Final answer:
Opportunistic pathogens like Legionella, Pseudomonas, and Mycobacterium can grow within protists, which aids their survival in water systems.
Step-by-step explanation:
Opportunistic pathogens such as Legionella, Pseudomonas, and Mycobacterium species have the ability to grow within protists. This relationship can enhance the pathogens' ability to survive in water distribution systems. Protists, particularly amoebae, often serve as a reservoir or host where these bacteria can reside, being sheltered from environmental stresses and treatments such as chlorination commonly used to disinfect water systems. For example, Legionella pneumophila, the causative agent of Legionnaires' disease, is known to thrive inside amoebae found in warm water systems, including air conditioning units of large buildings. The amoebae provide protection and also enhance the pathogenicity of the bacteria, thereby increasing the likelihood of an outbreak occurring wherever these conditions exist.