Microorganisms such as bacteria and fungi can be grown in the laboratory in a culture medium, a mixture that supplies everything they need to grow and divide.A culture medium contains:
• a carbon source, usually a sugar such as glucose, for respiration and for building molecules;
• a nitrogen source, such as amino acids or ammonium salts, to make proteins and nucleic acids;
• water, minerals (such as phosphate and magnesium ions) and sometimes vitamins.The medium can be a liquid nutrient broth in a flask or test tube, or it can be set as a solid gel with agar, an extract from seaweed. Most microorganisms cannot digest agar, and it stays solid at incubation temperatures, so it is poured into Petri dishes to make agar plates on which separate colonies can grow.Growth also depends on the conditions:
• Temperature: enzymes work fastest near an optimum. In school laboratories cultures are incubated at no more than 25 °C, which reduces the chance of growing human pathogens, which grow best at about 37 °C.
• pH: most bacteria grow best near pH 7; buffers in the medium keep the pH steady.
• Oxygen: obligate aerobes need oxygen, obligate anaerobes are killed by it, and facultative anaerobes grow with or without it.By controlling nutrients, temperature, pH and oxygen, a microbiologist can grow a chosen microorganism quickly. How to do this without contamination is covered in the next lesson.
• a carbon source, usually a sugar such as glucose, for respiration and for building molecules;
• a nitrogen source, such as amino acids or ammonium salts, to make proteins and nucleic acids;
• water, minerals (such as phosphate and magnesium ions) and sometimes vitamins.The medium can be a liquid nutrient broth in a flask or test tube, or it can be set as a solid gel with agar, an extract from seaweed. Most microorganisms cannot digest agar, and it stays solid at incubation temperatures, so it is poured into Petri dishes to make agar plates on which separate colonies can grow.Growth also depends on the conditions:
• Temperature: enzymes work fastest near an optimum. In school laboratories cultures are incubated at no more than 25 °C, which reduces the chance of growing human pathogens, which grow best at about 37 °C.
• pH: most bacteria grow best near pH 7; buffers in the medium keep the pH steady.
• Oxygen: obligate aerobes need oxygen, obligate anaerobes are killed by it, and facultative anaerobes grow with or without it.By controlling nutrients, temperature, pH and oxygen, a microbiologist can grow a chosen microorganism quickly. How to do this without contamination is covered in the next lesson.