A point to remember is that bacteria is EVERYWHERE! haha. Plus having bacteria and other microorganisms are beneficial to a healthy terrarium/vivarium. This is because these microbes help cycle the soil, which prevents/hinders the growth of potential pathogens and helps the plants grow healthy and strong. Plus live plants help increase humidity via transpiration as well as filtering the air by removing excess CO2, which can be toxic at high enough levels. Likewise, this decrease in CO2 will help prevent stagnant air from forming, which can make a nice growing environment for opportunistic microbes/pathogens.