Fermenting Yogurt at Home Lab Report
Fermenting Yogurt at Home Lab Report
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The primary objective of this report is to interpret results and discuss limitations/alternative
conclusions based on a culture of challenging bacteria and experimental results. This
experiment can vary from the starting material, and the results are primarily visual observations.
The last experiment focused on design, the experiment before the last focused on technique,
and this current experiment has a large focus on interpretations. The secondary objective is to
demonstrate lab report writing skills, this time with a more subjective results and variable
starting material. The latter will be done using a standard manuscript format, although scientific
writing is a universal skill for any format (e.g. charts, lab notes, memoranda).
CONTENTS
I. OVERVIEW
II. INSTRUCTIONS
III. SPECIFIC LAB REPORT & SUBMISSION INSTRUCTIONS
IV. EXAMPLE OUTLINE OF MATERIALS AND METHODS
V. APPROXIMATE GRADING
VI. GENERAL LAB REPORT INSTRUCTIONS
VII. EXAMPLE STUDENT LAB REPORT
I. OVERVIEW
Yogurt culture experiments can be done with safe household items. Although items are
generally regarded as safe, it is important to properly sanitize or disinfect equipment and
surfaces after the experiment. Do not allow children or others to come into contact with aspects
of the experiment, and never ingest any product of the experiment. Contact the instructor if
there is a safety concern or any other reason an alternative lab is needed.
This experiment is to culture yogurt from a commercial starter culture and observe changes in
the milk medium. The protocol will mostly follow lab exercise 7-8. Variations can come from
brand of starter yogurt used, milk type and quantity, temperatures used and method of
measuring, containers, and option to take pH measurement. You may follow the lab book or
example protocol. There are several examples on the internet if you need more examples, and
you are required to have a minimum of two references in your report. You will need to take
pictures of the observations and methods, and taking notes may be useful for writing the report.
The grade of this lab is NOT dependent on expected results. However, it is affected by
rationalizing methods, observations, and discussing your results.
II. INSTRUCTIONS
1) Read the lab book exercise 7-8, example outline, research the internet, and plan out your
experiment. At a minimum, you will need yogurt with active cultures, milk, bowl, stirring utensil,
and a heating source. Typically you would use a pot, stovetop, whisk, thermometer (of any kind,
as long as there are no safety concerns), plastic wrap, measuring cup, and a clear plastic bag.
Some students choose to measure pH (you could alternatively research the beginning and
ending expected pH measurements for the report).
Kitchen Yogurt Culture & Lab Report
2) Plan out your methods. Decide the amounts, temperatures, and time periods you will use.
You can follow one protocol or combine aspects, but remember to cite your sources and use
detailed descriptions. For example, a student who does not have a measuring cup may use a
standard size coffee mug to measure milk. A student who does not have a thermometer may
combine a specific amount of hot tap water (usually around 65°C/149°F) with part cold tap water
to estimate a temperature. Alternatives do not affect your grade, but lack of detail will.
3) Execute your experiment. Do not forget to make observations at the start. If you plan to
observe thickness in a plastic bag at the end, do the same at the beginning (and same thing if
you plan to let it fall off a spoon or fork). If you do not see results, this is not a problem.
However, the results, interpretation, and alternatives must be discussed in your report.
4) Finish and write report. When your last observations are made, materials can be discarded
and other items can be cleaned. Properly disinfect any working surfaces, and wash/sanitize any
containers and utensils. A small amount of bleach is recommended, but not needed if you have
other preferred methods. Do not ingest your experiment as part of this class.
III. SPECIFIC LAB REPORT & SUBMISSION INSTRUCTIONS
General lab report instructions for a manuscript format are below in another section. If you are
not familiar with this standard format, read those instructions first. These contain the second and
third most common mistakes, which are not citing sources and not using paragraph format like
listing items or using bullet points.
