The World of the Microorganism

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Microbiology is the study of microorganisms, which are tiny organisms that live around us and inside our body. An organism is a living thing that ingests and breaks down food for energy and nutrients, excretes undigested food as waste, and is capable of reproduction. You are an organism and so are dogs, cats, insects, and other creatures that you see daily.

A microorganism is simply a very, very small organism that you cannot see with your naked eye, but you sure feel its effect whenever your eyes fill with water and mucus flows like an open faucet from your nose. You call it a head cold. Actually, you are under siege by an army of microorganisms attacking membranes inside your body. Watery eyes and a runny nose are ways that you fight microorganisms by flushing them out of your body.

Microorganisms are a key component of biological warfare along with chemicals that disrupt homeostasis. The anthrax attack that followed the 9/11 terrorist attacks clearly illustrated how a dusting of anthrax in an envelope can be lethal to people in an office building. Anthrax is a disease caused by the microorganism Bacillus anthracis, a bacterium that forms endospores and infiltrates

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the body by ingestion, skin contact, and inhalation. An endospore is a bacterium in a dormant state that forms within a cell.

Fortunately, incidents of biological attacks using microorganisms have been infrequent. However, there are thousands of microorganisms all around us that can be just as deadly and debilitating as the microorganisms used in warfare throughout history.

Types of Microorganisms


An infection is caused by the infiltration of a disease-causing microorganism known as a pathogenic microorganism. Some pathogenic microorganisms infect humans, but not other animals and plants. Some pathogenic microorganisms that infect animals or plants also infect humans.

Pathogenic microorganisms make headlines and play an important role in history. Legendary gunfighter John "Doc" Holliday is famous for his escapades in the Wild West. He dodged countless bullets, showing that he was the best of the best when it came to gun fighting. Yet Mycobacterium tuberculosis took down Doc Holliday quietly, without firing a shot. Mycobacterium tuberculosis is the bacterium that causes tuberculosis (Fig. 1-1). This bacterium affects the lung tissue when droplets of respiratory secretions or particles of dry sputum from a person who is infected with the disease are inhaled by an uninfected person.

Fig. 1-1. Mycobacterium tuberculosis is the bacterium that causes tuberculosis.
Fig. 1-2. Yersinia pestis is the microorganism that caused the Black Plague.

Yersinia pestis nearly conquered Europe in the fourteenth century with the help of the flea. Yersinia pestis is the microorganism that caused the Black Plague (Fig. 1-2) and killed more than 25 million Europeans. You might say that Yersinia pestis launched a sneak attack. First, it infected fleas that were carried into populated areas on the backs of rats. Rodents traveled on ships and then over land in search of food. Fleas jumped from rodents and bit people, transmitting the Yersinia pestis microorganism into the person's blood stream.

In an effort to prevent the spread of Yersinia pestis, sailors entering Sicily's seaports had to wait 40 days before leaving the ship. This gave time for sailors to exhibit the symptoms of the Black Plague if the Yersinia pestis microorganism had infected them. Sicilians called this quarantenaria. Today we know it as quarantine. Sailors who did not exhibit these symptoms were not infected and free to disembark.

Campers and travelers sometimes become acquainted with Giardia lamblia, Escherichia coli, or Entameba histolytica whenever they visit tropical countries. Travelers who become infected typically do not die but come down with a bad case of diarrhea.


Not all microorganisms are pathogens. In fact many microorganisms help to maintain homeostasis in our bodies and are used in the production of food and other commercial products. For example, flora are microorganisms found in our intestines that assist in the digestion of food and play a critical role in the formation of vitamins such as vitamin B and vitamin K. They help by breaking down large molecules into smaller ones.

What Is a Microorganism?

Microorganisms are the subject of microbiology, which is the branch of science that studies microorganisms. A microorganism can be one cell or a cluster of cells that can be seen only by using a microscope.

Microorganisms are organized into six fields of study: bacteriology, virology, mycology, phycology, protozoology, and parasitology.


