A 28-year-old woman presents to the emergency department complaining of diarrhea with mild abdominal pain for the past two days. She describes the diarrhea as being watery and non-bloody, and also notes that she has had a decreased appetite since around the time the symptoms began. She reports that she recently returned from a trip to Thailand where she sampled a significant amount of local street food. Her vital signs are temperature of 98.7°F (37°C), blood pressure 122/62 mmHg, heart rate 64 beats/minute, and respirations 18/minute. On physical examination, she appears healthy. Abdominal examination is without guarding or focal tenderness to palpation. Gram-staining of the stool reveals gram negative rods.
Which of the following media and findings would correctly identify this organism?
D) MacConkey agar; pink colonies
The first step is to consider microorganisms that case watery diarrhea. These include Vibrio cholerae, Clostridium difficile, and ETEC (Enterotoxigenic Escherichia coli). V. cholerae causes severe, voluminous, watery diarrhea with a distinct and characteristic whitish, cloudy ("rice water") appearance. Patients infected with this organism lose an incredible amount of fluid in the process, and without prompt and proper treatment may die. While the patient does have watery diarrhea, it is not as severe as it would be in the case of an infection by V. cholerae, and the patient certainly would not show up looking "healthy." C. difficile causes pseudomembranous colitis which results in watery diarrhea that is occasionally bloody. This can be marked off the differential diagnosis simply because it is not a gram-negative rod. It is Gram positive). ETEC is associated with so-called "traveler's diarrhea" and presents as watery diarrhea of a milder course than that of V. cholerae. Patients may complain of abdominal pain, loss of appetite, fever, nausea, and bloating. Given the history of this patient who recently traveled overseas and the relative mildness of her symptoms, it can be concluded that she has contracted an ETEC infection.
Many bacteria can invade the GI tract, so determining the specific cause of a diarrheal illness takes some knowledge of selective and differential media, as well as microorganism characteristics. The enterics, as they are affectionately called, are all gram-negative rods which may be curved or "comma" shaped. Each has specific properties which can be utilized to differentiate one from the other.
Special media include:
- MacConkey agar is used to differentiate lactose fermenters from non-lactose fermenters. Indicator in the agar turns pink in the presence of lactose fermentation.
- Eosin-methylene blue (EMB) agar is also used to differentiate lactose fermenters from non-lactose fermenters. In this case, lactose fermenters produce a purple or black colonies.
- Hektoen enteric agar is used to differentiate Shigella from Salmonella on the basis of H2S production.
- TSI (triple sugar iron) slant is used to differentiate microorganisms which do or do not ferment sugars and/or produce H2S, and which are aerobic and anaerobic.
Answer A is incorrect; as explained above, EMB agar is used to differentiate lactose from non-lactose fermenters, but specifically, Escherichia coli (E. coli) produces metallic green colonies rather than purple or black colonies.
Answer B is incorrect; as explained above, Hektoen agar is used to differentiate Shigella and Salmonella, neither of which are the causative organism in this vignette. Shigella produces green colonies on Hektoen agar while Salmonella produces black colonies.
Answer C is incorrect; while E. Coli does grow on MacConkey agar, it does not produce mucoid colonies. This is a particular characteristic of Klebsiella pneumoniae, a gram-negative rod more associated with pneumonia and nosocomial urinary tract infections.
Key Learning Point
E. Coli, specifically, Enterotoxigenic E. Coli (ETEC) causes traveler's diarrhea, a relatively rapid-onset and mild watery diarrhea which tends to resolve on its own. Patients with traveler's diarrhea may present with symptoms following their period of travel or during. Other symptoms include abdominal pain, lack of appetite, nausea, and bloating. It is important to note differences between this and watery diarrhea caused by Vibrio cholerae, which causes a very severe watery diarrhea which can be fatal if left untreated.
