Colonization of tomato roots by some potentially human-pathogenic bacteria and their plant-beneficial properties

Many studies provide evidence for the occurrence of opportunistic pathogens in the rhizosphere of plants. The presence of a competitive microflora in soil may restrict root colonization by potentially pathogenic bacteria. The aim of the present work was to study the competitive tomato root tip colonization of potentially human pathogenic bacteria such as Bacillus cereus, Staphylacoccus saprophyticus, Pseudomonas aeruginosa, Acinetobacter sp. against best known root tip colonizer P. fluorescens WCS365 and to characterize for their ability to stimulate plant growth and to suppress tomato foot and root rot disease caused by Fusarium oxysporum f.sp. radicis-lycopersici. In competitive tomato root tip colonization assays all tested bacterial strains showed poor colonization of the tomato rhizosphere except P. aeruginosa which was better competitive colonizer than reference strains. All five bacterial strains stimulated shoot length (up to 35%) and dry matter (up to 58%) of tomato and only one strain B. cereus TSAU80 showed statistically significant disease control (27%) of tomato foot and root rot in comparison to the Fusarium-infected control plants (47%). These results show that the opportunistic pathogens possess beneficial effect on plant growth and are able to colonize the rhizosphere of tomato, potentially resulting risk of contamination for tomato greenhouses. However, their competitive colonization abilities in the rhizosphere are poor, whereas only P. aeruginosa appeared to be better competitive colonizer than reference strain.