With enough food and the right conditions, microbes will grow indefinitely. This can be useful in certain industrial processes, such as the production of insulin or biofuels. When the microbes cause diseases, however, uncontrolled growth can lead to people becoming more sick or dying. In laboratories that work with microbes, controlling their growth is essential for obtaining useful experimental results.
Sometimes controlling microbial growth means killing them outright. This is often done in hospitals to prevent infections caused by microbes living on medical equipment or other surfaces. These non-selective methods kill all microbes at once. Other times, specific microbes are targeted using a variety of chemicals and drugs. Laboratories use methods to kill microbes, but they also grow specific microbes under controlled conditions.
Heat Sterilization of Microbes
Sterilization is a method that kills or eliminates all of the microbes in a sample or on a surface. The purpose of sterilization is to kill all living microbes, along with any dormant ones. This makes the sample or surface “sterile.” Sterilization generally uses heat, chemicals, radiation or the direct removal of microbes. The method used depends upon the types of microbes present and the nature of the sample or surface. Radiation may work well in the laboratory, but may not be convenient — or safe — in the home.
Heat is the most common method. It is used in laboratories, hospitals and at home. Incineration uses open flames to kill microbes. This is done in laboratories to sterilize the equipment used to collect samples. Boiling is effective at killing microbes in water or other liquids. This can be used to sterilize drinking water while hiking. Steam heat is the basis of many sterilization procedures in laboratories and hospitals. This method uses an autoclave — like a pressure cooker — to kill microbes on equipment and clothing.
Chemical and Radiation Sterilization of Microbes
Many chemicals and gases are lethal to microbes. These can be used to sterilize equipment and surfaces in hospitals and laboratories. Some of these substances are also dangerous to humans, so special protection is required when using them. Ethanol — pure alcohol like that found in alcoholic drinks — is used to sterilize surfaces. Ethanol is also found in many commercial hand sanitizers and cleaning wipes. In order to be effective, though, the ethanol must be left on the surface long enough to kill the microbes.
Radiation is also effective at killing microbes. One common type of radiation used is ultraviolet, the same type that is found in sunlight. Special lightbulbs that produce only ultraviolet light are used. These may be installed in a laboratory, or used as part of a handheld device. The ultraviolet light, if strong enough, damages the DNA of the microbes beyond repair. This kills the microbes.
Antibiotics and Microbes
Antibiotics are drugs that kill bacteria. Some of these compounds occur naturally, while many are man-made. The first antibiotic, penicillin, was discovered in the 1930’s. Before this time, doctors had few tools to treat bacterial infections, such as pneumonia and tuberculosis. Like penicillin — which is made by a fungus — many other antibiotics are produced naturally by bacteria and plants. These protect them from attack by other bacteria.
Since the discovery of penicillin, scientists have developed many new types of antibiotics. Unlike sterilization procedures which kill all microbes in a sample, antibiotics target specific components or functions in a bacterium. Some antibiotics kill a wide range of bacteria, while others may kill one or a few types. Over time, however, bacteria can become resistant to the antibiotics, meaning the drug no longer kills them.
Antibiotic-Resistant Bacteria and Microbes
Antibiotic resistance in bacteria is a major problem for medicine and agriculture, where antibiotics are used widely to treat diseases. Normally, when an antibiotic is given to a patient, all of the bacteria that it targets are weakened or killed. Weakened bacteria are then killed by the patient’s immune system. Resistance to antibiotics results from antibiotics being overused or misused. This may occur when a patient does not take all of the antibiotics prescribed by a doctor, or uses them to treat a cold caused by a virus. Antibiotics are not effective against viruses.
Some bacteria are naturally resistant to antibiotics. Others develop resistance when antibiotics are not used long enough or in the right amount. When this happens, some bacteria may survive. The survivors have developed a resistance to the antibiotic. The next time the drug is used, the bacteria will not be killed. Over time, this resistance is passed onto the bacteria’s offspring. Bacteria can also transfer their resistance to other bacteria. Over time, more bacteria have grown resistant to one or more antibiotics. As this happens, doctors have fewer tools available to combat potentially-deadly infections.
Microbial Growth Laboratory Techniques
In addition to sterilization methods, scientists that work with microbes use other methods to grow microbes in laboratories. Microbes must be cultured — grown — in a liquid or on a solid surface. This solid or liquid medium contains nutrients that the microbes need to survive. A common type of medium is a gel-like substance — agar — kept in a round plate called a Petri dish. Scientists transfer samples of microbes to the medium using a laboratory tool, often a loop of wire called an inoculating loop.
A sample contains many microbes. To study each type in isolation, the sample is diluted to reduce the number of microbes growing in one flask or on one area of a plate. On agar, this is done by running the inoculating loop across the surface. When microbes, such as bacteria, reproduce on an agar plate, they form a colony — a clump — of identical microbes. In order to grow microbes in a laboratory, the conditions must be perfect for each microbe. This includes setting the temperature, humidity, heat, light and oxygen. To stall the growth of the microbes, the petri dish or flask is placed in a refrigerator.
