Health authorities are investigating an outbreak of respiratory infections among a group of military recruits. Fifteen recruits reported persistent cough, low-grade fever, and malaise. Apart from the low-grade fever, physical examination was largely unremarkable. Chest x-rays were all notable for pulmonary infiltrates that appeared more severe than what would have been expected based on assessment of the patients' clinical status. Sputum specimens were obtained, and the causative organism required a complex acellular medium enriched with cholesterol to grow. Which of the following organisms is the most likely cause of the outbreak?

Question

Health authorities are investigating an outbreak of respiratory infections among a group of military recruits.  Fifteen recruits reported persistent cough, low-grade fever, and malaise.  Apart from the low-grade fever, physical examination was largely unremarkable.  Chest x-rays were all notable for pulmonary infiltrates that appeared more severe than what would have been expected based on assessment of the patients' clinical status.  Sputum specimens were obtained, and the causative organism required a complex acellular medium enriched with cholesterol to grow.  Which of the following organisms is the most likely cause of the outbreak?

Quizplus · Accepted Answer

These patients have "walking pneumonia," an infection generally caused by Mycoplasma pneumoniae. Patients experience a low-grade fever, malaise, and a chronic, dry nagging cough. A classic sign is a chest x-ray revealing findings that are much worse than the clinical appearance of the patient. Another clue for M pneumoniae infection is that the organisms require cholesterol to grow because their cell membrane is composed of a single cholesterol-rich phospholipid bilayer. They completely lack a peptidoglycan cell wall, cell envelope, or capsule. (Fungi also incorporate sterols into their cell membranes; however, fungi use ergosterol, a target for antifungal therapy.) (Choices A and D) The fungus Coccidioides immitis causes San Joaquin Valley fever (known as "valley fever"). It is most common in the Southwest United States and Northern Mexico. Histoplasma capsulatum causes a similar illness, but it is typically localized to the Mississippi and Ohio River Valleys. Both organisms grow on standard fungal culture media. (Choice B) Coxiella burnetii causes Q-fever, a pneumonia-like illness resulting from inhalation of C burnetii spores that commonly contaminate animal hides. C burnetii is an obligate intracellular parasite that must be provided a cell culture to grow. (Choice C) Haemophilus influenzae requires chocolate agar (heat lysed blood agar), supplemented with factor X (hematin) and factor V (NAD+). (Choice E) Klebsiella pneumoniae pneumonia classically presents with cough productive of red jelly-like sputum ("currant jelly sputum") in patients with alcohol use disorder. K pneumoniae can be grown on standard agar, but MacConkey agar is often preferred because it contains bile, which will inhibit the growth of contaminant organisms. (Choice F) Legionella pneumophila requires an L-cysteine-supplemented agar to grow. (Choice H) The fungus Pneumocystis jirovecii can cause severe pneumonia in HIV+ patients with CD4 cell counts <200/mm³. Patients often have characteristic "ground glass" infiltrates on chest x-ray. P jirovecii pneumonia is diagnosed by silver staining the fluid collected during bronchoscopy. (Choice I) Streptococcus pneumoniae would cause lobar consolidation on chest x-ray. This organism can grow well on standard unenriched blood agar, but it is bile-soluble and cannot grow in the presence of optochin. Educational objective: Mycoplasma pneumoniae is the causative agent of "walking pneumonia," an infection typically characterized by a nagging nonproductive cough, low-grade fever, and malaise. Often, the chest x-ray suggests a severe pneumonia even though the patient appears relatively well. Mycoplasma species require cholesterol supplementation to grow on artificial media.

Health Authorities Are Investigating an Outbreak of Respiratory Infections Among