VIRULENCE, GROWTH, AND SURFACE CHARACTERISTICS OF ERWINIA-AMYLOVORA MUTANTS WITH ALTERED PATHOGENICITY

Erwinia amylovora CFBP1430 was mutagenized by phage MudllPR13 insertion. Thirty prototrophic pathogenicity mutants with a single insertion were isolated. Among these, 17 non-pathogenic (Path(-)) mutants were obtained; 11 of them were unable to induce a hypersensitive response (HR) on tobacco (Hrp(-) mutants), whereas the remaining six were still able to do so (Dsp(-) mutants). Thirteen other mutants showed reduced virulence (Rvi(-)) and were still able to induce an HR. One of them appeared to be Path(-) on apple seedlings and Rvi(-) on pear seedlings. All the Hrp(-) mutants and all but three of the Dsp(-) mutants mapped in the hrp-dsp gene cluster previously reported. Some Rvi(-) mutants also proved to map in this hrp-dsp region; most of them, as well as two Dsp(-) mutants map in an unknown genomic region. Cell-surface components thought to play a role in bacterial pathogenicity were examined, including exopolysaccharides (EPS), lipopolysaccharides (LPS), and outer-membrane proteins. One mutant only was found to be non-capsulated and unable to produce EPS. The insertion in this mutant mapped in a genomic cluster involved in amylovoran synthesis. Unlike the parental strain, some mutants exhibited sensitivity to the phage Ffm and this phenotype was associated with a modified LPS electrophoretic profile. Rvi(-) mutants and some Path(-) mutants were able to multiply in planta to some extent; other Path(-) mutants reached only a low population lever, except the non-capsulated one, which rapidly decreased to an undetectable level.