What defines pathogenic bacteria?

Pathogenic bacteria are defined as microscopic single-celled organisms that can cause disease in humans, animals, or plants. These bacteria thrive in various environments and can be found in soil, water, and even within the human body. Their presence can lead to infections, foodborne illnesses, and a range of health issues, making it critical to understand their behavior and transmission.

While some bacteria are indeed beneficial for digestion or used in food preservation, pathogenic bacteria are specifically associated with disease rather than beneficial processes. The option referencing bacteria that thrive in extreme environments is unrelated to pathogenicity, as these bacteria, known as extremophiles, typically do not cause disease in humans. Thus, the characteristics of pathogenic bacteria align with being single-celled organisms capable of causing harm, making the second choice the most accurate definition.