You can see by looking at this moldy apple that fungi like to feed on fruit sugars.Many forms of life, from bugs down to the tiniest microbes, like fruit as much as we do. Fruit is full of sugar, and many microorganisms use sugar as their food source.
How yeast ferments fruit
We saw earlier how yeast likes to feed on the starch in flour. Yeast cells will also feed on fruit, where you can see them form a powdery surface. When yeast consumes the sugar in fruit, it converts it into two products: carbon dioxide, which is a gas, and ethanol, which is a liquid alcohol. This is a form of fermentation. Ethanol is the alcohol in wine and other alcoholic drinks, so yeast is used to make these drinks. Wine, for example, is made from fermented grapes.
Viruses
The smallest of all known microbes are viruses. Viruses are not really organisms. They are chemical clusters that invade the cells of living things and eventually destroy them. Viruses can kill people, and they can also get into fruit crops and destroy them. Fruit viruses are often spread from one plant to another by aphids (tiny pests that feed on the plants).
There are many thousands of bacterial species. And those are just the ones scientists know about —there are more being discovered every year. Several kinds of bacteria help to turn milk into foods we eat every day.
This picture shows Streptococcus thermophilus (orange beads) and Lactobacillus bulgaricus (blue rods) bacteria in yogurt. Both these bacteria can fight bad bacteria that make us sick.
Making yogurt
Two bacteria often used to make yogurt are Streptococcus thermophilus and Lactobacillus bulgaricus. First the milk is heated to condense it (reduce its liquid content to make it thicker), and then the bacteria are added. With the milk sugars to feed on, the bacteria multiply quickly. They go to work on the milk, thickening it and giving it a yogurty taste.
Good bacteria in food can help us digest food. They also compete with bad bacteria in our bodies.
Ripening cheese
Cheese is made from the protein in milk. First, bacteria produce the acid that separates the protein from the liquid part of the milk. The protein forms soft, solid stuff called curd. Curd makes unripened cheeses, such as cottage cheese.
The curd can then be ripened with the help of other microbes that harden cheese and give it flavor. Different microbes—either bacteria or fungi—produce different kinds of cheese. The bacteria that make Swiss cheese, for example, are called propionibacteria. They release gas bubbles, and when the cheese hardens, there are holes where the bubbles were.
Blue cheese
Blue cheese ripens with the help of fungi. We learned earlier how fungi spread through bread. When the mold Penicillium roqueforti is put into cheese, it also starts to spread and form blue veins in the cheese.
The veins you see in blue cheese are the hyphae and spores of the fungus Penicillium roqueforti.
The smallest and most plentiful microorganisms in our food—and in the world—are bacteria. Even though you can’t see them without a microscope, bacteria are everywhere.
What are bacteria?
So what are these microscopic life-forms? Unlike plants and animals, bacteria are made of a single cell. You can see a typical bacterium below and take a look inside its cell. Bacteria often live in large clumps called colonies. They multiply constantly by dividing their cells.
Most bacteria are shaped like rods,but many are round. Others are bent or shaped like spirals. Bacteria also vary in size, but most measure between 1 and 4 micrometers across.
Whatever shape bacteria are, the insides of their cells hold the same basic parts. On the outside, some bacteria have pili to hold onto the cells of their food source. The larger hairs are flagella, which many bacteria use to move around.
Hungry bacteria
Bacteria need food to survive. Mostly they live on organic matter—food that comes from living things—just like we do. As bacteria digest food, they change the food they are eating.
Living in milk
Milk contains bacteria called lactobacilli that live on lactose, the sugar in milk. When milk goes sour, it’s because lactobacilli are producing a sour substance called lactic acid as they feed. Nobody wants to drink sour milk, but we can use the bacteria in milk to make food we do like. Next you will see how bacteria and fungi together help make some of our favorite foods!
Bacterial Names
Scientists use the term bacilli to describe rodshaped bacteria and cocci for bacteria shaped like spheres. These bacteria shapes are common in milk. Lacto means milk, so some milk bacteria are named lactobacilli and others are lactococci.
Fungi are a type of organism that includes the mushrooms we eat. But there are much tinier fungi that are part of some foods. These microscopic forms of fungi are known as microfungi.
Food invaders
First, let’s look at some microfungi that are invaders. Fungi spread through their food source with threads called hyphae. And their food source may well be your food source!
Fungi feed on living things and organic matter (things that were once alive or come from living things). As they do so, they break down their food by releasing chemicals into it.
Moldy bread
Mold is a type of fungus that invades food and breaks it down, causing it to spoil. If you look at a slice of bread and see a patch of mold, you are actually seeing a fungus feeding on the bread. The mold is a network of hyphae called a mycelium.
As the mycelium spreads, the hyphae grow fruiting bodies that hold spores, which are like tiny seeds. When the fruiting bodies release the spores, they travel in the air to find new food to grow on.
Making bread
Yeasts are different from regular fungi. They only have one cell. Carbohydrates—sugar and starch—are the yeast cell’s favorite food. When yeast is mixed with flour and water to make bread, the yeast cells begin to feed on the starch in the flour. This process, called fermentation, releases carbon dioxide bubbles. The bubbles make the bread swell up, or rise. After the bread is cooked, the bubbles appear as holes.
You can see the holes in this bread made by the yeast releasing gas bubbles.
These yeast cells are budding to reproduce themselves. The bud comes off to make a new yeast cell. These cells have been magnified 4,000 times.
How Small Is Small?
An average yeast cell is about 4 micrometers across. There are 1,000 micrometers in a millimeter, so you could line up 250 yeast cells alongside just 1 millimeter on your ruler. You could fit more than 6,000 alongside 1 inch.
You probably think you know what is in the food you are eating. You may be surprised, however, when you see your food under a microscope. Microscopes allow us to see things up close and look at them in a different way. Under the microscope, we can see the tiny parts that foods are made of. We can learn why foods change when we cook them or when they spoil.
Sharing your food
The microscope also shows us that you may not be the only one eating your food! In the refrigerator or on the kitchen counter, tiny organisms (living things) may also be munching away.
Microorganisms and microbes
Organisms too small to see are called microorganisms. They range from tiny animals that you could find with a magnifying glass to living things so small you could fit thousands of them on a grain of salt. Microorganisms that are not animals are often called microbes. Some of these make our food rot. Others make us sick if we eat them. Other microbes actually make food healthy or give it a good taste.
Food up close
Let’s take a look at your food under the microscope and discover what you are really eating. By looking first at two everyday foods—bread and milk—we’re going to learn about two important kinds of microorganisms with which we share our food and our world.
Micro-Monster
The flour mite could live in your kitchen cabinet in a package of flour. In real life, this mite is 0.3 to 0.5 of a millimeter long, so you could see it with a magnifying glass. This is what it looks like when it is magnified 200 times.