Details remain important in this experiment, specifically on observations, and the interpretations
by association. Having expected results is not a grading criterion for this activity, but discussing
any problems or other alternatives is. Specific to this report, you must include at least two
references. Below are other considerations specific to this report:
Introduction1) What was the purpose of this experiment (i.e. what objective do you accomplish)? What
bacteria are you trying to culture? What classification are these bacteria?
2) What conditions are optimal for this growth (temperature, pH, etc.)? What are potential
variables you can change in the culture?
3) Briefly preview (summarize) what was done in the experiment and the end results.
Materials & Methods1) What materials did you use (sentences, no listing!)? What brands (as applicable)? Was there
any preparation, such as letting the commercial yogurt incubate at room temperature?
2) Describe your methods as you did them. Do not personalize (I, my, we) and cover beginning
to end (preparation, execution, and analysis of results). Specifically mention the observation
method of beginning yogurt, mixture, temperature, pH, and incubation periods.
Kitchen Yogurt Culture & Lab Report
Results1) Describe observations at the beginning of the experiment. What color, thickness,
homogeneity, bubbles did you observe?
2) Describe all the changes in from the beginning of the experiment to the end. What were the
temperatures? What was the pH or expected pH? Was there any color change in the solution?
Was there a change in thickness or bubbles?
3) Focus only on observations, conclusions and interpretations belong in the next section.
Discussion1) What conclusions can you make from your observations? Can this be interpreted as growth
or no growth?
2) How did your results lead to your conclusion? Did you deviate or change something that
helped or hurt the culture?
3) Was there anything unexpected in the results? Were there other parameters that could have
been measured or measured better (e.g. pH strips or a thermometer)? Could your protocol be
modified for an anticipated better result?
References (Bibliography)
1) Reference each source you used in APA format. Then cite the references you used in the
text of report, generally at the end of the sentence it applies to. There is a minimum of two
references for this report. Refer to “General Lab Report Instructions” for information/links on
APA citations.
Figures & Legends
1) Post each picture you took here, after the references. Refer to each figure in the appropriate
section of your paper (such as the Results) by number based on when you discussed these (i.e.
Picture 1, Picture 2, etc.). Write a short description below it (legend), usually 1 or two sentences.
Activity submission: Save the report as one file (pictures embedded) and upload it to the
submission folder on the course website (preferably Word file, .doc .docx .rtf .pdf). You
are always welcome to email a backup copy to the instructor. Typical submission is
about 3 pages single spaced (excluding figures).
IV. EXAMPLE OUTLINE OF MATERIALS AND METHODS
The following is an example from a past student written as an outline, not a lab report. There are
notes from the instructor at the end.
Materials:
Milk (Whole, 1%, low-fat, or skim)
Starter yogurt (Great Value plain yogurt &Yoplait Light Vanilla yogurt)
Kitchen Yogurt Culture & Lab Report
Medium-size saucepan
Medium-size bowl
Wire whisk
Cooking thermometer
Measuring cup
Plastic wrap
pH paper
Materials used
Method: First Attempt
First step is to gather all materials needed to perform this project (get as close to
what is called for as possible) and prepare the stove for heating up the milk.
Slowly, heat 5 cups of milk in the saucepan to 185 degrees F with the heating
thermometer. Be careful not to rush this step because the milk will reach its
boiling point and may not work.
Once the milk has reached 185 degrees F, remove the milk and let it cool to 110
degrees F.
Next place ¼ cup of the starter yogurt in the medium sized bowl.
Slowly, stir in cooled milk at about 1/3 to ½ cup at a time. Mix after each addition
until the blend is smooth. If the mix is not completely smooth a nice thick texture
is also a good sign.
Cover the bowl with plastic wrap and puncture it several times to allow gases and
excess moisture to escape.
Take note of the cultures that are present in the yogurt starter.
Incubate for about 5-6 hours at 30-35 degrees C (85-95 degrees F).
Once incubation is complete the yogurt should be thick just like the starter yogurt.
