Antimicrobial applications of nanotechnology: methods and literature

Figures & data

Table 1 Comparison of commonly-used methods of quantifying bacteria viability

Assay	Advantages	Disadvantages
Optical density measurement	Quick, no reagents required	Spectrophotometer required, low accuracy
Cell counting devices	High accuracy	Costly device
Spread-plate (colony counts on agar)	High accuracy	Determines CFU count but not total cell population or size of CFUs, time consuming, large amounts of disposable materials required, cells must be removed from surfaces for measurement
Crystal violet staining	Quantifies biofilm formation	Spectrophotometer required, not suitable for planktonic bacteria growth
Live/dead fluorescent stain	Allows visualization of sample surface	Costly reagents, fluorescent plate reader or microscope required
MTS/MTT/XTT assays	Measures cell viability on surfaces and in solution	Spectrophotometer required, costly reagents

Abbreviation: CFU, colony forming unit.

Table 2 Summary of select studies concerning the antimicrobial effects of nanoparticles

Chemistry	Size (average)	Zeta potential	Organism tested	MIC	Proposed mechanism	Reference
ZnO	13 nm	N/A	Staphylococcus aureus	Reduced 95% at 80 μg/mL	ROS inhibition	Reddy^Citation10
ZnO	60 nm	N/A	S. aureus	Reduced 50% at 400 μg/mL	ROS inhibition	Jones^Citation2
ZnO	40 nm	Positive (no value)	S. aureus, Escherichia coli	Both species reduced 99% at 400 μg/mL	Membrane disruption	Nair^Citation11
ZnO	12 nm	N/A	E. coli	Reduced 90% at 400 μg/mL	Membrane damage due to particle abrasiveness	Padmavathy^Citation12
ZnO ions	N/A	N/A	Pseudomonas aeruginosa, S. aureus, Candida albicans	Reduced 100% at 1917, 9, and 39 μg/mL, respectively	ROS inhibition	McCarthy^Citation13
Silver	21 nm	N/A	E. coli, Vibrio cholerae, Salmonella typhi, P. aeruginosa	All reduced 100% at 75 μg/mL	Membrane disruption, Ag ion interference with DNA replication	Morones^Citation18
Silver	Triangles (50 nm)	Positive (no value, cationic surfactant)	E. coli	Reduced 99% with 0.1 μg/mL added to agar surface	Membrane disruption, Ag ion interference with DNA replication	Pal^Citation20
Silver	12 nm	Negative (no value)	E. coli	Reduced 70% with 10 μg/mL in agar	Membrane disruption, Ag ion interference with DNA replication	Sondi^Citation3
Silver	13.5 nm	−0.33 mV	S. aureus, E. coli	Inhibitory concentration of 3.56 μg/L and 0.356 μg/L, respectively, added to agar surface	Membrane disruption, Ag ion interference with DNA replication	Kim^Citation21
Cu	100 nm	N/A	E. coli, Bacillus subtilis	Reduced 90% at 33.40 μg/mL and 28.20 μg/mL, respectively	Protein inactivation via thiol interaction	Yoon^Citation22
Fe3O4	9 nm	−19.09 mV	S. aureus	Increased dead cells observed at 3 mg/mL	ROS, membrane disruption	Tran^Citation24
Fe3O4	8 nm	N/A	Staphylococcus epidermidis	Reduced 65% at 2 mg/mL	ROS, membrane disruption	Taylor^Citation25
Al2O3	11 nm	120 mV	E. coli	Reduced 35%, 70%, and 68% at 10, 100, and 500 μg/mL, respectively	Dose-dependent ROS, particle penetration	Simon-Deckers^Citation26
Al2O3	60 nm	30 mV	E. coli, B. subtilis, Pseudomonas fluorescens	Reduced bacteria species by 36%, 57%, and 70% at 20 μg/mL	Flocculation	Jiang^Citation9
TiO2	17 nm	12 mV	E. coli	Reduced 0%, 35%, and 80% with 10, 100, and 500 μg/mL, respectively	Membrane disruption	Simon-Deckers^Citation26
SiO2	20 nm	35 mV	E. coli, B. subtilis, P. fluorescens	Reduced bacteria species 58%, 40%, and 70% at 20 μg/mL	Flocculation, membrane disruption	Jiang^Citation9
Chitosan	40 nm	51 mV	E. coli, S. aureus	Reduced bacteria species 100% at 4 μg/mL and 8 μg/mL, respectively	Flocculation, membrane disruption	Qi^Citation27

Abbreviations: Ag, silver; Al₂O₃, aluminum oxide; Cu, copper; Fe₃O₄, iron oxide; MIC, minimum inhibitory concentration; N/A, not available; ROS, reactive oxygen species; SiO₂, silicon dioxide; TiO₂, titanium dioxide; ZnO, zinc oxide.

Figure 1 Zinc oxide (ZnO) nanoparticles (A), Escherichia coli bacteria prior to exposure to ZnO nanoparticles (B), and E. coli bacteria after exposure to ZnO nanoparticles (C). Membrane irregularities were observed in bacteria exposed to ZnO nanoparticles. With kind permission from Springer Science+Business Media: Journal of Nanoparticle Research. Investigation into the antibacterial behaviour of suspensions of ZnO nanoparticles (ZnO nanofluids). 9(3), 2007, page 483. Zhang L. Figure 2.⁸

Figure 2 Transmission electron microscope images of silver nanoparticles used (A). Scanning electron microscope image of Escherichia coli control group (B) and E. coli exposed to 50 μg/mL of silver nanoparticles in lysogeny broth medium for 4 hours (C). Transmission electron microscope image of E. coli exposed to 50 μg/mL of silver nanoparticles in lysogeny broth medium for 1 hour at low magnification (D) and high magnification (E). Reprinted from Journal of Colloid and Interface Science, 275(1). Sondi I, Salopek-Sondi B. Silver nanoparticles as antimicrobial agent: a case study on E. coli as a model for Gram-negative bacteria. 177–182. Copyright © (2004), with permission from Elsevier.³

Figure 3 Illustration comparing bacteria surface interactions with nanorough surfaces and conventional nanosmooth surfaces. Due to the high degree of roughness on nanomaterials, rigid bacteria cell membranes cannot lay flush against the material surface. This may inhibit the preliminary steps which lead to bacteria adhesion. As a result, bacteria activity on a nanomaterial surface may be reduced.

Figure 4 Atomic force microscopy images of particle compacts of microphase zinc oxide (ZnO) (A) and nanophase ZnO (B). Analysis indicated that compacts of nanophase ZnO had a 25% increase in surface area. Copyright © 2006, John Wiley and Sons. Adapted with permission from Colón G, Ward BC, Webster TJ. Increased osteoblast and decreased Staphylococcus epidermidis functions on nanophase ZnO and TiO2. J Biomed Mater Res A. 2006;78(3):595–604.⁷

Figure 5 X-ray electron microscopy image of silver nanoparticles (A) and a particle size distribution histogram (B) of those particles. Higher magnification reveals polyhedral structure (C). Nondisruptive electron transmission microscopy reveals an 80–120 nm coating of silver nanoparticles on the surfaces of a polymer catheter (D).

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