Editorial Feature

Antibacterial Abilities of Au Nanoparticles are Size-Dependent

Nanobiotechnology involves the application of nanoscale technologies to the field of biological and life science. Recently, researchers have investigated whether the size of gold nanoparticles (AuNPs) influences their antibacterial abilities on three strains of Streptococcus, a common dental bacteria. 

Antibacterial Abilities of Au Nanoparticles are Size-Dependent

Image Credit: Jezper/Shutterstock.com

Dental caries are the commonly seen human infectious disease, involving various pathogenic factors and microbes like Streptococcus salivarius (S. salivarius), Streptococcus mutans (S. mutans), and Streptococcus sanguinis (S. sanguinis), salivary related disorders, and individual diet.

Over the years, various preventive methods have been introduced for dental caries, such as metals, because of their antibacterial properties. 

A metal’s antibacterial properties are thought to be affected by how big its surface is; larger surface areas of metal nanoparticles result in more potent interactions with various other molecules.

A study published in the Journal of Dentistry has analyzed varying sizes of silver, zinc oxide and gold nanoparticles against strains of Streptococcus. Average sizes of 25, 125 and 80 nm were explored. 


This study had 75 children between the ages of three and five years participating for six months. Children’s dental caries were assessed utilizing bitewing radiographs and dental explorers.

Using a sterile toothpick, 75 specimens of dental plaque with dental caries were obtained. Moreover, unstimulated saliva from the sublingual region was collected using a sterile cotton swab.

The cultures were incubated anaerobically and S. sanguinis colonies were selected. The prototype strain of S. mutans (ATCC 25175), S. sanguinis (ATCC 10556), and S. salivarius (ATCC9759) were employed as standard species.

Polymerase chain reaction (PCR) was utilized for biochemical tests confirmation (respectively Figures 1, 2, and 3).

The Impact of Different-Sized Nanoparticles on Streptococcus Species

Figure 1. Image Credit: Lavaee, et al., (2021)

The Impact of Different-Sized Nanoparticles on Streptococcus Species

Figure 2. Image Credit: Lavaee, et al., (2021)

The Impact of Different-Sized Nanoparticles on Streptococcus Species

Figure 3. Image Credit: Lavaee, et al., (2021)

The gradient system confirmed the annealing temperature. PCR amplification of isolated S. mutans, S. sanguinis, and S. salivarius species are shown in Figures 1, 2, and 3. The antibacterial assessment was carried out on clinically isolated bacteria and standard species.

A water-based solution of nanoparticles was prepared, and the size distribution of the nanoparticles was confirmed by ultraviolet-visible spectroscopy and a particle size analyzer (see Figure 4). Mean size ranged from 25 to 90 nm for nanoparticles.

The Impact of Different-Sized Nanoparticles on Streptococcus Species

Figure 4. Image Credit: Lavaee, et al., (2021)

Colloidal solutions of nanoparticles with an initial concentration of 500 µg/mL, were sterilized, and the minimum bactericidal concentration (MBC) and minimum inhibitory concentration (MIC) were determined.

Evaluation of the MIC and MBC values of AuNPs (gold nanoparticles) various sizes against standard species of S. sanguinis, S. mutans, and S. salivarius were also performed, data were analyzed by SPSS version 18. To compare the MIC and MBC values, ANOVA and Post-hoc Tukey were used.


Tables 1 and 2 list the MBC and MIC of AuNPs against patient-isolated bacteria and standard species.

The team noted that the MIC and MBC of different sizes of AuNPs against different patient-isolated bacteria and standard species were statistically different.