Make sure to test the pH and check for any smells, color changes, bubbles, or
anything that wasn’t there before incubation.
Method: Second Attempt
First step is to gather all materials needed as specific as the directions call for
Kitchen Yogurt Culture & Lab Report
and prepare the stove for heating the milk.
Slowly heat 2 cups of milk in the saucepan to 185 degrees F with the
temperature thermometer. Be sure the milk doesn’t heat up too fast causing it to
boil.
Once the milk has reached 185 degrees F, remove the milk and let it cool to 110
degrees F.
Next place 2 TBS of the starter plain yogurt in a bowl. Slowly, stir in cooled milk
and mix after each addition until the blend is smooth or somewhat thick.
Place half of the mix in an air tight jar and the other half in another jar with a
plastic wrap cover with holes in it.
Incubate for about 15 hours at 85 degrees F.
Once the mix has reached a yogurt like texture, check the pH and for smells,
color changes, bubbles, or anything else that wasn’t there before incubation.
Yogurt Culture: (Same in both)
Active yogurt cultures, L. Acidophilus, bifidus, Lactobacillus bulgaricus and
Streptococcus thermophilus, lactic acid-producing bacteria
Observations: Second Attempt
My second attempt was much more successful to say in the least. Instead of
using 5 cups of milk I switched to only 2 cups of milk (mostly because that is
what I had left from attempt 1).
While boiling the milk I made sure that it did not reach boiling point but still made
it to the appropriate 185 degrees with the cooking thermometer. The milk will
start bubbling around the edges when it gets close to 185 degrees.
Once the milk reached this temperature it was cooled to 110 degrees.
Next, I mixed it in with 2 TBS of the Great Value plain yogurt. It wasn’t as smooth
as the instructions mention it will be but it was much thicker.
Then I divided the mix up into two little jelly jars, one with an air tight lid and the
other with the plastic wrap with holes.
On the side, I also placed some of the mix in a baggy to see if any air bubbles
would arise from fermentation. After setting out for 15 hours, bubbles did form
that were not originally there when I began, but it is hard to know for sure if the
yogurt made them or if they were already there.
I left the two jars to sit for 5 hours but the mix was still not set up like it should
have been, so I left the mix out all night and checked it in the morning.
I finally made yogurt!
Next I checked the smell, texture, and pH of both jars.
The jar with the air tight lid had a milky smell and was also thicker. I also noticed
condensation on the top of the lid and yellow bubbles on the top layer of the
yogurt. If I had to guess I think it was the whey created from the milk.
As for the second jar, it had a more mild smell, was a little thinner, had no
yellowish bubbles, and no condensation on the plastic wrap.
Both pHs were tested with aquarium pH strips and both had a pH of about 5.
Note, it is very hard to find pH paper but I did find some at Wal-Mart in the pet
department with the aquarium things. The pH should be some what acidic.
Also, there was no noted color change, except for the yellow bubbles that formed
in the yogurt in the jar with the lid. Everything was a milky white color all through
Kitchen Yogurt Culture & Lab Report
out.
Jar on the left has plastic wrap w/ holes
Jar on the right has an air tight lid
Kitchen Yogurt Culture & Lab Report
Jar with plastic wrap lid after This jar was a little thinner
Noticeable what could be fermentation after the yogurt was left out
Observations: First Attempt
When I began to heat my milk it slowly heated to the appropriate temperature,
but it also reached the boiling point (which it is said that it shouldn’t).
While adding the milk to the yogurt it never gave a smooth look or texture.
I then placed the plastic wrap over the bowl and punctured holes to the top.
After setting out for about 6 hours the mix never became smooth and began to
smell bad so I threw it out without testing anything else.
I think there a few reasons why my first attempt didn’t work, the first being that
the milk reached the boiling point. A second idea was possibly not enough, and
the wrong kind, of starter yogurt. Take note to what the directions say to use and
get as close or specific to them. A third idea is not leaving the mix out long
enough to set up properly. If the mix is not thick by the fifth or sixth hour leave it
out longer until it is.
