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Bacteria can be classified into aerobes and anaerobes. The main difference between the two is the fact that aerobic bacteria require oxygen to remain alive, while anaerobic bacteria do not rely on oxygen for metabolic processes and survival. While aerobes are able to thrive in habitats that have abundant oxygen, anaerobes may die in the presence of oxygen. This type of bacteria does have a growth advantage in areas of the body unexposed to oxygen, and they may become virulent pathogens. The difference in the capacity to utilize oxygen among aerobes and anaerobes is important in the treatment of bodily infections.
The classification of bacteria can be based not just on whether or not they require oxygen, but also on how they use it. Obligate aerobes are microorganisms that need oxygen to survive and die in its absence. An example is the bacterium Bacillus anthracis. Obligate anaerobes are organisms that die when exposed to oxygen, such as Clostridium tetani and Clostridium botulinum, which cause tetanus and botulism, respectively.
Facultative anaerobes can live in the presence or absence of oxygen, but prefer to use oxygen. Examples of this type include Escherichia coli (E. coli) and Staphylococcus, or simply staph. Subtypes of E. coli, such as the O157:H7, cause hemorrhagic diarrhea, while staph is known for causing skin infections such as boils, folliculitis, and impetigo. When a deep skin laceration becomes infected with staph, a more severe form of infection called cellulitis can occur.
The other two classifications are microaerophilic bacteria and aerotolerant bacteria. Microaerophiles can live in habitats that have lower levels of oxygen compared to the atmosphere. Examples of microaerophiles are Helicobacter pylori, which causes peptic ulcers, and Borrelia burgdorferi, which causes Lyme disease.
Aerotolerant anaerobic bacteria do not have any use for oxygen but are not adversely affected by its presence. An example is the genus Lactobacillus, which is normally found in the gut, skin, and vagina. When the populations of Lactobacillus in the vagina become depleted, bacteria such as Gardnerella vaginalis and Bacteroides multiply, leading to bacterial vaginosis.
Bacteria are cultured in a microbiology laboratory to provide an important clue of their identity. In particular, when grown in a test tube, the following observations may be documented. Obligate aerobes gather on the surface of the culture medium to maximize oxygen absorption, while obligate anaerobes gather at the bottom to keep away from oxygen. Facultative bacteria gather near the top, while microaerophiles gather near the upper part, but not on the surface. Aerotolerant anaerobes are evenly spread along the depth of the medium.
Identifying whether a bacterium is an aerobe or an anaerobe is important in the treatment of bacterial infections. Treatment of infections caused by anaerobic bacteria is often more challenging because they are resistant to usual antibiotic therapies. For instance, treatment of bacteria such as Bacillus fragilis usually includes combination antibiotics such as piperacillin/tazobactam, imipenem/cilastatin, amoxicillin/clavulanate, and metronidazole plus ciprofloxacin or gentamicin.
@croydon - My introduction to anaerobic bacteria was a science experiment my high school biology teacher had going. She had a jar which had been filled with some compounds (can't remember what they were) as well as the bacteria and it had been sealed completely so the only thing that ever got in was sunlight.
The bacteria had been living in there along with a couple of other species, for over two decades and were still going, because they had a perfect closed system going. They didn't need any oxygen, and the sunlight was enough to provide energy. I mean, it just looked like a pile of sludge from the outside, but it was amazing when you thought about it.
When I was studying environmental science at university one of the field trips we took was to a local farm to see how they processed their waste water. It was a dairy farm and they had a lot of contaminated water from the concentration of fecal matter at the sheds when the cows were there to be milked.
One of the things they had to do was account for the fact that there was both anaerobic and aerobic bacteria in the waste and both were needed to process it really efficiently into fertilizer. So, they had a series of ponds for the water, one of which was a still pond and others which churned up the water. In the still pond, the anaerobic bacteria would thrive and break down part of the waste and in the churning pond the aerobic bacteria would take over and do their part. It was kind of cool how they had worked it out.
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