What characteristic distinguishes pathogenic from non-pathogenic bacteria?

Pathogenic bacteria are specifically characterized by their ability to cause disease in a host organism. This infectious ability allows them to invade body tissues, evade the immune response, and lead to various health issues, ranging from mild to severe infections. The distinction arises because non-pathogenic bacteria are generally harmless and can even be beneficial to human health, aiding in processes like digestion and maintaining a balanced microbiome.

While size, color, and shape may vary among different types of bacteria, these characteristics are not reliable indicators of whether a bacterium is pathogenic or non-pathogenic. For instance, both types of bacteria can come in a range of sizes and shapes, and they may appear similar in color under certain conditions. Hence, the ability to cause illness is the defining trait that separates pathogenic from non-pathogenic bacteria.