Notes from the Instructor
-You will be submitting a manuscript-style lab report, not anything similar to this presentation
-Using a closed jar is interesting, but trying with and without is not needed.
-Avoid using flavors and added fruit. It likely will not help, and can make observations more
Kitchen Yogurt Culture & Lab Report
difficult.
-Look to see if your starter yogurt lists active cultures. If not, say so and discuss this in the
report.
-Boiling the milk can cause chemical changes to will affect the experiment.
-Pay close attention in any protocol if temperature is listed in Celsius or Fahrenheit.
-More starter yogurt and/or less milk can make the process faster, and feel free to scale down.
-Eating or any consumption of this yogurt is not part of this assignment or this class. There is a
slight risk of growing unintended microbes with the yogurt culture. Do not consume any starting
or ending product, and do not discuss any consumption or tasting of the products in the lab
report.
-Letting the commercial (starter) yogurt sit at room temperature for hours before adding can
activate growth.
-Grading will not be affected if pH measurements are not taken. However, pH should still be
discussed and expected values can be found from online resources.
-If there is any safety concern with doing this lab (allergies, hazards, etc.), please discuss this
with the instructor before starting. Alternative projects, such as writing the report without doing
the experiment, are allowed upon request.
V. APPROXIMATE GRADING
15% Introduction, detailed Method and Materials
15% Detailed observations
15% Discussion of results, alternative conclusions
15% Discussion of other potential experimental methods/alterations
20% General scientific writing
10% Minimum of two references and citations in text
10% Figures and legends, and referenced in the text
VI. GENERAL LAB REPORT INSTRUCTIONS
A. Lab reports are divided into sections. Each section has a heading and provides clarity to the report,
and serves a purpose (avoid the wrong information in the wrong section!). Reports in this course all
contain the sections (described below):
-Title/Byline
-Introduction
-Materials and Methods
-Results
-Discussion
-References
-Figures, Tables, and Legends
B. Lab reports are written in paragraph form. With the exception of the References Section, any pictures,
charts, tables, and lists go in the last section. Anything in the last section is referenced and described in
the paragraph text of the previous sections.
C. Length and word count ranges are only suggestive (no direct grade impact). Shorter papers often have
missing information and longer papers tend to be unfocused.
Kitchen Yogurt Culture & Lab Report
D. Materials and Methods/Results sections are written in past tense (what happened, not a procedure for
someone else).
E. A very fundamental aspect in science and this course is knowing the difference between observations
(results) and interpretations (conclusions). An observation is a measurement, detection (when equipment
is used), or something you see. An interpretation is what the observation implies, common knowledge
conclusions, or what you think it suggests. The lab workbook is full of examples. Keep in mind
interpretations are not 100% correct, and there is always some possibility of error (students often forget
this on common experiments, e.g. Gram stains).
F. Your grade is considered your primary feedback. Grades are usually posted within two weeks of the
due date. If you desire specific feedback on what impacted the grade, email the request to the instructor
and generally the three aspects that impacted your grade the most will be returned. You are always
welcome to ask for more feedback after. You can always ask for feedback on a draft before submission.
Generally give a few days for comments (start early, don’t wait until the deadline is near).
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G. Reports can always be resubmitted up to the deadline. Emailing copies to the instructor before the
deadline is encouraged. If your grade is a zero because it did not submit correctly, the instructor will
grade the emailed copy upon request (and no penalties applied if original email was received before
deadline). Please note that emailed copies are not automatically graded.
H. The following is more information on items for each section:
Title Section
-Title (come up with something relevant)
-Byline (your name, course section)
Introduction
1) Basic Introduction (1 paragraph)
What is the scientific objective of the experiment? What is the concept, or how do you plan on primarily
achieving this objective? How does this relate to other applications, theories, the environment, diseases,
industrial/lab/medical aspects, etc?
