Med Microbiol lmmunol (1990) 179:323-333 9 Springer-Verlag 1990 Interaction of lactoferrin with Escherichia coli ceils and correlation with antibacterial activity Paolo Visca 1, Claudia Dalmastri 1, Daniela Verzili 2, Giovanni Antonini 3, Emilia Chiaucone 2, and Piera Valenti 4 l Istituto di Microbiologia, Facolta' di Medicina e Chirurgia, and 2Centro di Biologia Molecolare del CNR, Dipartimento di Scienze Biochimiche, Universita' di Roma "La Sapienza", Piazzale A. Moro 5, 1-00185 Roma, Italy 3Dipartimento di Medicina Sperimentale e Scienze Biochimiche, Universita' di Roma "Tor Vergata', 1-00173 Roma, Italy 4Istituto di Microbiologia, Facolta' di Medicina e Chirurgia I, Universita' di Napoli, 1-80138 Napoli, Italy Received June 6, 1990 Abstract. It has been established that the antimicrobial activity of lactoferrin towards Escherichia coli is enhanced by a direct contact between the protein and the microbial cell and that, in the case of E. coli K-12 strains, an antibacterial activity of lactoferrin unrelated to iron withdrawal is present. Evidence is now reported that lactoferrin binds to surface structures expressed in E. coli K-12 strains grown in either an "excess" or "stress" of iron. Under the experimental conditions used, lactoferrin binding both in the apo and in the iron-saturated form yields a maximum of 1.6 • 105 bound molecules/E, coli K-12 cell; the amount of lactoferrin bound does not depend on the expression of the iron-regulated outer membrane proteins. In contrast, lactoferrin does not bind to E. coli clinical isolates. Apo-lactoferrin (at 500 gg/ml in a chemically defined medium) inhibits the growth ofE. coli K-12 strains but not of clinical isolates. These findings suggest that the antibacterial activity of the protein could be associated to its binding to the cell surface. Introduction The well-known antimicrobial activity of transferrins, such as apo-lactoferrin (Lf), apo-serotransferrin and apo-ovotransferrin, was ascribed originally to their iron- chelating properties (for a review see [11, 13, 30)]. However, experimental evidence obtained in recent years in several species points to the existence of additional antimicrobial mechanisms based on the interference between these proteins and microbial structures. Arnold and co-workers reported a bactericidal effect of human Lftowards a variety of species including Streptococcus mutans [2, 4], Vibrio cholerae [3] and Legionella pneumophila [6] and demonstrated by means of immunofluorescence experiments a direct binding of the protein (both in the apo Offprint requests to: P. Visca