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Background Escherichia coli is the most predominant Gram-negative bacterial pathogen associated with neonatal meningitis. Previous studies indicated that the prototypic neonatal meningitis E. coli (NMEC) strain RS218 (O18:K1:H7) harbors one large plasmid. Objectives of the present study were to analyze the complete nucleotide sequence of this large plasmid (pRS218) and its contribution to NMEC pathogenesis using in vitro and in vivo models of neonatal meningitis. Results The plasmid is 114,231 bp in size, belongs to the incompatibility group FIB/IIA (IncFIB/IIA), and contains a genetic load region that encodes several virulence and fitness traits such as enterotoxicity, iron acquisition and copper tolerance. The nucleotide sequence of pRS218 showed a 41- 46% similarity to other neonatal meningitis-causing E. coli (NMEC) plasmids and remarkable nucleotide sequence similarity (up to 100%) to large virulence plasmids of E. coli associated with acute cystitis. Some genes located on pRS218 were overly represented by NMEC strains compared to fecal E. coli isolated from healthy individuals. The plasmid-cured strain was significantly attenuated relative to the RS218 wild-type strain as determined in vitro by invasion potential to human cerebral microvascular endothelial cells and in vivo by mortalities, histopathological lesions in the brain tissue, and bacterial recovery from the cerebrospinal fluid of infected rat pups. Conclusions The pRS218 is an IncFIB/IIA plasmid which shares a remarkable nucleotide sequence similarity to large plasmids of E. coli associated with cystitis. Both in vitro and in vivo experiments indicated that pRS218 plays an important role in NMEC pathogenesis. [ABSTRACT FROM AUTHOR]