Table 1. Minimum inhibitory and minimum bactericidal concentration mean of gold nanoparticles against patient-derived S. mutans, S. salivarius, and S. sanguinis. Source: Lavaee, et al., (2021)

Size (nm) bacteria MIC mean (µg/ml) MBC mean (µg/ml)
25 S.mutans 1.73±1.23 4.05±2.68
S.sanguinis 3.17±1.50 6.46±2.98
S.salivarius 2.86±1.58 6.09±3.15
Total 2.51±1.55 5.41±3.09
60 S.mutans 91.61±46.39 184.65±91.37
S.sanguinis 148.21±64.46 289.28±124.15
S.salivarius 119.31±63.20 242.42±124.76
Total 117.46±61.82 234.37±119.97
90 S.mutans 232.95±124.53 217.26±236.59
S.sanguinis 353.57±130.34 329.53±326.91
S.salivarius 329.54±124.28 302.53±314.42
Total 299.10±136.42 277.47±294.03


Table 2. Minimum inhibitory and minimum bactericidal concentration mean of gold nanoparticles against standard S. mutans, S. salivarius, and S. sanguinis. Source: Lavaee, et al., (2021)

AuNPs S. salivarius 
(ATCC 9759)
S. sanguinis 
S. mutans 
(ATCC 25175)
MBC (µg/ml) MIC (µg/ml) MBC (µg/ml) MIC (µg/ml) MBC (µg/ml) MIC (µg/ml)
25 nm 1.95 0.97 3.9 1.95 7.81 1.95
60 nm 125 62.5 125 62.5 250 125
90 nm 500 250 1000 500 500 250
Chlorhexidine 50 50 50 25 50 50


The MBC and MIC of chlorhexidine against the three patient-isolated and standard species of streptococcus (see Table 2), were higher than values registered for 25 nm AuNPs. The MBC and MIC values of AuNPs against patient-isolated S. sanguinis, S. mutans, and S. salivarius are illustrated in Figures 5 and 6, respectively.

The Impact of Different-Sized Nanoparticles on Streptococcus Species

Figure 5. Image Credit: Lavaee, et al., (2021)

The Impact of Different-Sized Nanoparticles on Streptococcus Species

Figure 6. Image Credit: Lavaee, et al., (2021)

Smaller AuNPs were more potent antibacterial agents, and 25 nm AuNPs had the lowest MIC and MBC against both patient-isolated and standard bacteria. In many studies, nanoparticles have demonstrated a wide range of antibacterial activity against different bacteria, both gram-positive and gram-negative.

Nanoparticles can attach to the cell membrane and disrupt its permeability. By reducing the size of nanoparticles, more gold ions can be released, and antibacterial properties are enhanced.

Anaerobic bacteria may be prevented from releasing nanoparticles due to insufficient air; therefore, the different release of nanoparticle ions makes the difference in antimicrobial potencies for aerobic and anaerobic bacteria. The antibacterial effect of nanoparticles is related to their concentration.

However, almost all studies support the size-dependent antimicrobial effect of nanoparticles in a similar manner; as the size of nanoparticles decreases, the antimicrobial effect increases.


The current study evaluated the antibacterial potency of three different sizes of AuNPs on both clinically isolated and standard species. 

Here, a correlation was shown between the size of AuNPs and their antibacterial activity. As the size of AuNPs decreases, the antibacterial properties improve. The patient-isolated bacteria are more defiant to the antibacterial effect of AuNPs.

Continue reading: Improving Dental Restorative Materials Using Nanotechnology.

Journal Reference:

Lavaee, F., Ranjbar, Z., Modarezi, F., Keshavarz, F. (2021) The Effect of Gold Nano Particles with Different Sizes on Streptococcus Species. Journal of Dentistry, 4(73), pp. 235–242. Available online: https://dentjods.sums.ac.ir/article_47735.html.

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Megan Craig

Written by

Megan Craig

Megan graduated from The University of Manchester with a B.Sc. in Genetics, and decided to pursue an M.Sc. in Science and Health Communication due to her passion for combining science with content creation. As part of her studies, Megan partnered with Jodrell Bank Discovery Centre as a Digital Marketing Assistant, producing content and updating sections of their website. In her spare time, she loves to travel, exploring each location's culture and history - including the local cuisine. Her other interests include embroidery, reading fiction, and practicing her Japanese language skills.


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