease, dirt, and body fluids can interfere with the effectiveness of disinfection or sterilization methods. Therefore, it is crucial to consider these environmental factors and ensure that the conditions are optimal for the chosen microbial control method to work effectively.

Why is the composition of the item being treated important in selecting a microbial growth control method?

The composition of the item being treated is crucial because some physical and chemical microbial control methods can damage certain materials. For instance, heat and irradiation can deform or melt plastics, while liquid chemical disinfectants can harm electrical equipment. Therefore, the material composition must be compatible with the chosen microbial control method to avoid damage. For example, using gas-based methods instead of liquid disinfectants for electrical equipment can prevent damage while effectively controlling microbial growth. Understanding the material composition ensures that the selected method is both effective and safe for the item being treated.