Plasmids and the virulence of Proteus mirabilis

The effects of large plasmids on different virulence characteristics of Proteus mirabilis strains mostly from clinical origin were studied. Moreover the inhibitory effect of urea and its derivatives on the swarming property of the strains was investigated. A. All strains were screened for plasmid detection, antibiotic resistance and swarming ability. B. Three multiresistant plasmid-carrying strains (PM5, P49 and P991) were cured and two transconjugants (G9pPM5 and G9pP49) were obtained by conjugation between two p+ donors (PM5 and P49) and one p- recipient (G9). C. By comparing the virulence properties of cured and transconjugant strains with their parental isolates it was found that; 1. Plasmids confer resistance to P. mirabilis strains against one or more antibiotics. 2. The presence of most plasmids reduces the swarming ability of the strains. 3. Plasmids affect the motility and flagellation of P. mirabilis strains. 4. Plasmids enhance the adherence property of their host strains to inert surfaces and uroepithelial cells as well as autoagglutination. 5. Plasmids increase the hydrophobicity of P. mirabilis strains. 6. The presence of plasmids reduced the growth rate of the strains. This effect was more apparent in iron-restricted medium. 7. Plasmids reduced the growth rate of their host strains in the presence of detergent (SDS). 8. The presence of plasmids reduced the survival of P. mirabilis strains in human and rabbit serum. 9. Plasmids decreased the survival of the strains in aquatic systems. 10. Plasmids reduced the production of urease and increased some others such as haemolysin and protease. D. Urea and some of its relatives inhibited the swarming property of P. mirabilis strains and this effect was concentration dependent.