Effects of Alcoholic Dilutions on
Bacterial Growth
Project by: Lauren Pease
Grade 9
Problem
 What effect do different dilutions of alcohol have on bacterial
growth?
 Does alcohol have a greater inhibitory effect on the growth of
gram-negative or gram-positive bacteria?
Research of Bacterial Species
 Staphylococcus epidermidis is found on the outer layer of the
human skin and is the leading pathogen of foreign body
infections.
-it is typically immune to most antibiotics
 Escherichia coli lives in the intestinal tract of the human body
and many other animals.
Research of Gram-negative vs.
Gram-positive Bacteria
 Gram-positive bacteria
-cell wall contains a thick layer of peptidoglycan that
encompasses the entire cell wall
 Gram-negative bacteria
-cell wall contains a thin layer of peptidoglycan located
only between the cytoplasmic and outer membrane
 Peptidoglycan is a polymer that consists of glycan and peptide
chains which is responsible for inflexibility of cell walls in
prokaryotes
Research of Ethanol Alcohol
 Often referred to as “pure alcohol”
 Ethyl alcohol without additives or denaturants
 Is a group of chemical compounds whose molecules contain a
hydroxyl group bonded to a carbon atom
 Made by the fermentation of sugars
Hypothesis
 If there is a higher percentage of alcohol content, the alcohol
will greater inhibit the growth of the bacteria.
 Also, if there is an equal percentage of alcohol used on the
Staphylococcus Epidermidis and Escherichia Coli, then the
Staphylococcus Epidermidis will be greater affected by the
alcohol.
Materials
 5 Petri dishes of Staphylococcus epidermidis (gram-positive bacteria)
 5 Petri dishes of Escherichia coli (gram-negative bacteria)
 4 different dilutions of ethanol alcohol at 10 mL each with




dilutions of 25%, 50%, 75%, and 100%
40 sterile disks submerged in the dilutions of ethanol alcohol (4
on each dish)
Sharpie
Tape
Goggles and gloves were used for safety
Variables
 Independent Variables
-dilutions of ethanol alcohol
 Dependent Variables
-the effects of the alcoholic dilutions of the bacteria
 Constants
-the temperature of the incubator
-time each Petri dish spent in the incubator
 Controlled Variables
-Petri dish of Staphylococcus epidermidis and Escherichia coli
that contained two blank disks and two water submerged
disks
Procedure
1)
2)
3)
4)
5)
6)
7)
The dilutions of ethanol alcohol were created by using the alcohol
and a specified amount of water.
Aseptic technique was used to swab the two types of bacteria onto
five Petri dishes each.
The sterile disks were immersed into different dilutions (one disk for
each percentage of alcohol) and then four were placed on each Petri
dish.
For the controlled variable, two disks submerged in water, and two
blank disks were placed in each Petri dish.
The Petri dishes were then placed in the incubator and left there for
two days.
Observations on the dishes were made both days.
Then measurements were taken of which bacteria’s growth was
greater inhibited and which alcohol dilution caused it.
Data
Staphylococcus Epidermidis blank
control group
0
group 1
0
group 2
0
group 3
0
group 4
0
averages
0
standard deviation
0
water
0
0
0
0
0
0
0
25%
50%
75%
100%
0
0
0
0
0
0
0
0
0
0
0.5
0.5
0
0.25
0.5
1.5
2
2
0.5
1.5
0.4
0.45
0.3
0.7
0.894427 0.873212 0.273861 0.758288 0.466631
Escherichia Coli
control group
blank
0
water
0
25%
0
50%
0
75%
0
100%
0
group 1
0
0
1
1.5
2
2
group 2
0
0
1
1.25
1
1
group 3
0
0
0.75
1.25
1
1
group 4
0
0
0.5
0.5
0.5
0.5
averages
0
0
0.65
0.9
0.9
0.9
standard deviation
0
0
0.41833
0.627495 0.74162
0.74162
Data (continued)
Staphylococcus Epidermidis
Measure of Inhibition of Alcohol (mm)
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
blank
water
25%
50%
75%
Dilution of Ethanol Alcohol
100%
Data (continued)
Measure of Inhibiton of Alcohol (mm)
Escherichia Coli
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
blank
water
25%
50%
75%
Dilution of Ethanol Alcohol
100%
Data (continued)
Averages of Bacterial Inhibition
Measure of Inbition of Alcohol (mm)
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
blank
water
25%
50%
Dilution of Ethanol Alcohol
75%
100%
Conclusion
 The hypothesis was not supported by the data.
 I hypothesized that the dilution of 100% ethanol alcohol would
greater inhibit the growth of the Staphylococcus epidermidis more
than it would inhibit the growth of Escherichia coli. I also
hypothesized that there would be a significant difference between
the 25% and 100% dilutions of alcohol with the 100% being most
effective.
 The dilutions of alcohol had no significant difference in inhibiting
the bacteria’s growth regardless of the percent of alcohol content.
 The alcohol had a greater inhibitory effect on the Escherichia coli
than the Staphylococcus epidermidis.
 This is most likely the result of the overuse of alcoholic sterilizers
in hand sanitizers and hospitals because bacteria are becoming
immune to the alcohol’s ability to be lethal.
Conclusion (continued)
 Improvements could be made to the experiment by testing more
than one type of gram-negative and gram-positive bacterium.
 Also, one of the new forms of bacteria that could be tested
should be found somewhere besides the human body.
 Further experimentation could be done by testing ethanol
alcohol on other forms of bacteria that may be found in hospitals
to greater verify this immunity of Staphylococcus epidermidis to
ethanol alcohol
Works Cited
Bukhari, Mohammad. "Staphylococcus Epidermidis." University of ConnecticutDepartment of Molecular and Cell Biology. N.p., 27 Sept. 2004. Web. 10 Feb.
2011. <http://web.uconn.edu/mcbstaff/graf/Student%20presentations/
S%20epidermidis/sepidermidis.html >.
"The cell wall surrounds and holds in the microbe." The Microbial World.
University of Wisconsin-Madison, 2006. Web. 31 Jan. 2011.
<http://www.microbiologytext.com/
index.php?module=Book&func=displayarticle&art_id=60>.
"Comparative Characteristics of Gram-Positive and Gram-Negative Bacteria."
University of Maryland . N.p., n.d.Web. 10 Feb. 2011.
<http://www.life.umd.edu/classroom/bsci424/BSCI223WebSiteFiles/
GramPosvsGramNeg.htm>.
Works Cited (continued)
"Peptidoglycan." Dorland's Medical Dictionary for Health Consumers. The Free
Dictionary, 2007. Web. 31 Jan. 2011.
<http://medical-dictionary.thefreedictionary.com/peptidoglycan>.
"Pure Alcohol (Ethanol)." Pharmacology Company. N.p., n.d.Web. 30 Jan. 2011.
<http://www.pharmco-prod.com/pages/ep1.pdf>.
Seida, Caitlin. "How Alcohol Kills Bacteria." Associated Content from Yahoo.
N.p., n.d.Web. 1 Dec. 2010. <http://www.associatedcontent.com/article/
2066479/how_alcohol_kills_bacteria.html>.
Shakhashiri. "Ethanol." Science Fun. N.p., 5 Feb. 2009. Web. 29 Jan. 2011.
<http://scifun.chem.wisc.edu/chemweek/pdf/ethanol.pdf>.