ABSTRACT This study aims to evaluate the effectiveness of Bifidobacterium longum administration on the growth of Klebsiella pneumoniae in mice (Mus musculus). A true experimental design with a post-test approach was employed. Mice were divided into five treatment groups: preventive, curative, supportive, positive control, and negative control. Bifidobacterium longum was administered orally at a dose of 10⁷ CFU/ml, while Klebsiella pneumoniae was inoculated intraperitoneally at a dose of 10⁶ CFU/ml. Bacterial colony counts were determined using blood cultures on MacConkey agar medium and a colony counter. The results showed that Bifidobacterium longum was ineffective in preventing Klebsiella pneumoniae growth in a preventive manner. However, Bifidobacterium longum demonstrated effectiveness in inhibiting Klebsiella pneumoniae after infection, making it a promising potential curative and supportive therapy. Bifidobacterium longum shows potential as a probiotic agent to inhibit Klebsiella pneumoniae infections, supporting the development of alternative and supportive therapies to combat infections and antibiotic resistance. Keywords: Bifidobacterium longum, Klebsiella pneumoniae, Probiotics, Antibacterial, Mice. ABSTRAK Penelitian ini bertujuan untuk mengevaluasi efektivitas pemberian Bifidobacterium longum terhadap pertumbuhan Klebsiella pneumoniae pada mencit (Mus musculus). Kami menggunakan true experimental dengan desain post-test. Mencit dibagi menjadi lima kelompok perlakuan: preventif, kuratif, suportif, kontrol positif, dan kontrol negatif. Pemberian Bifidobacterium longum dilakukan secara oral dengan dosis 10⁷ CFU/ml, sedangkan Klebsiella pneumoniaediinokulasikan secara intraperitoneal dengan dosis 106CFU/ml. Jumlah koloni bakteri dihitung melalui kultur darah pada medium MacConkey menggunakan colony counter. Hasil menunjukkan bahwa Bifidobacterium longum tidak efektif dalam mencegah pertumbuhan klebsiella pneumoniae secara preventif. Namun Bifidobacterium longummenunjukkan efektivitas dalam menghambat klebsiella pneumoniae setelah infeksi terjadi, menjadikannya potensi terapi kuratif dan suportif yang menjanjikan. Bifidobacterium longum memiliki potensi sebagai agen probiotik untuk menghambat infeksi Klebsiella pneumoniae mendukung pengembangan terapi alternatif dan supportif untuk mengatasi infeksi dan resistensi antibiotik. Kata Kunci: Bifidobacterium Longum, Klebsiella Pneumoniae, Probiotics, Antibacterial, Mice.