AIM:To establish a severe pneumonia mouse model induced by bacterial infection.METHODS:A total of 102 male SPF C57BL/6J mice were randomly divided into a control group and a model group.Klebsiella pneu-moniae was administered via tracheal instillation at a concentration of 5×109 CFU.Mice were euthanized on days 1,2,4,8,and 14 post-infection to assess general condition,body weight,mortality,white blood cell and neutrophil counts,in-flammatory markers,and pathological changes in lung,heart,liver,spleen,kidney,and intestinal tissues.RESULTS:Mice in the model group exhibited symptoms such as dyspnea and huddling from 6 hours to 4 days post-infection,which progressively worsened,accompanied by continuous weight loss(P<0.01).These symptoms gradually resolved between days 5 and 14.Arterial oxygen saturation in the model group dropped to 80.7%from days 1 to 8(P<0.01)but returned to normal from days 9 to 14.A total of 23 model mice died between days 1 and 9,with no deaths thereafter,resulting in a mortality rate of 31.9%(P<0.01).Pathological examination revealed inflammatory cell infiltration,congestion,and ede-ma in lung tissue from days 1 to 2,with continued inflammatory cell infiltration,alveolar structural disorganization from days 4 to 8,and alveolar rupture and fusion by day 14(P<0.05 or P<0.01).Additionally,model mice showed significant increases in neutrophil count,white blood cell count,protein content in bronchoalveolar lavage fluid,total cell count,neutrophil ratio,and levels of inflammatory factors tumor necrosis factor-α(TNF-α),interleukin-1β(IL-1β)and IL-6 in peripheral blood from days 1 to 8(P<0.05 or P<0.01).No significant pathological changes were observed in heart and liver tissues,while spleen,kidney,and intestinal tissues exhibited notable pathological changes:indistinct boundaries be-tween red and white pulp in the spleen,significant congestion and edema around renal glomeruli,renal tubules,and col-lecting ducts,and extensive inflammatory cell infiltration in the colonic mucosa.CONCLUSION:Tracheal instillation of 5×109 CFU Klebsiella pneumoniae induces severe pathological changes in the lungs of mice,offering a robust model for studying the pathogenesis and treatment of severe pneumonia.