you dont need to take Tetnus inj at every cut or minor injury. the knife does carry tetnus bactria, it is mostly from soil/ dust  contaminated with tetanus spores , that can infect an open wound, mostly in road traffic accidents  or other major dirty  injuries . if you had Tetanus vaccination in your child hood and booster at 10 years interval , you dont need to take Tetanus inj. If you didnt take booster at 10 years ,you can take the booster now  as  your age is 15 yrs ( from your profile )