Lab – Diversity of Prokaryotes
#nevereatingyogurtagain
Introduction: Understanding the nature of cell structure and function is important
to an understanding of organisms. All organisms are composed of cells, whether
they exist as single cells, colonies of cells, or in multicellular form. Cells are usually
very small, and for this reason, a thorough understanding of subcellular structure
and function has been possible only through advances in microscopy and
molecular biology.
There are two general types of cells: prokaryotic and eukaryotic. These two words have
their root in the Greek word karyon (nut), which refers to a cell's nucleus. The prefix pro- means "before" or "prior to."
Thus, prokaryotic means "before having a nucleus." Prokaryotic cells do not have a membrane-bound nucleus and their
genetic material (DNA) is only loosely confined to a nuclear area within the cell. Bacteria are prokaryotes.
Prokaryotic and eukaryotic cells also differ in several other ways. Eukaryotic cells are generally larger and
contain additional specialized compartments (membrane-bounded organelles) in which cell functions such as energy
production may occur Prokaryotic cells lack membrane-bound organelles; their cell functions are carried out in the
cytoplasm.
Part One:
Bacteria have three basic shapes: Bacilli (rod shaped), Cocci (sphere shaped)
or Spirilla (spiral shaped).
Bacteria can also arrange themselves into colonies. Each arrangement is given
a special name.
- Diplo-, meaning paired
(ex: diplococcus, a pair of sphere-shaped bacteria)
- Strepto-, meaning chain
(ex: streptobacilli, a chain of rod-shaped bacteria)
- Tetrad, meaning group of four
(note: this is not a prefix. You would just say "tetrad cocci bacteria")
- Staphylo-, meaning random
(ex: staphylococcus, a random arrangement of sphere-shaped bacteria)
Scientists and doctors can identify bacteria by their shape and arrangement. For example, if you've ever had strep
throat, the doctor probably took a sample from your mouth or throat. They are looking for bacteria that are spherical
and in chains. The technical name of this is streptococcus. Hence the name: strep throat.
Part Two:
Yogurt is a fermented milk product which originated in Turkey. It is made by adding to
milk a bacterial culture which is a mixture of two species — Lactobacillus bulgaricus (or
Lactobacillus acidophilus) and Streptococcus thermophilus. The bacteria grow on the milk
sugar, or lactose. In the process of digesting the milk, the bacteria make an acid — lactic acid
— which curdles the milk protein, making the whole mixture thick. The lactic acid lowers the
pH of the milk and therefore gives the yogurt its tartness. Since the milk is partially digested,
yogurt is a mild food to eat because it is easily digestible.
In addition, the bacteria that live in the yogurt are some of the beneficial species that normally live in your
intestines. Because of this, people often eat yogurt after they have taken antibiotics (pills) for an infection. When you
take antibiotics, the medicine not only kills off the disease-causing bacteria in your infection, it also kills off the beneficial
bacteria in your intestines. So eating yogurt will allow you to replenish the beneficial bacteria in your digestive system
and help to decrease the digestive upset that often comes from taking antibiotics.
Part Three:
Bacteria can grow under very diverse conditions, which is why they are found nearly everywhere on Earth. Although
bacteria are good at adapting to their environments, certain conditions promote bacterial growth more than others.
These conditions include temperature, moisture, oxygen and pH. Knowing and avoiding these optimal conditions can
help prevent bacterial growth, bacterial infections and food poisoning.
Bacteria thrive in warm temperatures, especially those close to body temperature. The human body, therefore,
provides an ideal environment for many types of bacteria to grow. Certain strains of bacteria, however, can grow at
lower or higher temperatures.
Bacteria need water to grow and will die without a water source.
Moist areas are particularly prone to bacterial growth, such as bathrooms
and kitchens. Water content in food also provides an excellent
environment for bacteria to grow. Moist tissues in the body, such as the
mouth and nose, provide an excellent source of moisture for bacteria and
are particularly prone to bacterial growth.
The presence of oxygen can greatly affect the growth of bacteria.
Although some bacteria can survive without oxygen, many types of
bacteria require oxygen to grow. This is why many commercial foods are
vacuum-sealed. Once you open a food product and break this seal, exposure to the environment and oxygen limits the
shelf life of the product before it spoils. Keeping food properly sealed while it is being stored is a good preventive
measure against bacterial growth, because it restricts the amount of oxygen.
The pH of an environment -- a measure of its acidity or alkalinity -- is important for bacterial growth. Most
strains of bacteria prefer to grow in conditions with neutral pH, similar to the pH of the human body. Some strains of
bacteria, however, can live in slightly more acidic or more alkaline conditions. Cleaning solutions are typically highly
acidic or basic, which kills bacteria, because they cannot survive at these extremes of pH. The acidity of food is also
important for bacterial growth. More acidic foods can typically be stored longer without spoiling. Preserving agents that
increase the acidity of food are commonly added to help prevent bacterial growth and allow for longer storage.
Name ________________________________________________ Date _______________________ Period __________
Lab – Diversity of Prokaryotes
#nevereatingyogurtagain
Pre Lab Questions:
1. Describe the origin of the word prokaryotic.
2. Compare and contrast prokaryotic cells and eukaryotic cells.
3. How do prokaryotic cells and eukaryotic cells compare in size?
Part One:
1. List the three basic shapes of bacteria.
2. Using the naming rules of bacteria arrangements, draw the following:
Diplobacilli
Streptococcus
Tetrad spirilla
Staphylobacilli
3. Observe the slides of two preserved prokaryotic specimens. Do the following for each slide: identify the sample,
write the magnification, draw what you see on HIGH, describe and name the shape of the bacteria.
Specimen Sample: _________________________________
Total magnification (eyepiece x objective): ____________x
Bacteria description and name:
Specimen Sample: _________________________________
Total magnification (eyepiece x objective): ____________x
Bacteria description and name:
Part Two:
4. Summarize the process of how yogurt is made.
5. Why do most doctors suggest that you eat yogurt with probiotics when taking antibiotics?
6. Make a wetmount slide in order to observe the bacteria commonly found in yogurt.
a. With a toothpick, place a small drop of fresh yogurt onto a microscope slide.
b.
Add a small drop of water and then mix the sample with the toothpick.
c. Cover the sample with a coverslip.
d. Do the same thing for another type of yogurt.
Do the following for each slide: identify the sample, write the magnification, draw what you see on HIGH, describe and
name the shape of the bacteria.
Yogurt Sample: _________________________________
Total magnification (eyepiece x objective): ____________x
Bacteria description and name:
Yogurt Sample: _________________________________
Total magnification (eyepiece x objective): ____________x
Bacteria description and name:
Part Three:
7. Describe the ideal growing conditions for most species of bacteria.
You and your lab partners will be attempting to collect bacteria from an area of your choice, anywhere in the
school. The goal is to choose a location you think will grow a large amount of bacteria. When making your choice
you will want to keep in mind the ideal growing conditions for bacteria.
8. Where did you choose to collect your sample?
a. Describe your reasoning for choosing this location.
9. Petri dishes contain a substance called agar. What do you think agar provides to the bacteria?
10. Observe the samples collected from your class and determine which sample grew the greatest amount of
bacteria. Draw a conclusion regarding the ideal locations for bacterial growth within Lassiter High School.