Biology - HOT Science Lab

Student
Investigating Bacterial Growth
NGSSS:
SC.912.L.14.52 Explain the basic functions of the human immune system, including specific and
nonspecific immune response, vaccines, and antibiotics. (AA)
Background: Most bacteria (and other microorganisms) are harmless. In fact, many bacteria
are beneficial. Cheesemaking, decay, and soil building are a few of the important processes that
depend on the action of decomposing bacteria, which thrive on decaying organic matter.
Bacteria are also important in the process of digestion for many organisms, including humans.
Bacteria in termites and ruminants (such as cows) help break down the cellulose in the food
they eat so it can be used for energy and nutrients. Bacteria in the human digestive system help
with the synthesis of vitamin K, which is essential for blood clotting. However, some bacteria are
pathogens (disease causers). Tuberculosis, tetanus, strep, diphtheria, anthrax, syphilis, and
some pneumonias are a few of the serious diseases caused by bacteria.
Chemical substances that either kill bacteria or inhibit bacterial growth are called antimicrobial
agents. Alcohol and some mouthwashes are antiseptics, and are used on cuts or wounds to
inhibit bacterial infection. Others, like chlorine bleach, are too concentrated or toxic for use on
living tissue. These are called disinfectants and are used on clothes, surfaces, or other nonliving
objects. These agents generally work either by disrupting the cell membrane, causing the
bacterium to lyse, or binding to the bacterium enzymes, which inhibit its activity. Even though
antiseptics and disinfectants are very useful in helping to prevent infections, we cannot use
them internally to treat an infection. If bacteria enter our bodies, we rely on another class of
chemicals called antibiotics to kill them. Although their use is now commonplace, antibiotics
were only discovered about 85 years ago. Before then, more people died from infections than
from all the wars in history combined. Antibiotics were the first of the ―miracle drugs‖ and they
have permanently altered the course of history.
The effectiveness of each type of antimicrobial agent is influenced by many factors. Some of
these factors include the environmental conditions in which the agent is applied, the chemical
properties of the agent, how long the agent has been stored, and the rate of deterioration of the
agent.
The procedure of placing bacteria on agar plates is called inoculation. Organisms so small that
they can only be seen with a microscope are living all around, on, and in us. They include
bacteria, viruses, molds, and yeasts.
In the laboratory, you will test the effectiveness of antibiotics in inhibiting the growth of bacteria.
You will grow samples of bacteria until they form colonies so big that you can see them with the
naked eye. You will grow your samples on sterile nutrient agar in a sterile petri dish. ―Sterile‖
means that there is nothing alive in the agar or on the dish. Nutrient agar supplies the nutrients
that microorganisms (not just bacteria) need to live and reproduce.
When you allow bacteria to grow, they will grow uniformly wherever their growth is not
effectively inhibited by a bactericide. This uniform growth is called a bacterial lawn and the
regions where no growth occurs are zones of inhibition. A large zone of inhibition is created by a
Biology HSL Page 289
Curriculum and Instruction