Bacterial Oxygen Classification, Metabolism and ROS Defense

Name: Aerobic And Anaerobic Bacteria | Bacterial Oxygen Requirements | Part 1
Uploaded: 2023-07-23T12:00:09+00:00
Duration: 31 min 19 s
Channel: Dr. Najeeb Lectures
Description: Summary and key takeaways on Bacterial Oxygen Classification, Metabolism and ROS Defense, covering to Bacterial Classification Classifying bacteria according

Dr. Najeeb Lectures

Jul 23, 2023

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31 min video

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YouTube video ID: HUVsv-F8w9U

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Classifying bacteria according to their relationship with oxygen clarifies how they cause disease and how clinicians can treat infections. Five categories emerge because bacteria differ in energy production and in self‑defense against reactive oxygen species. As one lecturer notes, “It’s very important to classify bacteria according to that because these five categories they behave differently in the pathogenesis of infections and diseases.”

Energy Metabolism and Oxygen

Oxygen serves as the final electron acceptor in aerobic respiration, making this pathway the most efficient way to generate ATP. Aerobic respiration therefore produces the greatest amount of cellular energy. In contrast, anaerobic fermentation represents the most primitive metabolism, yielding only a small ATP surplus through partial breakdown of nutrients. Anaerobic respiration occupies an intermediate evolutionary step: it retains the Krebs cycle and electron transport chain but substitutes inorganic or organic compounds such as nitrates, sulfates, elemental sulfur or fumarate for oxygen. Bacteria that retain both aerobic and anaerobic pathways can thrive in fluctuating environments.

Metabolic Pathways

Aerobic respiration follows the sequence: glucose → glycolysis → pyruvate → Krebs cycle → electron transport chain → oxygen (final electron acceptor) → water + ATP.
Anaerobic fermentation proceeds as: glucose → glycolysis → pyruvate → organic compounds (e.g., lactate, ethanol) that act as final electron acceptors, without involving the Krebs cycle or electron transport chain.
Anaerobic respiration follows: glucose → glycolysis → pyruvate → Krebs cycle → electron transport chain → non‑oxygen electron acceptors (nitrates, sulfates, sulfur, fumarate), producing ATP while avoiding oxygen.

Reactive Oxygen Species (ROS)

Exposure to oxygen inevitably generates reactive oxygen species, unstable radicals that damage DNA, RNA, proteins, and membrane lipids. “Every good thing has a dark side,” the instructor reminds, referring to oxygen’s toxic byproducts. Survival in oxygenated habitats therefore demands enzymatic defenses. Dismutases convert superoxide radicals into hydrogen peroxide; catalases and peroxidases further break hydrogen peroxide down into water and oxygen, neutralizing the threat and preserving cellular integrity.

Mechanisms & Explanations

The detoxification cascade relies on three key enzymes: superoxide dismutases, catalases, and peroxidases. Together they prevent ROS‑induced destruction of essential biomolecules. The Great Oxidation Event, driven by cyanobacteria about three billion years ago, raised atmospheric oxygen to roughly 21 % and forced the evolution of both aerobic metabolism and ROS‑detoxifying systems across the tree of life.

Takeaways

Bacterial classification based on oxygen relationship guides understanding of pathogenesis and treatment strategies.
Aerobic respiration yields the most ATP because oxygen serves as the final electron acceptor in the electron transport chain.
Anaerobic fermentation is the most primitive metabolism, producing minimal ATP through partial nutrient breakdown without the Krebs cycle or electron transport chain.
Anaerobic respiration occupies an intermediate evolutionary position, using the Krebs cycle and electron transport chain but employing non‑oxygen electron acceptors such as nitrates, sulfates or fumarate.
Survival in oxygenated environments requires detoxification enzymes—dismutases, catalases and peroxidases—to neutralize reactive oxygen species that damage DNA, proteins, lipids and membranes.

Frequently Asked Questions

What role did cyanobacteria play in the Great Oxidation Event?

Cyanobacteria performed oxygenic photosynthesis, releasing molecular oxygen that accumulated in the atmosphere about 3 billion years ago, marking the Great Oxidation Event. This rise in atmospheric O₂ created new ecological niches and forced many organisms to evolve aerobic metabolism or protective mechanisms against reactive oxygen species.

How do bacteria detoxify reactive oxygen species?

Bacteria produce enzymes such as superoxide dismutases, catalases and peroxidases that convert reactive oxygen species into harmless molecules; dismutases transform superoxide radicals into hydrogen peroxide, which catalases and peroxidases then break down into water and oxygen, preventing damage to cellular components.

