WBR0086

Author	[[PageAuthor::Ogheneochuko Ajari, MB.BS, MS [1] (Reviewed by Will Gibson, Jad Al Danaf, Rim Halaby, M.D. [2], and Yazan Daaboul)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Microbiology
Sub Category	SubCategory::Head and Neck, SubCategory::Infectious Disease
Prompt	[[Prompt::A 38-year-old healthy man presents to the emergency department (ED) for fever, abdominal pain, vomiting, and watery diarrhea. He states that his symptoms have been present for the past 2 days and are progressively worsening. He recently returned from a trip to South America. He reports that during his travel, he ingested unpasteurized milk. Physical examination in the ED is unremarkable. Following appropriate management, the patient is prescribed antibiotics and is then discharged home. Two days later, his stool cultures reveal a gram-positive rod with tumbling motility. Which of the following organisms is most likely responsible for the patient's condition?]]
Answer A	AnswerA::''Actinomyces israelii''
Answer A Explanation	[[AnswerAExp::Actinomyces israelii is a gram-positive anaerobic rod that forms long branching filaments resembling fungi. It characteristically crosses tissue planes and causes oral/facial abscesses that drain through sinus tracts in skin.]]
Answer B	AnswerB::''Listeria monocytogenes''
Answer B Explanation	[[AnswerBExp::Listeria monocytogenes is a gram-positive rod with a characteristic tumbling motility. Infection is typically acquired commonly through ingestion of unpasteurized milk/cheese, poorly prepared meats.]]
Answer C	AnswerC::''Nocardia asteroides''
Answer C Explanation	[[AnswerCExp::Nocardia asteroides is a gram-positive rod and weakly acid-fast aerobe found in soil. It causes a clinical presentation similar to that of tuberculosis: Subacute pulmonary infections in immunocompromised patients.]]
Answer D	AnswerD::''Trichomonas spp.''
Answer D Explanation	AnswerDExp::Trichomoniasis is a sexually transmitted infection caused by ''Trichomonas vaginalis''. It is an anaerobic, flagellated parasite that has a characteristic "cork-screw" motility.
Answer E	AnswerE::''Shigella spp.''
Answer E Explanation	[[AnswerEExp::Shigella is a gram-negative, non-motile rod causing enterocolitis or shigellosis, a severe form of dysentery.]]
Right Answer	RightAnswer::B
Explanation	[[Explanation::Listeria monocytogenes is a ubiquitious gram-positive facultatively-intracellular glucose-fermenting motile pathogen that causes listeriosis. It is a foodborne pathogen that is often found in dairy products, meat, fruits, and vegetables, and is also transmitted by cattle. It infects refrigerated foods due to its unusual ability to slowly grow at low temperatures: -18 °C to 10 °C. This characteristic feature also makes the organism difficult to grow on regular cultures using usual incubation temperatures. Generally, it grows on blood agar, showing a narrow band of beta-hemolysis. L. monocytogenes induces its own uptake into non-phagocytic cells. It has the capacity to migrate in a "tumbling motility" fashion via a unique actin-polymerization process (rocket tails). Primarily, L. monocytogenes first enters the host through the intestine and directly affects the liver before involving other organs. It grows in the cytosol of infected cells and escapes phagocytosis by secretion of cholesterol-dependent cytolysin listeriolysin O (LLO). Manifestations of listeriosis may not be directly observed following infection. Many patients remain asymptomatic for prolonged period of time. L. monocytogenes may cause meningitis or granulomatosis infantiseptica in newborns and the elderly. It may also be responsible for septicemia in adults, especially among patients with immunocompromised states, such as patients on immunosuppression (eg transplant recipients) or patients with hematologic malignancies (eg. leukemia) or receiving antineoplastic therapy. Pregnant women are also especially susceptible to L. monocytogenes, who may suffer from amnionitis or spontaneous abortions. Even patients with other diseases, such as reduced gastric acidity, cirrhosis, hemochromatosis, and chronic renal failure, are at higher risk of L. monocytogenes infection. Otherwise, healthy children and adults rarely suffer from listeriosis. Manifestations may range from an asymptomatic course or febrile gastroenteritis to meningoencephalitis. Educational Objective: Listeria monocytogenes is a gram-positive rod with "tumbling" motility on light microscopy. In healthy adults, it usually causes symptoms of febrile gastroenteritis. References: Ramaswamy V, Sresence VM, Rejitha JS, et al. Listeria - review of epidemiology and pathogenesis. J Microbiol Immunol Infect. 2007;40:4-13. First Aid 2014 page 133]]
Approved	Approved::Yes
Keyword	WBRKeyword::Microbiology, WBRKeyword::Listeria, WBRKeyword::Listeria monocytogenes, WBRKeyword::Bacteria, WBRKeyword::Abdominal pain, WBRKeyword::Diarrhea, WBRKeyword::Infection, WBRKeyword::Food poisoning
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