While technology advancement and scientific innovation have created new topics and fields of inquiry in STEM education, external content experts such as university scientists/researchers have been increasingly involved to enhance K-12 teachers' disciplinary understandings and professional development (PD). However, few studies have scrutinized scientist-facilitated PD programs regarding teacher epistemology, about "how" and "in what ways" the programs impact teachers' epistemological understandings of disciplinary knowledge. To address the gap, this paper investigates the process by which teachers construct epistemological understandings and teaching practices in interacting with scientists. Informed by theories of epistemic cognition and social cognition, we conducted an interactional ethnography in a school-university partnered PD program with six primary teachers. Based on participant observation, teacher interviews, and classroom videos and artifacts, we identified three patterns of teacher-scientist negotiation: reciprocal negotiation of knowledge presentation, observation and interpretation of scientist practices, and inconsistency in knowledge translation. The teachers' professional responsibility and knowledge served as a critical filter in their decisions of selecting, interpreting, and rejecting scientist inputs, leading to respective epistemological stances and pedagogical actions. The research uncovers the situated and multifaceted negotiation of teacher epistemology and offers implications for researching and supporting their epistemological development.