Antimicrobial Activities of Some Actinomycetes Isolated from Different Rhizospheric Soils in Tunisia

Fifty four isolates of actinomycetes were collected from four different rhizospheric soils: 18 strains from palm tree bark and soil, 12 strains from an olive field soil, 9 strains from a coastal forest, and 15 strains from an agriculture soil situated in the Algerian-Tunisian border (Oum Tboul). Based on morphological and cultural characters, the isolates were classified as Streptomyces (42 strains), Micromonospora (4 strains), Pseudonocardia (1 strain), Actinomadura (1 strain), Nocardia (1 strain), and non-Streptomyces (5 strains). More than half of the isolates inhibited at least one tested pathogenic microorganisms in liquid culture. In addition, antimicrobial activities of some strains were tested on solid culture. Several bioactive compounds were identified by liquid chromatography joined with low-resolution mass spectroscopy (LC/MS) and analysed by MEDINA's database and by the dictionary of natural products Chapman & Hall. An interesting chlorinated compound with the molecular formula C20H37ClN2O4, produced by three different strains (SF1, SF2, and SF5), was subject of an attempted purification. However, it was demonstrated using confocal microscopy and LC/MS high resolution that this compound is produced only on solid culture. These three potential antimicrobial isolates showed high similarity with Streptomyces thinghirensis and Streptomyces lienomycini, in terms of morphological characteristics and 16S rRNA gene sequences (bootstrap 97 %). All these findings prove the high antimicrobial diversity of the studied soils. The potential of the selected and other relatively unexplored extreme environments constitute a source of interesting actinomycete strains producing several biologically active secondary metabolites.