__________ are harder to destroy than bacteria.

Spores are a highly resilient form of certain bacteria, allowing them to survive extreme conditions that typically would lead to the death of the bacterial cells. They can withstand heat, desiccation (drying out), and exposure to various chemicals and environmental stresses that would otherwise be lethal to the vegetative forms of bacteria. This ability to remain dormant and resistant makes spores significantly harder to destroy than their actively growing counterparts.

In contrast, cocci, bacilli, and spirilla are all classifications of bacteria based on their shape—spherical, rod-shaped, and spiral, respectively. While these forms can reproduce and cause infections, they do not possess the same level of resilience as spores when it comes to enduring harsh environments. Therefore, when considering which type of microbial life is particularly challenging to eliminate, spores emerge as the correct answer due to their specialized structure and dormant state that enhance their survival capabilities.