Microorganisms and Classifying Living Things

A tiger and a lion look alike. A tiger and a beetle look very different. Classification helps you describe these patterns using shared features, called characteristics.

Why scientists sort living things

Scientists have named more than a million kinds of living things. Groups help them identify discoveries, share information, and study relationships. A useful feature might be a backbone or a way of feeding. Living in the same place is a poor guide: a pond holds plants, insects, and fish.

Carl Linnaeus was a Swedish scientist who lived from 1707 to 1778. His book Systema Naturae first appeared in 1735. He organized living things into groups inside groups. His two-part naming method became standard in the 1750s. Scientists still use it, but update groups when new evidence appears.

Follow a lion through the levels

The seven main levels, from broadest to narrowest, are kingdom, phylum, class, order, family, genus, species. Remember them with “King Philip Came Over For Good Soup.” Linnaeus used five of these levels; phylum and family came later.

A tiger, Panthera tigris, shares the lion's genus. A house cat, Felis catus, shares its family but not its genus. The lion is more closely related to the tiger. Each narrower group brings together living things with more in common.

Name one species clearly

A species is one particular kind of living thing. In animals and plants that breed with a partner, members can produce young that can also have young. A horse and donkey can produce a mule. Mules almost never have young, so horses and donkeys are separate species. Bacteria split in two; scientists use shared features and genetic information to identify their species.

A scientific name uses two words. Capitalize the genus, keep the second word lowercase, and print both in italics: Homo sapiens. Underline both words when handwriting. Both words name the species; the second word alone is incomplete. A common seven-spotted ladybug has one scientific name worldwide: Coccinella septempunctata. Everyday names can change between languages.

A mushroom grows from the ground, just as a plant does. To place it in a group, look at how it gets food. Where it grows is not enough.

Compare the main groups

Plants make food from water and carbon dioxide using sunlight. This is photosynthesis. A green substance called chlorophyll captures light. Plants also grow and reproduce. Some make seeds; others make spores.

Animals cannot make food through photosynthesis. They eat plants, animals, or both. They usually sense changes and respond quickly. Movement helps identify many animals, but some adults stay fixed. A barnacle fixed to a rock still catches and eats food.

Fungi include mushrooms, molds, and yeasts. A fungus has no chlorophyll. It sends digestive juices into the material around it. Then it absorbs the dissolved food. Many fungi grow as a network of fine threads. A mushroom is the spore-making part of a much larger fungus.

Give a reason for your group

Try these three examples:

• “An oak is a plant because it makes food using sunlight.”
• “An earthworm is an animal because it eats dead leaves.”
• “Bread mold is a fungus because it digests and absorbs food from bread.”

A mushroom is not a plant. Its way of feeding places it with fungi. Spores alone would not settle this: ferns and mosses make spores too. Scientists once grouped fungi with plants. Evidence about feeding and other features changed that grouping. Fungi are more closely related to animals than to plants.

Size can give a second label

A microorganism, or microbe, is too small to see individually without a microscope. This label describes size, not one branch of life. Yeast is a fungus and a microorganism. A mushroom is a fungus, but it is large enough to see.

Some living things can fool you at first. A Venus flytrap catches insects for extra nutrients, yet still makes food by photosynthesis. That makes it a plant. A coral may look plantlike, but its tiny animals catch food with tentacles.

A whale swims, and a bat flies. Their homes and movements do not make them fish or birds. Use the features that define each group.

Animals with backbones

Vertebrates have a backbone. Invertebrates do not. Most named animal species are invertebrates; insects alone make up more than half.

A whale breathes with lungs and feeds its young milk. A bat has fur and feeds its young milk. Both are mammals. Flight is not the deciding feature for birds: penguins cannot fly but have feathers. Live birth is not the deciding feature for mammals: platypuses and echidnas lay eggs.

Animals without backbones

Adult insects have six legs and three body parts: head, thorax, abdomen. Arachnids, such as spiders, have eight legs and two main body parts. Crustaceans include crabs, shrimp, and land-living pill bugs. They have a hard outer covering and many legs. Myriapods, such as centipedes, have long bodies with many legs. These four groups have jointed legs and hard outer skeletons.

Mollusks have soft bodies. Many have shells, but slugs and octopuses show that shells are not required. Earthworms have long, soft bodies without legs. People and worms both belong to the animals.

Plants with seeds or spores

Flowering plants form seeds inside fruits. Grasses and oak trees belong here even when their flowers are easy to miss. Conifers, such as pines, form seeds in cones. Ferns and mosses make spores instead of seeds. Ferns have roots, stems, and leaves. Mosses are small, soft plants without true roots.