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Helpful resources related to this video

If you want to practice or explore the concepts discussed in the video, these commonly used tools may help.

Microbiology Textbook With Bacterial Metabolism Chapters Recommended

Provides in-depth visual and textual explanations of metabolic pathways like the Krebs cycle and electron transport chain discussed in the lecture.

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High Quality Compound Light Microscope

Allows students to observe bacterial morphology and perform staining techniques to identify oxygen-related characteristics in a laboratory setting.

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Microbiology Laboratory Starter Kit

Includes essential tools for culturing bacteria, which is necessary for testing oxygen requirements and metabolic behaviors.

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Full Transcript YouTube

right today we are going to talk about bacteria 
and all of you know that it's very important  
to classify bacteria as if they are Obligate 
Anaerobes or they are Aerotolerant anaerobes or  
they are Facultative Anaerobe or Micro-Aerophilics 
or Obligate Aerobes. It's very important to  
classify bacteria according to that because 
these five categories they behave differently  
in the pathogenesis of infections and diseases 
and also their treatment by the drugs right or  
by the pharmacological agents is different 
for anaerobes and different for aerobes now  
before we classify bacteria into these subclasses 
the very important thing is we should know the  
relationship of bacteria with oxygen right this 
classification is based on how bacteria deal  
with oxygen now there are two aspect related 
with bacteria dealing with oxygen number one  
that do bacteria use oxygen in energy production 
or they do not use oxygen and energy metabolism  
this is one thing right when we are classifying 
bacteria in such way we must be very clear that  
what is the relationship of bacteria with oxygen  
in energy metabolism in production of energy or 
ATP right secondly second aspect which is also  
extremely important that we must know what 
is the relationship of bacteria with oxygen  
related to the their self defenses against the 
reactive oxygen sufficients as you know that  
wherever oxygen is present right it can react 
with different biologically active molecules  
right it may be ourselves animal cells or plant 
cells or even microbe cells when these cells are  
exposed to the oxygen right different biochemical 
reactions which I will teach you later in this  
lecture those biochemical reactions produce 
highly reactive unstable dangerous oxygen derived  
radicals or reactive molecules as a group those 
molecules are called reactive oxygen species  
right because reactive oxygen species are produced 
in any biological system which comes in touch with  
oxygen to survive in oxygenated environment and to 
multiply in oxygenated environment the biological  
system must know how to deal with reactive oxygen 
species how to detoxify reactive oxygen species so  
whole this lecture will revolve around initially 
about two basic concepts that energy metabolism  
and role of oxygen in bacteria and secondly what 
are the bacteria having type of defenses against  
the reactive oxygen species and then eventually 
we will in the end we will classify bacteria  
according to the upper classification let's come 
back and deal with first thing right that what  
is the role of oxygen in different type of 
bacteria energy metabolism we say basically  
that there is anaerobic energy production 
right and there is aerobic energy production  
when we say anaerobic energy metabolism we mean 
that basically when nutrients are broken down  
when nutrients are catabolized or broken down to 
release energy and capture energy right oxygen is  
used or not as final electron 
acceptor right let me briefly tell you  
when let's suppose we have glucose right if a 
bacteria gets glucose now to extract energy out  
of glucose bacteria has to bacterial metabolic 
pathway has to break down the glucose into its  
components right and energy should be 
released in the form of electrons and  
those electrons in the end should be accepted by 
final electron acceptor if final electron acceptor  
in energy metabolism is 
oxygen we say there is aerobic  
respiration right but if final electron 
acceptor is not oxygen then we say the  
energy producing mechanism is not aerobic it is 
anaerobic it is not aerobic it is anaerobic now
billions of the years back right about 3 billion 
years back right when bacteria start life started  
and bacteria were there originating 
during the evolution right initially  
Earth was not having enough oxygen rather in the  
very beginning in a very ancient times 
there was no oxygen in the atmosphere  
there were no oxygen in the atmosphere so the life 
forms life forms which were evolving at that time  
they have to evolve and develop such mechanisms 
to develop energy which are independent of oxygen  
right then a very big event occurred and we call 
it Big oxidation event in the evolution history  
what was that event a special type of bacteria 
came into existent we call them cyanobacterium  
now cyanobacteria were able to produce oxygen 
