Bacteria Oxygen Requirements

Mohammed laqqan 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

Mohammed laqqan 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.

Mohammed laqqan Obligate or strict anaerobes 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 O 2 -+2H+ to H 2 O 2  catalase (H 2 O 2 to H 2 O + O 2 )  peroxidases (H 2 O 2 to H 2 O using NAD to NADH)

Mohammed laqqan Aerotolerant anaerobes 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 O 2 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 O 2 ) they grow by fermentation or anaerobic respiration, but in the presence of O 2 they switch to aerobic respiration.

Mohammed laqqan 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 % O 2,10 % CO 2 and 85 % N 2

Mohammed laqqan

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 ( O 2 - ) - 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

Mohammed laqqan 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 --- H 2 O 2 SOD- neutralizes oxygen free radicals  Catalase 2 H 2 O Catalase H 2 O + O 2 Obligate anaerobes lack catalase and superoxide dismutase- they die in the presence of oxygen

Mohammed laqqan Oxygen detoxification

Mohammed laqqan 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).

Mohammed laqqan The End