Antibiotics are given prophylactically to prevent infection in almost all cases of what is referred to as acute fight bite. The only two exceptions are: 1) patients with minimal superficial wounds (skin is not broken) and 2) those individuals who come for care 72 hours or more after the injury and have no signs of infection.
With up to 50 different types of bacteria in the human mouth, the concern for infection is very real. The physician may also surgically debride the wound (open the hand down to the bone/joint and clean out any microbes present).
Without knowing what may happen in those early hours after the contact with someone’s tooth, a wide-spectrum antibiotic is given. This type of medication covers most of the common bacteria (Staphylococcus, Streptococcus, Corynebacterium, Eikenella corrodens). Anyone with clinical signs of infection (fever; red, swollen, tender or painful skin/joint) will likely receive additional medications including intravenous antibiotics.
Even a small skin opening from such an injury can allow bacteria to enter the body. The joint can become infected to the point that finger amputation is the end-result. Not all complications from infection following an acute fight bite of the hand are as drastic as finger amputation. But loss of motion, decreased grip strength, tenosynovitis, osteomyelitis, and septic arthritis occur in up to one-third of all cases.
You did the right thing to get your brother medical care right away. The risk of amputation from infection increases dramatically with longer delays between injury and care.