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Case Study 3: The tacos are trouble

Page history last edited by Derek Weber 12 years, 1 month ago




Between 2006 November and December, 67 cases of individuals with bloody diarrhea accompanied by severe abdominal cramps were reported to the CDC from five states (NJ, NY, PA, DE, and SC).  Among the 67 ill persons, 51 (76%) were hospitalized and 8 (12%) developed a type of kidney failure called hemolytic-uremic syndrome (HUS).  Illness onset dates have ranged from November 20 to December 5.  The peak of the outbreak was in the last week of November.


Public health officials set out to determine the source of this pathogen.  The investigation revealed that all 67 of these individuals ate at Taco Bell restaurants in four states (NJ, NY, PA, and DE).  It is presumed that contaminated food or foods are the source of contamination, particularly foods or ingredients that were consumed raw.  A sample of chopped yellow onions collected on 4 December from an open bin in a Taco Bell restaurant in Nassau County, NY tested positive for this pathogen.  To determine if these yellow onions were the source of contamination, DNA fingerprinting was utilized.  It turns out that the onions were not the source of the infection.


Stool samples and rectal swabs were obtained from patients and cultured (see tube provided).  Using a series of test, you are responsible for identifying the cause of this outbreak and what antibiotic may be effective in treating the signs and symptoms.


Possible Culprits:

Escherichia coli

Pseudomonas aeruginosa

Salmonella enterica

Staphylococcus aureus

Streptococcus pyogenes

Haemophilus influenzae


Here is an outline of the next three weeks:


Week 1 (3/21 and 3/23):

1. Gram stain the provided culture


a. Write a two-paragraph summary on the structural and chemical differences between a Gram(+) bacteria and a Gram(-) bacteria.  Discuss the difference in susceptibilities to antibiotics. 

b. Write up the steps of a Gram-stain.  Explain the purpose of each step.

Experiment:  Perfrom a Gram stain.   I have provided two known bacteria for comparison, Escherichia coli and Staphylococcus aureus.  Record your experiment using video; obtain an image of your results at 1000x with oil.   Describe your results in your presentation.


2. Culture organism on selective/differential media


a.  Write a one-paragraph introduction about selective and differential media.  Describe the use of both selective and differential agents and how this technique is useful in identifying bacteria based on physical and chemical requirements for growth. 

b.  Write two paragraphs discussing two specific types of selective and differential media: MacConkey Agar and Mannitol Salt Agar (MSA).  Focus on which class of bacteria can grow on these diverse media and how they can be used to identify specific species as well. 

Experiment:  Based on your Gram-stain, you will select on of these two media to culture your organism.  Think about including controls to assist in your analysis the following week.  In your presentation, describe your results.


Week 2 (4/4 and 4/6): 

1.  Record results of the selective/differential plate -  The results from your selective/differential media experiments will be ready by week 2.   Obtain an image of your results.


2.  Antimicrobial susceptibility test-  Choosing the proper antibiotic is important in the treatment of any infection.  In this experiment, you will analyze the effectiveness of six different antibiotics in the treatment of your patient's infection.  The protocol for this experiment can be found here:  antimicrobial sus.pdf.  


Week 3 (4/11 and 4/13): 

1.  Record result of antimicrobial susceptibility test -  Measure a zone of inhibition (mm) for each of your antibiotics in your antimicrobial susceptibility test.  In your presentation.  discuss which antibiotic(s) would be most effective in treating your patient.  In your presentation, write a summary of the antibiotic, including its origin (where it comes from), chemical structure, route of administration, target within the bacterial cell, diseases used to treat and any possible side effects.


2.  Present your case to the rest of the physicians -  In lab you will present your case to a group of fellow physicians.  The first part of the presentation will involve outlining your case study, focusing on important information like patient history, background, signs and symptoms, etc.  Provide some anatomical overview of the affected system.  Next, review your results: Gram stain,  selective differential media, and antimicrobial susceptibility.  Provide background on each technique in your discussion.  Lastly, provide a detailed summary of the bacteria in question.  Focus on: cell wall, cell shape and arrangement, metabolism, where its found in nature and/or human body, mechanisms by which they damage the human host, how they are spread (epidemiology), etc.  You will have 15-20 minutes for your presentation.  Upload your PowerPoint or Prezi to the space provided here for future viewing.

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