Unicellular organisms fall into four general categories that describe how these microbes react to the presence of oxygen:
AEROBES are microorganisms that require the presence of oxygen to live and reproduce themselves. Strict aerobes cannot survive in the absence of oxygen and produce energy only by oxidative phosphorylation. (Oxidative phosphorylation is a biochemical process in cells. It is the final metabolic pathway of cellular respiration in which energy, as ATP, is created in the cell’s mitochondria.)
AEROTOLERANT ANAEROBES are microorganisms that do not require the presence or oxygen to live and reproduce, and are not destroyed if oxygen is present. They generate ATP only by fermentation and have mechanisms to protect themselves from oxygen.
STRICT ANAEROBES, in most cases, generate their energy by fermentation or by anaerobic respiration and are always killed in the presence of oxygen. These organisms are also called “obligate anaerobes”. Obligate anaerobes vary greatly in their sensitivity to oxygen. Extremely oxygen-sensitive anaerobes, such as spirochetes and some Clostridium species, cannot tolerate even 0.5% oxygen. Thus, oxygen is toxic for them.
FACULTATIVE ANAEROBES prefer to grow in the presence of oxygen, using oxidative
phosphorylation, but can grow in an anaerobic environment using fermentation.
The most virulent and destructive pathogens that affect mankind generally fall into the “strict anaerobe” category. They include bacteria like Staphylococcus aureus, Streptococcus
pneumoniae, Clostridium botulinum and Escherichia coli. Viruses include Mycobacterium bovis, Herpesviridae and Influenza A virus/Orthomyxoviridae.
Oxygen has a tendency to form very reactive by-products, (including hydrogen peroxide [H2O2] and O2-superoxide,) inside a cell. These by-products create havoc by reacting with protein and DNA, thus inactivating them. Cells that are able to live in the presence of oxygen have enzymes, (like Superoxide Disutase, Catalase and Peroxidase,) that help them cope with H2O2 and O2- and thus are not destroyed by the presence of oxygen.
Oxygen’s anti-microbial mechanisms are not completely understood. It is known that the cell envelopes surrounding many pathogen’s, like bacteria, are made up of polysaccharides and proteins. In gram-negative pathogenic organisms, fatty acid alkyl chains and helical lipoproteins are present. In acid-fast bacteria, such as Mycobacterium tuberculosis, one third to one half of the capsule is composed of complex lipids, (esterified mycolic acid, in addition to normal fatty acids), and glycolipids (sulfolipids, lipopolysaccharides, mycosides, trehalose mycolates).

It is this high lipid content of the cell walls of these pathogenic bacteria that may explain their sensitivity, and eventual destruction, when exposed to oxygen molecules. Oxygen molecules penetrate these cellular envelopes and affect the cytoplasmic integrity of these pathogenic organisms. In addition, oxygen disrupts the metabolic activity of these disease-causing cells.
Unlike aerobic organisms, anaerobic organisms do not possess enzymes that are able to deactivate oxygen. Thus, reactive toxic molecules containing oxygen,
damage the cells’ structural integrity, stop the metabolic processes, and bring about cellular destruction and death.
As mentioned above, the outer cytoplasmic membranes of unicellular pathogens are composed of lipids, proteins, and lipoproteins. These membranes act as a diffusion barrier for water, ions and nutrients. Research indicates that the membranes are actually a lipid matrix containing randomly distributed globular proteins that penetrate through the lipid bilayer.
Oxygen reacts with the unsaturated fatty acids of the lipid layer in cellular membranes, forming hydro-peroxides. There is a synergistic effect with cellular- formed H2O2. Lipid peroxidation products include alkoxyl and peroxyl radicals, singlet oxygen, ozonides, carbonides, carbonyls, alkanes and alkenes.
Oxygen disrupts the integrity of the bacterial cell envelope through oxidation of the phospholipids and lipoproteins. In fungi, oxygen inhibits cell growth at certain stages. With viruses, the oxygen damages the viral capsid and disrupts the reproductive cycle by disrupting the virus-to-cell contact with peroxidation. The weak enzyme coatings on cells that make them vulnerable to invasion by viruses make them susceptible to oxidation and elimination from the body, which then replaces them with healthy cells.
Basically, oxygen disorganizes membrane permeability so that the organism’s nucleic acids and cations leak out and the cell dies.
In addition, oxygen destroys pathogens in a number of different ways: oxygen shortcircuits the processes by which pathogens create energy; oxygen disturbs the structure of the bacterial cell wall; oxygen also interferes with the production of essential proteins.