Antibacterial activity of silver nanoparticles: sensitivity of different Salmonella serovars

Abstract

Salmonella spp. is one of the main causes of foodborne illnesses in humans worldwide. Consequently, great interest exists in reducing its impact on human health by lowering its prevalence in the food chain. Antimicrobial formulations in the form of nanoparticles exert bactericidal action due to their enhanced reactivity resultant from their high surface/volume ratio. Silver nanoparticles (AgNPs) are known to be highly toxic to Gram-negative and Gram-positive microorganisms, including multidrug resistant bacteria. However, few data concerning their success against different Salmonella serovars are available. Aims of the present study were to test the antimicrobial effectiveness of AgNPs, against Salmonella Enteritidis, Hadar, and Senftenberg, and to investigate the causes of their different survival abilities from a molecular point of view. Results showed an immediate, time-limited and serovar-dependent reduction of bacterial viability. In the case of S. Senftenberg, the reduction in numbers was observed for up to 4 h of incubation in the presence of 200 mg/l of AgNPs; on the contrary, S. Enteritidis and S. Hadar resulted to be inhibited for up to 48 h. Reverse transcription and polymerase chain reaction experiments demonstrated the constitutive expression of the plasmidic silver resistance determinant (SilB) by S. Senftenberg, thus suggesting the importance of a cautious use of AgNPs.

Keywords: Salmonella; antimicrobials; nanoparticles; silver; silver resistance.

FIGURE 1

Representative TEM micrograph (A) and particle size distribution obtained from TEM data (B) of NM-300K silver nanoparticles dispersed in water. Scale bar equals 100 nm.

FIGURE 2

Intensity-based particle size distribution of an aqueous suspension of NM-300K silver nanoparticles obtained from DLS analysis.

FIGURE 3

Growth curves of Salmonella Senftenberg (A,B), Salmonella Hadar (C,D), and Salmonella Enteritidis (E,F), in the presence of different doses of AgNPs (left side) or AgNO₃ (right side).

FIGURE 4

(A) One percent agarose gel showing the PCR product of SilB amplification from total DNA of Klebsiella pneumoniae, clone ST258 (lane 1), Salmonella Senftenberg (lane 2), Salmonella Hadar (lane 3), Salmonella Enteritidis (lane 4). In the lane 5 was run the negative control (DNA template replaced by sterile water). (B) One percent agarose gel showing the PCR product of SilB amplification from the plasmidic DNA fraction of Klebsiella pneumoniae, clone ST258 (lane 1) and Salmonella Senftenberg (lane 2). The asterisks indicate the 233 bp PCR product expected for SilB fragment amplification. MW indicates the Perfect DNA Markers (EMD, Millipore, USA), 0.05–10 kb ladder.

FIGURE 5

RT-PCR analysis. One percent agarose gel showing DNA bands originating from PCR amplification experiments referring to Klebsiella pneumoniae, clone ST258 (K) and Salmonella Senftenberg (S). The asterisk indicates the 233 bp PCR product expected for SilB fragment amplification. MW indicates the Perfect DNA Markers (EMD, Millipore, USA), 0.05–10 kb ladder.