You might have already heard that bacteria are found everywhere. Did you know that “everywhere” includes plants and animals, even humans? Various species interact and affect one another, forming symbiotic relationships. These partnerships can be categorized based on how species are affected- do they suffer, remain the same, or benefit? What if they both help each other? Three types of symbiotic relationships are parasitism, commensalism, and mutualism. Furthermore, you can classify the relationships as endosymbiosis (one organism lives inside the other) or ectosymbiosis (one organism lives on the surface of the other).

Parasitism is a relationship where one species benefits and the other is harmed. Let’s use an analogy of students: the kid who sits next to you in math class calls you at midnight to copy off of your homework. They benefit because they get the answers, but you suffer because you’re losing sleep! Salmonella bacteria, which cause salmonellosis (yucky), benefit by living in the intestines, multiplying, and spreading to another host. That is also an example of endosymbiosis. The host is harmed because they have food poisoning! Fire blight, a serious condition affecting apples and pears, occurs when the pathogenic bacteria Erwinia amylovora colonizes plants.

Commensalism means that one species benefits and the other is unaffected. Let’s say you’re acing mathematics, and a different classmate asks you (at a reasonable time of day!) to explain something before the next test. They may get a higher grade, but you’ll do about the same, because you’re already a pro. We host a variety of commensal bacteria, such as Staphylococcus epidermidis, on our skin. This is an example of ectosymbiosis. While the bacteria consume our dead skin cells for nutrients, they don’t usually affect us very much. However, they sometimes cause infections associated with indwelling medical devices (such as feeding tubes), and sometimes help prevent harmful bacteria from colonizing our skin.

With mutualism, both species mutually benefit from their relationship. You and another classmate are helping each other- they can give you a few pointers in writing, but need your expertise to understand math. You would both do better in your respective classes. Bacteria that live in cows’ digestive tracts help the cows digest components of plants. Ruminococcus breaks down cellulose, and Selenomonas breaks down starch. The cows are able to get more nutrition out of their food, while the bacteria have access to food and shelter.