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Bacterial Growth and Inhibition Lab V3
Name: _________________ Partner: ______________ Due Date: _______________
Complete a self-assessment before submitting the lab report. Be sure all work is attached.
Section
Pre-Lab Question
- materials list
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-storyboard
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- purpose
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- hypothesis
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- 6 variables affecting bacterial growth
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- independent variable
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- dependent variable
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- controlled variables
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- reason for observing a not inoculated plate
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0
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1
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2
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Conclusion
States whether or not the hypothesis was supported
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1
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Discussion and Application Questions
- conditions required for bacterial growth
- incubation direction
- alcohols effectiveness
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1
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2
Errors and Improvements
- two sources of error
- improvement to lab design
- future research directions
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Observations – must hand in original data
- Logical design of table for 2 days and 4 quadrants
- Observations include the colour, amount of growth(% of surface
area), surface texture of colonies, edges of colonies, height of
colonies
- The sketch follows all of the rules for a scientific drawing: uses
pencil, blank paper, title, labels, drawn to scale, stippling, neat
Overall Communication
a) The lab report is complete and in the correct order, is neat
and easy to read and on time. Label each section.
b) Grammar, spelling and sentence structure are correct
c) Uses formal laboratory language , written in the third
person past passive verb tense, “my bacteria growth was
excellent” vs “The bacteria growth was noticeable.”
TOTAL
2 3
2
2
2
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Bacterial Growth and Inhibition Lab - 2011 (V3)
Name: _________________
Partner: _________________
Introduction
You can find bacteria everywhere, even on your desk and on your skin. Although they are
much too small to be seen individually without the help of a powerful microscope, you can
see evidence of their presence by growing colonies in this lab. In the first part of this
investigation, you will collect bacterial samples from our school and investigate the effect of
an disinfectant on bacterial growth. After a few days of incubation, you will examine and
describe the bacterial colonies and possibly fungi growth.
There are many variables that can affect the growth of bacteria. In this lab bacteria from
surfaces in the school will be cultured and the use of alcohol to inhibit bacterial growth will
be examined. The lab answers will be written in a formal style with the use of the third
person past passive verb tense.
Safety:
You must use caution and good judgment when working with bacterial cultures. Always
wear gloves and wash your hands after handling any culture. Petri dish lids should remain
closed unless you are transferring samples. Once you have collected your samples, your
Petri dishes must be closed and never opened. You must follow your teacher’s instructions
for disposing of all swabs, cultures and Petri dishes.
Note: For safety reasons, you are not permitted to sample inside washrooms, or take
samples from individuals. Samples from here will receive a mark of zero.
Pre-Lab
1.
Carefully read the entire lab and the rubric before answering the following questions.
2.
Complete a two column list of the materials.
3.
Sketch a storyboard with 6 scenes showing the procedure.
4.
Make an observation table.
The following answers must be neatly written on a separate paper. Double space. Use your
text for support.
5.
What is the purpose of the lab?
6.
State a hypothesis.
7.a) List six variables that will affect bacterial growth.
b)
What is the independent variable in the lab?
c)
What is the dependent variable in the lab?
d)
What are three controlled variables in the lab?
e)
Why is a not inoculated plate required for observation?
Materials:
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Procedure:
1. Wipe your desk thoroughly with cleanser. Dry the desk with paper towels.
2. Turn the Petri dish upside down and using a grease pencil or permanent marker, label
the bottom as shown below. Add the date and your initials to the edge of the plate.
A
B
C
D
Note: Prior to collecting Samples B and D, the area will
be sterilized with alcohol.
3. Collect bacteria from 2 different locations in the school. Record the locations on your
observation table.
a) Sample Transfer technique
Transfer the sample immediately after collection. To transfer samples, slide off the
lid of your Petri dish, using a “Z” pattern, gently streak the surface of the agar with a
swab of each sample. Ensure that you place your sample in the center of the
quadrant.
b) Location 1
Sample A and B represent samples that will be taken from the first location. Sample
A will be collected by dipping the swab in the source agar bottle, and then gently
wiping the surface of the selected object. Next the swab will be gently swiped across
the agar plate quadrant A. Do not break the surface of the agar.
Prior to taking Sample B, the area will be disinfected with rubbing alcohol. Repeat
the same process as above and swiped on quadrant B.
c) Location 2:
Quadrants C and D represent samples that will be taken from the second location
Sample C will be collected by using the procedure outlined for Sample A. Sample D
will collected in the same manner as described for Sample B.
4. Bring all materials back to class. Put all materials in their designated spots. Dispose
of the swabs in the biohazard disposal bag provided by your teacher.
5. Tape the Petri dishes closed, turn it upside down and place it in the incubator.
6. Spray Enviroclean on desk and wipe. Wash your hands with soap and warm water.
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Observations
In two subsequent classes the petrie dishes will be examined. The following page provides
information about features of bacterial colonies, that can be observed. Record qualitative
observations of the appearance of the bacterial colonies.
Using the grid provided, determine the % of each quadrants’ surface that the total bacteria
colonies occupy. This is a quantitative observation.
On the second day of observations complete a sketch of the petrie dish as well as the day 1
observations.. Use the circle on the last page of the handout for the diagram.
Be sure to record observations of the Petri dish that was not inoculated with bacteria.
Observation Tables
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The following answers must be neatly written on a separate paper. Double space. Use your
text for support.
Conclusion
Write a statement about whether or not the hypothesis was supported.
Discussion and Analysis
1.
What general statements can be made about the conditions required for bacterial
growth?
2.
Why are petrie dishes incubated upside down?
3.
What general statements can be made about the effectiveness of alcohol as a
disinfectant based on the results of this lab?
Errors and Improvements
1.
Describe two possible sources of human error in this lab.
2.
What is one suggestion for improving the lab design? This is not asking about how to
correct the human errors.
3.
What are two future research questions that could be asked based on the results of this
lab?
Criteria Used to Describe Bacterial Colonies

