The purpose of this experiment was to determine which disinfectant would
most effectively kill bacteria.
I became interested in this idea when I was washing the counter and
I wondered if the disinfectant I was using was better than another was.
The information gained from this experiment would help parents, health
care providers, and food preparers to know which disinfectant to use to
the best results for killing bacteria.
My hypothesis was that bleach would work the best in killing the bacteria.
I based my hypothesis on Marie Clark, Microbiologist at Memorial Hospital,
who said, "Bleach will probably work the best because it is an oxidant."
The constants in this study were:
The manipulated variable was the type of disinfectant that was used.
The amount of disinfectant used
The type of agar plates used
The way the number of live colonies were measured
The temperature and time of incubation
The number of organisms there were to start
That the disinfectants were opened from a new bottle for the experiment
The time the bacteria and disinfectant were in contact before plating
The responding variable was how many bacteria were killed by the disinfectant
after one night of incubation.
To measure the responding variable the percentage of surviving colonies
||microliters Escherichia coli
||microliters Staphylococcus epidermidis
||microliters Staphylococcus aureus
||microliters Enterococcus faecalis
||microliters Pseudomonas aeruginosa
||microliters Bacillus sp. (not anthrax)
||microliters Lysol (from new bottle)
||microliters Bleach (from new bottle)
||microliters Krud Kutter (from new bottle)
||microliters 409 (from new bottle)
||Nutrient agar plates
1. Culture several species of bacteria including:
a. Escherichia coli,
b. Staphylococcus epidermidis,
c. Staphylococcus aureus,
d. Enterococcus faecalis,
e. Pseudomonas aeruginosa
f. Bacillus sp. (not anthrax)
2. Using a 10% solution of one bacterial species, and dispense 100
microliters into a sterile tube.
3. Dispense (by pipette) 10 microliters of disinfectant into tube of
4. Hold the mixture in the tube for 5 minutes.
5. Pipette 10 microliters of the mixture onto a nutrient agar plate.
6. Spread mixture with sterile loop to cover entire surface.
7. Incubate over-night at 35 degrees Celsius in air.
8. Use a percent to put down how many live colonies are on the plate.
9. Repeat steps 2-7 with the next type of disinfectant.
10. Repeat steps 2-8 with the next bacterial specie.
11. Repeat steps 2-9 with the control group (no adding of disinfectant.)
12. Record all data.
13. Auto-clave all materials used and wash hands.
The original purpose of this experiment was to determine which disinfectant
would most effectively kill bacteria.
The results of the experiment were that bleach worked the most effectively
to kill bacteria.
See table and graphs.
My hypothesis was that bleach would work most effectively to kill the
The results indicate that this hypothesis should be accepted.
Because of the results of this experiment, I wonder if bleach would
still work the best if there were different types of bacteria used. I also
wonder if there would be different results if I tested more disinfectants
for the experiment. I also wonder if one half or one tenth as much disinfectant
would show similar results.
If I were to conduct this project again, I would test more brands of
disinfectants for the experiment. I would also use more types of bacteria,
and I would do more than one trial for the each test to obtain more accurate
results and to get an average.
Health is one of the most important factors in human life. Contracting
an illness can be bad, and can even cause death. Using a disinfectant that
will effectively kill harmful bacteria can help people stay healthy.
Bacteria are "any of a large group of very small one-celled organisms that
reproduce by fission or by forming spores. Some kinds can cause disease,
while others are active in fermentation." The American Heritage Student
Dictionary, 1994 Edition
Bacteria are one-celled organisms that are classified as prokaryotes,
meaning they have no nucleus. They are approximately 2-3 microns in diameter
(a micron is 1/24,500 of an inch or 0.001 millimeters.) Bacteria are just
about everywhere, consist of about a thousand types, and can be either
harmful or harmless.
Types of Bacteria:
There are 4 types of bacteria. Those with a round shape are called "cocci",
the rod-shaped are "bacilli", the bent rods are "vibrios", and the spiral
shaped are "spirilla" or "spirochetes". They can either live alone, or
they may live in a pair, which is called "diplo", a cluster, which is "staphylo",
or a chain, which is "strepto".