Follow a key one choice at a time

A classification key uses clear questions to identify something. Here is a key for four garden creatures:

1. Does it have legs? Yes: go to 2. No: go to 3.
2. Does it have six legs? Yes: ladybug. No, it has eight: spider.
3. Does it have a shell? Yes: snail. No: earthworm.

For a snail, start at 1: no legs. Move to 3: a shell. You reach “snail.” Start at the beginning each time. “Is it pretty?” would be a poor key question because people can disagree.

Your hands can look clean while carrying microbes. Individual microorganisms are too small to see with your eyes alone. Large numbers can become visible together.

Measure a tiny living thing

A micrometer, written µm, is a tiny unit of length. There are 1,000 µm in 1 mm. Many bacteria are about 1–2 µm long. Imagine lining up 500 bacteria, each 2 µm long:

The line would be 1,000 µm long, or just 1 mm. A flu virus is smaller still: about 0.1 µm across.

A microscope makes an image look bigger. This is magnification. At ×400, the image of a 2 µm bacterium is 800 µm long, or 0.8 mm. The bacterium itself has not grown. A school light microscope can show many microorganisms. Viruses need much more powerful electron microscopes because they are too small for light microscopes.

Small individuals, visible crowds

A fuzzy mold patch contains many microscopic threads. A colony is a group of microbes growing together. A visible bacterial spot contains huge numbers of bacteria. A granule of dried yeast also contains many individual yeasts.

Microbes live in soil, water, and air, and on surfaces. They also live on and inside you. A pinch of soil can contain hundreds of millions of bacteria. Trillions live in your gut. Most do not cause disease. “Germ” is the everyday word for a disease-causing microbe.

One becomes two

Living microbes feed, grow, reproduce, release waste, and respond to conditions. Many bacteria reproduce by splitting in two. Some can split every 20 minutes with enough warmth, moisture, and food.

Start with one bacterium. After each split, double the total:

1 → 2 → 4 → 8 → 16 → 32 → 64 → 128 → 256 → 512.

Three hours contain nine 20-minute periods. That gives 512 bacteria in this example. Growth slows as food runs out or waste builds up; doubling cannot continue forever.

Most microbes need food, moisture, suitable warmth, and time to grow. Cold slows growth. Freezing stops it without killing most microbes. High heat kills most. A microbe is not another name for an insect. Some tiny pond animals, such as rotifers, are microorganisms too.

“Microbe” does not mean “bacterium.” Several different groups contain tiny members. Knowing the group helps you understand how a microbe lives.

Bacteria and microscopic fungi

Bacteria are very small living things with their own broad group. Some look like rods, balls, or spirals. Some help us: Lactobacillus bacteria help make yogurt. Others cause disease: Salmonella can cause food poisoning. A shape or the label “bacterium” does not tell you whether it is harmful.

Yeast and mold belong to the fungi. One yeast is microscopic, but a packet holds huge numbers. Mold grows fine threads across food. You see the fuzzy patch once enough threads and spores collect. Mushrooms belong to the same broad group, though they are much larger.

Protists

Protists are a mixed group of mostly tiny living things. They are not animals, plants, fungi, or bacteria. An amoeba changes shape as it moves and swallows food. Tiny algae, including diatoms, make food using sunlight. The parasite that causes malaria is also a protist. Members of this group can feed in very different ways.

Why viruses are different

A virus makes copies only inside the living thing it infects. Outside a host, it cannot feed, grow, or reproduce. Most scientists therefore do not count viruses as living things. Scientists still study them alongside microbes. Colds, flu, measles, and chickenpox are caused by viruses.

Some bacteria also need a host. The important difference is that every virus needs one to make copies.

Give both labels when they fit

Say, “Yeast is a microorganism and a fungus.” The first label tells you its size. The second tells you its biological group. Say, “An amoeba is a microorganism and a protist.” Mold does not become a plant when its patch grows large enough to see. Its way of feeding still places it with fungi.

Microbes do more than cause disease. Many help you every day. Others have no helpful or harmful effect that you notice; we call these neutral.

Food made with microbes

Yeast feeds on sugar. During fermentation, it makes carbon dioxide gas and alcohol. The gas forms bubbles trapped in bread dough, making it rise. Baking kills the yeast and drives off the alcohol. Holes in a bread slice show where gas bubbles were.

Yogurt uses bacteria instead. They change milk sugar into lactic acid. This acid makes milk thicken and taste sour. Bacteria also help make cheese. Some cheeses use selected harmless molds, such as the blue mold in blue cheese. Microbes also help make sauerkraut, soy sauce, and vinegar.

