case study 2: don't swim in the pool (final)

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  • 1. Dont Swim in the Pool
    Patrick Racine

2. Background
Several cases of skin rash infections reported at Hotel A in Bangor, Maine
Feb. 18-27, 2000
Infections related to low chlorine levels in the pool and hot tub
>1.0 mg/L, less than state required 1-3 mg/L
Sample of unknown pathogen taken from draining ear of 6 year old child and the pool filter
3. Patient History
9 patients infected
Had rash for 7 days (or less) or an outer ear infection
All had spent time in either the pool or hot tub
7 spent time in both
4. Signs and Symptoms
Skin rash (folliculitis)
Outer ear infection (otitisexterna)
5. Possible Culprits
Originally there were 6 potentialbacteria that could have caused the infections:
Escherichia coli
Pseudomonas aeruginosa
Salmonella enterica
Staphylococcus aureus
Streptococcus pyogenes
6. Gram Stain
A Gram stain differentiates bacteria based upon properties of their cell walls using a crystal violet stain
Separated into 2 categories:
Gram-positive (+) (dark blue/violet)
Gram-negative (-) (red/pink)
E. coli and S. aureus were Gram stained along with unknown bacteria for comparative reasons
7. Results of Gram Stain
Gram stain showed that the unknown bacteria is Gram-negative
Due to its red/pink color
This eliminates 2 bacteria from being considered the cause of the outbreak
Staphylococcus aureus
Streptococcus pyogenes
Both are Gram positive
8. Differential/Selective Media
Two types of growth media used to inhibit or isolate growth of a microorganism
Differential: Different microorganisms grown on the same media; distinguished by how each organism reacts to specific dyes and chemicals placed on media
Selective: Allows growth of specific organism, inhibits others
9. Type of Growth Media Used
Two options:
MacConkeys Agar
Mannitol Salt Agar
Because the suspect bacteria is Gram-negative, the best option was the MacConkey Agar
Allows for growth of Gram-negative bacteria and inhibits the growth of most Gram-positive bacteria
10. Determining the Culprit Bacteria
At this point, there were still four potential culprits remaining:
Escherichia coli
Pseudomonas aeruginosa
Salmonella enterica

  • By using the information from the results of the growth media and research into the 4 potential bacteria, it was concluded that the culprit bacteria is Pseudomonas aeruginosa

Pseudomonas aeruginosa
Aerobic respiration
Due to production of arginine, undergoes anaerobic respiration as well
Found in various environments including soil, water and hospitals
Most abundant organism on Earth
Rarely infects healthy individuals
Prefers individuals with unhealthy immune systems
Resistant to many antibiotics
11. Pseudomonas aeruginosa
12. Reasons for Selecting Culprit Bacteria

  • The infections occurred on the skin and outer ear, so Escherichia coli and Salmonella enterica were immediately eliminated 13. Escherichia coli and Salmonella enterica both cause infection of the digestive system 14. Although Staphylococcus aureusdoes cause various skin infections, including folliculitis, it does not cause the outer ear infection that is also associated with the culprit bacteria 15. This leaves Pseudomonas aeruginosa as the only logical culprit

Antimicrobial Susceptibility
In order to choose a proper antibiotic to treat the infected patients, an antimicrobial test was conducted
The effectiveness of 6 potential antibiotics were tested on the culprit bacteria
Based on the results itwas determined that the culprit bacteria was most sensitive to Gentamycin, making that antibiotic the best option
16. Antimicrobial Susceptibility Test
The antibiotic that creates the largest zone of inhibition would be considered the most effective.


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