Which bacteria are known to grow in clusters and are associated with abscesses and boils?

The presence of Staphylococci as the correct choice is based on their unique growth characteristics and their association with specific types of infections. Staphylococci are spherical bacteria that tend to group together in clusters, resembling grapes when viewed under a microscope. This clustering property is distinct and differentiates them from other types of bacteria.

Staphylococci are significant in the context of skin infections, as they are commonly linked to abscesses and boils. These infections occur when the bacteria invade skin tissue, leading to the formation of pus-filled pockets. The most well-known species, Staphylococcus aureus, is particularly notorious for causing such infections, as well as other serious conditions.

In contrast, Streptococci grow in chains or pairs rather than clusters, and although they can also cause skin infections, they are usually more associated with throat infections and deeper skin infections. Bacilli are rod-shaped bacteria and are not typically associated with abscesses in the same manner as Staphylococci. Spirilla, which are spiral-shaped bacteria, are also unrelated to the formation of boils and abscesses.

Thus, Staphylococci's clustering growth pattern and specific association with skin-related infections validate their selection as the correct answer in this context.