What distinguishes pathogenic bacteria from non-pathogenic bacteria?

The distinguishing feature of pathogenic bacteria is their ability to cause disease in a host. Pathogenic bacteria have specific mechanisms that allow them to invade tissues, evade the immune system, and disrupt normal physiological functions, leading to illness. This capability encompasses a variety of bacteria that can lead to a wide range of infections, from mild to severe.

The other options do not accurately capture the essence of what makes bacteria pathogenic. While some pathogenic bacteria can indeed be found in soil, many non-pathogenic bacteria also inhabit these environments. Size is not a consistent factor, as pathogenic and non-pathogenic bacteria can vary greatly in size, and size alone does not determine pathogenicity. Furthermore, not all pathogenic bacteria are anaerobic; in fact, many are aerobic and thrive in the presence of oxygen. Pathogenicity is specifically linked to the bacteria's ability to cause disease, rather than environmental conditions or physical characteristics.