Which of the following is NOT classified as a pathogen?

Non-pathogenic bacteria are categorized as those microorganisms that do not cause disease in humans or other hosts. Unlike pathogens, which include harmful bacteria, viruses, and fungi that can lead to infections and illnesses, non-pathogenic bacteria can even provide beneficial functions, such as aiding in digestion or protecting against harmful organisms.

Overall, the distinction lies in the ability of a microorganism to cause disease; thus, non-pathogenic bacteria serve a different role in the ecosystem compared to the other options presented, which are all classified as pathogens capable of leading to health issues. This is why the correct choice accurately identifies non-pathogenic bacteria as not fitting into the classification of pathogens.