Isolation, identification and antimicrobial activity of Pseudomonads isolated from the rhizosphere of potatoes growing in Algeria.

Fourteen bacterial isolates from the rhizosphere of potato plants growing near Setif, (Algeria) were characterised as fluorescent Pseudomonads by phenotypical methods and one was identified as Pseudomonas chlororaphis by sequencing ribosomal DNA. In dual culture, this isolate inhibited the growth of the phytopathogenic fungi Fusarium oxysporum f. sp. lycopersici, F. oxysporum f. sp. albedinis, F. solani and Rhizoctonia solani and the oomycete Pythium ultimum. Extracts of supernatants from liquid cultures of the Ps. chlororaphis isolate completely inhibited these organisms when incorporated into potato dextrose agar at a rate equivalent to 0.31 ml culture filtrate/ml, or greater. In a disc assay, extracts equivalent to 0.31 ml supernatant gave inhibition zones of 15 mm and 25 mm for the Gram-positive bacteria Bacillus subtilis and Paracoccus paratrophus, respectively. Fractionation of extracts of supernatants by TLC and HPLC with diode array detection allowed the identification of phenazine carboxylic acid as one of the antimicrobial compounds and the tentative identification of two others as 2-hydroxy phenazine carboxylic acid and 2-hydroxy phenazine.