FEMS Microbiology Letters 14 (1982) 171-175 171 Published by ElsevierBiomedicalPress Plasmid content and virulence properties of urinary Escherichia coli strains Cees Waalwijk *, Johannes F. van den Bosch ,,a, David M. MacLaren * and Johannes de Graaff ** Departments of * Medical and ** Oral Microbiology, School of Medicine, Free University, Amsterdam, The Netherlands Received22 February 1982 Accepted 4 March 1982 1. INTRODUCTION Several properties have been related to the abil- ity of Escherichia coli strains to infect the urinary tract. These properties include serotype, colicin V production, haemolysis, adherence and serum-re- sistance. The relationship between certain O-antigens and nephropathogenicity has been suggested by several authors [1,2], whereas a high content of K-antigen might facilitate an E. coli strain to infect the urinary tract [3]. The production of colicin V is closely associated with pathogenicity [4]. Haemolysin production is found far more frequently in clinical isolates as compared to faecal strains [5]. Furthermore, elimination of the haemolytic property from nephropathogenic strains leads to a loss of viru- lence [6]. In one case, the loss of haemolysin production was accompanied with the loss of a plasmid. When the plasmid DNA was reintro- duced into this cured mutant, the strain reverted to a haemolytic phenotype and the original viru- lence level was restored [7]. The ability of E. coli strains to adhere to mam- malian cells has been shown to be an important virulence factor for strains isolated from patients a Present address: Department of Epidemiology, Erasmus Uni- versity, Rotterdam, The Netherlands. with acute pyelonephritis [8]. Finally, resistance to the bactericidal effect of serum is found more often in urinary isolates than in faecal strains [9]. Some of these virulence factors have been re- ported to be plasmid-mediated [4,10-12]. In the present study the plasmid content of a number of urinary isolates of E. coli was investigated, in an attempt to correlate the presence of a certain plasmid species with one of these properties as well as with the virulence in an experimental mouse model. 2. MATERIALS AND METHODS A total of 23 E. coli strains have been investi- gated. All strains were isolated from females with urinary infections, mainly cystitis, and have been described previously [6,13]. To examine the viru- lence in the mouse model, mice were injected intravenously with 2.5 • 10 s late log phase cells and at various times after injection the kidney viable counts were determined. Group I strains are aviru- lent, with low viable counts in the kidney and other organs. Group II strains are nephropatho- genic, with high viable counts only in the kidney and group III strains have a general virulence with high counts in kidney, spleen and blood [13]. Colicin production was tested by overlaying chlo- roform-killed colonies of the strains with E. coli 0378-1097/82/0000-0000/$02.75 © 1982 Federation of European Microbiological Societies