There are trillions of living biological "computers" on, inside, and around the human body: microbes. Microbes have the potential to enhance human-computer interaction (HCI) in entirely new ways. Advances in open-source biotechnology have already enabled designers, artists, and engineers to use microbes in redefining wearables, games, musical instruments, robots, and more. "Living Bits," inspired by Tangible Bits, is an attempt to think beyond the traditional boundaries that exist between biological cells and computers for integrating microorganism in HCI. In this work we: 1) outline and inspire the possibility for integrating organic and regenerative living systems in HCI; 2) explore and characterize human-microbe interactions across contexts and scales; 3) provide principles for stimulating discussions, presentations, and brainstorms of microbial interfaces. We aim to make Living Bits accessible to researchers across HCI, synthetic biology, biotechnology, and interaction design to explore the next generation of biological HCI.