A 4-year-old boy is brought to the physician by his mother with decreased appetite, abdominal cramps, and diarrhea over the last 3 days. The stools were initially watery but have now become bloody. On physical examination, his temperature is 100.4 F (38 C). There are signs of dehydration and he has a mildly distended abdomen with diffuse pain elicited on palpation. Stool studies are positive for fecal leukocytes and occult blood. Fecal cultures grow non-lactose fermenting, gram-negative rods on MacConkey agar. The bacteria ferment glucose without gas production, do not generate hydrogen sulfide when grown on triple sugar iron agar, and cannot replicate at refrigeration temperatures. Which of the following bacterial factors is the most important during the pathogenesis of this patient's disease?

Question

A 4-year-old boy is brought to the physician by his mother with decreased appetite, abdominal cramps, and diarrhea over the last 3 days.  The stools were initially watery but have now become bloody.  On physical examination, his temperature is 100.4 F (38 C).  There are signs of dehydration and he has a mildly distended abdomen with diffuse pain elicited on palpation.  Stool studies are positive for fecal leukocytes and occult blood.  Fecal cultures grow non-lactose fermenting, gram-negative rods on MacConkey agar.  The bacteria ferment glucose without gas production, do not generate hydrogen sulfide when grown on triple sugar iron agar, and cannot replicate at refrigeration temperatures.  Which of the following bacterial factors is the most important during the pathogenesis of this patient's disease?

Quizplus · Accepted Answer

11ec160b_386c_5e12_9249_f38df99a2ef7_MD0001_00 This patient's fever, abdominal cramps, and bloody diarrhea (dysentery) are consistent with bacterial enteritis. His stool culture results are most specific for a Shigella species (Shigella sonnei being most common in the United States). Shigella is transmitted via the fecal-oral route and is never a component of the normal bacterial gut flora. Shigellosis is primarily a pediatric disease but also shows a predilection for men who have sex with men and adults in skilled nursing facilities. Shigella species are non-lactose fermenting organisms that produce acid (not gas) during glucose fermentation (in contrast to Escherichia coli). Furthermore, Shigella species are non-motile and do not produce H₂S (in contrast to Salmonella). Mucosal invasion is the essential pathogenic mechanism for Shigella infection. Shigella invades the gastrointestinal mucosa, particularly via the M cells that overlie Peyer's patches. After cell entry, Shigella is able to lyse its containment vacuole and enter the cytosolic compartment. It then can induce apoptosis of the host cell and spread to adjacent cells via protrusions created through host-cell actin polymerization. Shigella invasion triggers a robust host inflammatory response that is largely mediated by neutrophils. (Choice A) Some strains of Shigella produce shiga toxin, which damages the intestinal mucosa by inactivating the 60S ribosomal subunit, halting cellular protein synthesis. However, shiga toxin plays a minor, nonessential role in the disease process, as nontoxigenic strains also cause shigellosis. (Choice B) Intestinal colonization with Shigella does not occur as the bacterium is highly pathogenic; transmission of as few as 10-100 organisms is sufficient to cause disease. If Shigella is isolated from a stool culture, an active infection is taking place. (Choice D) Proliferation in lymph nodes is not a characteristic of Shigella. In contrast, both Salmonella typhi and Yersinia enterocolitica (grows at low temperatures like Listeria) can gain access to the lymphatics and proliferate in the mesenteric lymph nodes. Y enterocolitica can cause inflammation and enlargement of the lymphoid tissue around the appendix and terminal ileum ("pseudoappendicitis"), leading to right lower quadrant pain that can be confused with acute appendicitis. (Choice E) Shigella infections rarely cause bacteremia, as the bacterium is readily phagocytosed and destroyed after entering the bloodstream. Enteric bacteremia is more likely to be caused by E coli, S typhi (typhoid fever), Klebsiella, and Proteus. Educational objective: Shigella is a non-motile, non-lactose fermenting organism that does not produce H₂S when grown on triple sugar iron agar. Mucosal invasion of the M cells that overlie Peyer's patches is an essential pathogenic mechanism for Shigella infection. Shigella then escapes the phagosome and spreads laterally to other epithelial cells via actin polymerization. __________ References: Shigellosis (http://www.ncbi.nlm.nih.gov/pubmed/15880088)

A 4-Year-Old Boy Is Brought to the Physician by His