and over the millions of years they became so  
abundant that they produce huge amount of oxygen 
and now our atmosphere has about 21 percent oxygen  
is that right so basic thing is that when life 
evolved initially it evolved in the world where  
there was no free oxygen available so it means 
energy metabolism had to run without oxygen  
so they had to be anaerobic energy metabolism 
they had to be anaerobic energy metabolism but  
as organism evolved and cyanobacteria made lot 
of oxygen right then these primitive bacteria  
which were originally anaerobic metabolism 
running these bacteria had to evolve further  
right arrival of oxygen had one good news and 
one very bad news the good news was that if  
life forms could evolve in such a way that they 
could use oxygen in the energy metabolism as  
final electron acceptor then lot of ATP could 
be made lot of energy could be made right  
this was the advantage of using oxygen in energy 
metabolism it means the very big Advantage was  
that if bacteria could evolve in such a way 
that once oxygen is available right they should  
convert their anaerobic metabolism into aerobic 
metabolism and advantages they will produce lot of  
energy because anaerobic processes produce less 
energy less ATP and aerobic processes produce more  
energy right and if there's more energy there 
is more ATP then of course they could multi  
they could survive better they could multiply 
better and they could grow better so this was  
a big Advantage but oxygen also brought a very 
big disadvantage you know every good thing has a  
dark side these days the way we learn medical 
Sciences we are we think oxygen is always good  
but this is not true oxygen is good 
because when it is used in aerobic  
respiration it gives lot of energy so 
far for good but there is one trouble
the trouble is that exposure of metabolic pathways 
and biological system to oxygen produces lot of  
reactive oxygen species which I will explain 
later in this lecture those reactive oxygen  
species or reactive oxygen related molecules they 
react they are very unstable and they are very  
reactive and they damage DNA they damage RNA 
they damage proteins they damage lipids they  
destroy membranes and life is not compatible 
now we are in trouble what is the trouble  
that when living organisms microbes or plants 
or other or even animals when their cells are  
exposed to the oxygen right even though 
there is a big advantage that oxygen can  
produce help in producing lot of ATP but this 
is a disadvantage that oxygen can also produce  
reactive oxygen species and that can destroy 
the cells so what happened during the evolution  
all organisms were forced and selected in such 
a way that they should develop defenses against  
oxygen derived reactive radicals or molecules they 
should have defenses against what they should have  
defenses against reactive oxygen species and 
for those defenses many mechanisms evolved  
some of the mechanisms were they developed 
special enzymes which I will explain later  
like dismutases catalyzes peroxidases these 
enzymes could detoxify reactive oxygen species  
so in this way when our biological 
systems or microbes they developed  
evolved into such cells which were producing 
dismutases they were producing catalyzes  
they were producing peroxidases then they were 
able to neutralize or detoxify reactive oxygen  
species and then they could survive so what is 
the whole discussion up to this moment up to  
this moment I have only told you one thing that 
anaerobic organism anaerobic energy metabolism  
is a primitive metabolism less energy right 
once the oxygen appear into atmosphere oxygen  
new metabolic pathways develop to take the 
advantage of the presence of oxygen and  
those mechanisms were more efficient and oxygen 
dependent energy metabolism were producing high  
amount of energy ATP this was very good and the 
bad thing was that once oxygen developed and such  
cells or microbes when they were exposed to the 
oxygen reactive oxygen species dangerous molecules  
were made and if these organisms have to survive 
in the aerobic environment then or then they must  
develop defenses against reactive oxygen species 
which I will mention later in detail those are  
dismutases, catalyzes and peroxidases so first 
let me discuss about the differences in these  
mechanism let me tell you first of all aerobic 
respiration aerobic respiration is the most modern  
cellular respiration it's the most evolved 
mechanism it's the most efficient mechanism  
right and all of you know it very well 
but I will just review it very briefly  
that let's suppose if you are going to use 
the aerobic metabolism and you are going to  
utilize glucose as a substrate to make 
energy then you know that through the  
process of glycolysis glucose is broken down into 
Pyru...vate into pyruvate and this process where  
glucose is broken down into pyruvate this 
process is called glycolysis glyco....lysis
then you know that once pyruvate is made then it 
is taken into mitochondria right and there are  
Krebs cycle going on I will just make Krebs cycle 
here right it's a very simple diagram this is  
Krebs cycle and pyruvate will enter into it  
right and it will pass through 
Krebs cycle which is also called
