|MadSci Network: Microbiology|
Many of the bacteria found in cheese can grow under anaerobic conditions (in the absence of oxygen gas). The bacteria actually used to make the cheese (starter culture bacteria) are part of a group known generally as "lactic acid bacteria", and most lactic acid bacteria are capable of growth under anaerobic conditions (facultative anaerobes - grow with or without oxygen). In addition, yeasts may grow in the absence of oxygen, and you may occasionally get growth of yeasts.
The bacteria that you are most likely to find when you grow bacteria from a cheese sample without oxygen are lactic acid bacteria. These are fastidious bacteria, and usually require an enriched growth media to cultivate them. Other than these bacteria, and yeasts, you are unlikely to find anything more than a few random contaminants.
When you go to the store, most of the cheese you see is vacuum packaged. This effectively reduces the amount of oxygen in the package to a very, very low level. The reduced oxygen, in combination with refrigeration, keep the normal cheese bacteria from growing. Cheese itself restricts the growth of bacteria by a combination of low moisture (some water is removed during cheese making, and salt is usually added) and low pH. Most cheese is relatively acid, with a pH below 5.0. Occasionally you will see a package of cheese that has swelled up due to gas production. This is usually caused by an increase in storage temperature and may be caused by gas produced by the starter culture, gas produced by yeasts, or gas produced by other bacteria. The other bacteria may be normally present in low numbers, but will grow to higher populations if the temperature is increased.
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