Bacteriology is the study of bacteria. Bacteria are prokaryotic organisms. A prokaryotic organism is a one-celled organism that does not have a true nucleus. Many bacteria absorb nutrients from their environment and some make their own nutrients by photosynthesis or other synthetic processes. Some bacteria can move freely in their environment while others are stationary. Bacteria occupy space on land and can live in an aquatic environment and in decaying matter. They can even cause disease. Bacillus anthracis is a good example. It is the bacterium that causes anthrax.


Virology is the study of viruses. A virus is a submicroscopic, parasitic, acellular entity composed of a nucleic acid core surrounded by a protein coat. Parasitic acellular means that a virus receives food and shelter from another organism and is not divided into cells. An example of a virus is the varicella-zoster virus (Fig. 1-3), which is the virus that causes chickenpox in humans.

Fig. 1-3. The varicella-zoster virus causes chickenpox.


Mycology is the study of fungi. A fungus is a eukaryotic organism, often microscopic, that absorbs nutrients from its external environment. Fungi are not pho-tosynthetic. A eukaryotic microorganism is a microorganism whose cells have a nucleus, cytoplasm and organelles. These include yeasts and some molds. Tinea pedis, better known as athlete's foot, is caused by a fungus.


Phycology is the study of algae. Algae are eukaryotic photosynthetic organisms that transform sunlight into nutrients using photosynthesis. A eukaryotic photo-synthetic microorganism is a microorganism whose cells have a nucleus, nuclear envelope, cytoplasm, and organelles and that is able to carry out photosynthesis.


Protozoology is the study of protozoa, animal-like single-cell microorganisms that can be found in aquatic environments. Many obtain their food by engulfing or ingesting smaller organisms. Protozoa are found in aquatic and terrestrial environments. An example is Amoeba proteus.


Parasitology is the study of parasites. A parasite is an organism that lives at the expense of another organism or host. Parasites that cause disease are called pathogens. Examples of parasites are bacteria, viruses, protozoa, and many animals such as worms, flatworms, and arthropods (insects).

What's in a Name: Naming and Classifying

Carl Linnaeus developed the system for naming organisms in 1735. This system is referred to as binominal nomenclature. Each organism is assigned two latinized names because Latin or Greek was the traditional language used by scholars. The first name is called the genus. The second name is called the specific

Fig. 1-4. E. coli is a bacterium that lives in the colon.

epithet, which is the name of the species, and is not capitalized. The genus and the epithet appear underlined or italicized.

The name itself describes the organism. For example, Staphylococcus aureus is a very common bacterium. Staphylococcus is the genus and aureus is the epithet. In this case, the genus describes the appearance of the cells. Staphylo means a clustered arrangement of the cells and coccus signifies that the cells are spheres. In other words, this means a cluster of sphere-like cells. Aureus is the Latin word for golden, which means that the cluster of sphere-like cells has a golden hue.

Sometimes an organism is named for a researcher, as is the case with Escherichia coli (Fig. 1-4), better known as E. coli. The genus is Escherichia, which is named for Theodor Escherich, a leading microbiologist. The epithet is coli, which implies that the bacterium lives in the colon (large intestine).

Organisms were classified into either the animal kingdom or the plant kingdom before the scientific community discovered microorganisms in the seventeenth century. It was at that time when scientists realized that this classification system was no longer valid.

Carl Woese developed a new classification system that arranged organisms according to their molecular characteristics and then cellular characteristics. However, it wasn't until 1978 when scientists could agree on the new system for classifying organisms, and it took 12 years after this agreement before the new system was published.

Woese devised three classification groups called domains. A domain is larger than a kingdom. These are:


• Eubacteria: Bacteria that have peptidoglycan cell walls. (Peptidoglycan is the molecular structure of the cell walls of eubacteria which consists of N-acetylglucosamine, N-acetylmuramic acid, tetrapeptide, side chain and murein.)

• Archaea: Prokaryotes that do not have peptidoglycan cell walls.

• Eucarya: Organisms from the following kingdoms:


• Protista (Note: This is in the process of changing.)—algae, protozoa, slime molds.

• Fungi—one-celled yeasts, multicellular molds, and mushrooms.

• Plantae—moss, conifers, ferns, flowering plants, algae.

• Animalia—insects, worms, sponges, and vertebrates.