2) Background (1-3 paragraphs)
State any relevant microbiology background details, such as some information about a microbe genus
possibly involved. Then, provide information on how the experiment aspects work (not how to do it, but
what is actually happening during the experiment). Don’t forget details of aspects later in the report (e.g. if
you measure pH, mention here how pH could affect growth). [In a simple experiment, this can be short. In
complex experiments, touch as each aspect.]
3) Summarization overview (1 paragraph)
This is a summary of the next three sections. At a minimum, give a couple sentence summary of the
discussion including your main interpretation. Also, include any introduction, theory, or background details
that didn’t quite fit in before.
Materials and Methods
1) Materials (1 paragraph)
Describe the materials you used in this lab, and brands, sizes, and when it came from.
2) Methods (1 paragraph for each individual method, analysis method can be a separate paragraph)
Be comprehensive, start at the beginning and continue until the lab activity was complete. Remember to
right in past tense, or how it was done (not instructions for how to do it). Do not write “how to” do it, but
give enough detail that a classmate could repeat what you did based on your account. Finally, end each
method with how the experiment was analyzed (i.e. how results were observed, compared).
Results
Describe all beginning and end states, and any changes in between. (1 paragraph per experiment)
Note all observations and any measurements taken during the experiment (including no change). Make
statements regarding the consistency of observations (e.g. approximately half of the colonies appeared
Kitchen Yogurt Culture & Lab Report
golden, and these were not isolated from the red colonies). Remember that a result is a measurement or
observations (e.g. you do not see lysis of red blood cells, you see discoloration in the plate). Do not forget
any of the positive or negative control results, or results of any comparisons (including expected results
from the book, or from another external source).
Discussion
1) Interpretation of the results (1 paragraph or 1 paragraph per method)
Give a summary/recap of each result and state what you believe it indicates (e.g. The purple color of the
cells suggest these cells are Gram-positive.). Then state why this is the likely conclusion (e.g. The purple
Gram stain did not appear to wash out after alcohol was applied). Remember that every
interpretation/conclusion can have some degree of error. Never say “prove”, but instead use words like
suggest, indicates, etc.
2) Conclusion (1-2 paragraphs)
Summarize the conclusion(s). Start with the conclusion you are most confident with, and move to less
confident conclusions. This is often going from less to more specific (e.g. The Gram-stain results suggest
we isolated Gram-positive bacteria. Based the shape, these bacteria appear to be the Staphylococcus
genus. Based on where they were sampled from, the bacteria are likely Staphylococcus aureus.). In the
same or a new paragraph, state how much scientific confidence there is in the interpretations (e.g. Both
the positive and negative Gram stain controls were purple and red cells as expected, which suggests the
unknown sample was indeed Gram positive. The shape and arrangement of cells were more difficult to
observe, and it is possible that another genus was present.).
3) Summary of conclusion and limitations (1-2 paragraphs)
Recap the conclusions, what could not be strongly concluded, and add any limitations, while associating
the conclusions with the objectives of the experiment (e.g. The objective of this experiment was to identify
the genus of a skin sample. The results strongly indicated the bacteria were Gram-positive, but this
simplistic method was not able to determine the genus with high confidence based on equipment used.).
State an alternative conclusion, and compare these results to expected results of the alternative (e.g. The
cells could have also been Streptococcus, which are Gram-positive and spherical too). State further tests
that could provide more evidence, and how (e.g. The experiment could be repeated with a higher
powered microscope to see the arrangement detail, or a biochemical method such as streaking the
sample on an MSA agar plate and comparing growth/agar color would help differentiate these
alternatives.). Finally, state any issues or concerns with the experiments and how it may have impacted
the results (e.g. After the triple streak method, the colonies were not completely isolated for sampling.
