Closely related ST11 Klebsiella pneumoniae isolates with transient attenuation and re-emergence of carbapenem resistance during prolonged hospitalization: a case report.

BACKGROUND: Apparent transitions in carbapenem susceptibility among sequential Klebsiella pneumoniae isolates are often interpreted as strain replacement. However, phenotypic variation among closely related isolates may also produce similar phenotypic shifts, particularly in heavily treated patients.

CASE PRESENTATION: We report a 91-year-old long-term hospitalized patient from whom three sequential ST11 Klebsiella pneumoniae isolates were recovered from blood and urine over a 3-month period. The initial bloodstream isolate (KPB1) exhibited a hypermucoviscous phenotype, KPC production, and carbapenem resistance. A subsequent urinary isolate (KPU2) lost the hypermucoviscous phenotype and showed attenuation of carbapenem resistance, becoming susceptible to imipenem, meropenem, and doripenem while remaining resistant to ertapenem. A later urinary isolate (KPU3) reverted to carbapenem resistance while remaining non-hypermucoviscous. Repeat antimicrobial testing confirmed these findings. Core-genome SNP analysis identified 16-20 pairwise differences among the three isolates, supporting close genomic relatedness despite their phenotypic discordance.

CONCLUSION: This case illustrates marked differences in carbapenem susceptibility and hypermucoviscosity among closely related ST11 K. pneumoniae isolates recovered during a single patient's clinical course. KPU2 retained ertapenem resistance despite susceptibility to other tested carbapenems. The genomic findings were more consistent with closely related isolates than with replacement by an unrelated strain, although direct ancestry and continuous persistence could not be established. Phenotypic changes should be interpreted alongside repeat microbiological testing, available genomic evidence, and the clinical context.