CHROMIUM-RESISTANT MICROORGANISMS ISOLATED FROM EVAPORATION PONDS OF A METAL PROCESSING PLANT

Chromium occurs naturally at trace levels in most soils and water, but disposal of industrial waste and sewage sludge containing chromium co mpounds has created a number of contaminated sites, which could pose a major environmental threat. This study was conducted to enumerate and isolate chromium-resistant microorganisms from sediments of evaporati on ponds of a metal processing plant and determine their tolerance to other metals, metalloids and antibiotics. Enumeration of the microbiot a of Cr- contaminated sediments and a clean background sample was cond ucted by means of the dilution-plate count method using media spiked w ith Cr(VI) at concentrations ranging from 10 to 1000 mg L-1. Twenty Cr (VI) tolerant bacterial isolates were selected and their resistance to other metals and metalloids, and to antibiotics was assessed using a plate diffusion technique. The number of colony-forming units (cfu) of the contaminated sediments declined with increasing concentrations fr om 10 to 100 mg L-1 Cr(VI), and more severely from 100 to 1000 mg L-1 Cr(VI). The background sample behaved similarly to 100 mg L-1 Cr(VI), but the cfu declined more rapidly thereafter, and no cfu were observed at 1000 mg L-1 Cr(VI). Metals and metalloids that inhibited growth (f rom the most to least inhibitory) were: Hg > Cd > Ag > Mo = As(III) at 50 mug mL-1. All 20 isolates were resistant to Co, Cu, Fe, Ni, Se(IV) , Se(VI), Zn, Sn, As(V), Te and Sb at 50 mug mL-1 and Pb at 100 mug mL -1. Eighty-five percent of the isolates had multiple antibiotic resist ance. In general, the more metal-tolerant bacteria were among the more resistant to antibiotics. It appears that the Cr-contaminated sedimen ts may have enriched for bacterial strains with increased Cr(VI) toler ance.