Final answer:
Option (B), Water distribution systems often contain biofilms which may house opportunistic pathogens and also include grazing protists that consume bacteria.
Step-by-step explanation:
Water distribution systems often contain biofilms that may harbor opportunistic pathogens. Biofilms are complex and dynamic ecosystems that form on various surfaces in watery environments. These structured microbial communities offer a protective environment, allowing organisms to survive in challenging conditions, such as exposure to antimicrobial agents.
In addition to harboring pathogens, biofilms can also support numerous grazing protists that consume bacteria, playing a role in the ecosystem's balance. Biofilms are very difficult to eradicate due to their resistance to methods typically used to control microbial growth, such as antibiotics. The resilience of biofilms and their ability to foster pathogenic organisms make them a significant concern in both healthcare settings and water distribution systems.
Moreover, waterborne diseases linked to pathogenic microorganisms present a serious health risk, especially in underdeveloped countries where they can cause a high number of fatalities annually. Proper sewage treatment is crucial in controlling the spread of these waterborne pathogens.