How Small Is a Microorganism?

Microorganisms are measured using the metric system, which is shown in Table 1.1 In order to give you some idea of the size of a microorganism, let's compare a microorganism to things that are familiar to you.

German shepherd

Human gamete (egg) from a female ovary A human red blood cell A typical bacterium cell A virus An atom

1 meter 1 millimeter 100 micrometers 10 micrometers 10 nanometers 0.1 nanometer

Your Body Fights Back

Immunology is the study of how an organism specifically defends itself against infection by microorganisms. When a microorganism such as the bacterium Streptococcus pyogenes, which can cause strep throat, invades your body, white blood cells engulf the bacterial cells and digest it in an immune response called phagocytosis. Phagocytosis is the ability of a cell to engulf and digest solid materials by the use of pseudopods or "false feet."

Phagocytosis was discovered in 1880 by Russian zoologist Elie Metchnikoff, who was one of the first scientists to study immunology. Metchnikoff studied the body's defense against disease-causing agents and invading microorganisms. He

Table 1-1. Quantity and Length: Metric and English Equivalents


Fraction of Standard

English Equivalent

meter (m)

3.28 feet

centimeter (cm)

0.01 m = 10_2

0.39 inch

millimeter (mm)

0.001 m = 10 3

0.039 inch

micrometer (|lm)

0.000001 m = 10_6

0.000039 inch

nanometer (nm)

0.000000001 m = 10 9

0.000000039 inch

discovered that leukocytes (white blood cells) defended the body by engulfing and eating the invading microorganism.


Invading microorganisms activate your body's immune system. It is at this point when you experience a fever and feel sick. In an effort to help your immune system, physicians prescribe drugs called antibiotics that contain one or more antimicrobial agents that combat bacteria. An antimicrobial agent is a substance that specifically inhibits and destroys the attacking microorganism.

One of the most commonly used antimicrobial agent is penicillin. Penicillin is made from Penicillium, which is a mold that secretes materials that interfere with the synthesis of the cell walls of bacteria causing "lysis," or destruction of the cell wall, and kills the invading microorganism.


Our bodies have a wide range of body responses in the fight against pathogens. These responses are referred to as nonspecific resistance. Resistance is the ability of the body to ward off disease. The lack of resistance is called susceptibility. When your immune system is compromised you become susceptible to pathogens invading your body where they divide into colonies causing disease, making you sick.

Generally, your first line of defense is to use mechanical and chemical means to prevent a pathogen from entering your body. Skin is the primary mechanical means to fight pathogens; it acts as a barrier between the pathogen and the internal structures of your body. Mucous membranes are another mechanical barrier;

they move the pathogen using tears and saliva (to flush it) and mucus cilia in the respiratory track to physically move it. Urination, defecation, and vomiting are other mechanical means to combat a pathogen by forcefully removing the pathogen from your body.

Chemical means attack a pathogen by changing its pH properties. Skin sebum is an important defense. Sebum is a thick substance secreted by the sebaceous glands; it consists of lipids and cellular debris that have a low pH, enabling it to chemically destroy a pathogen.

Sweat contains the enzyme lysozyme, which attacks the cell walls of bacteria. Hyaluronic acid found in areolar connective tissue sets up a chemical barrier that restricts a pathogen to a localized area of the body. Likewise, gastric juice and vaginal secretions have a low pH that is a natural barrier to many kinds of pathogens.

History of the Microscope

Diseases are less baffling today than they were centuries ago, when scientists and physicians were clueless as to what causes disease. Imagine for a moment that a close relative had taken ill. One day she was well and the next day she was sick for no apparent reason. Soon she was dead if her body couldn't fight the illness. You couldn't see whatever attacked her—and neither could the doctor.


In 1590, Zacharias Janssen developed the first compound microscope in Middleburg, Holland. Janssen's microscope consisted of three tubes. One tube served as the outer casing and contained the other two tubes. At either ends of the inner tubes were lenses used for magnification. Janssen's design enabled scientists to adjust the magnification by sliding the inner tubes. This enabled scientists to enlarge the image of a specimen three and nine times the specimen's actual size.