Return nutrients to the soil

Many fungi and bacteria are decomposers. They break down dead living things and waste into simpler substances. Nutrients return to soil, where plants can take them up. Without this recycling, dead material would build up and soil would lose nutrients.

Think about a compost bin. It holds fruit peels, vegetable scraps, and dead leaves. Keep it moist and turn it to let air in. Bacteria and fungi feed on the scraps. Over months, they help make dark, crumbly compost. A very cold or dry bin changes more slowly because decomposers grow more slowly.

Helpers inside you

Helpful gut bacteria break down some food your body cannot digest alone, including some fiber. They make certain vitamins, such as vitamin K. By taking up space, they also make it harder for harmful microbes to settle. Trying to remove every bacterium would remove helpers too.

From microbes to medicines

In 1928, Alexander Fleming noticed something around a Penicillium mold. Bacteria had stopped growing nearby. This discovery led to penicillin, the first widely used antibiotic. It was produced in large amounts in the 1940s. Many antibiotics come from microbes. Scientists also use specially changed microbes to make medicines such as insulin.

Bacteria have other useful jobs. Water treatment plants use them to break down waste. Bacteria in small lumps on bean and pea roots turn nitrogen from the air into a form plants can use. Whether a microbe helps or harms depends on its kind and what it is doing.

A pathogen causes disease. You may call it a germ. An infectious disease starts when a pathogen enters your body and multiplies there.

Match the cause to the disease

A parasite lives on or inside a host, takes food from it, and harms it. Some parasites are microscopic. Others, including lice and tapeworms, are large enough to see. “Parasite” describes a way of living, so it overlaps other groups. The malaria parasite is a protist.

Many infections spread between people or animals. Some come from the environment. Tetanus bacteria can enter a wound from soil. Asthma and diabetes are not infections; you cannot catch them from another person.

A carrier is not always the cause

An animal that carries a pathogen between hosts is a vector. Certain female mosquitoes carry malaria parasites. A mosquito can pick up parasites when it bites an infected person. It can pass them on in a later bite. The mosquito is the carrier; the parasite causes malaria. Malaria does not spread through ordinary touching or coughing. Mosquitoes can also carry dengue viruses. Ticks can carry the bacteria that cause Lyme disease.

Symptoms are changes such as a fever, cough, or rash. Several diseases can share a symptom. A doctor may need tests to find the cause. Cold weather itself does not cause a cold: cold viruses do.

Spoiled food and unsafe food

Microbes can change food's color, smell, taste, or texture. Mold grows on bread and fruit. Milk may become sour and lumpy. Yet some food-poisoning bacteria cause no visible change or bad smell. Looking and smelling fine does not make food safe.

If bread or another soft food grows mold, throw away the whole item. Fine mold threads reach beyond the visible patch. Do not sniff it; spores can irritate your breathing. Mold chosen for cheese making is different from unwanted mold growing on stored food.

Most kinds of E. coli live harmlessly in the gut. Some kinds cause serious illness in contaminated food or water. A microbe's name alone may not tell the whole story.

Food preservation helps food last safely for longer. Different methods change what microbes can do. Some slow growth; some kill microbes; some keep new ones out.

Cold and heat work differently

A refrigerator at 40°F (4°C) or below slows most microbial growth. A freezer at 0°F (−18°C) stops growth. Most microbes survive freezing and can become active after thawing. Some harmful bacteria can still grow slowly in a refrigerator, so chilled food does not last forever.

Cooking thoroughly kills most harmful microbes. Pasteurizing heats milk or juice to a set temperature for a set time, then cools it quickly. This kills most harmful microbes without boiling. Canning combines heating with a sealed container that keeps new microbes out afterward.

Change water or acidity

Raisins and dry rice keep well because they have little available water. A large amount of sugar helps preserve jam. A little sugar has a different effect: yeast can feed on it. The amount and the conditions matter.

Packaging has a job too

Sealed packaging keeps out new microbes. Some packaging also removes air. It does not remove microbes already inside. Some breads contain preservatives that slow mold growth. Follow the storage directions on a package. Many food dates describe quality; a date alone cannot guarantee safety after poor storage.

Compare three loaves. An open loaf may dry out but collect spores. A bagged loaf stays softer, yet mold can grow in the warm, moist bag. Freezing stops mold growth during storage. Refrigeration slows mold, though bread goes stale faster there.

