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Peek into a clean room with no one in it. There are no pets in the room. There are no plants in the room. It looks like there is nothing alive in the room. The room, however, is swarming with life. Billions of tiny life forms called bacteria cover the tables and chairs and the floor. You can’t see them, but they stick to the windows, they cling to the ceiling, and they float through the air. Bacteria constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a number of shapes, ranging from spheres to rods and spirals.Bacteria are tiny single-cell microorganisms, usually a few micrometers in length that normally exist together in millions. A gram of soil typically contains about 40 million bacterial cells. A milliliter of fresh water usually holds about one million bacterial cells.Bacteria as prokaryotes. All living organisms on Earth are made up of one of two basic types of cells: eukaryotic cells, in which the genetic material is enclosed within a nuclear membrane, or prokaryotic cells, in which the genetic material is not separated from the rest of the cell.
Suppose you looked at part of a chair in the room through a microscope. Microscopes magnify bacteria. You would see pale forms moving and bumping into each other like ghosts. These tiny life forms are bacteria. They live everywhere in the world. Billions of them even live inside of you!

WHAT DO BACTERIA LOOK LIKE?
Bacteria come in three basic shapes. Some are round or shaped like a jellybean. Some are spiral-shaped like a corkscrew. Some are long, like rods.
Each sphere or spiral or rod is called a cell. Animals have millions of cells, but bacteria have only one cell. This single bacterial cell is called a bacterium. An outer wall surrounds the cell and protects it. A substance called DNA floats around inside the cell.
Some bacteria have hairlike parts called flagella. The flagella help the bacteria move around in search of food. Flagella also move bacteria away from things that could harm them.

WHAT DO BACTERIA DO?
Some bacteria help you. Bacteria in the body help fight off disease and help you digest your food. Some bacteria that live in soil help plants by producing substances plants need. They break down dead plants and animals and animal waste. They make a gas called carbon dioxide from decaying material. Other bacteria help plants take a gas called nitrogen out of the air. Plants need carbon dioxide and nitrogen in order to grow.
Some bacteria can harm you. There are bacteria that cause food poisoning, pneumonia, tuberculosis, and other sicknesses. Some bacteria cause tooth decay. Bacteria can also infect farm animals and wild animals.
Disease-causing bacteria are different from helpful bacteria. You can swallow disease-causing bacteria in unclean food or water. Bacteria that cause infections can get into cuts and sores. Bacteria can get on your hands and go from your hands into your nose, eyes, or mouth. That is why it is so important to wash your hands often.

HOW DO BACTERIA GROW AND SPREAD?
Most bacteria simply split in two. Bacteria reproduce very rapidly. One bacterial cell can become two in just a few minutes. Two bacteria become four bacteria and then four become eight bacteria. Bacteria keep multiplying this way until there are billions of them.
Different kinds of bacteria must compete for food. This competition keeps bacteria from overrunning Earth.

WHO DISCOVERED BACTERIA?
No one knew about bacteria until the microscope was invented. Bacteria are so small that they must be magnified at least 500 times their actual size for us to see them.
A Dutch microscope-maker named Antoni van Leeuwenhoek in the 1670s was the first person to see bacteria under a microscope. He saw tiny life forms swimming around in drops of rainwater and in scrapings from his teeth. He called the bacteria “animalcules,” meaning tiny animals.
French scientist Louis Pasteur in the 1800s proved that some bacteria are germs that cause disease. He found that heat kills bacteria. Heating surgical instruments to kill bacteria is called sterilization. Heating milk and other foods to kill bacteria is called pasteurization, a word named for Pasteur.
A German scientist named Robert Koch also made important discoveries about bacteria. He founded a field of science called bacteriology, the study of bacteria.

WHY DO WE STUDY BACTERIA?
Scientists study bacteria to find out how these tiny life forms behave. Scientists also want to know what diseases bacteria cause. They look for ways to kill disease-causing bacteria.
In the mid-1800s, scientists learned that killing bacteria can stop the spread of some diseases. Doctors learned that sterile surgical instruments and operating rooms help prevent infection after operations. Scientists learned that having clean drinking water and food prevents the spread of deadly diseases such as cholera and typhoid.
Scientists also learned how to make vaccines that protect against some diseases caused by bacteria. They made vaccines from dead or weakened bacteria or from poisons bacteria produce. For example, a shot of tetanus vaccine protects you from the bacteria that cause tetanus. Tetanus bacteria live in the soil. They can get into even small cuts on your body. Tetanus causes muscles to tighten. It can be deadly.
In the mid-1900s, scientists discovered antibiotics, drugs that kill bacteria. Before antibiotics, many people died from pneumonia and other infections caused by bacteria. Some bacteria have become resistant to antibiotics. The antibiotics no longer kill them. Scientists are working to make new kinds of antibiotics that will kill resistant bacteria.

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