The study of bacteria is important in modern medicine and microbiology as it allows us to observe the behaviour and growth of the microscopic organisms that can be the cause of some infectious diseases. "Culturing" is simply the process of growing samples of bacteria for observation and study. Bacteria are usually cultured in laboratories using solid growth media in Petri dishes or liquid growth media in test tubes.
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Solid growth medium
The most common medium used for culturing bacteria is based on agar, a substance derived from red algae. The medium also contains nutrients that bacteria need to grow and multiply. Agar is gelatinous and is used to set the solution. The medium is usually set in a Petri dish (also known as a "plate") and the bacteria are grown either on the surface, inside the medium or both.
Solid growth methods
Three of the most common ways of using a solid growth medium are the streak plate, spread plate and pour plate methods. In the streak plate method a sterile instrument is used to create streaks on the medium's surface. Bacteria are then deposited on top of the streaks and allowed to grow and multiply. In the spread plate method the bacteria are spread across the surface of the medium. The pour plate method involves heating the agar medium until it melts and pouring it into the plate. Heat resistant bacteria can be added before the solution sets, allowing them to grow throughout the medium.
Liquid growth medium
A liquid medium can also be used to culture bacteria. Liquid media are often referred to as "nutrient broths." Like agar-based solid media, they contain all the nutrients bacteria need to grow. Bacteria are usually added to liquid media in test tubes. They start off clear but the presence of bacteria will turn the solution cloudy.
Batch and continuous culture
In a closed system, such as a solid medium set in a Petri dish or a liquid medium in a test tube, the bacteria will eventually exhaust the source of nutrients or multiply so much that they run out of "biological space" and start to die. This is known as batch culturing. In continuous culturing, more of the growth medium is steadily fed into a growth chamber. To maintain the volume of growth medium in the chamber, the excess or "overflow" is simultaneously fed away. This allows the bacteria to continue to grow and multiply in a way that is closer to how they would in a natural environment.
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