Final answer:
The interaction described is known as mutualism, a symbiotic relationship where both species benefit, such as bacteria in termite guts aiding digestion or gut bacteria in humans producing vitamins.
Step-by-step explanation:
The type of ecological interaction that fits the description where some bacteria can only live inside of insects, and those insects do much better when they are colonized by these bacteria, is known as mutualism. In a mutualistic relationship, both species involved gain benefits that contribute to their survival and fitness. This is evident in the way certain bacteria live in symbiotic harmony with their insect hosts, such as the bacteria in the guts of termites that help digest cellulose, an ability termites lack on their own. The bacteria benefit by residing in a consistent and protective environment, while the termites gain energy from the food they consume. Likewise, bacteria living in the roots of legumes fix nitrogen for the plant, receiving nutrients in return, and human gut bacteria such as Bacteroides thetaiotaomicron help in digesting complex carbs and produce essential vitamins for the host.