Identification and investigation of some virulence factors of Pseudomonas tolaasii isolated from mushroom in Iran

In recent years brown blotch of cultivated mushroom (Agaricus bisporus) has been observed as a prevalent disease in Iran. The disease symptoms of the cultivated agaric include brown blotch, brown pit spots, rotting, weeping and drippy gill. Pseudomonas tolaasii has been cited as the causal agent of bacterial blotch, but much controversy exists regarding the involvement of this bacterium alone or blotch may be caused by more than one organism. In 2011, the diseased samples of mushroom were collected from Khorasan Razavi, Alborz, Tehran, Semnan, Ghazvin and Golestan provinces. Strains which were pathogenic on the cap of mushroom (pileus) were selected for further analysis. Biochemical, physiological and nutritional tests together with amplification of 1Kb region of 16S rDNA using Ps forward and reverse primers showed that all strains belong to Pseudomonas tolaasii. The isolated strains displayed different degrees of pathogenicity on mushroom. The presence of different virulence factors were evaluated among the strains. The results indicated that those strains which showed enhanced pyoverdin production had higher pathogenicity on agaric caps. There was no correlation between the existence of other virulence factors such as biofilm formation, motility, or PQS production and pathogenicity. Our results indicated that in P. tolaasii similar to other Pseudomonads, PQS exists but the actual role of this signal is not so clear. PQS dependent quorum sensing in P. tolaasii could not affect pathogenicity of this bacterium, thus most of the virulence determinants in this bacterium should be PQS independent.