Enterobacteriaceae

CASE 1

• An 55 year old American tourist develops abdominal cramps and watery diarrhea on her 10th day in Ghana. She has no fever or chills. Her stool has no blood or mucus. She is not otherwise unwell.

• By the time she seeks medical attention, she has had these symptoms for 3 days. She is advised to maintain her hydration. No medications are prescribed. Her symptoms resolve the following day.

Questions:

1. What is the most likely cause of this episode of travelers' diarrhea?
2. What are the possible mechanisms of E. coli-associated diarrhea? What toxins are sometimes produced by E. coli?
3. Can any E. coli cause these diarrheal illnesses?
4. Do the same E. coli also cause urinary tract infection?
5. How would you identify E. coli in culture? What features does it share with other Enterobacteriaceae?
6. If you culture this patient's stool and recover E. coli, would you assume that it caused the diarrhea?

CASE 2

• A 48 year old woman has had recurrent urinary infections for several years. The infections typically respond to antibiotic therapy, but they eventually recur. She has been having symptomatic infections at a rate of 6-8 per year. Recently, the frequency has increased.

• A urinalysis is performed. The urine is cloudy, the pH=8.0, blood-trace, glucose-negative, 10-25 leukocytes per high-power field (hpf), 3-5 erythrocytes per hpf. There are no casts. The physician suspects an infection with Proteus. A urine culture is performed, and it grows Proteus mirabilis in large numbers.

• An abdominal xray is performed to look for a kidney stone. It shows a huge opaque stone in the left renal pelvis (below)

Questions:

1. What clinical clue made the physician suspect a Proteus infection (hint: check the urinalysis)?
2. What product of Proteus causes the urine to be alkaline?
3. The presistence of ammonia is responsible for the precipitation of certain soluble compounds from the urine and the gradual development of the stone. How does the presence of the stone facilitate the recurrence of the urinary tract infections in this case?
4. E. coli is a much more common cause urinary infection and of recurrent urinary infection in younger women and older men, but the mechanism is different. How do E. coli cause recurrent urinary infections in these situations?
5. In the urine culture, what feature of Proteus distinguishes it immediately from E. coli?

CASE 3

• A 57 year old man is admitted with fever and cough productive a thick, sticky sputum that contains abundant dark red blood. The patient has been previously admitted for other alcohol-related illness in the past, but this is his first episode of respiratory illness.

• A chest xray show a right upper lobe infiltrate that causes the middle lobe fissure to sag.

• A sputum sample show numerous neutrophils and large Gram-negative bacilli. The sputum culture grows a predominance of lactose-positive, highly mucoid colonies.

Questions:

1. What are these mucoid bacteria? Are they Enterobacteriaceae?
2. What causes the mucoid appearance of the colonies?
3. How common is pneumonia due to Enterobacteriaceae in pneumonia that arises in the community? in the hospital?
4. What role did the patient's alcoholism play in the development of this pneumonia?