In this design case, we describe our work to design and develop a socio-scientific issue (SSI) based unit of instruction for use in high school biology. Our team includes university based science educators, an experienced classroom teacher, and a microbiologist. The unit focuses on antibiotic resistant bacteria as a context for student exploration of natural selection and engagement in modeling practices. Our team recently presented a model for SSI instruction that highlights: (a) a focal issue, (b) interaction of science ideas and practices, (c) social considerations, (d) use of information and communications technologies, and (e) a culminating experience that encourages students to synthesize their ideas. We use this model to frame the design case and discuss key decision points that influenced design and development of the unit. The design product is a three-week unit that we implemented in the spring of 2014. Key challenges presented in the case include continually evolving notions of scientific modeling practices and implications for related learning activities, developing supports for student negotiation of the social dimensions of antibiotic resistance, and determining how much emphasis to place on student use of information and communications technologies.