Bacterial Taxonomy 2 – Classification of Bacteria Based on Metabolic Characteristics
We look at plants and animals, and agree that we all need oxygen to survive and metabolize. So what about bacteria? Bacteria must metabolize in some way, right?
Bacteria can be divided into 2 major groups, based on their metabolic properties. The two most important metabolic properties used to classify bacteria into groups include:
1) How the organism deals with oxygen.
2) What the organism used as a carbon and energy source.
There are other properties used for classification, such as the different metabolic end products that bacteria produce, but these are less important.
How Bacteria Deal With Oxygen
Oxygen, is in fact, a very dangerous chemical. We know that it is highly reactive, and easily reacts with electrons to be oxidized into dangerous radicals such as hydrogen peroxide (H2O2), superoxide radicals, (O2-), and hydroxyl radicals (•HO).
These chemicals, being very dangerous and reactive, must quickly be removed from the body. It is clear that as humans, we adore oxygen – we need it to live after all! So we obviously have ways to protect against these radicals. This protection comes in the form of a 3 enzyme defense system (man, that sounds cool).
Catalase: Catalase breaks down hydrogen peroxide (H2O2) in the following reaction:
Peroxidase: Peroxidase also breaks down Hydrogen Peroxide (H2O2). It’s in the name, after all! Peroxidase requires an electron donor molecule, and then transfers the electron over from the electron donor to the H2O2 to form water.
Superoxide Dismutase: Superoxide Dismutase (SOD) breaks down the Superoxide radical (O2-). …. like a superhero fighting a supervillain? No? Okay.
After producing H2O2 in the SOD reaction, peroxidase and catalase can take over and reduce the superoxide into something harmless.
I’ll just leave this here….
So we’ve established that humans clearly love oxygen. Thus, we possess all 3 enzymes necessary to keep oxygen in check and keeping us alive.
Bacteria, on the other hand, are arranged on a spectrum, based on how well they are able to react to oxygen radicals when they do appear. On one end of the spectrum, are bacteria that have all 3 enzymes and will die in the absence of oxygen. On the other end, are organisms that have none of the required enzymes, and die pretty much immediately even in the presence of oxygen.
This spectrum is as follows:
Obligate aerobes have all the faculties to carry out oxidative phosphorylation to obtain energy with oxygen quite perfectly. They use glycolysis, the Krebs Cycle and the electron transport chain, just as we do, to obtain the energy they need for their metabolism.
Obviously then, all obligate aerobes contain all 3 enzymes – Catalase, Peroxidase and Superoxide Dismutase.
Noteworthy is the fact that they contain no faculty to carry out anaerobic respiration, and thus, they will definitely die in the absence of oxygen.
Bacteria that are obligate aerobes include:
- Bacillus cereus
- Leptospira Interrogans
- Branhamella catarrhalis
- Burkholderia cepacia
- Francisella tularensis
- Spirillum minus
- Coxiella burnetti
Facultative anaerobes are the closest analogy to humans. They are able to carry out aerobic respiration quite perfectly, possessing both superoxide disputes and catalase (not peroxidase). However, their most noteworthy feature is that they are also able to carry out anaerobic respiration.
Thus, they are mainly aerobic, but they have the faculty to carry out anaerobic respiration. This is why they are called facultative anaerobes. When the need arises, they have the faculty to carry out fermentation to obtain energy in the absence of oxygen. This is very similar to the anaerobic respiration carried out by human muscle cells during strenuous activity like sprinting.
Facultative Anaerobes include:
- Bacillus anthracis
- Most other gram negative rods
To know the facultative anaerobes, you must know a list of your ABC’S.
List – Listeria
A – Actinomyces
B – Bacillus anthracis
C – Corynebacterium
‘S – Staphylococcus
Microaerophilic bacteria are aerobic bacteria that require only a very small amount of oxygen to survive, and are poisoned by excessively high oxygen tension.
This is because they only have 1 defense enzyme, our superhero, Superoxide Dismutase (but no catalase or peroxidase).
The microaerophilic bacteria include:
- some Streptococci (although some species of streptococci are facultative anaerobes)
- Helicobacter pylori
- Leptospira (except Leptospira interrogans)
These guys.. they really don’t like oxygen. You can imagine since they’re on the extreme end of the spectrum. They have no electron transport chain, and have no enzymes to prevent against oxidative stress. Thus, if they are exposed to oxygen, they due.
Obligate Anaerobes include:
- Streptobacillus moniliformis
There is also a division of obligate anaerobes, known as aerotolerant anaerobes. These bacteria require no oxygen as they respire anaerobically, but unlike obligate anaerobes, they CAN survive in the presence of oxygen.
NOTE: Rickettsia and Chlamydia are both energy parasites, and steal their host’s ATP rather than utilizing aerobic or anaerobic respiration. These organisms are known as obligate intracellular organisms, and live in host cells.
The most important of these bacteria are listed in the table below.
(courtesy Clinical Microbiology Made Ridiculously Simple, 6th ed.)
How Bacteria Deal with their Carbon and Energy Source
Some organisms use light as an energy source (phototrophs), and some use chemical compounds as an energy source (chemotrophs).
Of the chemotrophs, organisms that use inorganic sources such as ammonium and sulphide are autotrophs. Those that use organic carbon sources are called heterotrophs.
However, most bacteria, and in fact, all medically important bacteria are chemoheterotrophs, because they use both chemical and organic compounds such as glucose for energy.
That’s all guys! Hope you got all of this! It may be a lot of bacteria to remember, but the most important ones you should keep in mind are summarized neatly in the table just before I speak about classification based on Carbon and Energy Source.
Hope this is of good use to you guys!