Aquatic microcosm tests are widely used to assess the toxic effects of chemical contaminants on higher pelagic and benthic organisms, but rarely take into account bacteria and other microorganisms. In this study, the bacterial compartment of freshwater/formulated sediment laboratory microcosms was monitored in 4-week tests. In a first experiment, the influence of various organic matter compositions (cellulose + TetraMin®, cellulose + spirulin + meat extract, peat moss + TetraMin®) on bacterial communities was studied. In a second experiment, the effect of fluoranthene on bacterial communities was estimated. The following bacterial parameters were measured: total bacteria counts (epifluorescence microscopy) and enzymatic activities (INT-reductase and β-glucosidase). Compared to natural sediments, the artificial sediment was a satisfactory support for bacterial development. The organic composition of the formulated sediments had no apparent influence on bacterial populations. This result could be due to the high variability of parameters, such as the using 2-p-iodophenyl-3-p-nitrophenyl-5-phenyltetrazolium chloride-reductase activity, and to the C/N ratio similarity between the tested organic matters. The fluoranthene-contaminated sediment had no apparent effect on bacterial populations. The absence of toxicity could be due in part to the uncontaminated surficial sand layer of the sediment which might have protected pelagic organisms by limiting the rate of suspended contaminated particles and creating an uncontaminated refuge for benthic organisms.