right so this is Krebs cycle now after 
that electrons which are released from  
the breakdown of glucose and its breakdown 
products they are taken to what is this  
electron transport chain and eventually they 
will be going to the final electron acceptor  
right and in Aerobic metabolism final electron 
acceptor is yes this is oxygen molecular oxygen  
molecular oxygen so what we say that aerobic 
cellular aerobic respiration is the most  
developed most modern most efficient right 
why because it produces maximum amount of ATP  
it consists of glycolysis then Krebs 
cycle then electron transport chain
Krebs cycle and then electron transport chain and 
once the while we are breaking down these products  
finally these products will be glucose and its 
derivative products will be eventually broken down  
into carbon dioxide and water and lot of energy 
will be produced and that energy will be trapped  
in the form of ATP but when these glucose and its 
breakdown products are breaking down progressively  
they are releasing electrons and these electrons 
will pass through electron transport chain and  
eventually these electrons will be accepted by 
molecular oxygen these electrons will be accepted  
by molecular oxygen and this oxygen then will 
react with protons and make water with hydrogen  
react with hydrogen and make water and energy 
will be trapped I will not go into detail of the  
pathway now this is fully developed structure 
how it Compares with anaerobic fermentation  
and anaerobic respiration that is aerobic 
respiration now the most most ancient  
pathway primitive Pathway to produce energy 
by the Microbes was anaerobic fermentation  
this is the most primitive of most I must say 
rudimentary metabolic Pathway to produce ATP  
then there were little more advanced that 
was still anaerobic pathway right and that  
is anaerobic respiration now what is the 
difference between anaerobic fermentation  
and anaerobic respiration let me explain in 
anaerobic fermentation if glucose is taken right  
glucose will pass through which is passing through 
glycolysis and making pyruvate pyruvic acid now  
this glycolysis pathway is common 
in all of them again here is also  
glucose and going through glycolysis pathway and 
eventually producing pyruvate now this pathway  
right this is common in all three of them it means 
it is present in anaerobic fermentation process  
glycolysis is also present in anaerobic 
respiration cellular respiration and it  
also it is also present in aerobic cellular 
respiration now some differences will occur  
actually in case of anaerobic fermentation there 
is no Krebs cycle there is no Krebs cycle there  
is no electron transport chain and as a final 
electron acceptor oxygen is not used right is not  
used no Krebs cycle no electron transport chain 
right and not utilization of oxygen as a final  
so what happens in anaerobic fermentation 
that nutrients are partially broken down  
right and this last product of glycolysis 
undergoes some other reaction and it  
can be converted let's suppose it 
can be converted into lactic acid  
lactate or it can be converted or 
it can be converted into ethanol
and carbon dioxide and what is the final electron 
acceptor here it is not oxygen that is why it is  
called anaerobic pathway because those metabolic 
energy Pathways which do not use oxygen as the  
final electron acceptor they are said anaerobic 
pathway now in anaerobic fermentation the final  
oxygen sorry final electron acceptor 
are organic compounds organic  
compounds and these compounds can be 
pyruvate pyruvate or they can be acetaldehyde
acetaldehyde  
now these are the final what is this electron 
acceptor right so electrons come over here now
we move to anaerobic this produces very little ATP 
why it produces so little ATP because nutrient is  
only partially broken down it is not completely 
broken down right now we move to another anaerobic  
mechanism of producing energy this mechanism is 
called anaerobic respiration now what I'm going  
to do I'm going to tell you the difference between 
anaerobic fermentation and anaerobic respiration  
in anaerobic respiration actually their glycolysis 
is there and also what is this Krebs cycle is also  
there even electron transporter chain is 
also there transport chain is also there
so but there's a big change the final electron 
acceptor is not oxygen that is why it is called  
anaerobic respiration not aerobic cellular 
respiration here the final electron acceptor
is yes not oxygen these are usually nitrates 
or sulfates or Elemental sulfur or Fumarate
if you cannot remember them no problem at least 
remember that in anaerobic respiration the final  
electron acceptor is not oxygen in anaerobic 
fermentation final electron acceptor is not  
oxygen because in these two condition oxygen 
is not the final electron acceptor that is why  
they are called anaerobic but they are 
even having difference from each other  
out of them anaerobic fermentation is most 
primitive anaerobic respiration is slightly  
more evolved and aerobic respiration is most 
evolved most advanced this has only glycolysis  
right fermentation has only anaerobic fermentation 
has only glycolysis no Krebs cycle no electron  
transport chain no utilization of oxygen at the 
