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In a recent study, we have shown that the mixtures of graphene oxide (GO) nanosheets and Pluronic F-127 (PLU) polymers (GO-PLU) exhibit superior antimicrobial activity compared to GO alone on physiologically naive bacteria challenged with hypoosmotic stress, particularly on Gram-negative species. Further, we have more recently demonstrated that the antibacterial activity of GO (alone) is significantly lower against physiologically mature bacteria. As inhibiting mature pathogens is important for practical biomedical applications, in this work, we studied the antibacterial performance of GO-PLU nanodispersions on mature Pseudomonas aeruginosa (P. aeruginosa) harvested at stationary phase. Colony counting and turbidimetric results showed that the antibacterial behavior of GO-PLU mixtures on mature bacteria displays a complex behavior showing dependence on both the environmental salinity and PLU concentration. At high osmolarity (i.e. low osmotic drop), PLU acts merely as an osmoprotectant rather than a synergistic killing agent in a concentration-dependent manner. At a higher level of osmotic drop, lower concentrations of PLU make almost no or a slight improvement in the antibacterial activity of GO, suggesting a competition in between the counteracting factors. Further tests using Escherichia coli (E. coli) have shown that the effect of GO-PLU on stationary-phase bacteria is also species(/strain)-specific. We expect these observations to pave the way for designing GO-based broad-spectrum disinfectant formulations, with the aid of suitable polymeric stabilizers. Given the high molecular weight polymers exist naturally in the ecological niches, the insights gained by current findings may also help to better understand the environmental fate of nanocarbons.