A Neonatal Model of Intravenous Staphylococcus epidermidis Infection in Mice <24 h Old Enables Characterization of Early Innate Immune Responses

Staphylococcus epidermidis (SE) causes late onset sepsis and significant morbidity in catheterized preterm newborns. Animal models of SE infection are useful in characterizing disease mechanisms and are an important approach to developing improved diagnostics and therapeutics. Current murine models of neonatal bacterial infection employ intraperitoneal or subcutaneous routes at several days of age, and may, therefore, not accurately reflect distinct features of innate immune responses to bacteremia. In this study we developed, validated, and characterized a murine model of intravenous (IV) infection in neonatal mice 6, 107, 108 colony-forming units (CFU) of SE 1457, a clinical isolate from a central catheter infection. A prospective injection scoring system was developed and validated, with only high quality injections analyzed. Newborn mice were euthanized between 2 and 48 h post-injection and spleen, liver, and blood collected to assess bacterial viability, gene expression, and cytokine production. High quality IV injections demonstrated inoculum-dependent infection of spleen, liver and blood. Within 2 h of injection, SE induced selective transcription of TLR2 and MyD88 in the liver, and increased systemic production of plasma IL-6 and TNF-α. Despite clearance of bacteremia and solid organ infection within 48 h, inoculum-dependent impairment in weight gain was noted. We conclude that a model of IV SE infection in neonatal mice表皮葡萄球菌（Staphylococcus epidermidis，SE）可导致留置导管的早产新生儿发生迟发性脓毒症，并引发显著的致病负担。针对SE感染的动物模型，可用于阐明疾病发病机制，亦是开发优化诊断方法与治疗手段的重要途径。当前用于新生儿细菌性感染的小鼠模型，多在小鼠出生数日后采用腹腔或皮下给药途径，因此可能无法准确反映机体针对菌血症的固有免疫应答特征。本研究中，我们构建、验证并表征了一种新生小鼠静脉（IV）感染模型：以临床分离自中心导管感染的SE 1457菌株，按6、107、108菌落形成单位（CFU）的剂量进行静脉接种。本研究建立并验证了前瞻性注射评分体系，仅纳入高质量注射操作的实验数据开展分析。分别于注射后2至48小时对新生小鼠实施安乐死，采集脾脏、肝脏与血液样本，以检测细菌存活率、基因表达及细胞因子生成情况。高质量静脉注射操作的实验结果显示，脾脏、肝脏与血液的感染程度呈接种剂量依赖性。注射后2小时内，SE可诱导肝脏中Toll样受体2（TLR2）与髓样分化蛋白88（MyD88）的选择性转录，并提升血浆中白细胞介素6（IL-6）与肿瘤坏死因子α（TNF-α）的系统性生成水平。尽管在注射后48小时内即可清除菌血症与实体器官感染，但仍观察到接种剂量依赖性的体重增长受损现象。我们认为，新生小鼠静脉SE感染模型