These groups of bacteria are harmless EXCEPT

Analyze the question: We need the group of bacteria that is not harmless, meaning the one that can cause harm (disease) to other organisms.

Explain the concept: Bacteria are grouped by how they get their nutrition and by their role in the environment:

• Nitrifying bacteria (e.g., Nitrosomonas, Nitrobacter) convert ammonia to nitrites and then nitrates. This is useful because it makes nitrogen available to plants.
• Nitrogen-fixing bacteria (e.g., Rhizobium, Azotobacter) convert atmospheric nitrogen into nitrogen compounds. They are beneficial and live in the root nodules of legumes like beans and groundnut.
• Saprophytic bacteria feed on dead organic matter and cause decay. They recycle nutrients and are generally harmless to living organisms, though they can spoil food.
• Denitrifying bacteria (e.g., Pseudomonas denitrificans) convert nitrates back to nitrogen gas. They reduce soil fertility, but they do not attack living organisms, so they are not "harmful" in the sense of causing disease. In this question, they are grouped with the harmless ones.
• Parasitic bacteria live on or in a living host and obtain nutrients from it, harming the host. Examples are Mycobacterium tuberculosis (tuberculosis), Vibrio cholerae (cholera) and Salmonella typhi (typhoid).

Solution: Going through the options, A, B, C and E are groups whose activities are part of the natural nutrient cycle and do not cause disease in living things. Only the parasitic bacteria harm their hosts by causing disease.

Answer: D. parasitic

Tip: In "harmless EXCEPT" questions, look for the option linked to disease. Parasite = harm to the host. Nitrifying, nitrogen-fixing and saprophytic bacteria are the "helpers" of the ecosystem.