This could have lead to a mixture of cells, however the colonies appeared to have the same morphology
characteristics and the cells after the Gram stain were consistent across the microscope slide.).
References
List out a bibliography of all resources used. The APA format is required, but any other format that has
the same information will not lose points (i.e. use any format you want, but if something is missing the
APA requirement will be referred to for why the reference is incorrect). Be sure that all citations and cited
in the text. At a minimum, you will generally need to cite the lab manual and text. It is alright to reference
a book and cite different chapters and sections separately in this course. Choose good external
resources, and do not directly use anonymous resources (see Wikipedia Policy in First Day Handout).
APA examples can be found all over the internet, including:
http://www.liu.edu/CWIS/CWP/library/workshop/citapa.htm
Citation in the Text:
During the turbulent 1960s, science fiction programs on television reflected the public’s attitudes toward
the older generation (Hodges, 2000).
Bibliography Listing for a Book Article or Chapter:
James, N. E. (1988). Two sides of paradise: The Eden myth according to Kirk and Spock. In D. Palumbo
(Ed.), Spectrum of the fantastic (pp. 219-223). Westport, CT: Greenwood.
Another link for a brief tutorial and basics of APA format:
Kitchen Yogurt Culture & Lab Report
http://flash1r.apa.org/apastyle/basics/index.htm
Additional links on citations:
http://www.mhhe.com/biosci/genbio/maderinquiry/writing.html
http://www.csub.edu/chemistry/331/laboutline.html
Figure, Table, and Legend
In this section, place all figures, tables, etc. in the order the item is referred to in the report. Remember
that any item in this section must be mentioned in one of the previous sections of the lab report. These
items are intended as aids, and do not replace descriptions in the report (i.e. a picture of a result does still
needs to be described in the Results section). Each item must have a title (Words, numbers, or letters)
and a legend (e.g. Figure 2: Isolated colonies on the agar plate after incubation period).
I. The following is a list of general lab manuscript considerations:
1. Write information in a concise, organized, and easy to read manner. The report should “flow”.
2. Do not personalize (e.g. I, my, mine, we, us, you, your).
3. Do not use contractions (e.g. Don’t, can’t).
4. Do not give possession to materials (e.g. yeast’s, bacteria’s, plate’s).
5. Declare all abbreviations.
(e.g. Gram-positive (Gram+) bacteria can contain toxins. Not all Gram+ bacteria have toxins.)
6. Repetition is encouraged, but make sure each aspect is repeated appropriately. Example:
Introduction: Gram staining is a differential stain.
Materials and Methods: Gram staining was finished by adding crystal-violet and iodine.
Results: Gram staining of a sample from the colony resulted in a purple color.
Discussion: Gram staining the unknown bacteria suggested it was Gram-positive because it appeared to
retain the purple color.
7. The introduction should introduce the rest of the paper (i.e. overall project, microorganisms possibly
involved, and each experiment should be covered with background information and purpose).
8. Avoid quoting other sources when it is something you can paraphrase. Remember, this is an
evaluation of your knowledge.
9. Any other subheadings (beyond main headings like Introduction, Materials & Methods, Results,
Discussion, References) should come at the beginning of the paragraph; never bold and underline
something inside a paragraph (with the exception of using italics or underlining the genus and species of
a microorganism).
10. Proper names are capitalized, including brand names. Sometimes a dye or a test is capitalized to
because it is named after a proper noun. (e.g. Identification is made by a Gram stain, first invented by
Hans Gram. We use mannitol salt agar to differentiate bacteria that can tolerate a high salt environment.
If you are unsure about capitalization, at least be consistent (your book gets confused too).
11. Provide alternative conclusions in your discussion. These can be based on a tests with unclear
results, tests that you were not able to perform, or additional tests that could have confirmed conclusions.
