As oxygen gas enters into cellular reactions, it can be transformed into several toxic products. Singlet oxygen (1O2) is an extremely reactive molecule produced by both living and nonliving processes.
Notably, it is one of the substances produced by phagocytes to kill invading bacteria. The buildup of singlet oxygen and the oxidation of membrane lipids and other molecules can damage and destroy a cell. The highly reactive superoxide ion (O2−), peroxide (H2O2), and hydroxyl radicals (OH) are other destructive metabolic by-products of oxygen. To survive these toxic oxygen products, many microorganisms have developed enzymes capable of scavenging and neutralizing these chemicals. The complete conversion of superoxide ion into less harmful oxygen gas involves a two-step process and at least two enzymes:
In this series of reactions essential for aerobic organisms, the superoxide ion is first converted to hydrogen peroxide and normal oxygen by the action of an enzyme called superoxide dismutase. Because hydrogen peroxide is also toxic to cells (it is used as a disinfectant and antiseptic), it must be degraded by an enzyme— either catalase or peroxidase—into water and oxygen. If a microbe is not capable of dealing with toxic oxygen by these or similar mechanisms, it will be restricted to habitats free of oxygen.
With respect to oxygen requirements, several general categories are recognized. An aerobe (aerobic organism) can use gaseous oxygen in its metabolism and possesses the enzymes needed to process toxic oxygen products. An organism that cannot grow without oxygen is an obligate aerobe. Most fungi and protozoa, as well as many bacteria (genera Micrococcus and Bacillus), have strict requirements for oxygen in their metabolism.
A facultative anaerobe is an aerobe that does not require oxy gen for its metabolism and is capable of growth in its absence. This type of organism metabolizes by aerobic respiration when oxygen is present, but in its absence, it adopts an anaerobic mode of metabolism such as fermentation. Facultative anaerobes usually possess catalase and superoxide dismutase. A number of bacterial pathogens fall into this group. This includes gram-negative intestinal bacteria and staphylococci. A microaerophile does not grow at normal at mospheric concentrations of oxygen but requires a small amount of it (1%–15%) in metabolism. Most organisms in this category live in a habitat such as soil, water, or the human body that provides small amounts of oxygen but is not directly exposed to the atmosphere.
A true anaerobe (anaerobic microorganism) lacks the metabolic enzyme systems for using oxygen gas in respiration. Because strict, or obligate, anaerobes also lack the enzymes for processing toxic oxygen, they cannot tolerate any free oxygen in the immediate environment and will die if exposed to it. Strict anaerobes live in highly reduced habitats, such as deep muds, lakes, oceans, and soil.
Determining the oxygen requirements of a microbe from a biochemical standpoint can be a very time-consuming process. An initial clarification comes from cultures made with reducing media that contain an oxygen-removing chemical such as thioglycollate. The location of growth in a tube of fluid thioglycollate medium is a fair indicator of an organism’s adaptation to oxygen use (figure 1). Growing strictly anaerobic bacteria usually requires special media, methods of incubation, and handling chambers that exclude oxygen.
Fig1. Use of thioglycollate broth to demonstrate oxygen requirements. Thioglycollate is a reducing medium that can establish a gradation in oxygen content. Oxygen concentration is highest at the top of the tube and absent in the deeper regions. When a series of tubes is inoculated with bacteria that differ in O2 requirements, the relative position of growth provides some indication of their adaptations to oxygen use. Tube 1 (far left): aerobic (Pseudomonas aeruginosa); Tube 2: facultative (Staphylococcus aureus); Tube 3: facultative (Escherichia coli); Tube 4: obligate anaerobe (Clostridium butyricum). Terese M. Barta, PhD
Even though human cells use oxygen, and oxygen is found in the blood and tissues, some body sites present anaerobic pockets or microhabitats where colonization or infection can occur. Dental caries are partly due to the complex actions of aerobic and anaerobic bacteria in plaque. Most gingival infections consist of similar mixtures of oral bacteria that have invaded damaged gum tissues. Another common site for anaerobic infections is the large intestine, a relatively oxygen-free habitat that harbors a rich assortment of strictly anaerobic bacteria. Anaerobic infections can occur following abdominal surgery and traumatic injuries (gas gangrene and tetanus).
Aerotolerant anaerobes do not utilize oxygen gas but can survive and grow in its presence. These anaerobes are not harmed by oxygen, and some of them possess alternative mechanisms for breaking down peroxide and superoxide. For instance, lactobacilli, which are common residents of the intestine, inactivate these compounds with manganese ions.
Although microbes require some carbon dioxide in their metabolism, capnophiles grow best at higher CO2 concentrations (3%–10%) than are normally present in the atmosphere (0.033%). This becomes important in the initial isolation of some pathogens from clinical specimens, notably Neisseria (gonorrhea, meningitis), Brucella (undulant fever), and Streptococcus pneumoniae. Incubation is carried out in a CO2 incubator that provides the correct range of CO2. Keep in mind that CO2 is an essential nutrient for autotrophs, which use it to synthesize organic compounds.
