NCCD: a unique combination of MOF-scaffolded stable luminescent copper nanoclusters (Cu NCs) and carbon dots (Cdots) as a potent antimicrobial agent against <i>Pseudomonas aeruginosa</i>

Bacterial biofilm infections cause huge cases of morbidity and mortality globally. Cells in bacterial biofilms can easily overcome host immune response and become resistant to conventional antibiotics. Hence, the concurrent issues of biofilm infections demand the urgent development of new therapeutic strategies other than the conventional antibiotic treatment. Herein, we have developed an exclusive combination of fluorescent copper nanoclusters (Cu NCs) with biocompatible and bactericidal fluorescent carbon dots (Cdots) coined as nanocluster-carbon dots (NCCD) as a wide-range antibacterial and antibiofilm agent against Gram-negative Escherichia coli and Gram-positive Bacillus subtilis. Interestingly, NCCD was found effective even against the pathogenic strain of Pseudomonas aeruginosa. CuNCs showed a MIC90 value of 20 ± 2, 7.5 ± 2, and 10 ± 3 µg mL-1 against E. coli, P. aeruginosa and B. subtilis respectively while Cdots showed a MIC90 value of 200 ± 5 and 160 ± 2, against E. coli and B. subtilis respectively. NCCD inhibited the biofilm production in B. subtilis and P. aeruginosa to 40.27% and 68.10%, respectively and that of reduced the mass of established biofilm to 53.09% and 68.43%, respectively. The antibiofilm activity of NCCD was found to increase considerably in combination with biofilm-degrading enzymes like α-amylase and/or DNase. The acidic microenvironment and presence of chelating enzymes in biofilm considerably lower the antibiofilm activity of antibiotics. However, a significant increase in the susceptibility of the biofilm to antibiotics was observed in presence of NCCD. The loss of cell membrane permeability was found to be one of the probable causes of cell death by NCCD. Further, it was demonstrated that NCCD-mediated biofilm degradation may be due to the regulation of bacterial chemotaxis leading to transition of cells from biofilm to planktonic state. Thus, it is conceivable that NCCD could be a potent agent against various antibiotic-resistant bacterial biofilm infections.