In 1665, Robert Hooke, an English scientist, popularized the use of the compound microscope when he placed the lenses over slices of cork and viewed little boxes that he called cells. It was his discovery that led to the development of cell theory in the nineteenth century by Mathias Schleiden, Theodor Schwann, and Rudolf Virchow. Cell theory states that all living things are composed of cells.


Hooke's experiments with a crude microscope inspired Antoni van Leeuwenhoek to further explore the micro world. Van Leeuwenhoek, an amateur lens grinder, improved Hooke's microscope by grinding lenses to achieve magnification. His microscope required one lens. With his improvement, van Leeuwenhoek became the first person to view a living microorganism, which he called Animalcules.

This discovery took place during the 1600s, when scientists believed that organisms generated spontaneously and did not come from another organism. This sounds preposterous today; however, back then scientists were just learning that a cell was the basic component of an organism.

Italian physician Francesco Redi developed an experiment that demonstrated that an organism did not spontaneously appear. He filled jars with rotting meat. Some jars he sealed and others he left opened. Those that were open eventually contained maggots, which is the larval stage of the fly. The other jars did not contain maggots because flies could not enter the jar to lay eggs on the rotting meat.

His critics stated that air was the ingredient required for spontaneous generation of an organism. Air was absent from the sealed jar and therefore no spontaneous generation could occur, they said (Fig. 1-5). Redi repeated the experiment except this time he placed a screen over the opened jars. This prevented flies from entering the jar. There weren't any maggots on the rotting meat.

Until that time scientists did not have a clue about how to fight disease. However, Redi's discovery gave scientists an idea. They used Redi's findings to conclude that killing the microorganism that caused a disease could prevent the disease from occurring. A new microorganism could only be generated by

How Do Organisms Appear?


Open Sealed

No spontaneous generation occurred in the sealed jar.

the reproduction of another microorganism. Kill the microorganism and you won't have new microorganisms, the theory went—you could stop the spread of the disease. Scientists called this the Theory of Biogenesis. The Theory of Biogenesis states that a living cell is generated from another living cell.

Although the Theory of Biogenesis disproved spontaneous generation, spontaneous generation was hotly debated among the scientific community until 1861 when Louis Pasteur, a French scientist, resolved the issue once and for all. Pasteur showed that microorganisms were in the air. He proved that sterilized medical instruments became contaminated once they were exposed to the air.

Pasteur came to this conclusion by boiling beef broth in several short-necked flasks. Some flasks were left open to cool. Other flasks were sealed after boiling. The opened flasks became contaminated with microorganisms while no microorganisms appeared in the closed flasks. Pasteur concluded that airborne microorganisms had contaminated the opened flasks.

In a follow-up experiment, Pasteur placed beef broth in an open long-necked flask. The neck was bent into an S-shape. Again he boiled the beef broth and let it cool. The S-shaped neck trapped the airborne microorganisms (see Fig. 1-6).


Pasteur placed beef broth into a long-necked flask, then bent the neck into an S-shape.

Pasteur placed beef broth into a long-necked flask, then bent the neck into an S-shape.

The beef broth remained uncontaminated even after months of being exposed to the air. The very same flask containing the original beef broth exists today in Pasteur Institute in Paris and still shows no sign of contamination. Pasteur's experiments validated that microorganisms are not spontaneously generated.

Based on Pasteur's findings, a concerted effort was launched to improve sterilization techniques to prevent microorganisms from reproducing. Pasteurization, one of the best-known sterilization techniques, was developed and named for Pasteur. Pasteurization kills harmful microorganisms in milk, alcoholic beverages, and other foods and drinks by heating it enough to kill most bacteria that cause spoilage.


The work of John Tyndall and Ferdinand Cohn in the late 1800s led to one of the most important discoveries in sterilization. They learned that some microorganisms are resistant to certain sterilization techniques. Until their discovery, scientists had assumed that no microorganism could survive boiling water, which became a widely accepted method of sterilization. This was wrong. Some ther-mophiles resisted heat and could survive a bath in boiling water. This meant that there was not one magic bullet that killed all harmful microorganisms.

