Characterisation of Pectinolytic Bacillus pumilus and Paenibacillus amyloliticus Strains, New Pathogens of Potato in Tunisia

Soft rot disease in potato is a major problem in fields and warehouses all over the world. Although it is known that bacteria from the genera Pectobacterium and Dickeya are the main causative agents of soft rot diseases, recent studies indicate the involvement of pectinolytic Bacillus and Paenibacillus in this disease. In the present research, samples of potato with soft rot symptoms were collected from eight governorates of Tunisia. Two hundred and seventy bacterial isolates were acquired from tubers. Twenty of the isolated strains indicted pectinolytic activity by forming deep cavities on crystal violet pectate medium. All pectinolytic isolates were able to macerate potato tuber tissue. Phenotypic characterisation showed that these isolates were Gram-positive bacilli, exhibiting pectinolytic, cellulolytic, proteolytic and amylolytic activity. The majority of the isolates indicated swimming and swarming motility. The application of API test, MALDI-TOF MS and 16S rDNA sequencing allowed for the assignment of nineteen of the tested isolates to the species Bacillus pumilus and one to the species Paenibacillus amyloliticus. To the best of our knowledge, this is the first report of soft rot in potato caused by pectinolytic B. pumilus in Tunisia.