Streptococcus salivarius (left image) is a bacterium that promotes oral health, and could potentially be linked to decreased risk of rheumatic heart disease. S. salivarius is a bacterium that lives in your saliva and acts as a probiotic in the prevention of oral infections, especially from Strep Throat. Since the same virus that causes Strep Throat has also been linked to the cause of rheumatic heart disease, S. salivarius could be at the forefront for heart disease prevention. The bacterium does have some negative effects, though. It has been found to accelerate colonization of dental plaque, and if it enters the blood stream, it could lead to septicemia (body sepsis), though cases of this are rare. 

Deadly or friendly: Mostly friendly, but with a dark side.

Alcanivorax borkumensis (right image) is bacterium that lives in aquatic environments and feeds on carbon chains. Because of this, A. borkumensis is found primarily in polluted waters, notably in areas that have experienced an oil spill, which serves as an ideal energy source for the bacterium. It has been shown to help wildlife in the ecosystem; when ingested by an animal, the bacterium could lessen the negative effects of oil in the digestive tract. The bioremediation of A. borkumensis holds incredible potential to the future of ecology as off-shore and deep sea drilling become more and more prevalent.

 
Deadly or friendly: Friendly – it helps clean up oil spills!