Final answer:
A vaccine made from killed bacteria is best defined as a bacterin, which is an inactivated vaccine that triggers an immune response without causing the disease. Bacterins differ from toxoid vaccines, which contain only inactivated bacterial toxins.
Step-by-step explanation:
Definition of a Vaccine Prepared from Killed Bacteria
The term that best defines a vaccine prepared from killed bacteria is Option 1: Bacterin. A bacterin is an inactivated vaccine which is composed of whole bacterial cells that have been killed or inactivated through treatment methods such as heat, radiation, or chemicals. Unlike toxoid vaccines, which are inactivated bacterial toxins used to neutralize toxins in diseases where toxins play a significant role in pathogenesis, bacterins contain the whole killed bacteria and are designed to elicit an immune response without causing the disease. This is because the key antigens remain intact during the inactivation process, allowing the immune system to recognize them and produce defensive mechanisms, such as memory cells, which will recognize and respond to future infections by the same bacteria. Therefore, vaccines play an essential role in preventing disease and have led to considerable reductions in morbidity and mortality from infectious diseases.