Which of the following bacteria are commonly found in the environment and are usually harmless?

Coliform bacteria are commonly found in the environment, particularly in soil, water, and vegetation, as well as in the intestines of warm-blooded animals. The presence of coliform bacteria is often used as an indicator of water quality and the potential presence of pathogenic microorganisms. While some types of coliform bacteria are harmful, the majority found in the environment are non-pathogenic and considered harmless. They play a significant role in the environment and are part of the natural microbial community.

In contrast to coliform bacteria, the other options represent bacteria that are more often associated with illness. E. coli includes both harmless strains and pathogenic strains that can cause serious gastrointestinal distress. Salmonella is a well-known pathogen responsible for foodborne illness. Legionella is associated with Legionnaires' disease, which can lead to severe respiratory problems. Therefore, the significance of coliform bacteria lies in their environmental prevalence and their generally non-harmful nature, making them a key focus in water quality assessments.