Evaluation of antagonistic mechanisms of bacterial species for their biocontrol activity against fire blight (Erwinia amylovora)

The paper presents an attempt to explain the potential mechanisms related to the biocontrol capacity of the four strains representing bacterial species in which this activity was observed for the first time (Pseudomonas vancouverensis L16, Pseudomonas chlororaphis subsp. aureofaciens 3M, Enterobacter ludwigii 43M and Pseudomonas protegens 59M). The P. protegens 59M strain showed the highest effectiveness in protecting pear fruitlets against fire blight. The phenotypic analyses of tested mechanisms showed that all strains demonstrated the ability of motility, hydrogen cyanide production and degradation of nicotinic acid. Also, all strains except E. ludwigii 43M, produced siderophores and P. vancouverensis L16 was able to produce biosurfactant, salicylic acid and indole-3-acetic acid (IAA) while the E. ludwigii 43M and P. chlororaphis subsp. aureofaciens 3M strains produced IAA and homoserine lactones (AHL), respectively. A study on the detection of genes encoding antibiotics characteristic of pseudomonads showed the presence of prnD in the P. chlororaphis subsp. aureofaciens 3M strain and phlD, pltC, pltB and pltC in P. protegens 59M, which suggested that the latter strain had the largest number of antibiotic-coding genes among all the strains tested. The regulatory gene gacA was also present in P. protegens 59M and P. chlororaphis subsp. aureofaciens 3M strains. However, none of the genes sought were detected in the L16 strain. It was assumed that the ability of strain P. protegens 59M to completely protect slices of pear fruitlet against E. amylovora infection is related to the production of antibiotics and possibly also to the other detected mechanisms of antagonism, such as siderophore, IAA and HCN production.