Final answer:
Spirochetes are gram-negative bacteria with a distinctive spiral shape, and some species are human pathogens causing diseases like syphilis and Lyme disease. They are best viewed using darkfield microscopy due to their thin structure.
Step-by-step explanation:
Spirochetes are a distinct group of gram-negative bacteria known for their long, spiral shape and motility. The genus Treponema includes various species of spirochetes, such as T. pallidum, which is infamous for causing the sexually transmitted infection syphilis. These bacteria are typically very thin, making them a challenge to view under standard brightfield microscopy, thus darkfield fluorescent microscopy is preferred. Spirochetes possess an axial filament that wraps around the cell and runs in the periplasmic space, giving them a unique mode of movement. While many spirochetes are free-living anaerobes, some are notable human pathogens, leading to diseases such as syphilis, Lyme disease, and leptospirosis.