Prevalence and features of hypervirulent Klebsiella pneumoniae in respiratory specimens at a US hospital system

Hypervirulent Klebsiella pneumoniae (hvKp) strains are considered to be relatively rare in the United States, but cases are increasingly reported. We prospectively and serially collected K. pneumoniae clinical isolates identified in respiratory specimens at a health system in Western Pennsylvania between 2020 and 2022. A total of 273 K. pneumoniae isolates from 216 unique patients were analyzed for markers of hypervirulence by both string test for a hypermucoid phenotype and multiplex PCR to detect isolates carrying cardinal virulence genes rmpA, rmpA2, iutA, and iro. Of the 273 isolates, 13 (4.8%) tested positive by string test including 11 nonduplicate K. pneumoniae isolates, and two of these (0.7%) were positive by PCR for virulence genes rmpA, rmpA2, iutA, and iro. The latter two putative hvKp strains, belonging to sequence types ST23-K1 and ST86-SLV-K2, possessed pLVPK-like plasmids, and were collected from community-associated infections in individuals without known travel histories. Both putative hvKp strains and two additional string test-positive strains were resistant to killing by human serum. The hvKp strains caused significant pneumonia in mice infected by oropharyngeal aspiration, with significantly higher weight loss and increased bacterial burden in the lungs of mice infected with the KL1 (ST23) strain compared to the KL2 (ST86-SLV) strain. We also observed decreased survival of mice infected with the KL1 strain compared to the KL2 strain. These findings add to the growing body of evidence suggesting that hvKp strains, once considered endemic to Asia, may now be circulating in North America.