Lactococcus lactis as a safe and inexpensive source of bioactive silver composites

Abstract

This research develops a safe, inexpensive, and more accessible source for synthesis of silver nanoparticles. The bioactive silver composites synthesized by Lactococcus lactis 56 KY484989 (LCLB56-AgCs) were characterized by various physico-chemical techniques and investigated for their antimicrobial activity and cytotoxicity. The average amount of nanoparticles was 0.363 ± 0.09 mg from 50 mL of culture medium. The synthesis efficiency varied from 71 to 85%. Synthesized silver nanoparticles with spherical in shape were found to be of 5-50 nm and average diameter 19 ± 2 nm. Based on the shape of isotopic pattern of d-electrons metals, the signals of silver isotopes [¹⁰⁷Ag]⁺ at m/z 106.905 and [¹⁰⁹Ag]⁺ at m/z 108.910 were confirmed. Moreover, LCLB56-AgCs exerted an inhibitory effect against all tested bacterial strains (Pseudomonas aeruginosa ATCC10145, Proteus mirabilis ATCC25933, Staphylococcus epidermidis ATCC49461, MSSA ATCC29213, and Staphylococcus aureus ATCC6338). More pronounced antimicrobial effect was noticed for 15 μg/well. Minimum inhibitory concentration required to inhibite the growth of 90% organism (MIC₉₀) of synthetized LCLB56-AgCs was in a range of 3.125-12.5 μg/mL. The concentration at which the viability of the L929 cells was reduced to 50% was above 200 μg/mL for LCLB56-AgNCs. These results open up possibilities for many applications of bioactive silver composites (BioAgCs) synthesized by L. lactis 56 in food and pharmaceutical industries.

Keywords: Antibacterial activity; Biosynthesis; Cytotoxicity; Lactococcus lactis; Silver composites.

Fig. 1

a Phylogenetic relationships of Lactococcus lactis 56 strain. Neighbor-joining analysis of 16S rDNA sequences, using the number of differences method (Nei and Kumar 2000) combined with bootstrap analysis from 1000 replicates (bootstrap values < 50% not shown). All positions containing gaps and missing data were eliminated. There were a total of 1440 positions in the final dataset. Analyses were performed using MEGA7 software (Kumar et al. 2016). b The confirmation spectrum of L. lactis 56 performed using the MALDI–TOF MS technique on HCCA matrix

Fig. 2

EDX spectra of a LCLB56-AgNCs (a), TEM micrograph (b), and FFT image (c); SAED (d) and XRD (e) patterns of bioactive silver nanoparticles

Fig. 3

Infrared spectrum of LCLB56-AgNCs registered in MIR range (a) and using thin layer method in DirectDetect® Infrared Spectrometer (b)

Fig. 4

Fluorescence of LCLB56-AgNCs nanocomposites

Fig. 5

Molecular fingerprint of synthesized LCLB56-Ag biocolloids (a) using MALDI–TOF MS technique and HCCA matrix with spotted 1 mM silver nitrate as control (b)

Fig. 6

The inhibition zones of LCLB56-AgNPs provided by well diffusion method against a MSSA, b Staphylococcus aureus, c Proteus mirabilis, d Pseudomonas aeruginosa, e Staphylococcus epidermidis

Fig. 7

Fluorescence microscopy detection of living (green-labeled) and dead (red-labeled) S. aureus cells after 3 h (a) and 24 h (b) of treatment with LCLB56-AgNPs 12.5 μg/mL and after 3 h (c) and 24 h (d) with 100 μg/mL (color figure online)

Fig. 8

Cytotoxicity effect of silver nanocomposites synthesized by Lactococcus lactis 56