Bacteria can also be classified by whether they do or donít need air
to live. If they do, they are called aerobic bacteria, or aerobes. If the
bacterium can live without oxygen, it is call an anaerobic bacterium, or
There are a lot of types of bacteria that help in everyday life. Bacteria
that live in the intestines help digest food, kill harmful bacteria cells,
and break down nutrients the body needs to stay healthy. There are also
bacteria that live in soil and water, and they recycle carbon, nitrogen,
sulfur, and chemical elements. Others break down dead organisms and animal
waste into chemical elements that plants and animals can use. Some bacteria
make fermentation possible, while others help clean the water at sewer
plants. There are even some types of bacteria that help create medications,
antibiotics, and drugs.
Out of thousands of types of bacteria, there are only a few that are harmful.
These harmful bacteria get into the body system by openings like the nose,
mouth, or cuts in the skin. They can cause disease like cholera, gonorrhea,
leprosy, pneumonia, syphilis, tuberculosis, typhoid fever, and whooping
cough. Harmful bacteria are transmitted from person to person by air, food,
or water. Bacteria can also get into plants and animal body systems. They
can create diseases like anthrax in livestock (mostly cows and sheep),
fire blight in fruit trees, soft rot in fruits and vegetables, and crown
galls in various different plants.
Some bacteria that get into the body can make poisons and cause diseases
like diphtheria, scarlet fever, or tetanus. These poisons may be given
off while the bacterium is alive or dead. One type of harmful bacteria
lets out poisons in poorly canned foods and can create food poisoning called
botulism. Once harmful bacterial cells get inside a personís body system,
they make the body operate improperly, and can also create a weak immune
system and turn healthy and harmless cells into harmful cells.
Structure of Bacteria:
"A bacterial cell may have up to three protective layers, these layers
surround the cytoplasm, which contains the cellís nucleoid. Hairlike flagella
extend through the layers in many types of bacteria. The flagella help
the bacterium to move by means of a whiplike motion." The World Book Encyclopedia,
Volume 2 (B), 1998 Edition
Bacteria consist of a single cell. Some have a capsule on the outside
of their cell wall, which helps resist destructive chemicals. If there
is a capsule on the bacterium, it is outside of the cell wall. The cell
wall gives the bacterium its shape and allows it to live in all types of
environments. The cell wall covers the cell membrane, which is an elastic,
baglike structure. The cell membrane holds the cytoplasm, a soft, jellylike
substance. This cytoplasm has lots of enzymes, which break down food and
build cell parts. There may also be flagella on the bacterium. It may have
flagella on one end, both ends, or all over. They help the bacterium move.
It also has the nucleoid, which is formed from the bacteriumís deoxyribonucleic
acid (DNA). The DNA controls the growth, reproduction, and all activities
of the bacterium.
To reproduce, bacteria use a process called binary fission. This process
includes the bacteriumís chromosome duplicating, and the cell dividing
into two identical cells. This is why scientists and microbiologists donít
reproduce bacteria for getting different genes. Higher organisms get and
have different genes because of the difference in their parentís reproductive
cells. Since bacteria have only one cell, and use binary fission to reproduce,
they cannot have different genes or reproductive cells. They have their
DNA mixed by transformation, conjugation, or transduction. When a bacteriumís
DNA is transformed, it means that they get DNA parts in soil or water as
dead bacteria are decayed. If it is conjuguated, the donor hooks onto the
receiver, makes a pilus (tube), and gives their plasmid DNA parts to the
receiver. When a bacterium is transducted, there is a transfer of DNA parts
through cells by a bacteriophage (a virus that infects). Doing any of these
to mix genes can give bacteria new traits. They can include withstanding
acidity or temperature changes, and antibiotic resistance.
There are only a couple of species of bacteria that cause disease out of
the thousands of species that are living. Some of them are Yersina pestis,
which caused plague. Plague was feared because it often resulted in widespread
death. Epidemics of plague have killed hundreds of millions of people.
There is also Vibrio cholerae, the bacteria that causes cholera. This caused
many cholera epidemics in Europe and Asia in the 1800ís, and thousands
of deaths. There was also a cholera epidemic in South America in the 1900ís,
which took about 6,000 lives.
Mycobacterium tuberculosis, or the tuberculosis causing bacterium,
is a big problem in the world. Streptomycin, the cure for tuberculosis,
doesnít work anymore because of the overuse of antibiotics making resistant
bacteria. There are also bacteria and viruses that cause forms of pneumonia
and strep throat. Pneumonia is a term for lung diseases including swelling,
and strep throat is a disease that affects the throat and tonsils.
