Occurrence of plasmids in pathogenic strains of Nocardia

The purpose of the present study was to determine the relative distribution of plasmids in 87 clinical isolates of Nocardia, belonging to the five major pathogenic species. A correlation between plasmid content and the site of infection within the host, resistance to antibiotics and enzymic profiles was also investigated. The plasmid extraction procedure of Kado and Liu was used. Electrophoretic analysis revealed one-to-four plasmid bands, ranging in size from <8 to >50 kb, in 27 strains (31%). Based on the number of isolates tested, the incidence of plasmid-bearing strains was significantly higher among N. farcinica than N. asteroides strains. Within N. farcinica, the incidence of plasmids was higher among strains isolated in the Paris area than in strains isolated elsewhere, such as in the French provinces or outside France. A statistically significant correlation was demonstrated between the cutaneous localisation of infections and the incidence of plasmid-bearing strains. The presence of plasmids in nocardiae could not be associated with specific phenotypic traits such as resistance to antibiotics or enzymic activity. The fact that the majority of Nocardia clinical isolates (60 of 87) did not contain plasmids suggests that plasmids are not involved directly in virulence and that there is no selective pressure for plasmid acquisition.