Antimicrobial activity of actinobacteria from a hypersaline area of the Dead Sea

Extremophiles are specialized microorganisms which produce special primary and secondary metabolites to cope with the extreme conditions including biotic competitions. Therefore, there is a great potential for discovering novel anti-infective secondary metabolites especially as these extreme organisms have not been widely explored. In this study, soil and black mud samples were collected from the Dead Sea area which is characterized by high salinity, high temperature, high barometric pressure, and paucity of rain to isolate actinobacteria and test their antimicrobial activity. Actinobacteria were successfully isolated from the samples using different types of selective media. The isolates were further identified by biochemical tests and their physiological properties (production of soluble pigments, tolerance to sodium chloride, utilization of different carbon sources, and enzymatic activity) were studied. As a result, a total of 13 different isolates of Actinobacteria (11 from the soil samples and 2 from the black mud samples) were obtained. Soil isolates were assigned as D11, D12, D21, D22, D23, D25, D31, D32, D33, D34, and D43, whereas black mud isolates were assigned as M12 and M13. Isolates D11, D32, D33 and M12 were identified as Brevibacterium sp., isolates D12, D21, D22, and M14 as Rhodococcus sp., isolate D23 as Leifsonia aquatica, isolate D25 as Arthrobacter sp., isolate D31 as Propionibacterium sp., isolate D34 as Corynebacterium pseudotuberculosis, and isolate D43 as Trueperella pyogenes. Some isolates have been detected for the first time in the studied area. Evaluation of the antimicrobial activity of the crude extracts prepared from the isolates against different bacteria and fungi was determined by micro-broth dilution method. Results showed that isolate D11 was active against Micrococcus luteus DSM1790 (minimal inhibitory concentration; MIC= 0.1 ?L/?L [crude extract: growth medium]), Mucor hiemalis DSM2656 (MIC= 0.1 ?L/?L), Bacillus subtilis DSM10 (MIC= 0.075 ?L/?L), and Staphylococcus aureus (MIC= 0.025 ?L/?L). Isolate D12 was found to be active against Mucor hiemalis DSM2656 (MIC= 0.1 ?L/ ?L), Mycobacterium smegmatis ATCC700084 (MIC= 0.1 ?L/?L), and Bacillus subtilis DSM10 (MIC= 0.1 ?L/?L). Isolate D22 was found to be active 108 against Mucor hiemalis DSM2656 (MIC= 0.025 ?L/?L) and Bacillus subtilis DSM10 (MIC= 0.1 ?L/?L). Isolate D25 was found to be active against Micrococcus luteus DSM1790 (MIC= 0.1 ?L/?L), Mucor hiemalis DSM2656 (MIC= 0.05 ?L/?L), Chromobacter violaceum DSM30191 (MIC= 0.05 ?L/?L), Mycobacterium smegmatis ATCC700084 (MIC= 0.1 ?L/?L), and Bacillus subtilis DSM10 (MIC= 0.1 ?L/?L). Isolates D31 and D43 were found to be active against Mycobacterium smegmatis ATCC700084 (MIC= 0.1 ?L/?L), and Bacillus subtilis DSM10 (MIC= 0.1 ?L/?L).