Synergistic Toxicity of Phenols and Cadmium Ion Binary Mixtures on Bacillus Sp and Pseudomonas Sp

Toxicity of Cadmium (Cd), 4-chlorophenol (4-CP), and 2,4-dichlorophenol (2,4-DCP) as single compounds and binary mixtures on Bacillus sp and Pseudomonas sp isolated from garden soil was assessed using inhibition of total dehydrogenase as toxicity response. Binary mixtures of metal and chlorophenol were composed using arbitrary concentration ratios (%) corresponding to metal/phenol mixtures of 20/80; 40/60; 50/50 and 30/70; 45/55, 50/50 for the chemical pairs: Cd/4-CP and Cd/2,4-DCP respectively. Results obtained showed that the binary mixtures of Cd/4-CP and Cd/2,4-DCP all exhibited a dose-dependent inhibition of dehydrogenase activity in the test isolates. The binary mixture of CD/4-CP exhibited higher toxicity in ratio 40/60 and 20/80 for the isolates Bacillus sp and Pseudomonas sp with IC50 values of 0.212±0.002 and 0.158± 0.008mM respectively. While for the mixture of Cd/2,4-DCP, toxicity was highest at the 50/50 and 45/55 ratio for Bacillus sp and Pseudomonas sp with IC50 values of 0.069±0.001mM and 0.068±0.001mM respectively. The binary mixtures of the chemicals evaluated showed a progressive inhibition of dehydrogenase activity with Pseudomonas sp showing a higher susceptibility. Isobolographic analysis of binary mixture interaction against Bacillus sp and Pseudomonas sp showed largely synergistic interactions. The combination of Cd with the chlorophenols resulted in a synergistic increase in the toxicity of the compounds to the test isolates. The toxicity of Cd/4-CP binary mixture ratios to dehydrogenase activity of Bacillus sp and Pseudomonas sp was in the order 40:60>50:50>20:80 and 20:80>50:50>40:60 respectively; similarly that of Cd/2,4-DCP mixtures were 50:50>45:55>30:70 and 45:55>50:50>30:70 for Bacillus and Pseudomonas sp respectively. The trend and implications of the toxicity interactions are further discussed.