The majority of clinical isolates of Klebsiella are which species?

• A. K. ozaenae
• B. K. pneumoniae
• C. K. aerogenes
• D. K. oxytoca

Answer: (B)

Klebsiella pneumoniae is the species most commonly isolated from clinical infections. It dominates because it’s a highly virulent member of the genus, with a thick polysaccharide capsule that helps it resist phagocytosis and establish infections in the respiratory tract, urinary tract, bloodstream, and other sites. This species is frequently recovered from hospitalized patients and those with weakened defenses, making it the principal cause of most Klebsiella-related infections seen in the lab.

Other Klebsiella species do exist and can cause disease, but far less often. For example, K. oxytoca can cause similar infections but is less common; K. ozaenae is more associated with chronic nasal disease (ozena) than invasive infections; K. aerogenes (historically grouped with Klebsiella in some settings) is less frequently recovered as a major pathogen compared to K. pneumoniae.