Silver-coated carbon nanotubes downregulate the expression of Pseudomonas aeruginosa virulence genes: a potential mechanism for their antimicrobial effect

Abstract

The antimicrobial activity of silver-coated carbon nanotubes (AgCNTs) and their potential mode of action against mucoid and nonmucoid strains of Pseudomonas aeruginosa was investigated in vitro. The results showed that AgCNTs exhibited antimicrobial activity against both strains with minimum inhibitory concentrations of approximately 8 µg/mL, indicating a high sensitivity of P. aeruginosa to AgCNTs. AgCNTs were also bactericidal against both strains at the same minimum inhibitory concentration. Scanning and transmission electron-microscopy studies further revealed that a majority of the cells treated with AgCNTs transformed from smooth rod-shape morphology to disintegrated cells with broken/damaged membranes, resulting in leakage of cytoplasmic contents to produce ghost cells. The molecular effects of AgCNTs on P. aeruginosa genes involved in virulence and pathogenicity, stress response, and efflux pumps were evaluated for changes in their expression. Quantitative real-time PCR (qRT-PCR) showed that after exposure to AgCNTs, the expression levels of the rpoS, rsmZ, and oprD genes were significantly downregulated in both strains of P. aeruginosa compared to the untreated samples. These results suggest that the mechanism of action of AgCNTs may be attributed to their effect on cell-membrane integrity, downregulation of virulence-gene expression, and induction of general and oxidative stress in P. aeruginosa.

Keywords:

• Pseudomonas aeruginosa
• AgCNT
• antimicrobial activity
• qRT-PCR

Supplementary material

Table S1 Gene and primers used for quantitative reverse-transcriptase polymerase chain reaction (sequence 5′ → 3′)

Acknowledgments

This research was supported by grants from the National Science Foundation Centers of Research Excellence in Science and Technology (HRD-1241701), NSF-HBCU-UP (HRD-1135863), and National Institutes of Health Minority Biomedical Research Support Research Initiative for Scientific Enhancement (1R25GM106995-01). We thank Yvonne Williams, Lashaundria Lucas, and Amber Grace for excellent administrative assistance.

Disclosure

The authors report no conflicts of interest in this work.