Answer:
1) Mutations: Bacterial mutations can occur spontaneously and alter the DNA of the bacterium, leading to changes in its characteristics and potentially leading to the development of a new strain.
2) Horizontal gene transfer: Bacteria can exchange genetic material with other bacteria through mechanisms such as conjugation, transduction, or transformation. This can result in the transfer of virulence factors, antibiotic resistance genes, or other traits that contribute to the development of new strains.
3) Antibiotic resistance: The overuse and misuse of antibiotics can lead to the selection and spread of antibiotic-resistant strains of bacteria. This can result in the emergence of new strains that are resistant to multiple antibiotics and pose a significant public health threat.