Protein electrophoretograms for the identification of ralstonia solanacearum in potato tubers

Densitometric analysis was performed on the whole cell proteins of 53 strains of Ralstonia solanacearum (Rs) isolated from different host plants and geographical areas. This study also included 18 unidentified, endophytic potato bacteria, non-pathogenic on tomato and eggplant, forming colonies similar to Rs (Rs-like bacteria) on differential or selective media. The comparison of the electrophoretograms was performed by calculating the correlation coefficient (r) and by principal component analysis. Visual comparison generally made it possible to distinguish between the Rs strains and nonpathogenic Rs-like bacteria. Densitometric analysis of the electrophoretograms revealed a main group of strains (r=0.76) including 4 subgroups. Subgroup 1 (r=0.92) included potato and tomato strains attributable to race 3 biovar 2, including all those of Mediterranean origin. Principal component analysis again included the Mediterranean strains in a single group characterised by just one of the two bands identified as differential in the Rs profile. Principal component analysis revealed that one Rs-like bacterium, with a markedly higher correlation coefficient (r=0.83), was clearly distinct from Rs. The results confirmed the reliability of the European diagnostic protocol as a basis for identification at the species level, that must be based on the pathogenicity test combined with whole cell protein profiles when these are used as a single phenotypic fingerprint.