Antibacterial effects of carbon dots in combination with other antimicrobial reagents

Abstract

This study was designed to investigate the antimicrobial effects of CDots in combination with other antimicrobial reagents, including H2O2, Na2CO3, and AcOH (acetic acid). CDots were synthesized and passivated with 2,2'-(ethylenedioxy)bis(ethylamine) (EDA). The minimal inhibitory concentration (MIC) of CDots was 64 μg/mL on both Gram negative bacteria E.coli cells and Gram positive bacteria Bacillus subtilis cells. When CDots were combined with H2O2, antibacterial synergistic effects were observed based on the fractional inhibitory concentration (FIC) index, and further confirmed by an isobologram analysis and viable cell number counting methods. With the combination treatment of 10 μg/mL CDots with 8.82 mM H2O2, the viable E.coli cell numbers decreased 2.46 log, which was significant lower than the log reduction from 8.82 mM H2O2 (1.57 log) or 10 μg/mL CDots (0.14 log) treatment alone. However, the combination of CDots with Na2CO3 or AcOH did not show synergistic effects, instead, exhibiting indifference effects according to the FIC index. This study indicated that the combination of CDots with their synergistic antimicrobial reagents, such as H2O2, could reach the goal of inhibiting bacteria growth by using lower concentration of each individual chemical in the combination than using one chemical treatment alone, reduce the risks imposed on environmental health and the possibilities of the development of microbial resistances.

Fig 1

Minimum inhibitory concentration (MIC) of EDA-CDots on E. coli (A) and B. subtilis cells (B) after 24 h at 37°C. Data is presented as the mean values with ±SD as Error bars. Different letters above the columns indicate statistically significant differences (p<0.05).

Fig 2. Inhibitory effects of H₂O₂ alone and H₂O₂/CDots combination on E. coli cells.

CDots concentration was 8 μg/mL or 16 μg/mL. Bacterial cell growth was measured by OD value at wavelength 595 nm. Data is presented by the mean of 3–5 replicated samples and Error bars are ±SD of the replicated measurements. Different letters above the columns indicate statistically significant differences (p<0.05).

Fig 3. Isobologram of the interaction between CDs and H₂O₂ against E.coli cells.

Fig 4. Viable E.coli cell numbers after treated with 1.76 mM H₂O₂, 8.82 mM H₂O₂, or 10μg/mL CDots alone or H₂O₂/CDots combination.

Different letters above the columns indicate statistically significant differences (p<0.05).

Fig 5. SEM images of E.coli cells with different treatments.

(A) Untreated control samples; (B) CDots (10 μg/mL) treated samples; (C) H₂O₂ (8.82 mM) treated samples; (D) CDots (10 μg/mL) and H₂O₂ combination treated samples.

Fig 6

Antimicrobial effects of Na₂CO₃ alone on E.coli cells (A) and B.subtilis cells (B), and in combination with 10 μg/mL CDots (C: on E.coli cells). Different letters above the columns indicate statistically significant differences (p<0.05).

Fig 7. Antimicrobial effects of Na₂CO₃ in combination with 10 μg/mL CDots on E.coli cells.

Different letters above the columns indicate statistically significant differences (p<0.05).