Lecture 3 - Metabolism and bacterial pathogenesis: respiration and stress Flashcards
What determines the environment that is occupied by a bacterial species?
Depends on the metabolic capabilities of that species -the microbe can grow if it has the appropriate metabolic machinery However: the microbe may affect its environment to suit its needs
Describe the relationships as demonstrated by the three arrows on this image of a microbe interacting with its host and environment
- The micro-organism interacting with its abiotic environment. Bacteria recurit carbon sources from the environment and change their environment e.g. A large number of bacteria present in an environment would alter the oxygen levels available to bacteria
- The micro-organism interacting with other bacteria in its environment. May have a competitive or co-odinated relationship
- The micro-organism interacting with its host. May be seen as a 2-way process but in reality whilst the microbe recieves things from its host it also actively changes the environment of the host to suit its needs, e.g. resulting in tissue degradation or diahrea (Shigella dysenteriae)
Give an example of a major determinant of pathogen nutrition and lifestyle
Oxygen availability
How is oxygen availability a major determinant of pathogen nutrirtion and lifestyle?
- the human body is carbon rich (therefore an attractive site for micro-organisms) and therefore the oxygen levels often become a limiting factor for growth
- Each cell of a species has the same demand for oxygen but as they grow and multiply, diffusion is unable to supply sufficient oxygen to support the metabolism of carbon substrates to CO2
-In this way, the bacterial metabolism has defined the environmental oxygen limitation
- colon, oral cavity
- O deficient inside biofilms
Give an example of a site in the human body where Oxygen is often a limiting factor in microbial nutrition and lifestyle
In the large intestine
In the oral cavity
Aside from locations within the human body, where does low oxygen availability also occur
Inside bacterial biofilms
What is respiration?
A highly efficient way of conserving biological energy using ATP from Glucose
Typically with the intake of oxygen and the release of carbon dioxide from the oxidation of complex organic substances.
What is the basic process of respiration?
Glucose is converted to pyruvate which enters the citric acid cycle to produce Carbon dioxide and NADH which is oxidised to produce:
Probably 38 ATP per 1 glucose
but possibly
30 ATP per 1/2 glucose
What is oxidation ?
The gain of oxygen or the loss of electrons
What is reduction?
The loss of oxygen or the gain of electrons (reduction in the charge)
What occurs (in respiration) in the absence of oxygen?
In the absence of O2 all NADH that would otherwise have been reduced accumulates and remains as NADH
Bacteria may use the process of fermentation instead
When does fermentation occur?
When there is no net electron acceptor (O2) and the cell cannot dispose of the accumulated NADH from the citric acid cycle
- cell must instead recycle the NADH
- results in a limited ability to make ATP
What is the process of fermentation?
- Glucose is converted to Pyruvate in the process of glycolysis, creating one ATP molecule and an NADH, which cannot be used as there is no end electron acceptor
- Pyruvate can then either be:
a. Converted to lactate and NADH oxidised to NAD+
b. Reduced to acetate and ethanol
c. Reduced to just ethanol - These processes return a smaller amount of ATP compared to aerobic respiration
What is produced when Glucose is converted to Lactate?
This is a redox balanced system in that:
1 glucose results in 1 lactose molecule and 1 ATP molecule
Why is aerobic respiration good?
Can makes lots of ATP from one glucose molecule, 38 ATP from 1 glucose molecule
-probably-
Basically a lot more than fermentation
Why is fermentation not as good as respiration?
Produces only 2 ATP per glucose molecule compared to the 38 produced by respiration
-Probably-
How is motive force generated by oxygen respiration?
- redox reductions are coupled to proton motive force
- proton motive force is coupled to ATP production
Where is the arrangement to generate proton motive force found?
Found in the mitochondria and many bacteria
How does the respiratory chain differ in E.coli?
E.coli has a simplified version of the respiratory charin
- no bc1 complex or cytochrome c
- retains NADH dehydrogenase, quinol and oxidase (oxygen reductase)
What are essentials of the stress response in bacteria?
Dispatch, destroy, diversify or die
And
Respond and regulate
Explain the dispatch, destroy, diversify or die response of bacteria to stress
Dispatch and destroy: When bacteria use enzymes or a release of proteins to mediate stress e.g. enzymes to degrade antibiotics, proteins to stabilse structures under heat stress
Diversify: When bacteria alter themselves in response to a stress to be able to survive under that stressor
What are the three ways a bacteria can react to stressors?
- Use enzymes to degrade stressor e.g. antibiotic
- transport stressor out of the cell
- Alter themselves
What is oxidative stress?
Every aerobic organism undergoes oxidative stress when electrons leak from the respiratory chains and catalyse O2 to O2- . These are the highly toxic intermediates of the conversion of O2 to water and include superoxide O2- peroxide O22- and an hydroxyl radical OH. . These are short lived radicals with an unpaired e- . These can be formed internally (O2 to H2O, or in a macrophage oxidative burst) or taken up externally.
What are the possible origins of oxidative stress?
- leaking of e- from the respiratory chain catalysing O2 to O2-
- Generated purposely by host macrophages (the oxidative burst by NADH oxidase)
- By other bacteria e.g. Streptococcus which produces pyruvate oxidase to outcompete other bacteria
How does oxidative stress cause damage?
The short lived oxygen radicals are highly reactive and can cause cellular damage. Superoxide can react with itself to form peroxide which then binds to amino acids and results in DNA damage
How is macrophage attack a source of oxidative stress?
- Macrophages identify and kill invading pathogens through the innate immune response
- Get the generation of an oxygen superoxide (O2-) via the NADPH oxidase in an oxidative burst
- generation of radical nitric oxide (NO-) by inducible NO synthase (nitosative burst)
What is the response of enteric bacteria to the toxic intermediates superoxide, peroxide and hydroxyl radicals?
Have sensory systems for superoxide (SoxRS) and peroxide (OxyR) to up-regulate superoxide dismutase and peroxidases/catalases
When does nutrient depravation occur in bacteria?
In the absence of sufficient oxygen, bacteria may then become nutrient starved
How do facultative anaerobes (e.g. the enteric bacteria) respond to nutrient depravation?
Regulate gene expression to synthesise alternative respiratory pathways under anearobic conditions
e.g. the anaerobic regulator in E.coli is FNR in E.coli
E.coli use up oxygen first then must switch on the expression of other enzyme genes using FNR
What is the FNR enzyme in bacteria?
The fumerate and nitrate reductase regulator
How does the FNR enzyme act?
- Aerobically, FNR exists as a monomeric protein
- Anaerobically, FNR dimerises and contains an oxygen sensitive iron sulfur cofactor
- FNR binds to promoters and activiates the expression of genes required under anaerobic conditions
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