Final answer:
Tightly coiled, spiral-shaped bacteria are called spirochetes, which are distinct from spirilli. Spirochetes are specialized spiral bacteria best viewed with darkfield microscopy.
Step-by-step explanation:
Tightly coiled, spiral-shaped bacteria are called spirochetes. This term refers specifically to the spiral-shaped bacterium which is often gram negative-like and can be better viewed by darkfield microscopy due to their very thin structure. Among other bacterial shapes such as the spherical cocci and the rod-shaped bacilli, spirochetes have a distinctive spiral form. Bacteria can be identified and classified based on their shape, and these various shapes can include coccobacillus which is a combination of spherical and rod-shaped.
Spirilli is another term that also describes spiral-shaped bacteria; however, it is different from spirochetes. The term 'spirilli' often refers to a broader category of spiral bacteria, of which spirochetes are a specific kind. It's important to distinguish between these terms when classifying bacteria by shape.