Which term describes microbes that reside on the host without causing disease under normal conditions?

• A. The transient microbiota
• B. The normal microbiota (normal flora)
• C. Pathogenic microbiota
• D. Commensal viruses

Answer: (B)

Normal microbiota (normal flora) are the microorganisms that regularly inhabit the body without causing disease in a healthy person. They take up niches on skin, mucous membranes, and in the gut, and they can help by occupying space that might otherwise be taken by pathogens and by producing beneficial compounds. This steady, resident population stays in place under normal conditions; problems occur if balance is disrupted—such as by antibiotics, immune suppression, or a breach of barriers—allowing these organisms to cause disease or enabling opportunistic pathogens to invade. Transient microbiota are the short-term visitors that don’t establish lasting colonies, while pathogenic microbiota are those capable of causing illness.