Abstract
A lot of effort has been dedicated recently to provide a better insight into the mechanism of the antibacterial activity of silver nanoparticles (AgNPs) colloidal suspensions and their released silver ionic counterparts. However, there is no consistency regarding whether the antibacterial effect displayed at cellular level originates from the AgNPs or their ionic constitutes. To address this issue, three colloidal suspensions exhibiting different ratios of AgNPs/silver ions were synthesized by a wet chemistry method in conjunction with tangential flow filtration, and were characterized and evaluated for their antimicrobial properties against two gram-negative, Escherichia coli (E. coli) and Pseudomonas aeruginosa (P. aeruginosa), and two gram-positive, Staphylococcus aureus (S. aureus) and Staphylococcus epidermidis (S. epidermidis), bacterial strains. The produced samples contained 25% AgNPs and 75% Ag ions (AgNP_25), 50% AgNPs and 50% Ag ions (AgNP_50), and 100% AgNPs (AgNP_100). The sample AgNP_100 demonstrated the lowest minimum inhibitory concentration values ranging from 4.6 to 15.6 ppm for all four bacterial strains, while all three samples indicated minimum bactericidal concentration (MBC) values ranging from 16.6 ppm to 62.5 ppm against all strains. An increase in silver ions content results in higher bactericidal activity. All three samples were found to lead to a significant morphological damage by disruption of the bacterial cell membranes as analyzed by means of scanning electron microscopy (SEM). The growth kinetics demonstrated that all three samples were able to reduce the bacterial population at a concentration of 3.1 ppm. SEM and growth kinetic data underline that S. epidermidis is the most sensitive among all strains against the investigated samples. Our results showed that all three AgNPs colloidal suspensions exhibited strong antibacterial properties and, thus, they can be applied in medical devices and antimicrobial control systems.
Highlights
In recent years, a significant increase in the number of bacterial strains resistant to commercial antibiotics has been observed
These results are in good agreement with the morphological observation by scanning electron microscopy (SEM) images showing the complete damage of the bacterial cell membranes (Figure 5)
As dissolution of silver ions directly released from the AgNPs into the samples may affect their biocidal profile, the determination of the colloidal silver content is of significant importance; it was the subject of this study
Summary
A significant increase in the number of bacterial strains resistant to commercial antibiotics has been observed. Over 70% of bacterial infections are caused by strains resistant to at least one type of the most commonly used drugs [1]. AgNPs coexist with their silver ions, and silver content is conventionally defined by both the nanoparticles and the ions, rarely providing insight to their exact percentile allocation. This complicates the comparative evaluation of literature reports on the effectiveness of nanosilver due to an apparent lack of consensus as to whether this originates from AgNPs [6] or their ionic constitutes [7]
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