View source for WBR0049

 {{WBRQuestion
|QuestionAuthor=William J Gibson (Reviewed by  {{Rim}} and Yazan Daaboul)
|ExamType=USMLE Step 1
|MainCategory=Microbiology
|SubCategory=Pulmonology
|MainCategory=Microbiology
|SubCategory=Pulmonology
|MainCategory=Microbiology
|SubCategory=Pulmonology
|MainCategory=Microbiology
|MainCategory=Microbiology
|MainCategory=Microbiology
|SubCategory=Pulmonology
|MainCategory=Microbiology
|SubCategory=Pulmonology
|MainCategory=Microbiology
|SubCategory=Pulmonology
|MainCategory=Microbiology
|SubCategory=Pulmonology
|MainCategory=Microbiology
|SubCategory=Pulmonology
|MainCategory=Microbiology
|SubCategory=Pulmonology
|MainCategory=Microbiology
|MainCategory=Microbiology
|SubCategory=Pulmonology
|Prompt=A 69-year-old man is brought to the emergency department by his wife for worsening fever, vomiting, and cough for the past 3 days. The patient's wife explains that they recently returned from a cruise trip on a nearby lake. His past medical history is significant for well-controlled essential hypertension managed by lisinopril. He has smoked one pack of cigarettes daily since the past 40 years. He denies drinking alcohol or any use of illicit drugs. On admission, his blood pressure is 134/88 mmHg,  heart rate is 86/min, temperature is 103 °F (39.5 °C), and respiratory rate is 25/min. Initial work-up reveals the following set of electrolytes: Na=129 mEq/L, K=4.5 mEq/L, Cl=108 mEq/L, and HCO3=21 mEq/L. Which of the following media may be used to grow the infectious agent responsible for this patient's condition?
|Explanation=[[Legionnaires' disease]] is a severe pulmonary infection caused by ''[[Legionella pneumophila]]'', a gram-negative rod. Legionnaires' disease typically affects smokers, elderly, or patients who are immunocompromised. Healthy individuals often have a less severe flu-like ''Legionella'' infection called [[Pontiac fever]].

''Legionella pneumophila'' should be suspected in patients who were recently exposed to water sources, such as rivers, lakes, or even air conditioners. It classically causes a fever-heart rate dissociation, whereby the patient is febrile but not as tachycardic as it would be expected in a febrile patient. Additionally, hyponatremia is characteristically observed in patients with ''Legionella'' infection. The mechanism of hyponatremia in ''Legionella'' infections was once thought to be due to SIADH, but more recent data suggests ADH does not play a major role and other mechanisms are yet to be elucidated. Finally, a mild but clinically insignificant decrease in bicarbonate may be evident in this patient as a compensatory mechanism of the patient's hyperventilation.

''Legionella'' grows on charcoal yeast extract using iron and cysteine. Because gram stains poorly in cultures of ''Legionella'', silver stain is used instead. A more rapid urinary antigen for ''Legionella'' is also available. Treatment is by antibiotics with atypical coverage, such as macrolides.
|AnswerA=Eaton’s agar
|AnswerAExp=Eaton’s agar is used to culture ''[[Mycoplasma pneumoniae]]'', a common cause of [[atypical pneumonia]].
|AnswerB=Charcoal yeast extract
|AnswerBExp=Charcoal yeast extract is used to culture ''[[Legionella pneumophila]]'', the organism responsible for [[Legionnaires' disease]].
|AnswerC=Tellurite Agar
|AnswerCExp=Tellurite agar is used to culture ''[[Corynebacterium diphtheriae]]''. ''C. diptheriae'' is largely eliminated from the developed world and therefore would be unlikely to be present in this patient. ''C. diptheriae'' infection causes severe pharyngitis accompanied by the development of greyish pseudomembranes that coat the throat and tonsils. The pulmonary symptoms and [[fever]] observed in this patient are inconsistent with ''C. diptheriae'' infection.
|AnswerD=MacConkey’s agar
|AnswerDExp=MacConkey's agar (MAC) is used to culture a variety of gram-negative organisms. Lactose-fermenting enterics (eg ''[[E. coli]]'') will appear as pink colonies on MacConkey's agar.
|AnswerE=Sabouraud’s agar
|AnswerEExp=Sabouraud’s agar is used to culture [[fungi]].
|EducationalObjectives=''[[Legionella pneumophila]]'' causes an acute respiratory disease called Legionnaires' disease, classically associated with fever-heart rate dissociation, and hyponatremia. It can be cultured using charcoal yeast agar.
|References=Schuetz P, Haubitz S, Christ-Crain M, et al. Hyponatremia and anti-diuretic hormone in Legionnaires' disease. BMC Infect Dis. 2013;13:585 
First Aid 2014 page 138
|RightAnswer=B
|WBRKeyword=Legionnaires', legionnaire, legionnaire's, microbiology, Bacteria, Pneumonia, Laboratory, gram, negative, rod, charcoal, yeast, extract
|Approved=Yes
}}