วันเสาร์ที่ 12 มิถุนายน พ.ศ. 2553

Inside Food Cells

When you look at food under the microscope you can see the parts it is made of. Food from living things, such as plants or animals, is made up of cells.

Looking inside a cell

If you magnify food even more, you can see the parts inside cells. Earlier, we looked at DNA inside a bacterial cell. Although DNA is unimaginably tiny, it is important. DNA is a molecule that controls what a cell becomes.

This is the nucleus of a cell in animal liver, magnified thousands of times. The blue areas inside the nucleus are chromosomes.

DNA and genes

Every cell in plants and animals also has DNA inside its nucleus. Long strings of DNA are packaged into microscopic units called chromosomes. The DNA in chromosomes forms sets of instructions, called genes, for the cell to follow. Because of genes, tomato cells form a tomato, and chicken cells turn into a chicken. Because of genes, the tomatoes might be bright red or pale orange.


  • antibiotic—medicine used to fight infections and illnesses caused by bacteria
  • bacteria—microorganisms with only one cell that are the smallest and most numerous life-forms on Earth
  • cell—tiny unit that all livings things are made of contaminated—infected with harmful organisms or chemicals
  • cyst—dormant (inactive) stage of a parasite that becomes active when it finds its way into a living host
  • fermentation—process during which microbes feed on sugars and release products such as alcohol
  • fungi—organisms similar to plants but with no ability to make food, so they live on other organisms (living or dead).
  • hyphae—threads that most fungi use to grow and spread through their food source
  • larva—stage of an insect after it has hatched from an egg but is not yet an adult. Larvae is the plural of larva.
  • microanimal—tiny bug or other animal too small to be seen clearly without a microscope
  • microbe—microorganism that is not a microanimal. Microbes also include viruses even though they are not really organisms.
  • microfungi—fungi that are microscopic or are made up mostly of microscopic parts
  • micrometer—measurement of length that is one-thousandth of a millimeter
  • microorganism—any living thing that is too small to be seen clearly without a microscope
  • mites—tiny creatures related to spiders, many of which live on animals, plants, or food
  • molecule—microscopic part that makes up all living and nonliving things. All cells are made of molecules.
  • nanometer—measurement of length that is one-millionth of a millimeter
  • nucleus—part of a cell that controls the cell’s form and functions
  • organism—any living thing, such as a plant, animal, or bacterium
  • parasite—organism that lives on or in another living thing and feeds off it
  • protist—usually single-celled microorganism that lives in water or damp places. Protists can be plant-like (algae) or animal-like (protozoa).
  • species—group of living things of the same kind. For example, bears are a kind of animal, but grizzly bears are a species.
  • spore—reproductive part of a fungus
  • starch—form of carbohydrate found in plants
  • virus—microbe that can only multiply by infecting living cells

Keeping Food Safe

It’s important to keep kitchen cloths clean to avoid spreading bacteria from one food to another.

After learning all about microscopic life in your food, you might be getting a little concerned. Don’t worry—most food is safe and healthy most of the time. And there are things you can do to make sure you keep your food free from microscopic invaders.

Keeping clean

Many kitchen bacteria are helpful or at least harmless. They will compete with harmful bacteria to keep food safe. You can help, too, by keeping things clean, and the most important thing to keep clean is you! Bacteria will travel quickly from your skin onto food. It’s really important to wash your hands after petting animals, using the bathroom, and playing outside. Never forget to wash your hands before touching food.

You may be surprised to know that the sponges and cloths we use for cleaning hold more bacteria than anything else! Bacteria can spread from a sponge to a dish and then to your food. Sponges and cloths need to be washed in very hot water or even heated in the microwave for two minutes to kill bacteria.

Hot and cold

Bacteria multiply faster in warm conditions. To slow decay, keep meat, produce, and dairy foods in the refrigerator or freezer. Proper heating and cooking will kill bacteria altogether. For example, Salmonella in eggs, chicken, and red meat die when these foods are thoroughly cooked.

What’s in the Water?

We rely on water just as much as we rely on food. And we take it for granted that the water in our homes is pure and safe to drink. Mostly it is, but not always. And there are many places in the world where drinking water can be as contaminated as some of the foods we’ve looked at.


Many of the microscopic life-forms that find their way into drinking water are protists. Protists are usually singlecelled microorganisms that live in water or damp places. You will remember we read about parasites that can get into our bodies from food and live inside us. Some protists can become parasites inside us if we drink the water that contains them.

The light blue circle is an amoeba called Entamoeba histolytica. It can get into people’s bodies from drinking water


Amoebas are one kind of protist. Even though they consist of just one cell and look like a shapeless blob, amoebas can move around and even catch food to eat! Like bacteria, amoebas can reproduce just by dividing themselves.


