A 37 year old man is admitted to the hospital with shoness of breath, cyanosis, and fever. Chest x-ray films reveal consolidation of the right lower lobe with relative sparing of the remaining lobes.A clinical diagnosis of lobar pneumonia is made and suppoed by the results of sputum cultures. Which of the following is the genus of the bacterium most likely to be isolated from this patient’s sputum?
Correct Answer: Streptococcus, alpha-hemolytic
Description: The clinical and radiologic characteristics of this patient's condition are consistent with lobar pneumonia, a respiratory infection that, in its classic presentation, involves a single pulmonary lobe. In 90% to 95% of cases, the etiologic agent is Streptococcus pneumoniae (AKA pneumococcus), an alpha-hemolytic streptococcus present in the throat of 40% to 70% of healthy individuals. A small minority of cases of lobar pneumonia are due to Klebsiella pneumoniae, staphylococci, streptococci other than pneumococcus, Haemophilus influenzae, Pseudomonas, and Proteus. Penicillin is the drug of choice for pneumococcal pneumonia, but sputum cultures are necessary to identify the infectious agent and determine its antibiotic sensitivity. S. pneumoniae is a major cause of purulent meningitis in the elderly. Other alpha-hemolytic streptococci (viridans streptococci) cause subacute endocarditis in patients with previously altered cardiac valves. Streptococcus mutans and other oral streptococci have been associated with dental caries. Bacteria belonging to the genera Haemophilus and Klebsiella cause respiratory infections; however, in immunocompetent individuals, these bacterial infections usually result in bronchopneumonia, which leads to multilobar, and often bilateral, pulmonary involvement. Beta-hemolytic streptococci cause countless infections in humans. Recall that classification of beta;-hemolytic streptococci is based on their surface antigens known as Lancefield antigens. Human diseases caused by this group of bacteria include: Streptococcus pyogenes (group A): pharyngitis, scarlet fever, erysipelas, impetigo, rheumatic fever, and glomerulonephritis. Streptococcus agalactiae (group B): neonatal sepsis and urinary infections - Enterococcus faecalis (group D): endocarditis and urinary infections. Ref: Levinson W. (2012). Chapter 15. Gram-Positive Cocci. In W. Levinson (Ed),Review of Medical Microbiology & Immunology, 12e.
Category:
Microbiology
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