Protected environments allow parallel evolution of a bacterial pathogen ina patient subjected to long-term antibiotic therapy

Long-term antibiotic treatment offers a rare opportunity to study the evolu tion of bacteria within the same individual. The appearance of new variants has been suggested to take place via the selection of enhanced resistance in compartments of the body in which the antibiotic concentration is low. L aboratory models of protected compartments have elegantly demonstrated thei r potential in selecting novel variants. However, comparable data from pati ents have been rare. In this study, extended antibiotic therapy in a single patient suffering from multiple infected liver cysts has provided the oppo rtunity to observe and analyse the molecular evolution of antibiotic resist ance. Each isolate has the same basic ompC gene sequence that is distinct f rom other Escherichia coli isolates, which suggests that they derive from t he same founder population. However, the isolates differ in their auxotroph ic markers, in the pl values of their dominant P-lactamase activities and i n the mutations in the promoter region of the ampC gene leading to increase d expression of the AmpC enzyme. The data provide strong evidence for a sin gle focal infection expanding via parallel pathways of evolution to give a range of anti biot ic-resistant isolates. These data suggest that the infec ted cysts provide numerous protected environments that are the foci for the separate development of distinct variants.