Protein misfolding and aggregation into amyloid structures are associated with dozens of human diseases. Recent studies have provided compelling evidence for the existence of highly ordered, amyloid-like conformations in the insoluble inclusion bodies produced during heterologous protein expression in bacteria. Thus, amyloid aggregation seems to be an omnipresent process in both eukaryotic and prokaryotic organisms. Amyloid formation inside cell factories raises important safety concerns with regard to the toxicity and infectivity of recombinant proteins. Yet such findings also suggest that prokaryotic cells could be useful systems for studying how and why proteins aggregate in vivo, and they could also provide a biologically relevant background for screening therapeutic approaches to pathologic protein deposition. © 2009 Elsevier Ltd. All rights reserved.