Kitchen Yogurt Culture & Lab Report
12. Conclusions are usually not absolute. Apply words such as suggests, probably, most likely…
13. Italicize the genus and species of a microorganism. (e.g. Escherichia coli)
14. The genus of a microorganism is abbreviated by the first letter every time it is mentioned after the first
time. (e.g. E. coli). The genus and species are always italicized (underline if italics are not an option).
15. Book and periodical citations hold more credit than general websites. If you find the book online, cite it
the same as if you had it physically. Avoid using anonymous sources such as Wikipedia.org.
16. Use past tense when describing what you did in the lab and present tense when making statements
of facts and chemical properties.
17. Write the report to the audience of your peers in terms of level.
18. Introduction, Materials and Methods, and Discussion sections will always have citations. Results
section will sometimes have citations.
VII. EXAMPLE STUDENT LAB REPORT
The following is a past example of a lab report. They had different instructions and many mistakes, but
may serve as examples for you. If you can identify the mistakes, then you are on the right track. Some
parts have been omitted, such as references and figures.
Introduction
The purpose of this experiment was to determine the causative agent of bacterial
contamination of a plastic bottle filled with drinking water, belonging to a college student. Upon
drinking the contaminated water the student presented with gastrointestinal problems and was treated
with antibiotics. After treatment with antibiotics that work on Gram positive and Gram‐negative
bacteria, the student recovered and symptoms disappeared. Based upon the symptoms the student
presented with, it is suspected that the culprit of contamination and illness is Escherichia coli.
The experiment began with a 10‐mL sample of cloudy water obtained from the plastic bottle.
The sample was inoculated on a nutrient agar plate, incubated, heat‐fixed, and Gram stained. The size,
shape, color, and morphology of the microorganisms were then studied for determination of bacterial
contamination.
A microbial culture can be a mixed culture or pure culture. A mixed culture is a microbial culture
consisting of two or more different types of species. A pure culture consists of only one type of species.
When dealing with a mixed culture, it is important to isolate the individual species so that proper
identification can be made. A common technique used for isolating species in a mixed culture is the
streak plate method. “In the streak plate method of isolation, a bacterial sample is streaked over the
surface of a plated agar medium” (Leboffe & Pierce, 2008). Cells that have been isolated grow into
colonies consisting only of the original cell type.
Basic stains are used to help identify bacterial species. A basic stain causes the bacterial cells to
become colored. Prior to staining, the bacteria are prepared into a bacterial smear and heat‐fixed.
Kitchen Yogurt Culture & Lab Report
Heat‐fixing kills the bacteria, allowing them to stick to the slide, and coagulates cytoplasmic proteins to
make them more visible (Leboffe & Pierce, 2008). Staining the cells with colored dye make them more
visible under the microscope.
Gram stain is a differential stain. A differential stain allows a microbiologist to detect
differences between organisms or differences between parts of the same organism. In a gram stain, the
primary stain crystal violet, is applied to the cells. Iodine is then added to enhance the crystal violet
staining. Decolorization then follows. Applying alcohol or acetone causes decolorization. Gram‐
negative cells become decolorized, whereas Gram‐positive cells do not. Gram‐positive cells remain
crystal violet.
Materials and Methods
Equipment used: one 10‐mL sample of cloudy water, one sterile inoculating loop, one nutrient
agar plate, one Bunsen burner, one clean glass microscope slide, Gram stain crystal violet, Gram stain
safranin, Gram stain iodine, 95% ethanol, bibulous paper, squirt bottle with distilled water, slide holder,
disposable gloves, staining tray, staining screen, one compound microscope with oil objective lens,
immersion oil, lens paper
To isolate the species in the mixed culture, a sample was aseptically obtained from the mixed
culture with a sterile loop. To aseptically obtain the sample the tube containing the 10‐mL sample of
cloudy water was held at an angle to minimize the chance of airborne microbial contamination. The
inoculating loop was then carefully dipped into the tube and a sample was obtained from the cloudy
water. The lid of the sterile agar plate was then lifted and used to shield the agar plate from airborne
contamination. The quadrant streak method was used to transfer the sample from the inoculating loop
to the agar plate. Beginning in one quadrant of the agar plate, using the inoculating loop, the mixed
culture was streaked back and forth. Light pressure, and the side of the end of the loop were used to
reduce the chances of cutting the agar. The loop was removed from the agar plate and the lid replaced
onto the agar plate. The loop, from base to tip, was then sterilized by passing it through the flame of a
Bunsen burner. The agar plate was rotated 90 degrees. Using the sterile loop, the agar plate was again
streaked, beginning at one end of the first streak pattern, in a back and forth fashion. This procedure
was repeated twice more until there were four streaks on the agar plate. After each streak the loop was
sterilized and the agar plate rotated 90 degrees. The plate was labeled with the date, time, and sample
inoculated. The plate was then inverted and incubated at 25 C for 48 hours.