Form - What is the basic shape of
the colony? For example, circular,
filamentous, etc.
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Elevation - What is the cross
sectional shape of the colony? Turn
the Petri dish on end.
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Margin - What is the magnified
shape of the edge of the colony?

Surface - How does the surface of
the colony appear? For example,
smooth, glistening, rough, dull
(opposite of glistening), rugose
(wrinkled), etc.

Opacity - For example,
transparent (clear), opaque,
translucent (almost clear, but
distorted vision, like looking
through frosted glass), iridescent
(changing colors in reflected light),
etc.

Pigmentation - For example,
white, buff, red, purple, etc.
From: http://www.sciencebuddies.org/science-fair-projects/project_ideas/MicroBio_Interpreting_Plates.shtml
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Sketch of Petrie Dish – Observation Day 2
Follow all the rules for a good scientific diagram listed on the rubric.
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Bacteria Lab Answer Key
This has not been updated to reflect the revised lab (Ellen march 2011)
Pre-Lab
- cold, antibiotics, disinfectants like alcohol or hydrogen peroxide, sterilization (heat), lack of nutrients, excess
waste in environment
Purpose
To study the effects of alcohol on the growth of bacteria
Hypothesis
Using alcohol to disinfect a surface area will decrease the number of bacterial cultures that grow from a swab
of that area.
Observations
- neat table, set up to observe twice, Day 1 and Day 2 and within that by 4 different quadrants
- well labelled observation criteria could include colour, texture, edge, height, % surface area growth, form,
opacity
Conclusion**
The hypothesis that disinfecting a surface area with alcohol would decrease bacterial growth (was or was not)
supported. Do not say proven. Proof is used for scientific laws where an incredible amount of repeated data
has been gathered in many ways that shows the result is always the same. Ex. The Law of Gravity has been
proven.
Discussion
a) **Controlled variables – incubation temperature and light, same type of agar, same plate, same amount of
oxygen, incubation time. Mention of alcohol will lose a mark or locations.
b) For bacteria to grow they require some warmth, nutrients and moisture, often oxygen, correct pH
c) In general alcohol is a good disinfectant and kills bacteria on surfaces like door knobs, hands, taps,
Errors
a) Possible sources of error in this lab include:
- contamination of plates before culturing,
- inconsistency in observations due to absences
- lids being left off while swabs were taken so airborne bacteria landed on plates,
-researchers did not allow alcohol to dry before taking swab
- agar was contaminated
- lack of repetition of the experiment many times to get better data
- researchers put samples in the wrong place in the petrie dish
- breaking surface of agar and having bacteria grow sideways so inaccurate % surface area measurement can
be taken
b) Improvement
- two plates per team to increase reliability of data
- incubate at a warmer temperature
- better locations for swabbing
c) Which type of bacteria have their growth inhibited by alcohol?
Would washing an area with soap and water inhibit bacterial growth?
Would incubating the bacteria at a warmer temperature have shown more bacterial growth inhibition in the
alcohol swabbed areas?
Would antibiotics inhibit the growth of bacteria
Anything about fungi vs bacteria
Communication
- ** scientists think, belief is for philosophers
- formal science writing should be like your textbook. Never use I, you, he, she, me and rarely use it.
- the third person past passive verb tense changes “ I swabbed the desk” to “ The desk was swa bbed”
Note: disinfectant used to clean a surface while antiseptic use to clean tissue, alcohol can be both but not
ammonia, both are to prevent infection.