Development of a mouse model for Klebsiella pneumoniae-associated neonatal sepsis

Klebsiella pneumoniae is a major cause of neonatal sepsis in low- to middle-income countries. With the proportion of multidrug-resistant K. pneumoniae strains increasing globally and the lack of novel antibiotics in the pipeline, vaccination to prevent infections due to K. pneumoniae is an alternative strategy to antibiotics. Several K. pneumoniae vaccines are currently in development; however, there are no small animal models of K. pneumoniae-associated neonatal sepsis that can be used to evaluate vaccine efficacy. Therefore, we developed and characterized a neonatal murine model of K. pneumoniae infection. We found that neonatal C57BL/6 mice were highly susceptible to K. pneumoniae B5055, a serotype O1:K2 hypervirulent strain, via peroral inoculation, and the susceptibility of mice to K. pneumoniae infection was age and dose dependent. Mice aged 2-3 days consistently exhibited 88%-100% mortality when infected with 10⁸-10⁹ colony-forming units of K. pneumoniae B5055. This susceptibility progressively decreased during the neonatal period such that K. pneumoniae B5055 at this dose was avirulent in 10-day-old and older C57BL/6 mice following peroral administration. Two-day-old mice were bacteremic as early as 2 h post-infection, which was accompanied by systemic dissemination of K. pneumoniae beyond the gastrointestinal tract into the brain, liver, lungs, and spleen. However, only the liver and lungs displayed inflammatory infiltrate around the portal tract and neutrophilic exudate in the alveoli and air spaces, when compared to uninfected mice, which showed no pathological effects. Collectively, these results show that we have generated a novel and reproducible animal model for K. pneumoniae neonatal sepsis.IMPORTANCEThe development of appropriate vaccines for K. pneumoniae relies on suitable animal models for in vivo efficacy testing. However, to date, there are no small animal models of K. pneumoniae-associated neonatal sepsis. We have established a neonatal mouse model of lethal K. pneumoniae infection that is age dependent and mimics the heightened susceptibility to Klebsiella spp. observed in human neonates. This newly discovered mouse model represents a valuable tool to study the pathogenesis of invasive K. pneumoniae infections in the neonate and to develop novel vaccines aimed at minimizing morbidity and mortality associated with K. pneumoniae neonatal sepsis.