The increasing consumption of graphene derivatives leads to greater presence of these materials in wastewater treatment plants and ecological systems. The toxicity effect of graphene oxide (GO) on the microbial functions involved in the biological wastewater treatment process is studied, using Pseudomonas putida and salicylic acid (SA) as bacterial and pollutant models. A multiparametric flow cytometry (FC) method has been developed to measure the metabolic activity and viability of P. putida in contact with GO. A continuous reduction in the percentages of viable cells and a slight increase, lower than 5%, in the percentages of damaged and dead cells, suggest that P. putida in contact with GO loses the membrane integrity but preserves metabolic activity. The growth of P. putida was strongly inhibited by GO, since 0.05 mg mL(-1) of GO reduced the maximum growth by a third, and the inhibition was considerably greater for GO concentrations higher than 0.1 mg mL(-1). The specific SA removal rate decreased with GO concentration up to 0.1 mg mL-1 indicating that while GO always reduces the growth of P. putida, for concentrations higher than 0.1 mg mL(-1), it also reduces its activity. Similar behaviour is observed using simulated urban and industrial wastewaters, the observed effects being more acute in the industrial wastewaters. (C) 2016 Elsevier B.V. All rights reserved.