Oxygen Requirements and Nitrate Test

  • Oxygen Tolerance and Bacterial Classification

    • Understanding different oxygen requirements helps in identifying bacterial species.
    • Classified based on their ability to tolerate or require oxygen:
    • Obligate Aerobes:
      • Require oxygen for growth.
      • Participate in aerobic respiration.
      • Grow at the top of culture media where [O2] is high.
      • Have both catalase and superoxide dismutase to detoxify harmful forms of oxygen.
    • Facultative Anaerobes:
      • Prefer aerobic conditions but can switch to anaerobic respiration or fermentation when oxygen is absent.
      • Grow throughout the medium but denser at the top due to higher ATP yield from aerobic respiration.
      • Also have catalase and superoxide dismutase.
    • Obligate Anaerobes:
      • Cannot survive in oxygen; even small amounts are lethal.
      • Grow only at the bottom of culture media.
      • Lack catalase and superoxide dismutase.
    • Microaerophiles:
      • Survive in lower than atmospheric oxygen levels (less than 21% O2).
      • Grow in the upper middle region of the medium.
    • Aerotolerant Anaerobes:
      • Do not utilize oxygen but are not affected by its presence.
      • Grow uniformly throughout the medium.
      • Possess both catalase and superoxide dismutase.

  • Testing Oxygen Tolerance

    • Fluid Thioglycollate Broth:
    • Used to assess oxygen tolerance in bacterial species.
    • Sodium thioglycolate and L-cystine reduce oxygen to water, assisting in the creation of an oxygen gradient.
    • Resazurin serves as the oxygen indicator; red when oxidized, colorless when reduced.

  • Methods for Creating Anaerobic Environments

    • Gas Pak Jar:
    • Contains a packet with ascorbic acid, reducing O2 to create an anaerobic environment (0.1% O2).
    • Candle Jar:
    • A lit candle consumes some oxygen and creates a microaerophilic environment (approx. 10% O2).
    • The combustion process generates CO2, beneficial for capnophilic microorganisms.

  • Reactive Oxygen Species (ROS) and Detoxification

    • During aerobic respiration, toxic reactive oxygen species can be produced.
    • Superoxide Radicals (O2-): Produced when oxygen is only partially reduced.
    • Hydrogen Peroxide (H2O2): Can form when electrons are transferred directly to oxygen.
    • Bacteria produce enzymes to detoxify ROS, such as:
    • Superoxide Dismutase: Converts O2- to H2O2.
    • Catalase: Converts H2O2 into harmless water and oxygen.

  • Catalase Test

    • Determines the presence of catalase in bacteria.
    • Reactions:
    • Magically distinguishes catalase + organisms (e.g., Micrococcus) from catalase - (e.g., Streptococcus).
    • Equation: 2H2O2→Catalase2H2O+O22H2O2 \xrightarrow{Catalase} 2H2O + O2

  • Nitrate Reduction Tests

    • Participants can reduce nitrate (NO3-) during anaerobic respiration.
    • Steps in the test:
      • Test for Nitrogen Gas (N2):
      • Check for gas in the Durham tube. Indicates presence of both enzymes (nitrate & nitrite reductase).
      • Test for Nitrite (NO2-):
      • If no gas, add Nitrate Reagents A & B. A red color indicates nitrate reductase; clear indicates need for further testing.
      • Test for Nitrate (NO3-):
      • If still clear after reagents, add zinc to the tube to reduce any unreacted nitrate to nitrite.
      • Color change confirms presence of nitrate. \n - Different bacteria can perform dissimilatory (reducing NO3- to N2) or assimilatory (reducing NO3- to NH4+) nitrate reduction.
    • Final step of nitrate reduction confirms if ammonium (NH4+) is present after eliminating alternatives.