Which statement best describes bacteria?

Bacteria are incredibly diverse microorganisms that can have both beneficial and harmful effects on humans, other organisms, and the environment. For instance, certain bacteria play a crucial role in processes such as digestion, where they help break down food in the intestines, and in various ecological systems, where they contribute to nutrient cycling. Conversely, some bacteria are pathogens that cause diseases in humans and animals. This versatility in function and impact is what makes the statement that they can be both beneficial and harmful the most accurate descriptor of bacteria.

In contrast, bacteria are not solely harmful to humans, as noted in the first option. While some can cause illness, many types are vital for health and ecological balance. The idea that all bacteria are infectious agents oversimplifies their role; in reality, many bacteria are harmless or even beneficial. Additionally, stating that they exist only in multicellular forms is incorrect, as bacteria are primarily unicellular organisms, with each individual bacterium consisting of a single cell.