Which of the following is the most likely causative organism? A 60-year-old alcoholic smoker abruptly develops high fever, shakes, a severe headache, and muscle pain. Heinitially has a dry, insignificant cough, but over the next few days he develops marked shortness of breathrequiring assisted ventilation. Chest x-ray demonstrates homogeneous radiographic shadowing that initiallyinvolves the left lower lobe but continues to spread until both lungs are extensively involved. Culture ofbronchoalveolar lavage fluid on buffered charcoal yeast extract (BCYE) demonstrates a coccobacillary pathogen.

Legionella pneumophila

The correct answer is A. The patient has a severe, potentially fatal, pneumonia with prominent systemic symptoms. Culture on BCYE is the specific clue that the organism is Legionella pneumophila. The disease is respiratory Legionellosis, also known as Legionnaires disease, because the disease was first described when it occurred in epidemic form following an American Legion convention at a Philadelphia hotel. Patients tend to be older (40-70 years old) and may have risk factors including cigarette use, alcoholism, diabetes, chronic illness, or immunosuppressive therapy. Listeria monocytogenes(choice B) causes listeriosis and is not a notable cause of pneumonia. Spirillium minus (choice C) is a cause of rat-bite fever and is not a notable cause of pneumonia. Staphylococcus aureus(choice D) can cause pneumonia, but is easily cultured on routine media. Streptococcus pneumoniae(choice E) can cause pneumonia, but is easily cultured on routine media.