A small drop of distilled water was placed on a clean slide using a sterile inoculating loop. A
sample of bacteria was aseptically obtained from the agar plate using an inoculating loop. The
inoculating loop was sterilized by flaming it using the Bunsen burner. The lid of the agar plate was lifted
and used to prevent airborne contamination. The loop was touched to an uninoculated portion of the
agar plate to cool. After cooling, a small amount of bacterial growth was obtained by gently touching a
colony with the wire tip of the loop. The loop was carefully removed from the plate and the lid was
replaced. Using the loop, the bacteria were then added to the slide and mixed with the water on the
slide. The loop was then flamed. The smear was allowed to air dry. To heat‐fix the preparation, a slide
Kitchen Yogurt Culture & Lab Report
holder was used to pass the smear through the upper part of a Bunsen burner flame two times. The
slide was then allowed to cool.
Wearing gloves, and holding the slide over the staining tray, the smear was covered with Crystal
Violet stain for one minute. Grasping the slide with a slide holder, the slide was gently rinsed with
distilled water. The smear was then covered with Iodine stain for one minute. The slide was then gently
rinsed with distilled water. The smear was then decolorized by applying 95% ethanol to the slide until
the run‐off was clear. The slide was then gently rinsed with distilled water. The smear was then
counterstained with Safranin stain for one minute. The slide was then gently rinsed with distilled water.
The slide was then gently blotted dry with bibulous paper. The slide was then observed under oil
immersion on a compound microscope with oil objective lens.
Results
Nothing was visible on the Nutrient Agar plate immediately after streaking. After incubation at
25 C for 48 hours, one distinct bacterial colony was observed on the nutrient agar plate. The
characteristics and morphology of the cultured microbes were as follows: Shape – circular, Type –
bacteria, Margin – entire, Elevation ‐ raised, Texture ‐ mucoid, Pigment production – opaque.
After applying a sample of the bacteria onto a slide plate and performing a Gram stain the
sample of bacteria were observed under a compound microscope with an oil objective lens. The
bacteria on the slide appeared reddish‐pink in color and were rod‐shaped. The shape of the bacteria on
the slide was consistent with the shape of the bacteria on the nutrient agar plate.
Discussion
The fact that only one distinct bacterial colony appeared on the nutrient agar plate, suggests
that the water in the bottle was contaminated with only one type of bacterium. After Gram staining the
bacteria, the resulting reddish‐pink appearance of the bacteria indicates that the bacteria are Gram
negative.
It is suspected that the causative agent of contamination is Escherichia coli. The findings of the
lab tests are consistent with this assumption. E. coli is a Gram negative, rod‐shaped bacterium. There
are many different strains of E. coli, most of which are harmless. One stain, of E. coli O157:H7, is
pathogenic and can cause gastrointestinal infection in humans.
A fermentation test could be used to confirm or deny these suggestions. E. coli ferments the
carbohydrate sorbitol (Tortua, Funk, Case, 2010). Pathogenic E. coli O157:H7, however, does not
ferment sorbitol.
[Reference List Omitted]