General Microbiology Laboratory Bacteria Oxygen Requirements Bacterial requirements

General Microbiology Laboratory Bacteria Oxygen Requirements

Bacterial requirements for growth v Source of energy v Source of "organic" carbon § § sugars fatty acids v Source of metal ions (e. g. iron) v Optimal temperature v Optimal p. H v The need or lack of need for oxygen

Oxygen Requirements v. 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 O 2 as a final electron acceptor in aerobic respiration.

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 v superoxide dismutase • O 2 -+2 H+ to H 2 O 2 v catalase • (H 2 O 2 to H 2 O + O 2) v peroxidases • (H 2 O 2 to H 2 O using NAD • to NADH)

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.

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

Why is oxygen is harmful? Toxic forms of oxygen v Singlet oxygen-normal molecular oxygen that has been boosted into high energy state. v Superoxide free radicals ( O 2 -) - formed by all living organisms during respiration. v Peroxide anion-found in hydrogen peroxide. v Toxic forms of oxygen • It is a strong oxidizing agent • It pulls electrons off other molecules • toxic to cells • must be neutralized

Oxygen detoxification v • • • v 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. v Superoxide dismutase (SOD) • O 2 + 2 H+ -- SOD --- H 2 O 2 • SOD- neutralizes oxygen free radicals Note: ok, but hydrogen peroxide also toxic… v Catalase • 2 H 2 O 2 ------Catalase---- 2 H 2 O + O 2 • Obligate anaerobes lack catalase and superoxide dismutase-they die in the presence of oxygen

Oxygen detoxification

The requirements for growth v SOD • • Obligate aerobes. Facultative anaerobes. Aerotolerant anaerobes. (Not obligate anaerobes). v CATALASE • Obligate aerobes. • Facultative anaerobes. • (not aerotolerant anaerobes). • (Not obligate anaerobes).

End of lecture