Oxygen Requirements for Microbial Growth - Online Tutor, Practice Problems & Exam Prep

Microbes are classified based on their oxygen requirements into five groups: obligate aerobes, facultative anaerobes, microaerophiles, obligate anaerobes, and aerotolerant anaerobes. Obligate aerobes need high oxygen levels, while facultative anaerobes thrive better with oxygen but can survive without it. Microaerophiles require low oxygen concentrations, obligate anaerobes cannot tolerate oxygen, and aerotolerant anaerobes grow equally in both environments. Aerobes generate reactive oxygen species (ROS) like superoxide and hydrogen peroxide, which can damage cells. They utilize enzymes such as superoxide dismutase, catalase, and peroxidase to neutralize ROS, ensuring cellular protection.

Oxygen Requirements for Microbial Growth

In this video, we're going to begin our lesson on oxygen requirements for microbial growth. It turns out that all organisms that utilize chemical energy require what's known as a final electron acceptor for their electron transport chain. Later in our course, in a different video, we'll discuss these final electron acceptors and the electron transport chain more extensively. But for now, here in this video, you can just imagine that the final electron acceptor and electron transport chain are crucial for generating energy for the cell.

In many microbes, the final electron acceptor in this electron transport chain is going to be oxygen gas, or O₂. Organisms that utilize oxygen gas as the final electron acceptor are referred to as aerobes. These are microbes that require oxygen gas to act as the final electron acceptor in the electron transport chain. Therefore, these aerobes require oxygen to be abundant in order for them to grow, and they only grow where oxygen is abundant. We call these areas, these environments that have lots of oxygen, aerobic environments. Aerobes require oxygen for growth, so they grow only in aerobic environments where there's lots of oxygen.

On the other hand, anaerobes are microbes that grow where little to no oxygen is present. When little to no oxygen is present, we refer to these as anaerobic environments. If we look at our image below, you'll notice on the left-hand side we are focusing on aerobic environments. In aerobic environments, there is plenty of oxygen, and aerobes are capable of surviving in them. Here we're showing you an aerobe because notice he's saying "I can't live without you" speaking to the oxygen gas. An example of this is Mycobacterium tuberculosis, which is a bacterium that causes tuberculosis, a lung condition.

On the right-hand side, we are focusing on the anaerobic environment. The anaerobic environment is the opposite of the aerobic environment. Anaerobic environments do not have a lot of oxygen. Over here, what we have is an anaerobe saying "No, thank you. I do not need oxygen to grow." An example of this anaerobe is Bacteroides fragilis, which is a bacterium that survives in the gastrointestinal tract of humans.

This concludes our brief introduction to oxygen requirements for microbial growth and how aerobes grow in aerobic environments with lots of oxygen, and anaerobes grow in anaerobic environments with little to no oxygen. We'll be able to learn more about the oxygen requirements for microbial growth as we move forward in our lesson. So, I'll see you all in our next video.

Oxygen Requirement Classes of Microbes

In this video, we're going to begin our lesson on oxygen requirement classes of microbes. Microbes can really be classified into 5 groups based on their requirement for oxygen gas or O₂. Notice down below in this table, we're showing you the oxygen requirement classes of microbes. They're organized into these 5 groups that we have numbered 1 through 5. One thing that's very important to note here is that each of these 5 groups has test tubes. These test tubes that you see here are really special because they allow for really high concentrations of oxygen towards the top of the test tube, represented by these light blue backgrounds. As you start to go towards the bottom of the test tube, you start to get lower oxygen concentrations. At the very bottom of each of these test tubes is the lowest oxygen concentration. This is something that's very important to keep in mind as we move forward throughout the rest of this video.

The very first group that we have over here on the far left are the obligate aerobes. As their name implies, these are aerobic organisms, or organisms that are obligated to aerobic environments or environments that have high oxygen concentrations. Notice that all of the brown speckles that you see throughout here represent microbial growth. There is only microbial growth here, in the region of the test tube that has the highest oxygen concentration. Obligate aerobes cannot live without oxygen. Notice here we have an obligate arrow saying "I can't live without you" to the oxygen gas because they absolutely require oxygen gas and cannot live without the oxygen gas.

