Oxygen profoundly affects bacterial growth, with its presence or absence determining which types of bacteria can thrive.
Different bacterial species have evolved various strategies for dealing with oxygen, resulting in several categories:
-
Strict Aerobes: These bacteria require oxygen for growth. They use oxygen as the final electron acceptor in aerobic respiration, a process that generates energy (ATP). Without oxygen, these bacteria cannot produce sufficient energy and will not grow. An example is Mycobacterium tuberculosis, the bacterium that causes tuberculosis.
-
Strict Anaerobes: Oxygen is toxic to these bacteria. They lack the enzymes to detoxify harmful reactive oxygen species (ROS) like superoxide radicals and hydrogen peroxide, which are produced in the presence of oxygen. They use anaerobic respiration or fermentation for energy production. Clostridium botulinum, the bacterium that produces botulinum toxin, is a strict anaerobe.
-
Facultative Anaerobes: These bacteria can grow with or without oxygen. In the presence of oxygen, they will use aerobic respiration, which is the most efficient energy-producing pathway. In the absence of oxygen, they can switch to anaerobic respiration or fermentation. Escherichia coli is a well-known facultative anaerobe.
-
Microaerophiles: These bacteria require oxygen for growth, but at lower concentrations than those found in the atmosphere (typically 2-10% oxygen). Higher concentrations of oxygen can be toxic to them. Helicobacter pylori, which causes stomach ulcers, is a microaerophile.
-
Aerotolerant Anaerobes: These bacteria do not require oxygen for growth, but they can tolerate its presence. They don't use oxygen for metabolism but possess enzymes that allow them to detoxify ROS. They typically use fermentation for energy production. Streptococcus species are often aerotolerant.
The effect of oxygen on bacterial growth is primarily determined by a bacterium's metabolic pathways and its ability to detoxify ROS. The presence or absence of oxygen dictates which bacteria can colonize specific environments.
