Harmful bacteria: 1  E. Coli can also be harmful if food or water is contaminated with it and then eaten. 2  Listeriosis can cause individuals to become very ill and can even be potentially deadly. 3  Examples of where it can be found are in shellfish, cold cuts, and unpasteurized milk.Answer #1). Helpful bacteria: 1  E. Coli are found in the intestines of humans and aid in digestion. 2  Streptomyces is used in making antibiotics. 3  Rhizobium are helpful bacteria found in the soil.