A 62-year-old man comes to the emergency department due to sudden-onset of high fever, shaking chills, shortness of breath, and productive cough. The patient has smoked a pack of cigarettes daily for 40 years. Examination shows dullness to percussion over the left side of the chest. Chest x-ray reveals consolidation in the left upper lobe. Gram stain of the sputum shows numerous polymorphonuclear leukocytes and gram-positive cocci in pairs. When cultured on a blood agar plate under aerobic conditions, the bacterial colonies are surrounded by a zone of incomplete hemolysis. The organism responsible for this patient's condition is most likely to exhibit which of the following virulence features?

Question

A 62-year-old man comes to the emergency department due to sudden-onset of high fever, shaking chills, shortness of breath, and productive cough.  The patient has smoked a pack of cigarettes daily for 40 years.  Examination shows dullness to percussion over the left side of the chest.  Chest x-ray reveals consolidation in the left upper lobe.  Gram stain of the sputum shows numerous polymorphonuclear leukocytes and gram-positive cocci in pairs.  When cultured on a blood agar plate under aerobic conditions, the bacterial colonies are surrounded by a zone of incomplete hemolysis.  The organism responsible for this patient's condition is most likely to exhibit which of the following virulence features?

Quizplus · Accepted Answer

Lobar pneumonia typically presents with acute-onset fever, cough, and lobar consolidation on chest x-ray. Although a number of different bacterial pathogens cause lobar pneumonia (eg, Haemophilus influenzae, Moraxella catarrhalis, Staphylococcus aureus), the most common cause is Streptococcus pneumoniae. S pneumoniae is a gram-positive diplococci that exhibits partial (α) hemolysis on blood agar (green colonies) and is bile-soluble and Optochin-sensitive. Its major virulence factor is a thick polysaccharide capsule that encases the organism and prevents phagocytosis and complement binding. Under the microscope, the capsule swells and appears as a halo around the blue-stained bacterial cells when specific anti-capsular antibodies and methylene blue dye are added ("quellung reaction"). Infection with S pneumoniae leads to the generation of anti-capsular antibodies that are protective against future infection with that strain. However, over 90 different antigenic variations (serotypes) of capsular polysaccharides have been identified, so future infection can occur with a strain that has not yet been encountered. Therefore, patients at high risk for invasive pneumococcal disease (eg, HIV, extremes of age) are generally given the pneumococcal vaccine, which induces immunity against the most common strains. Other virulence factors of S pneumoniae include IgA protease (inactivates secretory IgA), adhesins (necessary for adhesion to epithelial cells), and pneumolysin (cytotoxin that causes pores in cell membranes and cell lysis). (Choice A) Certain bacterial exotoxins (eg, Shiga toxin) enter the host cell and cleave a nucleobase from the host ribosome, thereby inhibiting host protein synthesis. Others (eg, diphtheria toxin) prevent protein elongation. S pneumoniae does not directly inhibit host protein synthesis. (Choice B) Motility within host tissue can be provided by flagella. S pneumoniae is a nonmotile organism. (Choice C) Facultative intracellular bacteria such as Mycobacterium tuberculosis and Listeria monocytogenes have virulence factors that prevent the bacteria from being destroyed by the phagolysosome of macrophages. S pneumoniae is not an intracellular pathogen. (Choice E) Spores are resistant to chemical disinfectants, irradiation, desiccation, and temperatures as high as 120 C (248 F). Bacillus and Clostridium species commonly produce spores, but S pneumoniae does not. Educational objective: The primary virulence factor of Streptococcus pneumoniae is a polysaccharide capsule that inhibits opsonization and phagocytosis. The polysaccharide capsule of the most virulent strains is targeted by the pneumococcal vaccine, which confers immunity against those subtypes. __________ References: The pneumococcus: epidemiology, microbiology, and pathogenesis. (http://www.ncbi.nlm.nih.gov/pubmed/23818515)

A 62-Year-Old Man Comes to the Emergency Department Due to Sudden-Onset