Final answer:
The Horace Silver Quintet and the Jazz Messengers co-led by Art Blakey and Horace Silver were the two major bands that defined hard bop, known for their unique styles and contribution to the jazz genre.
Step-by-step explanation:
The two major bands that defined hard bop during the mid-1950s were the Horace Silver Quintet and the Jazz Messengers co-led by Art Blakey and Horace Silver before Silver left to focus on his own group. The Horace Silver Quintet was significant for its soulful infusion into jazz, characterized by catchy compositions and Silver's distinctively funky piano style. On the other hand, the Jazz Messengers were known for their driving rhythms, bluesy melodies, and the promotion of young talent, making a considerable impact on the advancement of the hard bop style.