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

Patrick Racine

Siddarth Santhebennur

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

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

Signs and Symptoms Skin rash (folliculitis)

Outer ear infection (otitis externa)

Possible Culprits Originally there were 6 potential bacteria that could

have caused the infections:

Escherichia coli

Pseudomonas aeruginosa

Salmonella enterica

Staphylococcus aureus

Streptococcus pyogenes

Haemophilus influenzae

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

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

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

Type of Growth Media Used Two options:

MacConkey’s 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

Determining the Culprit Bacteria At this point, there were still four potential culprits

remaining:

Escherichia coli

Pseudomonas aeruginosa

Salmonella enterica

Haemophilus influenzae

• 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 Rod-shaped

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

Opportunistic Rarely infects healthy individuals

Prefers individuals with unhealthy immune systems

Resistant to many antibiotics

Pseudomonas aeruginosa

Reasons for Selecting Culprit Bacteria The infections occurred on the skin and outer ear, so

Escherichia coli and Salmonella enterica were immediately eliminated

Escherichia coli and Salmonella enterica both cause infection of the digestive system

Although Staphylococcus aureus does cause various skin infections, including folliculitis, it does not cause the outer ear infection that is also associated with the culprit bacteria

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 Chlorampheicol Gentamycin Penicillin Streptomycin Tetracycline Vancomycin

Based on the results it was determined that the culprit bacteria was most sensitive to Gentamycin, making that antibiotic the best option

Antimicrobial Susceptibility Test

The antibiotic that creates the largest zone of inhibition would be considered the most effective.