Antoni van Leeuwenhoek made microscopes in Holland during the late 17th
century. He was the first person to study bacteria and worked hundreds
of hours to make ground glass lenses for his microscopes. A lot of people
say that he was the founder of microbiology. Leeuwenhoek was the first
person to discover bacteria as well as study them.
Louis Pasteur proved that microbes donít come from dead matter, and
Robert Koch proved that bacteria are disease causing. Sergei Winogradsky
explained about enery-yielding metabolic reactions and found out that there
were anaerobic microorganisms. Martinus Beijerinck found out microorganisms
help cycle nutrients like nitrogen. Selman Waksman discovered that bacteria
in soil made antibiotics. Lynn Margulis showed the prokaryotic nature of
eukaryotes like plants and animals. Lynnís studies made the hypothesis
of endosymbiosis, which stated the "key eukaryotic features such as the
energy-generating centers called mitochondria in all animals, plants, and
fungi and the photosynthesizing centers called chloroplasts in all algae
and plants were derived from ancient bacteria." (Microsoft Encarta 2001)
Carl Woese discovered there is one bacteria, called archae bacteria, that
are prokaryotes and eukaryotes at the same time.
A disinfectant is "a substance that kills microorganisms that cause disease."
The American Heritage Student Dictionary 1994 Edition
Disinfectants are liquid substances that kill germs or bacteria on non-living
objects. They are usually chemicals used to clean clothes, rooms, dishes,
and utensils. Disinfectants are often put into water and sewer systems
to help stop citywide diseases. They are also used to help hospitals and
care units stay clean and free of germs. The common disinfectants used
in households arenít as effective as the ones used in hospitals or care
units to stop diseases. There are even some disinfectants that are largely
useless and could be replaced with soap and water to do the job.
There are seven main types of disinfectants. They include:
2)formaldehyde and glutaradlehyde,
6)pine oil disinfectants, and
7)quaternary ammonium compounds.
There are two different types of alcohol disinfectants. There are ethyl
and isopropyl alcohols. These disinfectants are used to clean plastic and
rubber. They also clean thermometers used to take a personís body temperature.
Formaldehyde and Glutaraldehyde:
Formaldehyde and glutaraldehyde are fast-acting disinfectants. They disinfect
quickly and effectively. They are used mostly by hospitals to clean the
surgical tools and other medical devices.
Hypochlorites are disinfectants that have chlorine bleach and chlorinated
lime, usual ingredients in disinfectants and deoderizers. They are used
to treat water and sewage systems and to clean eating utensils.
Iodophors are disinfectants that include iodine. They are used to clean
hospital surfaces like tables and beds, and also to disinfect food preparation
Phenols are disinfectants that include carbolic acid, creosote, and hexachlorophene.
They are used to clean floors, trash cans, bathrooms, and other large surfaces.
Pine Oil Disinfectants:
Pine oil disinfectants are mixed with detergents. They are most commonly
used to clean floors, walls, and bathroom fixtures, like toilets and sinks,
and have a pine-like smell to them.
Quaternary Ammonium Compounds:
Quaternary ammonium compounds are used in lots of common household cleaners.
They are used as disinfectants and as detergents.
Health is very important. Getting a sickness from bacteria can happen in
an unhealthy environment. Using a disinfectant to kill harmful bacteria
is an important way to stay healthy.
American Society for Microbiology "Bacteria" October 18, 2002
"Bacteria." Microsoft Encarta. 2001 edition. CD-ROM. Microsoft Corporation,
Egmond, Wim van and Parmentier, Jan. "Bacteria." November 8, 2002.
Facklam, Howard and Margery. Bacteria. Canada Twenty First Century Books
Rodowski, Jr., Christopher A. "Disinfectant" The World Book Encyclopedia.
1995 vol. 5 (D)
Schlessinger, David "Bacteria" The World Book Encyclopedia. 1999 vol.
"Whatís a Microbe?." October 6, 2002
Todar, Kenneth University of Wisconsin-Madison Department of Bacteriology.
I would like to thank the following people:
My mom for driving me to the hospital so I could do my experiment.
Mr. Newkirk for correcting my journal, abstract, etc.
Mrs. Helms for helping me with my board.
The Memorial Hospital Microbiologist for helping me with my experiment.
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