Giardia lamblia is another protist that infects people’s intestines. It can exist in water in a dormant (inactive) stage called a cyst. When it gets into your body, it becomes an active parasite and can give you a nasty illness called giardiasis. Infections from Giardia can last for a long time.

The Giardia lamblia protist has several flagella— the whip-like tails it uses to move around.

Pantry Pests

As we’ve seen, many microorganisms like to eat the same stuff as we do. Even when food is stored in a package or a can, bugs and microbes can find ways to get at it.

Pests in the package

Take a look at the packages in your kitchen cabinets or pantry. You’ll probably see flour, cereals, rice, pasta, baking mix, crackers, and maybe some nuts and popcorn. Bugs love to get into dried foods like these. If you see tiny beetles, moths, or worms on your counter, you may find they are also in your food.

This weevil was found in a package of rice. It is covered in rice dust.

Packages and containers don’t have to be open for bugs to get in. Many bugs can chew through packaging, and arrive in your kitchen already inside the package. Beetles, moths, and mites lay their eggs where there will be a good food source for larvae when they hatch. They often do this while food is stored in warehouses or on its way to the store.

Canned bacteria

Clostridia are bacteria that thrive without oxygen. Cans of food can be infected by Clostridia if their contents aren’t properly processed. The Clostridia produce a gas that makes cans swell up. Some Clostridia can also cause botulism, a very serious kind of food poisoning.

Meat Microbes

As we saw earlier, bacteria absorb their food from other livings things or from decaying matter. Many bacteria can digest protein, so they live on meat. With the right supply of food, bacteria multiply quickly, so a small piece of meat can hold millions of bacteria.

Food poisoning

We know that most bacteria are harmless and even useful. But some of these microscopic organisms can also be very harmful. Several bacteria that infect meat can give people food poisoning. Most kinds of food poisoning have the same horrible symptoms. People feel nauseous, and they vomit a lot. They get really bad stomachaches and diarrhea. People usually recover after a few days, but sometimes they develop more serious illnesses.

Bad bacteria

Let’s look at some of the bacteria in meat that make us sick. Salmonella bacteria can invade all kinds of meat.

They are common in chicken, so eggs can be infected, too. It takes fewer than 20 Salmonella cells to make someone sick. Campylobacter jejuni is even more common than Salmonella in chicken. People infected with this bacterium get terrible diarrhea. Usually it goes away by itself, and people may not know what caused the illness.

Good and bad E. coli

Some forms of the bacterium Escherichia coli (E. coli for short) are healthy. They live in our intestines and fight other, harmful bacteria. But one type of E. coli that lives in cattle intestines can make people sick. When beef is ground into hamburger, the E. coli often infects meat and gets passed on to humans. Other harmful types of E. coli come from infected chicken or even vegetables.

Antibiotics are substances that can kill bacteria in your body. We use them as medicines when we get sick from bacterial infections. In the photo below, an antibiotic is destroying an E. coli cell wall. Farmers and ranchers add antibiotics to animal feed to prevent illness in cows, pigs, and sheep. But some bacteria are getting so used to antibiotics that the antibiotics can’t kill them anymore. So when bacteria cause infections, antibiotics may not be able to kill the bacteria and make people better.

Magnified Meat

There are lots of microorganisms that feed and grow on meat. When you see flies buzzing around your kitchen, you should make sure your meat is covered up!


Flies like to lay their eggs on meat, and you probably won’t notice them because they are so tiny. When the eggs hatch, the maggots that come out will start eating the meat. They do this by squirting their saliva on the meat to soften it. Then they suck up the meaty liquid.


Organisms that live on or in other living things are called parasites. Some microanimals are parasites that live in people’s bodies. The meat we eat may contain microscopic parasite eggs. When you eat meat containing live eggs or larvae, a parasite can hatch, mature, and make its home inside your intestine.

Maggots start out tiny, but they are easy to see after they have been feeding and growing for a few days.

Vegetables and Grains

All kinds of microorganisms live on vegetables and grains. They are found on growing crops and on produce in your home.


Unfortunately, there are many insects, bacteria, fungi, and viruses that destroy grain and vegetable crops. Insects too small to see can eat their way through a whole field of vegetables. More than 5,000 types of fungi attack food crops with diseases such as rust, smut, blight, and mildew.

Helping crops grow

Other bacteria, microfungi, and microanimals have useful roles to play when crops are growing. These microorganisms keep soil healthy and prey on pests. The bacterium Bacillus thuringiensis protects crops because it makes a substance that harms pests. Farmers and gardeners can spray the bacteria on plants without any harm to people, animals, plants, or the environment.

Phytophthora infestans is a fungus that attacks potatoes and gives them blight. Infected potatoes that seem healthy when they are growing may rot later after you bring them home from the store.