Aims: The recent EU ban of growth-promoting antibiotics in animal production was based on fears concerning antibiotic resistance being transmitted to human pathogens. This paper explores the adaptation mechanism of a common ruminal bacterium, Prevotella bryantii, to one of the banned compounds, flavomycin (flavophospholipol). Methods and Results: Growth in the presence of flavomycin (2 and 20 mu g ml(-1)) was characterized by a concentration-dependent increase in the length of the lag phase, which decreased after previous flavomycin exposure. From growth patterns on solid medium, decreased sensitivity appeared to be due to a whole-population adaptation. Proteomic analysis indicated upregulation of three native proteins occurred following flavomycin adaptation. Further analysis of two of these proteins resulted in no database matches, suggesting that they may be species-specific. Flavomycin adaptation also resulted in co-adaptation to bacitracin and vancomycin. Conclusions: Adaptation of P. bryantii to flavomycin, which also resulted in co-adaptation to bacitracin and vancomycin, may involve an increased availability of undecaprenyl pyrophosphate. Significance and Impact of the Study: The use of flavomycin, and similar growth-promoting antibiotics, in animal production may prompt adaptive responses in ruminal bacteria which can significantly change their antibiotic sensitivity.