Response of planktonic bacteria of New Calabar River to zinc stress

Abstract

Toxicity of Zn2+ on four planktonic bacteria isolated from New Calabar River water was assessed via dehydrogenase
assay. Pure cultures of the bacterial strains were exposed to various Zn2+ concentrations (0.2 - 2.0 mM) in a nutrient
broth amended with glucose and TTC. The responses of the bacterial strains to Zn2+ is concentration- dependent. At
0.2 mM, Zn2+ stimulated dehydrogenase activity in Proteus sp. PLK2 and Micrococcus sp. PLK4. In all strains,
dehydrogenase activity was progressively inhibited at concentrations greater than 0.2 mM. The IC50 ranges from
0.236  0.044 to 0.864  0.138 mM. Total inhibition occurred at concentrations ranging from 1.283  0.068 to 2.469 
0.045 mM. The order of zinc tolerance is: Micrococcus sp. PLK4 > Proteus sp. PLK2 > Pseudomonas sp. PLK5 >
Escherichia sp. PLK1. The result of the in vitro study indicated that the bacterial strains are sensitive to Zn2+ stress.
Therefore, Zn2+contamination would pose serious threat to their metabolism in natural environments.