Germ Theory

Until the late 1700s, not much was really known about diseases except their impact. It seemed that anyone who came in contact with an infected person contracted the disease. A disease that is spread by being exposed to infection is called a contagious disease. The unknown agent that causes the disease is called a contagion. Today we know that a contagion is a microorganism, but in the 1700s many found it hard to believe something so small could cause such devastation.


Opinions changed dramatically following Robert Koch's study of anthrax in the late 1800s. Koch noticed a pattern developing: Anyone who worked with or ingested animals that were infected with anthrax contracted the disease. In fact,

Fig. 1-7. Bacillus anthracis rapidly multiplies in the active state and becomes infectious.

people who simply inhaled the air around an infected animal were likely to inhale the anthrax bacterium spores and come down with the disease. Koch's investigations into anthrax led him to discover how microorganisms work.

Anthrax is caused by Bacillus anthracis (Fig. 1-7), which is a bacterial type of microorganism consisting of one cell. Bacillus anthracis, whether in a dormant or an active state, is called a spore. A spore is not infectious. However, under the right conditions, the Bacillus anthracis spores germinate and enter the active state and rapidly multiply and become infectious.

The question that Koch raised is: Would taking active Bacillus anthracis from one animal and injecting it into a healthy animal cause the healthy animal to come down with anthrax? If so, then he could prove that a microorganism was actually the cause of disease.

Bacillus anthracis was present in the blood of infected animals, so Koch removed a small amount of blood and injected it into a healthy animal. The animal came down with anthrax. He repeated the experiment by removing a small amount of blood from the newly infected animal and gave it to another healthy animal. It, too, came down with anthrax.

Koch expanded his experiment by cultivating Bacillus anthracis on a slice of potato. He then exposed the potato to the right blend of air, nutrients, and temperature. Koch took a small sample of his homegrown Bacillus anthracis and injected it into a healthy animal. The animal came down with anthrax.

Based on his findings, Koch developed the Germ Theory. The Germ Theory states that a disease-causing microorganism should be present in animals infected by the disease and not in healthy animals. The microorganism can be cultivated away from the animal and used to inoculate a healthy animal. The healthy animal should then come down with the disease. Samples of a microorganism taken from several infected animals are the same as the original microorganism from the first infected animal.

Four steps used by Koch to study microorganisms are referred to as Koch's Postulates. Koch's Postulates state:

1. The microorganism must be present in the diseased animal and not present in the healthy animal.

2. Cultivate the microorganism away from the animal in a pure culture.

3. Symptoms of the disease should appear in the healthy animal after the healthy animal is inoculated with the culture of the microorganism.

4. Isolate the microorganism from the newly infected animal and culture it in the laboratory. The new culture should be the same as the microorganism that was cultivated from the original diseased animal.

Koch's work with anthrax also developed techniques for growing a culture of microorganisms. He eventually used a gelatin surface to cultivate microorganisms. Gelatin inhibited the movement of microorganisms. As microorganisms reproduced, they remained together, forming a colony that made them visible without a microscope. The reproduction of microorganisms is called colonizing. The gelatin was replaced with agar that is derived from seaweed and still used today. Richard Petri improved on Koch's cultivating technique by placing the agar in a specially designed disk that was to later be called the Petri dish (Fig. 1-8). It, too, is still used today.


The variola microorganism was one of the most feared villains in the late 1700s. The variola virus causes smallpox. If variola didn't kill you, it caused pus-filled blisters that left deep scars that pitted nearly every part of your body. Cows were also susceptible to a variation of variola called cowpox. Milkmaids who tended to infected cows contracted cowpox and exhibited immunity to the smallpox virus.


Edward Jenner, an English physician, discovered something very interesting about both smallpox and cowpox in 1796. Those who survived smallpox never contracted smallpox again, even when they were later exposed to someone who was infected with smallpox. Milkmaids who contracted cowpox never caught smallpox even though they were exposed to smallpox.

Jenner had an idea. He took scrapings from a cowpox blister found on a milkmaid and, using a needle scratched the scrapings into the arm of James Phipps, an 8-year-old. Phipps became slightly ill when the scratch turned bumpy. Phipps recovered and was then exposed to smallpox. He did not contract smallpox because his immune system developed antibodies that could fight off variola and vaccinia.

