Some bacteria are aerobic, that is, they require oxygen for growth. Other grow best when oxygen concentration is low (microaerophilus).
Still others can grow either in the present or absence of oxygen (facultative aerobes or facultative anaerobes).
A number of bacteria species will not grow in the presence of oxygen because it is toxic to them, and these are called anaerobes.
It should be pointed out that while oxygen is toxic to anaerobic bacteria, they sometimes can grow under conditions that appear to include the presence of oxygen.
The explanation for this is that organic material contain compounds that themselves tie up the available oxygen.
Also in many instances, aerobic bacteria first grow and consume the oxygen and produce compounds that also combine with available oxygen, thus producing conditions suitable for the growth of anaerobic bacteria.
In contrast to this, certain aerobic bacteria can utilize the oxygen in certain oxygen bearing-compounds, such as nitrates.
Yeasts grow best in aerobic conditions, but some grow anaerobically (e.g. fermentative types) although more slow.
Oxygen Requirements by Microorganisms