Next, the facultative anaerobes, as their name implies, can survive in both aerobic and anaerobic environments. You should notice that there are brown speckles throughout the entire test tube. However, there are a lot more brown speckles towards the top of the test tube than towards the bottom. This is because facultative anaerobes, although they can grow in both the presence and absence of oxygen, actually grow a lot better in the presence of oxygen gas. That's why there are more microbes in the aerobic region of the test tube, where the oxygen concentration is higher, and fewer facultative anaerobes in regions with lower oxygen. Notice that the microbe, where there's no oxygen, is saying "this is ok, I guess," and really, this is its second option when no oxygen is present. But when in aerobic environments where oxygen is present, the microbe says "this is amazing," and the presence of oxygen is going to be its number one option to be able to grow best. So there is microbial growth throughout the entire test tube; however, there's going to be a lot more towards the regions that have higher oxygen because oxygen allows for the generation of more ATP, more energy. More energy means more growth, and that's why they're able to grow more and better in aerobic environments.

The third group we have here are the microaerophiles. The root "micro" means small. These aerobes, microaerophiles, do not need much oxygen at all. In fact, too much oxygen is actually toxic to these microaerophiles. They require oxygen; however, they need only small amounts of oxygen because, if it's too high, it's going to be toxic to them. They grow right here in this very specific region where there's a small amount of oxygen. But notice that they're not at the very top where there's the most amount of oxygen, and they also do not grow where there's no oxygen. They do require oxygen but only a small amount of oxygen gas.

The next group we have here are the obligate anaerobes. Obligate anaerobes are practically the opposite of obligate aerobes. That's because obligate anaerobes can only survive where there is no oxygen. Oxygen is pretty much toxic to these obligate anaerobes. Notice that this obligate anaerobe is saying "get away from me," and he's got his protective shield and sword because oxygen is toxic to these obligate anaerobes. They cannot grow in the presence of oxygen. They require anaerobic environments, environments that have no oxygen, in order to grow. That's why all of the microbial growth is towards the very bottom for the obligate anaerobes.

The fifth and final group we have here are the aerotolerant anaerobes. The aerotolerant anaerobes really grow equally in oxygen and areas that have no oxygen. They grow equally in aerobic and anaerobic environments. Notice that the microbial growth is evenly distributed. They do not grow better in one or the other; they grow equally in both. So here what we're saying is that these aerotolerant anaerobes are tolerant to both oxygen and regions that have no oxygen as well. So they grow in the presence or the absence of oxygen equally.

This wraps up our introduction to oxygen requirements for microbes, and 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.

Organisms that require oxygen for metabolism are referred to as…

• A. Facultative aerobes.
• B. Obligate aerobes.
• C. Facultative anaerobes.
• D. Microaerophiles.

Organisms that are indifferent to the presence of oxygen are…

• A. Aerotolerant anaerobes.
• B. Facultative anaerobes.
• C. Obligate aerobes.
• D. Microaerophiles.

Reactive Oxygen Species

In this video, we're going to begin our lesson on reactive oxygen species. Aerobes, which you recall from our previous lesson videos are microbes that require oxygen gas, tend to generate toxic derivatives of oxygen called reactive oxygen species. Reactive oxygen species are commonly abbreviated as just ROS. Reactive oxygen species, or ROS, are highly reactive oxygen molecules that can cause damage to the cell. Reactive oxygen species, or ROS, present a problem for aerobes. Examples of reactive oxygen species, or ROS, include superoxide, which is O₂^-, as well as hydrogen peroxide, which is H₂O₂. Superoxide and hydrogen peroxide are capable of causing damage to the cell. These aerobes need to have a way to manage these reactive oxygen species, and all aerobes produce enzymes that protect the cell from these reactive oxygen species, or ROS.

