The synergistic toxicities of senary mixtures of an anionic surfactant, Sodium Dodecyl Sulfate (SDS) with five divalent metal ions, Pb2+, Cd2+, Ni2+, Zn2+ and Co2+, to Serratia marcescens (SerEW01) and Acinetobacter seifertii respectively isolated from water and sediments in Otamiri River, Owerri, Imo State, Nigeria were critically analyzed with dehydrogenase activity inhibition as the response. The EC50S observed for the individual toxicants for S. marcescens (Ser EW01) was between 0.046 ± 0.003mm (Zn2+) and 2.329 ± 0.092mm (SDS) and between 0.011 ± 0.00mm (Cd2+) and 2.810 ± 0.140mm (SDS), for A. seifertii. At p < 0.05, the EC50S for individual toxicant were significantly different for each organism. To analyze the senary mixtures effects against the bacteria, fixed ratio mixtures of arbitrary combined ratios (ABCR) and EC50 equi-effect concentration (EECR50) were designed. Logical function was used to describe the dose-response relationships between the individual toxicants and the mixtures. Based on the independent actions (IA) and the concentration addition (CA) models, there was a significant difference between the predicted and experimental toxicities. However, there was an underestimation of the mixture toxicities in both organisms by the predicted models, at high concentration and slight overestimation against A. seiferii at low concentration. Furthermore, CA-model made a better prediction of the mixture toxicities than IA- model at low concentrations, especially in ABCR 2 and 3 mixture ratios for A. seiferii. The Toxic Index (TI) and Model Deviation Ratio (MDR) analyses indicate synergistic interaction of the mixtures against both bacteria. Thus, in natural environment, mixtures of metals and surfactant could potentially harm the aquatic microbial ecosystems.