Use the methods together

Clean hands help keep germs away from food. Keep raw meat apart from food that will be eaten without cooking. Cook food thoroughly and refrigerate leftovers within two hours. On days hotter than 90°F (32°C), use the shorter limit of one hour. A sealed lunch box does not replace keeping perishable food cold.

A packet of dried yeast contains living but inactive yeast. Water, food, and warmth help it become active. The carbon dioxide it releases gives you something to measure.

Ask a question and predict

Try, “How does water temperature affect gas made by yeast?” A prediction includes a reason: “I predict warm yeast will make more gas because cold slows its activity.”

Use identical bottles and balloons. Put equal amounts of yeast, sugar, and water into each bottle. Change the water temperature. Collect the gas in balloons and measure each balloon around its widest part. This distance is its circumference. Record the increase every ten minutes for thirty minutes.

Keeping the other conditions alike helps you connect the result to temperature. Different balloon stretchiness can affect measurements, so use matching balloons. Circumference compares gas production; it is not a direct measure of gas volume.

Add a control comparison

A bottle with sugar and warm water but no yeast checks whether yeast causes the gas. Another with yeast and warm water but no added sugar checks the effect of added food. The controls leave out different ingredients, so each answers a different question.

In one class's test after thirty minutes, balloons increased by 1 cm at 10°C (50°F), 18 cm at 40°C (104°F), and 0 cm at 70°C (158°F). The no-sugar bottle increased by 0 cm. The warm bottle made the most gas. Cold slowed yeast; very hot water killed it. Yeast begins dying around 55–60°C (131–140°F).

Repeat and find a mean

Three warm bottles gave increases of 17, 18, and 19 cm. Add the measurements, then divide by the number of tests:

The mean increase was 18 cm. Check a result far from the others and repeat if needed. Compare the results with your prediction.

You can also test sugar amounts while keeping temperature fixed. Flour supplies usable sugars more slowly as substances in flour break down starch. A lot of salt slows yeast. For practical work, an adult handles hot water; do not drink the mixtures.

Tiny mold spores land on bread. With suitable conditions, they grow threads that absorb food from it.

Make one clear comparison

You could test moisture, temperature, light, or bread type. Choose one. To test moisture, compare damp and dry slices from the same loaf. Keep temperature, light, slice size, and observation time the same. Comparing warm damp bread with cold dry bread changes two things at once.

For practical work, seal each slice in its own clear, labeled bag. Observe through the bag and never open it. Spores can cause breathing problems. Anyone with asthma or a mold allergy should not handle the bags. An adult throws them away still sealed, and everyone washes their hands afterward.

Count coverage the same way each day

Place a transparent grid over each bag. Count equal squares more than half covered by mold. Use the same counting rule every day. If 18 of 50 squares count as covered:

About 36% of the slice is covered. This is an estimate because patches do not fit square edges exactly. Record daily results in a table. Draw a line graph with days along the bottom and coverage up the side.

One class found these results after seven days: damp bread at room temperature had 41 of 50 squares covered, or 82%. Dry bread at room temperature had 7 of 50, or 14%. Damp refrigerated bread had none. The room-temperature pair shows that moisture helped mold grow. The damp pair shows that cold slowed it.

Mold gets food from bread, so it can grow without light. A sunny windowsill may be warmer than a dark cupboard. To test light fairly, keep their temperatures equal too.

What stays inside the bag?

A sealed bag acts as a closed system. Material stays inside, although heat and light can pass through. Bread changes as mold feeds. Water may form droplets inside. Weigh the whole sealed bag at the beginning and end. Its mass stays almost the same because material changes form without leaving.

Germs need a route to reach another person. Hygiene helps interrupt those routes. Clean-looking hands and objects can still carry microbes.

Follow the route

Germs can travel through the air when people breathe, talk, cough, or sneeze. They can also pass by touch. A hand may pick up germs from a handle, then carry them to the eyes, nose, or mouth. Food, water, blood, and bites are other routes. Different germs use different routes.

Imagine Jada sneezes into her hands and opens a door. Tom touches the handle, then rubs his eyes. Jada could break this chain with a tissue or her elbow. Washing her hands would also help. Tom could wash his hands before touching his face.

Wash every part of your hands

Hygiene means habits that keep you and your surroundings clean to reduce germ spread. Handwashing is one of these habits:

1. Wet your hands with clean running water and add soap.
2. Rub palms, backs, between fingers, and under nails.
3. Scrub for at least 20 seconds.
4. Rinse under clean running water and dry with a clean towel or air-dry.

Soap loosens grease, dirt, and germs so water can carry them away. It also breaks apart some viruses. A quick water rinse leaves more behind. Wash before eating or preparing food, after using the toilet, after coughing or blowing your nose, and after touching animals, waste, or garbage.

