Synthesis, characterization, and evaluation of antibacterial effect of Ag nanoparticles against Escherichia coli O157:H7 and methicillin-resistant Staphylococcus aureus (MRSA)

Figures & data

Figure 1 Characterization of AgNPs synthesized by cysteine reduction at low agitation (200 rpm) for 1 hour. (A) TEM images; (B) DLS; (C) UV-Vis spectrum; (D) zeta potential at pH =7.0; (E) DLS of AgNPs after 3 months of synthesis; (F) zeta potential at pH =7.0 after 3 months of synthesis.

Abbreviations: AgNPs, silver nanoparticles; DLS, dynamic light scattering; TEM, transmission electron microscopy.

Figure 2 XRD spectra for the three samples of AgNPs.

Abbreviations: AgNPs, silver nanoparticles; XRD, X-ray diffraction.

Figure 3 Sedimentation of AgNPs at different times. AgNPs were synthesized by cysteine reduction at high agitation (800 rpm) for 4 hours. (A) 1 day; (B) 8 days; (C) 15 days; (D) 30 days.

Abbreviation: AgNPs, silver nanoparticles.

Table 1 Characterization of AgNPs synthesized by cysteine reduction at different reaction conditions at pH =4.0

Type of AgNPs	Stabilization method	Average size (nm)	PdI	Surface charge (mV)	λmax (nm)
1	Low agitation: 200 rpm for 1 hour	20.49± 1.00	0.223	+23.37	405
2	High agitation: 800 rpm for 4 hours	97.22±5.38	0.281	+11.4	416
3	Stabilization 1 hour by ultrasound	64.77±3.12	0.409	+18.3	404

Abbreviations: AgNPs, silver nanoparticles; PdI, polydispersity index.

Table 2 Characterization of AgNPs synthesized by cysteine reduction at different reaction conditions at pH =7.0

Type of AgNPs	Stabilization method	Average size (nm)	PdI	Surface charge (mV)	λmax (nm)
1	Low agitation: 200 rpm for 1 hour	36.16±0.72	0.434	−0.42	405
2	High agitation: 800 rpm for 4 hours	113.37±4.65	0.852	−8.70	416
3	Stabilization 1 hour by ultrasound	78.50± 1.42	0.906	−2.29	404

Abbreviations: AgNPs, silver nanoparticles; PdI, polydispersity index.

Table 3 MIC of AgNPs against Escherichia coli O157:H7 and MRSA determined by the microdilution method

Microorganism	Type of AgNPs	MIC50 μg/mL	MIC90 μg/mL	MIC99 μg/mL
E. coli O157:H7	1	<0.25	0.25	0.50
	2	1	10	20
	3	0.50	10	10
Methicillin resistant	1	<0.25	0.25	0.50
S. aureus (MRSA)	2	<0.25	0.50	10
	3	<0.25	0.25	0.50

Abbreviations: AgNPs, silver nanoparticles; MIC, minimum inhibitory concentration; MRSA, methicillin-resistant Staphylococcus aureus.

Table 4 Inhibitory activities of AgNPs against Escherichia coli O157:H7 and MRSA

Microorganism	Type of AgNPs	Concentration (μg/mL)	Inhibition zone (mm)
E. coli O157:H7	1	0.25	–
		1	7
	2	10	5
		20	7
	3	10	5
		20	7
Methicillin resistant	1	0.25	–
S. aureus (MRSA)		1	9
	2	10	10
		20	12
	3	10	11
		20	12

Note: These tests were carried out by diffusion disc tests.

Abbreviations: AgNPs, silver nanoparticles; MRSA, methicillin-resistant Staphylococcus aureus.

Figure 4 Microbial growth kinetics of Escherichia coli O157:H7 in the presence of AgNPs synthesized by cysteine reduction at different conditions. (A) Type 1; (B) Type 2; (C) Type 3.

Abbreviations: AgNPs, silver nanoparticles; OD, optical density; h, hours.

Figure 5 Microbial growth kinetics of MRSA in the presence of AgNPs synthesized by cysteine-reduction at different conditions: (A) Type 1; (B) Type 2; (C) Type 3.

Abbreviations: AgNPs, silver nanoparticles; MRSA, methicillin-resistant Staphylococcus aureus; h, hours; OD, optical density.

Figure 6 Tests of diffusion discs showing antimicrobial activity against Escherichia coli O157:H7 and MRSA of nanoparticles synthesized at different conditions. AgNPs were used at 10 μg/ml. (A) Type 1; (B) Type 2; (C) Type 3.

Abbreviations: AgNPs, silver nanoparticles; MRSA, methicillin-resistant Staphylococcus aureus.

Table 5 MBC of AgNPs against Escherichia coli O157:H7 and MRSA

Microorganism	Type of AgNP	MBC μg/mL
E. coli O157:H7	1	1
	2	20
	3	10
Methicillin resistant	1	0.50
S. aureus (MRSA)	2	10
	3	1

Abbreviations: AgNPs, silver nanoparticles; MBC, minimal bactericide concentration; MRSA, methicillin-resistant Staphylococcus aureus.

Figure 7 Bactericidal effect of AgNPs against Escherichia coli O157:H7 and MRSA using different types of AgNPs. (A) Type 1; (B) Type 2; (C) Type 3.

Abbreviations: AgNPs, silver nanoparticles; MRSA, methicillin-resistant Staphylococcus aureus.

Figure 8 Proposed reaction mechanism for the formation of Ag–cysteine complex.

Figure 9 Microbial growth kinetics of: (A) Escherichia coli O157:H7, and (B) MRSA in the presence of 1 μg/ml AgNPs.

Abbreviations: AgNPs, silver nanoparticles; MRSA, methicillin-resistant Staphylococcus aureus; OD, optical density; h, hours.