The study tested whether growing Escherichia coli (E. coli) with other bacterial strains, Staphylococcus epidermidis or Streptococcus salivarius, would cause the E. coli to develop antibiotic resistance faster than growing E. coli alone. Over multiple generations, the study found no significant difference in the development of antibiotic resistance between the pure and mixed bacterial cultures. While all cultures showed decreasing effectiveness of the antibiotics over time, mixing bacterial strains did not accelerate the mutation rate leading to antibiotic resistance in E. coli.

1. The Culturing of Staphylococcus
epidermidis and Streptococcus salivarius
with Escherichia coli Does Not Increase
Antibiotic Resistance in Escherichia coli
By: Sarah Weber

2. INTRODUCTION
• Antibiotic resistance is important -- antibiotics are
used to treat bacterial infections
• When testing effectiveness of an antibiotic --
performed on individual strains
• Not realistic -- hundreds of strains interact in the
environment
• Bacteria intermingle and transfer genes between each
other in order to survive

3. INTRODUCTION
• Escherichia coli
– Member of a large and diverse group of gram
negative bacteria
– Normally live in human and animal intestines
– Most strains are harmless, some are pathogenic
– Can be used to indicate if water is contaminated
– Rod-shaped (bacillus) bacteria

4. INTRODUCTION
• Streptococcus salivarius
– Gram positive
– Principal bacterium living in human oral cavity
without causing harm
– Pioneer in colonizing dental plaque
– Can cause disease if it enters the blood stream
– Spherical shaped (coccus) bacteria

5. INTRODUCTION
• Staphylococcus epidermidis
– Gram positive
– Part of normal skin flora in humans
– Leading cause of hospital born infections in
immune compromised patients
– Spherical shaped (coccus) bacteria in grape-like
clusters

6. INTRODUCTION
• Penicillin
– One of the earliest discovered and widely used
antibiotics
– Used to treat many different types of infections
caused by bacteria
– Kills bacteria by interfering with the ability to
synthesize the cell wall

7. INTRODUCTION
• Erythromycin
– Macrolide antibiotic
– Used to treat many different types of infections
caused by bacteria
– Slows the growth of sensitive bacteria
– Reduces the production of important proteins
needed by the bacteria to survive

8. INTRODUCTION
• Streptomycin
– Aminoglycoside antibiotic
– Used to treat many different kinds of bacterial
infections
– Cannot be given orally; injected intramuscularly
– Prevents growth of bacteria by protein synthesis
inhibition

9. INTRODUCTION
• Mutation rate
– Bacteria mutate at a rapid rate, some faster than
others
– Bacteria that survive in the presence of antibiotics
acquire resistance through:
• Resistance genes
• Recombination with foreign DNA
– Presence of antibiotics induces mutations causing a
slow mutation rate

10. AIM
• The aim of the research was to study the speed at
which bacterial resistance occurred in a pure culture
versus mixed cultures.

11. HYPOTHESIS
• I hypothesized that the antibiotic resistance in the E.
coli with S. epidermidis and E. coli with S. salivarius
would increase faster than in the E. coli alone.

12. METHODS
• Cultures made of parent strains
• Parent strains plated to see zone of inhibition
• Plates were made of E. coli only, mixtures of E. coli
with S. salivarius, and mixtures of E. coli with S.
epidermidis
• Test plates of individual strains
– Left to Right: E. coli, S. epidermidis, S. salivarius

13. METHODS
• Every other day a culture was made using the
previous growth plate to make a new generation using
only the E. coli with S. salivarius and E. coli with S.
epidermidis plates
• The cultures were incubated, then plated onto blood
agar with one disk each of penicillin, erythromycin,
and streptomycin

14. 0
0.5
1
1.5
2
2.5
3
3.5
4
E. coli S. salivarius S. epidermidis
ZoneofInhibition(mm)
Parent Generation
Diameter of the Zone of Inhibition for the Parent
Generation
Penicillin
Erythromycin
Streptomycin

15. • Generation Number 1
‾ Left to right: E. coli, E. coli with
S. salivarius, E. coli with S.
epidermidis
• Generation Number 16
‾ Left to right: E. coli, E. coli with S.
salivarius, E. coli with S.
epidermidis

16. 0
0.2
0.4
0.6
0.8
1
1.2
1.4
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
ZoneofInhibition(mm)
Generations
Diameters of the Zone of Inhibition for E. coli Control
Penicillin
Streptomycin

17. 0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
ZoneofInhibition(mm)
Generations
Diameters of the Zone of Inhibition for S. salivarius
and E. coli
Penicillin
Erythromycin
Streptomycin

18. 0
0.5
1
1.5
2
2.5
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
ZoneofInhibition(mm)
Generations
Diameters of the Zone of Inhibition for S. epidermidis
and E.coli
Penicillin
Erythromycin
Streptomycin

19. RESULTS
• An ANOVA (General Linear Model) was performed
• No significant difference between the two different
bacterial combinations on the rate of change in the
zone of inhibition (p> 0.05)
• Significant difference in generations (p< 0.05)
Generation Mean (mm)
1 0.5875a
17 0.3167c,d

20. RESULTS
• Significant difference with respect to inhibition of
bacterial growth (p< 0.05)
Antibiotic Mean (mm)
Streptomycin 0.4371a
Erythromycin 0.3400a,b
Penicillin 0.2961b

21. CONCLUSION
• Hypothesis of antibiotic resistance in the E. coli with
S. epidermidis and E. coli with S. salivarius would
increase faster than in the E. coli alone was rejected
• Significance: two bacteria grown together did not
selectively mutate any faster than individual strains
• Change in the zone over time showed the antibiotics
are not as effective as when first introduced

22. LIMITATIONS OF RESEARCH
• Too many similarities between the two treatment
bacteria (S. salivarius and S. epidermidis)
• Too many similarities in the zones of inhibition
• The number of generations was insufficient
• The recombination of the two bacteria did not
occur

23. FURTHER RESEARCH
• Allow for more generations to determine how many
are needed for the bacteria to become completely
desensitized to the antibiotics
• Change the bacteria used
• Change the antibiotics used

24. QUESTIONS