Here, what we're showing you are three enzymes that these aerobes can use to protect themselves from the reactive oxygen species. The first enzyme that we have here is superoxide dismutase, abbreviated as SOD. This enzyme converts two superoxide molecules into oxygen gas and hydrogen peroxide. If we take a look at our image down below, reactive oxygen species, or ROS, include oxygen, O₂^-, which is superoxide, as well as hydrogen peroxide or H₂O₂. Once again, these can cause damage to the cell. These three enzymes help to protect the aerobes from these reactive oxygen species.

Superoxide dismutase converts two superoxide molecules, or two O₂^- molecules, and converts them with hydrogen ions into hydrogen peroxide and oxygen gas. The next enzyme is catalase. Catalase converts two hydrogen peroxide molecules into oxygen gas and two molecules of water. Notice that catalase converts two hydrogen peroxide molecules into two water molecules and oxygen gas. Water and oxygen gas are non-toxic to aerobes. By using these enzymes, aerobes are able to convert toxic reactive oxygen species into non-toxic components.

Sometimes, they need to work together to convert these toxic components, these toxic reactive oxygen species, into non-toxic molecules of water and oxygen gas. The third and final enzyme that we're going to talk about here is peroxidase. Peroxidase converts hydrogen peroxide, and two hydrogen ions, into two water molecules. Again, converting toxic reactive oxygen species into non-toxic components such as water and oxygen. This concludes our brief lesson on reactive oxygen species, these toxic components that tend to be generated by aerobes, and the enzymes that aerobes use to protect themselves from their own reactive oxygen species that they generate. We'll be able to get some practice applying these concepts as we move forward in our course. I'll see you all in our next video.

The enzyme(s) that deal with toxic oxygen-containing molecules is/are

• A. Glycolase.
• B. Catalase.
• C. Superoxide dismutase.
• D. Cytochrome oxidase.
• E. B & C.
• F. B & D.

Here’s what students ask on this topic:

What are the different oxygen requirement classes of microbes?

Microbes are classified into five groups based on their oxygen requirements: obligate aerobes, facultative anaerobes, microaerophiles, obligate anaerobes, and aerotolerant anaerobes. Obligate aerobes require high oxygen levels to grow. Facultative anaerobes grow better with oxygen but can survive without it. Microaerophiles need low oxygen concentrations, as high levels are toxic to them. Obligate anaerobes cannot tolerate oxygen and grow only in its absence. Aerotolerant anaerobes grow equally well in both oxygen-rich and oxygen-poor environments.

How do aerobes protect themselves from reactive oxygen species (ROS)?

Aerobes generate reactive oxygen species (ROS) like superoxide (O₂⁻) and hydrogen peroxide (H₂O₂), which can damage cells. To protect themselves, aerobes produce enzymes such as superoxide dismutase, catalase, and peroxidase. Superoxide dismutase converts superoxide into oxygen gas (O₂) and hydrogen peroxide. Catalase then converts hydrogen peroxide into water (H₂O) and oxygen gas. Peroxidase also converts hydrogen peroxide into water, ensuring that these toxic ROS are neutralized and do not harm the cell.

What is the difference between obligate aerobes and obligate anaerobes?

Obligate aerobes require high levels of oxygen to grow and cannot survive without it. They use oxygen as the final electron acceptor in their electron transport chain. In contrast, obligate anaerobes cannot tolerate oxygen and grow only in its absence. Oxygen is toxic to obligate anaerobes, and they thrive in environments where oxygen is not present.

Why do facultative anaerobes grow better in the presence of oxygen?

Facultative anaerobes can grow in both the presence and absence of oxygen, but they grow better in the presence of oxygen because it allows for more efficient energy production. Oxygen enables the generation of more ATP through aerobic respiration, providing more energy for growth. In anaerobic conditions, facultative anaerobes switch to less efficient anaerobic respiration or fermentation, resulting in slower growth.

What are microaerophiles and where do they thrive?

Microaerophiles are microbes that require low concentrations of oxygen for growth. High levels of oxygen are toxic to them. They thrive in environments where oxygen is present in small amounts, such as specific layers of soil or water where oxygen diffusion is limited. Microaerophiles grow best in these microaerobic conditions, avoiding both high oxygen and completely anaerobic environments.