Microorganism requiring lower levels of oxygen than normally found in atmosphere
Anaerobicbacteria can be identified by growing them in test tubes ofthioglycollate broth: 1:Obligate aerobes need oxygen because they cannot ferment or respire anaerobically. They gather at the top of the tube where the oxygen concentration is highest. 2:Obligate anaerobes are poisoned by oxygen, so they gather at the bottom of the tube where the oxygen concentration is lowest. 3:Facultative anaerobes can grow with or without oxygen because they can metabolise energy aerobically or anaerobically. They gather mostly at the top because aerobic respiration generates more ATP than either fermentation or anaerobic respiration. 4:Microaerophiles need oxygen because they cannot ferment or respire anaerobically. However, they are poisoned by high concentrations of oxygen. They gather in the upper part of the test tube but not the very top. 5:Aerotolerant organisms do not require oxygen as they metabolise energy anaerobically. Unlike obligate anaerobes however, they are not poisoned by oxygen. They can be found evenly spread throughout the test tube.
Amicroaerophile is amicroorganism that requires environments containing lower levels ofdioxygen than those present in theatmosphere (i.e. < 21% O2; typically 2–10% O2) for optimal growth.[1] A more restrictive interpretation requires the microorganism to be obligate in this requirement.[2][3] Many microaerophiles are alsocapnophiles, requiring an elevated concentration ofcarbon dioxide (e.g. 10% CO2 in the case ofCampylobacterspecies).[4]
The original definition of amicroaerophile has been criticized for being too restrictive and not accurate enough compared to similar categories. The broader termmicroaerobe has been coined to describe microbesable to respire oxygen "within microoxic environments by using high-affinity terminal oxidase".[1]
Microaerophiles are traditionally cultivated in candle jars. Candle jars are containers into which a litcandle is introduced before sealing the container'sairtight lid. The candle's flame burns until extinguished by oxygen deprivation, creating a carbon dioxide-rich, oxygen-poor atmosphere.[5]
Newer oxystat bioreactor methods allow for more precise control of gas levels in the microaerobic environment, using a probe to measure the oxygen concentration or redox potential in real time.[6] Ways to control oxygen intake include gas-generating packs and gas exchange.[4]
As oxystat bioreactors are expensive to buy and run, lower-cost solutions have been devised. For example, the Micro-Oxygenated Culture Device (MOCD) is a system involving ordinary flasks, oxygen-permeable tubes, sensors, and water pumps. Aeration is done by pumping the culture medium through the tubes.[1]
A wide variety of microaerobic conditions exist in the world: in human bodies, underwater, etc. Many bacteria from these sources are microaerobes, some of which are also microaerophiles.
Many members ofLactobacillus sensu lato (seeLactobacillaceae) are microaerophiles. As facultative anaerobes, they do survive anaerobic conditions, but grow better with a little oxygen.[9]