Jenner's experiment discovered how to use our body's own defense mechanism to prevent disease by inoculating a healthy person with a tiny amount of the disease-causing microorganism. Jenner called this a vaccination, which is an extension of the Latin word vacca (cow). The person who received the vaccination became immune to the disease-causing microorganism.


Elie Metchnikoff, a nineteenth-century Russian zoologist, was interested by Jenner's work with vaccinations. Metchnikoff wanted to learn how our bodies react to vaccinations by exploring our body's immune system. He discovered that white blood cells (leukocytes) engulf and digest microorganisms that invade the body. He called these cells phagocytes, which means "cell eating." Metch-nikoff was one of the first scientists to study the new area of biology called immunology, the study of the immune system.

Killing the Microorganism

Great strides were made during the late 1800s in the development of antiseptic techniques. It began with a report by Hungarian physician Ignaz Semmelweis on a dramatic decline in childbirth fever when physicians used antiseptic techniques when delivering babies. Infections become preventable through the use of antiseptic techniques.


Joseph Lister, an English surgeon, developed one of the most notable antiseptic techniques. During surgery he sprayed carbolic acid over the patient and then bandaged the patient's wound with carbolic acid-soaked bandages. Infection following surgery dramatically dropped when compared with surgery performed without spraying carbolic acid. Carbolic acid, also known as phenol was one of the first surgical antiseptics.


Antiseptics prevented microorganisms from infecting a person, but scientists still needed a way to kill microorganisms after they infected the body. Scientists needed a magic bullet that cured diseases. At the turned of the nineteenth century, Paul Ehrlich, a German chemist, discovered the magic bullet. Ehrlich blended chemical elements into a concoction that, when inserted into an infected area, killed microorganisms without affecting the patient. Today we call Ehrlich's concoction a drug. Ehrlich's innovation has led to chemotherapy using synthetic drugs that are produced by chemical synthesis.


Scientists from all over set out to use Ehrlich's findings to find drugs that could make infected patients well again. One of the most striking breakthroughs came in 1929 when Alexander Fleming discovered Penicillin notatum, the organism that synthesizes penicillin. Penicillium notatum is a fungus that kills the Staphylococcus aureus microorganism (Fig. 1-9), and similar microorganisms.

Fig. 1-9. Pénicillium notatum is a fungus that kills the staphylococcus aureus.

Fig. 1-9. Pénicillium notatum is a fungus that kills the staphylococcus aureus.

Fleming grew cultures of Staphylococcus aureus, a bacterium, in the laboratory. He was also conducting experiments with Pénicillium notatim, a mold. By accident the Staphylococcus aureus was contaminated with the Pénicillium notatum, causing the Staphylococcus to stop reproducing and die. Pénicillium notatum became one of the first antibiotics. An antibiotic is a substance that kills bacteria.

A summary of the scientists and their contributions can be found in Table 1-2.

Table 1-2. Scientists and Their Contributions





Zacharias Janssen

Developed the first compound microscope.


Robert Hooke

Observed nonliving plant tissue of a thin slice of cork.


Francesco Redi

Discovered that microorganisms did not spontaneously appear. His contribution led to the finding that killing the microorganism that caused the disease could prevent the disease.


Antoni van Leeuwenhoek

Invented the single-lens microscope, grinding the microscope lens to improve magnification. First person to view a living organism.


Edward Jenner

Developed vaccinations against disease-causing microorganisms.

Table 1-2. Scientists and Their Contributions (Continued)





Mathias Schleiden, Theodore Schwann, Rudolf Virchow

Developed cell theory.


Ignaz Semmelweis

Reported a dramatic decline in childbirth fever after physicians used antiseptic techniques when delivering babies.


Louis Pasteur

Discovered that microorganisms were everywhere, living on organisms and in nonliving things such as air. His work led to improved sterilization techniques called pasteurization. One of the founders of bacteriology.


Joseph Lister

Reduced infections after surgery by spraying carbolic acid over the patient before bandaging the wound. This was the first surgical antiseptic.


