Changes in Plasmid Profile of Several Indiginous Pseudomonas Fluorescent Isolates, as Affected by Different Levels of Zn and Cd

The species of Pseudomonas genus show different levels of resistance to soil''s heavy metals pollution. Since، there exists is a relationship between heavy metal resistance and plasmid profile of bacteria، in this study، plasmid profile of Plant Growth Promoting Rhizobacteria (PGPR) Pseudomonas fluorescence isolates were studied in the presence of cadmium and zinc. The bacteria were initially isolated from soybean rhizosphere and characterized as based on biochemical and biological properties. Then، different concentrations of Zn and Cd، in the H. E. P. E. S and M. E. S solid mediums، were employed to evaluate the resistance of the isolates to these heavy metals. Based on the growth in the presence of Zn and Cd، the isolates were classified into 4 groups. Two isolates taken from each group were exposed to different concentrations of Zn and Cd for a period of 2 months. Then، the isolates were sub-cultured and their plasmids isolated using alkaline lysis procedure. The plasmid profile of isolates indicated that the toxic effect of the cadmium was higher than that of zinc even when at lower concentrations. At concentrations of cadmium، all the isolates lost their plasmids. Three isolates also showed multiple resistances to heavy metals.