When soap and water are unavailable, sanitizer with at least 60% alcohol kills many germs. It works poorly on visibly dirty or greasy hands. Soap and water are the better choice then.

Match the habit to the route

Cover a cough or sneeze with a tissue, discard it, and wash your hands. Use your elbow if you have no tissue. Staying home when sick reduces contact that spreads infection. Clean food, safe water, cooking, and chilling help interrupt the food-and-water route.

If local officials issue a boil-water advisory, an adult can follow their instructions. CDC's usual germ-killing advice is a rolling boil for one minute, or three minutes above 6,500 feet (about 2,000 m). Boiling does not remove harmful chemicals.

Where bites spread disease, use repellent, long sleeves and pants, and mosquito nets. Empty standing water where mosquitoes lay eggs. Check for ticks after time in woods or long grass.

Your body has several defenses working at once. Some stop germs entering. Others deal with germs that get inside. Together they form your immune system.

Barriers at the entrances

Your skin is a tough barrier that most germs cannot cross. A cut opens a route inward. Blood clots and forms a scab, helping seal the gap while the skin heals. Cleaning and covering a cut helps keep germs out.

Your nose hairs catch dust and some germs. Sticky mucus traps more in the nose and airways. Tiny hairs in the airways sweep mucus toward the throat. You swallow or cough it out. Coughing and sneezing help clear material, though released germs can spread to others.

Tears wash your eyes. Saliva washes your mouth. Both contain substances that destroy some bacteria. Strong stomach acid kills many swallowed germs. Helpful gut bacteria also make it harder for harmful microbes to settle. These defenses reduce entry; they do not catch every pathogen.

Defenses inside

White blood cells travel in your blood. Some surround and swallow germs, then break them down. Others make antibodies. These substances stick to matching features on germs and help the body deal with them.

An antibody is like a key that fits a particular lock. Different germs have different features, so one antibody does not match everything. After an infection, the immune system can remember the germ. If it returns, the body can respond faster, often before you feel sick. This protection is called immunity. People usually do not catch chickenpox again after recovering.

Signs of a response

A cut can become red, warm, and swollen as defenses respond. Extra mucus can appear during a cold. A fever is a temperature around 100.4°F (38°C) or higher. Body temperature varies; about 98.6°F (37°C) is a common reference value. Fever often signals the body fighting an infection. It can help defenses work and slow some germs.

Getting over one cold does not protect you against every cold. More than 200 different viruses can cause colds. Memory against one does not cover all the others. The same reason explains why the immune system needs to recognize many different germs.

Prevention acts before a disease starts. Treatment helps someone who is already ill. Vaccines and medicines do different jobs, so the cause of an illness matters.

Vaccines prepare your defenses

A vaccine presents a safe way to recognize a germ. It may contain a weakened or killed germ, a harmless piece, or instructions for making a harmless piece. Your immune system responds and remembers. It can react faster to the real germ later. Vaccines protect against disease without first having that disease; protection is not perfect.

Most vaccines are shots. Some are swallowed or sprayed into the nose. A sore arm, tiredness, or a mild fever can happen for a day or two. Serious reactions are very rare. Having no side effects does not mean the vaccine failed.

For infections that spread between people, vaccination can reduce spread. With fewer people becoming infected, germs have fewer routes through the community. This community immunity helps people who cannot receive certain vaccines. It does not apply to every disease: tetanus comes from the environment.

A discovery that changed health

In 1796, Edward Jenner used material from a cowpox sore to vaccinate eight-year-old James Phipps. Jenner later tested his protection against smallpox. The word “vaccine” comes from vacca, Latin for cow. After worldwide vaccination, the World Health Organization declared smallpox eradicated in 1980. It is the only human disease eradicated worldwide.

Match medicine to the cause

Antibiotics kill bacteria or stop them growing. They cannot cure viral colds or flu. A doctor may prescribe one for bacterial strep throat. Antifungal medicines treat fungal infections such as athlete's foot. Antiviral medicines treat some viral infections. Pain relievers ease symptoms without killing germs. Rest and fluids help during many infections while your defenses work.

Some bacteria have features that let them survive an antibiotic. When susceptible bacteria die, resistant ones can survive, multiply, and spread. This is antibiotic resistance. The bacteria are resistant; your body has not become resistant to the medicine. Unneeded or incorrect antibiotic use speeds the problem.

Take medicine only when a parent, caregiver, doctor, or pharmacist gives it to you. Use prescribed antibiotics exactly as directed; never share them or use leftovers.