Agar plates are inoculated with bacteria using various techniques to introduce the microorganisms onto the nutrient-rich surface, allowing for growth and study.
There are several common methods for inoculating agar plates:
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Streak Plating: This is a common technique for isolating pure bacterial colonies. A sterile loop is used to pick up a sample of bacteria and streak it across a portion of the agar surface. The loop is then sterilized, and the streaking process is repeated several times, each time dragging fewer bacteria from the previous streak. This dilution effect eventually leads to individual cells being deposited on the agar, which will grow into isolated colonies.
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Spread Plating: In this method, a small volume of bacterial suspension is pipetted onto the center of the agar plate. A sterile spreader (usually a glass or plastic rod bent into a hockey stick shape) is then used to evenly distribute the suspension across the entire surface of the agar. This results in a uniform layer of bacterial growth.
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Pour Plating: Here, a known volume of bacterial suspension is mixed with molten agar at a temperature that won't kill the bacteria (usually around 45-50°C). This mixture is then poured into a sterile Petri dish and allowed to solidify. The bacteria are thus distributed throughout the agar medium. Colonies will grow both on the surface and within the agar.
Factors Affecting Inoculation and Growth:
- Sterility: All materials and equipment must be sterile to prevent contamination.
- Incubation Temperature: Most bacteria are incubated at 25°C in school laboratories for no more than 24–48 hours. To avoid the growth of human pathogens, it is not recommended to incubate plates at body temperature (37°C).
- Incubation Time: The incubation period depends on the bacteria and the desired outcome.
- Nutrient Availability: The agar medium must provide the necessary nutrients for the bacteria to grow.
After inoculation, the agar plates are incubated under appropriate conditions (temperature, humidity, atmosphere) to allow the bacteria to grow and form colonies. The resulting colonies can then be studied and used for further experiments.
