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The Factors Affecting Growth on Pour Plates

Pour plates contain bacteria embedded in an agar medium rich in the nutrients they need to grow. Microbiologists often use pour plates to determine how many bacteria were present in a sample. A variety of factors, such as temperature, oxygen, pH and availability of appropriate nutrients, help to determine whether the bacteria will thrive. Ultimately, the most important consideration, however, is the type of bacteria because different bacteria need different conditions.
  1. pH and Temperature

    • The pH or acidity/alkalinity is crucial. Most bacteria thrive only within a particular pH range -- usually no more than 1 unit or so. Some bacteria prefer highly alkaline environments, while others are fond of acidic solutions. Most bacteria, however, like a neutral pH. Temperature preferences also vary among bacteria. Some bacteria like it hot; thermophiles prefer temperatures above 40 degrees Celsius, while mesophiles like more moderate temperatures, in the 20 to 40 degree Celsius range. Psychrophiles would rather you keep the temperature between -10 and 20 degrees Celsius.

    Oxygen and Osmotic Strength

    • Oxygen is another very important consideration. Some bacteria die without it, while others die with it. Oxygen is lethal for bacteria called obligate anaerobes. Aerobic bacteria, by contrast, need oxygen to live and grow. Facultative anaerobes are more flexible; they can grow with oxygen or without it and adjust to their environment. The salt content of the medium is also important. Most bacteria can tolerate moderate salt levels but die at high salt concentrations. Halophilic bacteria are a little more unusual; they require high salt concentrations and will die without them.

    Nutrition

    • Some bacteria are autotrophs; just like plants, they make their own food from sunlight and CO2. Other bacteria are heterotrophs and need sugars or food they can digest in the medium. Regardless of whether they are heterotrophic or autotrophic, bacteria need various other nutrients, such as nitrogen, sulfur, potassium and phosphorus -- the same kinds of nutrients that you require for your own growth, although some bacteria can exploit different chemical forms of these nutrients.

    Other Factors

    • Just as you are unable to synthesize vitamin C and must eat food containing this nutrient, so some bacteria have compounds they are unable to synthesize and must obtain these from their environment. Some bacteria require certain amino acids, for example. Trace elements like iron and zinc are essential to bacterial growth. As you can see, the requirements for growth on a pour plate vary depending on the species, so it is important to understand bacteria and what they need when you are trying to grow them.


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