Transcript Oxygen Requirements

Bacteria Oxygen Requirements
Bacterial requirements for growth
1. Source of energy
2. Source of "organic" carbon
 sugars
 fatty acids
3. Source of metal ions (e.g. iron)
4. Optimal temperature
5. Optimal pH
6. The need or lack of need for oxygen
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Oxygen Requirements
Bacteria classified based on their oxygen requirements
into 5 main classifications:
1- Obligate or strict aerobes
• lack of oxygen kills the bacteria;
• ex. Bacillus & Pseudomonas
• they can not carry out fermentation
• they use O2 as a final electron acceptor in
aerobic respiration.
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2- Obligate or strict anaerobes
• do not carry out oxidative phosphorylation
• Obligate anaerobic may live by fermentation,
anaerobic respiration, bacterial photosynthesis.
• oxygen kills the bacteria; ex. Clostridium tetani
& Bacteroides
• lack certain enzymes
 superoxide dismutase
O2-+2H+ to H2O2
 catalase
(H2O2 to H2O + O2)
 peroxidases
(H2O2 to H2O using NAD to NADH)
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3- Aerotolerant anaerobes
• the bacteria don’t use oxygen, but oxygen
doesn’t harm them; ex. Lactobacillus
• They live by fermentation alone whether
or not O2 is present in their environment.
(respire anaerobically)
4- Facultative anaerobes
• can shift their metabolism (anaerobic if
oxygen is absent or aerobic if oxygen is
present);
• ex. E. coli, Staphylococcus
• Under anaerobic conditions (no O2)
they grow by fermentation or anaerobic
respiration, but in the presence of O2
they switch to aerobic respiration.
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5- Microaerophilic bacteria
• grow well in low oxygen concentrations
and higher carbon dioxide
concentrations; ex.
Campylobacter & Heliobacter pylori
• killed by higher concentrations of
oxygen
• mixture of 5 % O2 ,10 % CO2 and 85 %
N2
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Why is oxygen is harmful?
Toxic forms of oxygen
 Singlet oxygen-normal molecular oxygen that has been
boosted into high energy state.
 Superoxide free radicals ( O2-) - formed by all living
organisms during respiration.
 Peroxide anion-found in hydrogen peroxide.
 Toxic forms of oxygen
• It is a strong oxidizing agent
• It pulls electrons off other molecules
• toxic to cells
• must be neutralized
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Oxygen detoxification
 Toxic forms of oxygen need to be neutralized by enzymes
• Superoxide dismutase
• Catalase
• Peroxidase
 If microbe does not produce these enzymes, must have
anaerobic conditions.
 Superoxide dismutase (SOD)
• O2 + O2 + 2 H+ -- SOD --- H2O2
• SOD- neutralizes oxygen free radicals
 Catalase
• 2 H2O2 ------Catalase---- 2 H2O + O2
• Obligate anaerobes lack catalase and superoxide dismutasethey die in the presence of oxygen
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Oxygen detoxification
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The requirements for growth
SOD
• Obligate aerobes.
• Facultative
anaerobes.
• Aerotolerant
anaerobes.
• (Not obligate
anaerobes).
CATALASE
• Obligate aerobes.
• Facultative
anaerobes.
• (not aerotolerant
anaerobes).
• (Not obligate
anaerobes).
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The End
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