Influence of exposure time on the response of pure cultures of bacteria and microbial community of petroleum refinery wastewater to toxicity of phenol was assessed through TTC-dehydrogenase activity (DHA) inhibition test. At sufficient concentrations, exposure of these bacterial cells to phenol resulted in inhibition of dehydrogenase activity. InPseudomonas sp. RWW2 and Escherichia sp. DISK2, phenol progressively inhibited dehydrogenase activity at 200 - 1400 mg/l at all the exposure time. However, in Bacillussp. DISK1, Pseudomonas sp. DAF1 and microbial community, increase in exposure time resulted in stimulation of dehydrogenase activity at lower concentrations of phenol. The toxicity threshold concentrations of phenol vary among the bacterial strains and the exposure time and indicate that bacteria could acclimate to phenol with increase in exposure time. At concentrations higher than 800 mg/l, phenol toxicity was not overcome in Pseudomonas and Escherichia species as well as the microbial community. It is suggested that for acute TTC-dehydrogenase assay involving bacteria, reliable and reproducible result would be best achieved within 48 h.
Key words: Dehydrogenase activity, petroleum refinery wastewater, dose-response models
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