Robert Koch

Discovered how microorganisms spread contagious diseases by studying anthrax. Developed the Germ Theory. Developed techniques for cultivating microorganisms.


John Tyndall, Ferdinand Cohn

Discovered that some microorganisms are resistant to certain sterilization techniques. One of the founders of bacteriology.


Elie Metchnikoff

Discovered that white blood cells (leukocytes) engulf and digest microorganisms that invade the body. Coined the word phagocytes. Founded the branch of science called immunology.


Richard Petri

Developed the technique of placing agar into a specially designed dish to grow microorganisms, which was later called the Petri dish.


Paul Ehrlich

Developed the first drug to fight disease-causing microorganisms that had already entered the body.


Alexander Fleming

Discovered Penicillium notatum, the fungus that kills staphylococcus aureus, a microorganism that is a leading cause of infection.


1. What is a microorganism?

(a) A microorganism is a small organism that takes in and breaks down food for energy and nutrients, excretes unused food as waste, and is capable of reproduction.

(b) A microorganism is a small organism that causes diseases only in plants.

(c) A microorganism is a small organism that causes diseases only in animals.

(d) A microorganism is a term that refers to a cell.

2. What is a pathogenic microorganism?

(a) A microorganism that multiplies

(b) A microorganism that grows in a host

(c) A microorganism that is small

(d) A disease-causing microorganism

3. Name the parts of this microorganism using the nomenclature system:

Mycobacterium tuberculosis.

(a) A bacterium is a one-cell organism that does not have a distinct nucleus.

(b) Mycobacterium is the presemous and tuberculosis is the specific postsemous.

(c) Mycobacterium is the epithet and tuberculosis is the specific genus.

(d) Mycobacterium is the genus and tuberculosis is the specific epithet.

4. Why is a bacterium called a prokaryotic organism?

(a) A bacterium is a one-cell organism that does not have a distinct nucleus.

(b) A bacterium is a one-cell organism that has a distinct nucleus.

(c) A bacterium is a multicell organism that does not have a distinct nucleus.

(d) A bacterium is a multicell organism that has a distinct nucleus.

5. Why is a fungus called a eukaryotic microorganism?

(a) Fungus has cells that have a nucleus, nuclear envelope, cytoplasm, and organelles.

(b) Fungus has cells that have a nucleus and no nuclear envelope.

(c) Fungus has cells that have a nucleus, nuclear envelope, cytoplasm, but no organelles.

(d) Fungus has cells that have no nucleus, no nuclear envelope, no cytoplasm, and no organelles.

6. What is Archaea?

(a) Archaea is a classification for organisms that have two nuclei.

(b) Archaea is a classification for organisms that use phagocytosis.

(c) Archaea is a classification of an organism that identifies prokaryotes that do not have peptidoglycan cell walls.

(d) Archaea is a classification of an organism that identifies prokaryotes that have peptidoglycan cell walls.

7. What is phagocytosis?

(a) The ability of a cell to reproduce.

(b) The ability of a cell to move throughout a microorganism.

(c) The ability of a cell to engulf and digest solid materials by use of pseudopods, or "false feet."

(d) The ability of a cell to change shape.

8. What is a compound microscope?

(a) A microscope that has one lenses.

(b) A microscope that has two sets of lenses: an ocular lens and an eyepiece.

(c) A microscope whose lenses are concave.

(d) A microscope whose lenses are convex.

9. What is Germ Theory?

(a) Germ Theory states that a disease-causing microorganism should be present in animals infected by the disease and not in healthy animals.

(b) Germ Theory states that a disease-causing microorganism should be present in healthy animals and not in infected animals.

(c) Germ Theory states that a disease-causing microorganism should be destroyed.

(d) Germ Theory states that a disease-causing microorganism cannot be destroyed.

10. What is Edward Jenner's contribution to microbiology?

(a) Edward Jenner discovered the Germ Theory.

(b) Edward Jenner discovered how to create vaccinations to trigger the body's immune system to develop antibodies that fight microorganisms.

(c) Edward Jenner discovered the compound microscope.

(d) Edward Jenner discovered the compound nomenclature system.

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