end is that right it produces electron acceptor  
pyruvate or acetaldehyde because this partial 
breakdown so there is very little ATP made  
right then later on bacteria evolved more 
right and more advanced anaerobic mechanism was  
anaerobic respiration now here glycolysis 
mechanism is there Krebs cycle is there  
electron transport chain is there right 
so it is much evolved much better evolved  
as compared to fermentation right but anaerobic 
fermentation but still it is considered anaerobic  
even though it has glycolysis as a Krebs cycle 
electron transport chain still it is considered  
anaerobic mechanism because it is not using 
oxygen as a terminal electron acceptor it is using  
some other inorganic or sometimes organic 
compound inorganic like nitrate sulphate or  
Elemental sulfur or fumarate right but then 
is the most advanced mechanism what is that  
aerobic respiration aerobic cellular 
respiration has glycolysis pathway as our cells  
glycolysis pathway they are having Krebs cycle 
they have electron transport chain and in the end  
as final electron acceptor we use oxygen 
and this helped to produce lot of energy  
right so now we will see some of the bacteria 
will have only energy producing only by anaerobic  
mechanisms right and we will talk about later 
there are some bacterias which have evolved  
further and they are having anaerobic mechanisms 
still with them and also they are having aerobic  
mechanism of energy production now if a bacteria 
has energy production only by anaerobic mechanism  
and bacteria cannot use oxygen for energy 
purpose it means such bacteria which are  
having only anaerobic mechanisms like they 
can do anaerobic fermentation or anaerobic  
respiration such bacteria will only able to 
survive and multiply in anaerobic environment  
for example medically such bacteria are present in 
big number between the teeth and gingival crevices  
this is anaerobic environment right then anaerobic 
are also present in Colon large intestine  
right because those bacteria know how to survive 
without oxygen right those bacteria know that they  
can produce energy in an anaerobic fashion is that 
right then there is another group of bacteria we  
will talk later they are really very very sharp 
you know what they are doing they have evolved  
fully to develop the aerobic respiratory 
pathway but the clever thing is they still  
maintain anaerobic metabolic energy 
pathways so if this is a bacteria  
which is maintaining its primitive metabolic 
pathway for energy like anaerobic fermentation  
as well as anaerobic respiration and it is 
evolved so much that it has also evolved  
the biological mechanism right for aerobic 
cellular respiration this is a very lucky  
bacteria because if environment is if oxygen 
is not present still it can survive well  
it can produce enough energy at least by anaerobic 
mechanisms and if there is oxygen available then  
it will multiply more efficiently in the presence 
of oxygen as oxygen is providing at the end  
with more ATP but if we let's suppose there is 
another bacteria this is very primitive bacteria  
in its biochemical reactions that it can only do 
anaerobic fermentation and anaerobic respiration  
right but it cannot use the molecular oxygen 
as final electron acceptor then this bacteria  
Will Survive well in anaerobic environment 
and multiply well in anaerobic environment  
but this bacteria cannot take extra advantage 
of oxygen if oxygen is present it cannot take  
the advantage of oxygen to utilize oxygen as a 
more efficient energy producing mechanism right  
so this was one thing right the relationship 
of metabolism energy metabolism of bacteria  
and the relationship with oxygen right 
then second relationship with oxygen that  
is extremely important as I mentioned in the 
beginning of the lecture that when microbes are  
exposed it does not matter microbe is having what 
energy metabolism once they are exposed to oxygen  
right they will make lot of reactive oxygen 
species which I will explain that how it is  
after the break I will explain how reactive oxygen 
species are made and reactive oxygen species are  
such atoms, molecules or radicals which are oxygen 
derived but they are very very reactive they  
will react with DNA they will react with proteins 
they will react with the what is this your lipids  
right so they are able to destroy nucleic acids 
DNA and RNA they can destroy proteins they can  
destroy lipids and of course if they keep on doing 
these things they destroy the cell or microbes  
so as more and more oxygen was available in 
the atmosphere right bacteria had to evolve  
the mechanism to develop a defense against 
reactive oxygen species bacteria were forced  
to develop defenses against reactive  
oxygen species let's have a break and after 
that we'll deal with four things number one  
in the presence of oxygen how reactive 
oxygen species are made number one number two  
why they are so dangerous number three that how 
bacteria more evolved bacteria have developed  
defenses against the reactive oxygen species 
like enzymes dismutases, catalyzes, peroxidases  
right and I will also discuss those bacteria 
which fail to develop these defenses against  
reactive oxygen sufficient what kind of troubles 
those bacteria have let's have a break now.