Evaluation of Heavy Metals Resistance in Biofilm Cells of Native Rhodococcus spp. Isolated from Soil

Background & Aims of the Study: Rhodococci according to possess large genome, their activemetabolism and survival under extreme conditions, are highly regarded for biodegradation andbioremediation of different pollutants especially heavy metals in the environment. Biofilms aremore resistant to heavy metals than free-swimming organisms. The aim of this study was biofilmformation of two native Rhodococcus strains (Rhodococcus rhodochrous and Rhodococcusrhodnii) and comparative evaluation effects of heavy metals such as lead, copper, zinc,chromium and cadmium in various concentrations against biofilm and planktonic cells thatcarried out at the first time.Materials & Methods: Bacterial strains used in this investigation were isolated fromagricultural soils in Qom, Iran. Biofilm formation was performed at 30 °C and 37 °C in trypticsoy broth (TSB) and brain heart infusion (BHI) broth, at 24,48,72 and 96 hours in 96-wellmicroplate. Minimum inhibitory concentration (MIC) of heavy metals in various concentrationsfor planktonic cells was determined, using microdilution and macrodilution methods. Minimumbiofilm inhibitory concentration (MBIC) and Minimum biofilm eradicating concentration(MBEC) in various concentrations of heavy metals for biofilm cells were evaluated bymicrodilution method. The result of this study confirmed, using scanning electron microscope(SEM).Results: The best condition for biofilm formation of native R.rhodochrous isolate wasdetermined after incubation at 37°C in BHI broth, at 96 hours and the best condition for biofilmformation of native R.rhodnii isolate was determined at 30°C in BHI broth, at 96 hours. MIC ofheavy metals for planktonic cells of isolates for cadmium, zinc and lead was 8 mM and assayedfor copper and chromium respectively 4 and 1 mM. MBEC of heavy metals for biofilm cells ofisolates for cadmium, zinc and lead was 16 mM and assayed for copper and chromiumrespectively 8 and 4 mM. The results of present study showed biofilm cells of nativeRhodococcus isolates were 2 times more resistance to lead, copper, zinc and cadmium thanplanktonic cells while biofilm cells were 4 times more resistance than planktonic cells tochromium.Conclusions: Biofilm formation of R.rhodochrous and R.rhodnii and their high resistance tovarious concentrations of heavy metals especially cadmium show that inoculation of these nativeRhodococcus isolates to contaminated agricultural soils with heavy metals, have an effective rolefor bioremediation.

Keywords:

Rhodococcusrhodochrous , Rhodococcusrhodnii , Biofilm , Soil PollutantHeavy metals resistanceIran