Diversity of plasmids in Xanthomonas campestris pv. vesicatoria

Plasmid profiles were determined for 522 strains of Xanthomonas campestris pv. vesicatoria, the causal agent of bacterial spot of tomato (Lycopersicon esculentum) and pepper (Capsicum annuum). The numbers and sizes of plasmids in strains from culture collections, as well as in strains recently isolated from plants in commercial fields of pepper or tomato, were diverse. Each plasmid was placed into one of 13 classes based on size, as determined by electrophoretic mobility in agarose gels. Plasmid sizes ranged from about 3 to 300 kb. Ten strains had no detectable plasmid. Seventy-one different plasmid profiles were observed, with a maximum of five plasmid size classes in a profile. No plasmid or plasmid profile was characteristic for X. campestris pv. vesicatoria. Mobility of plasmids was demonstrated in field experiments after inoculation of pepper plants with donor strains containing plasmids in five classes and an antibiotic-resistant recipient strain that contained one unique plasmid. All strains recovered on an antibiotic-amended medium contained the unique plasmid present in the recipient strain. Twenty-nine percent of the 199 cultures recovered on the antibiotic-amended medium in one experiment contained one or more plasmids that were similar in size to those in the donor strains ; seven different plasmid profiles were observed. The diverse plasmid content of strains of X. campestris pv. vesicatoria in nature may be the result of frequent transfer of the plasmids between strains.