Obligate aerobe in the context of "Bacillus subtilis"

An aerobic organism or aerobe is an organism that can survive and grow in an oxygenated environment. The ability to exhibit aerobic respiration may yield benefits to the aerobic organism, as aerobic respiration yields more energy than anaerobic respiration. Energy production of the cell involves the synthesis of ATP by an enzyme called ATP synthase. In aerobic respiration, ATP synthase is coupled with an electron transport chain in which oxygen acts as a terminal electron acceptor. In July 2020, marine biologists reported that aerobic microorganisms (mainly), in "quasi-suspended animation", were found in organically poor sediments, up to 101.5 million years old, 250 feet below the seafloor in the South Pacific Gyre (SPG) ("the deadest spot in the ocean"), and could be the longest-living life forms ever found.

Thioglycolate broth is a multipurpose, enrichment, differential medium used primarily to determine the oxygen requirements of microorganisms. Sodium thioglycolate in the medium consumes oxygen and permits the growth of obligate anaerobes. This, combined with the diffusion of oxygen from the top of the broth, produces a range of oxygen concentrations in the medium along its depth. The oxygen concentration at a given level is indicated by a redox-sensitive dye such as resazurin that turns pink in the presence of oxygen.

This allows the differentiation of obligate aerobes, obligate anaerobes, facultative anaerobes, microaerophiles, and aerotolerant organisms. For example, obligately anaerobic Clostridium species will be seen growing only in the bottom of the test tube.

Obligate anaerobes are microorganisms killed by normal atmospheric concentrations of oxygen (20.95% O₂). Oxygen tolerance varies between species, with some species capable of surviving in up to 8% oxygen, while others lose viability in environments with an oxygen concentration greater than 0.5%.

Obligate anaerobes, which die when normal amounts of oxygen are present, are contrasted with obligate aerobes, which die without oxygen. Bacteria that fall in between these two extremes may be classified as either facultative anaerobes, which can use oxygen but also survive without it, or microaerophiles, which need lower levels of oxygen. Aerotolerant organisms are indifferent to the presence or absence of oxygen.

A facultative anaerobic organism is an organism that makes ATP by aerobic respiration if oxygen is present, but is capable of switching to fermentation if oxygen is absent.

Some examples of facultatively anaerobic bacteria are Staphylococcus spp., Escherichia coli, Salmonella, Listeria spp., Shewanella oneidensis and Yersinia pestis. Certain eukaryotes are also facultative anaerobes, including pupfish, fungi such as Saccharomyces cerevisiae and many aquatic invertebrates such as nereid polychaetes.

A microaerophile is a microorganism that requires environments containing lower levels of dioxygen than those present in the atmosphere (i.e. < 21% O₂; typically 2–10% O₂) for optimal growth. A more restrictive interpretation requires the microorganism to be obligate in this requirement. Many microaerophiles are also capnophiles, requiring an elevated concentration of carbon dioxide (e.g. 10% CO₂ in the case of Campylobacter species).

The original definition of a microaerophile has been criticized for being too restrictive and not accurate enough compared to similar categories. The broader term microaerobe has been coined to describe microbes able to respire oxygen "within microoxic environments by using high-affinity terminal oxidase".

Aerotolerant anaerobes use fermentation to produce ATP. They do not use oxygen, but they can protect themselves from reactive oxygen molecules. In contrast, obligate anaerobes can be harmed by reactive oxygen molecules.

There are three categories of anaerobes. Where obligate aerobes require oxygen to grow, obligate anaerobes are damaged by oxygen, aerotolerant organisms cannot use oxygen but tolerate its presence, and facultative anaerobes use oxygen if it is present but can grow without it.

Bacillus, from Latin "bacillus", meaning "little staff, wand", is a genus of Gram-positive, rod-shaped bacteria, a member of the phylum Bacillota, with 266 named species. The term is also used to describe the shape (rod) of other so-shaped bacteria; and the plural Bacilli is the name of the class of bacteria to which this genus belongs. Bacillus species can be either obligate aerobes which are dependent on oxygen, or facultative anaerobes which can survive in the absence of oxygen. Cultured Bacillus species test positive for the enzyme catalase if oxygen has been used or is present.

Bacillus can reduce themselves to oval endospores and can remain in this dormant state for years. The endospore of one species from Morocco is reported to have survived being heated to 420 °C. Endospore formation is usually triggered by a lack of nutrients: the bacterium divides within its cell wall, and one side then engulfs the other. They are not true spores (i.e., not an offspring). Endospore formation originally defined the genus, but not all such species are closely related, and many species have been moved to other genera of the Bacillota. Only one endospore is formed per cell. The spores are resistant to heat, cold, radiation, desiccation, and disinfectants. Bacillus anthracis needs oxygen to sporulate; this constraint has important consequences for epidemiology and control. In vivo, B. anthracis produces a polypeptide (polyglutamic acid) capsule that kills it from phagocytosis. The genera Bacillus and Clostridium constitute the family Bacillaceae. Species are identified by using morphologic and biochemical criteria. Because the spores of many Bacillus species are resistant to heat, radiation, disinfectants, and desiccation, they are difficult to eliminate from medical and pharmaceutical materials and are a frequent cause of contamination. Not only are they resistant to heat, radiation, etc., but they are also resistant to chemicals such as antibiotics. This resistance allows them to survive for many years and especially in a controlled environment. Bacillus species are well known in the food industries as troublesome spoilage organisms.