4.3. Systemic Infection Mouse Model

For the systemic infection model, ICR male mice (four-week-old male weighing 18 to 20 g) (Daehan Bio Link Co., Ltd., Eum sung Gun, Korea) were used. Mice were maintained in animal chambers kept at 23 ± 2 °C with 55% ± 20% relative humidity. A systemic infection mouse model was established as described previously [20]. K. pneumoniae strains were cultured in CAMHB for 18 h at 37 °C. For inoculation, the cultured bacteria were suspended in 0.9% NaCl containing 5% gastric mucin (Sigma-Aldrich). Groups of five male ICR mice were intraperitoneally (i.p.) injected with a single 0.5 mL dose of the bacterial suspension. The challenge inoculum was sufficient to kill 100% untreated control mice within 24 h after infection. LCB10-0200 and ceftazidime were subcutaneously administered at 1 and 4 h after the bacterial infection. Mortality was recorded for 7 days, and the median effective dose required to protect 50% of mice (ED₅₀) was calculated using the probit method with BioStat version 6 (Walnut, CA, USA).