Influence of extracellular polysaccharides on the toxicity of copper and cadmium towardKlebsiella aerogenes

Abstract

Many investigations have reported the effects of heavy metals on pure cultures of bacteria (14, 21, 30), algae (3, 22, 25), and on natural microbial populations (1, 2, 17). Most of the studies have focused on copper toxicity because copper sulfate has been used as an algicide since the turn of the century (3, 22, 24, 25). The assessment of the toxicity of heavy metals toward microorganisms must take into account the physicochemical parameters of the water (17, 23, 24) and the presence of chelators in natural waters. Natural (humic acids, amino acids, polypeptides) and artificial (EDTA, Nitroloacetate) chelators form organic complexes with metallic ions and thus alter the availability of these toxic metals to aquatic microorganisms (10, 19, 20, 24, 28). Microorganisms are known to produce large quantities of extracellular polysaccharides which are organized in well-defined capsules around the cells, or are excreted as loose materials into the growth medium. The ecological significance of these polymetric materials has been previously discussed (13, 15, 27). However, among the various functions assigned to extracellular polysaccharides, their ability to bind metallic ions has received little attention (11, 26). In this study we report the effect of capsular polysaccharides on copper and cadmium toxicity toward Klebsiella aerogenes. This particular bacterium was chosen because we had a noncapsulated mutant which could be compared to the encapsulated strain.