The emergence of MDR Gram-negative bacteria prompted the use of colistin, as the last resort in the treatment of severe infections by these pathogens. Although uncommon, colistin resistance is increasing and its spread is being considered a global health threat.
Due to the disruptive action of silver on bacteria reviewed by Barras et al. and its ability to bind to the sulfur atoms present in sulfhydryl groups of proteins and enzymes located on the bacterial cell surface [
4], we hypothesized that colloidal silver may present antibacterial activity against Gram-negative and Gram-positive bacteria. In this study, we showed that colloidal silver presented bactericidal activity with MIC
90 values between 4 and 8 mg/L against a collection of 270 isolates of
A. baumannii,
P. aeruginosa,
E. coli,
S. aureus,
S. epidermidis, and
Enterococcus spp. These data are consistent with previous work reporting that colloidal silver was active against three reference strains of
A. baumannii,
P. aeruginosa, and
S. aureus [
27]. In other work, using Kirby–Bauer disc diffusion test, colloidal silver at 30 ppm was shown to be active against
S. aureus,
S. epidermidis, and
Bacillus subtilis but not against
E. coli [
28].
It is noteworthy to mention that sub-MICs of colloidal silver did not significantly affect the membrane permeability of both Gram-negative and Gram-positive bacteria, in accordance with previously published data by Fenq et al. which showed that
S. aureus is less permeable to silver ions when compared with
E. coli [
8]. In contrast, other studies showed that silver enhanced the cell permeability of a reference
E. coli strain [
7]. This difference in the membrane permeability results between both studies could be due to the fact that, in our study, we used MDR clinical isolates of
E. coli that may have reduced membrane permeability.
Although, in our study, the colloidal silver at sub-MIC against the studied strain could not alter the bacterial membrane permeability, colloidal silver may increase the production of ROS such as ampicillin [
29], ciprofloxacin [
30,
31], and vancomycin [
32]. Barras et al. reported in their review that silver is a non-redox active metal that cannot directly produce ROS [
4]. The production of ROS by silver occurred through the perturbation of the respiratory electron transfer chain [
9], Fenton chemistry following destabilization of Fe–S clusters, or displacement of iron [
7], and inhibition of anti-ROS defenses by thiol–silver bond formation [
33]. We observed in our study that colloidal silver at lower and higher concentration produced ROS in Gram-negative bacteria and to a much lesser extent in Gram-positive bacteria, especially in
E. feacalis, which may explain the lower bactericidal activity of colloidal silver against Gram-positive bacteria in time–kill curves assays. Similar results were observed by Kim et al., who reported that the differences in structure, thickness, and composition of cells between Gram-negative and Gram-positive can explain why
E. coli shows substantial inhibition by silver nanoparticles, whereas
S. aureus is less inhibited [
34]. The antimicrobial potential of silver ions is influenced by the thickness and composition of the cell wall of the microorganisms, and the difference in the organization of the peptidoglycan layer [
35]. Gram-negative bacteria contain lipopolysaccharides (LPS) in the cell membrane, which contributes to structural integrity of the membrane, in addition to protecting the membrane from chemical attacks. However, the negative charge of LPS promotes the adhesion of silver and renders the bacteria more susceptible to antimicrobial therapy [
35]. Several studies showed the pronounced adhesion and deposition of silver onto the cell surface of Gram-negative bacteria in particular, due to the presence of LPS in their cell membrane [
36]. In Gram-positive bacteria, the cell wall is composed of a negatively charged peptidoglycan layer, and the amount of peptidoglycan is comparatively higher in Gram-positive bacteria than Gram-negative bacteria [
35]. The lower susceptibility of Gram-positive bacteria to antibiotic therapy can be explained on the basis of the fact that their cell wall is comparatively much thicker than that of Gram-negative bacteria [
37]. The thicker cell wall of Gram-positive bacteria, as well as the negative charge of the peptidoglycan layer, allows the adhesion of silver ions. For this reason,
S. aureus, which possesses a thick cell wall and more peptidoglycan molecules, prevents the action of the silver ions and renders the bacterium comparatively more resistant to silver [
8].