Green synthesis, characterisation and antibacterial activities of Strobilanthes crispus-mediated silver nanoparticles (SC-AGNPS) against selected bacteria

Figures & data

Figure 1. (A) Strobilanthes crispus leaves that were used in the study; (B) biosynthesis of AgNP mediated with S. crispus at 24h.

Figure 2. Characterization of SC-AgNPs. (A) UV-Vis spectra of S. crispus leaves water extract at 4.5 mg/mL; (B): UV-Vis spectra of SC-AgNPs at 4.5 mg/mL; (C) FESEM images of SC-AgNPS at 50,000 × magnification; (D) XRD spectra of SC-AgNPs; (E) FTIR analysis of commercial AgNPs, S. crispus aqueous extract and SC-AgNPs.

Table 1. Particle size and zeta potential analysis.

Polydispersity index (PDI)	Particle size (nm)	Zeta potential (mV)
0.373	75.25	−35.6

Table 2. Zone inhibition of bacteria after exposure to SC-AgNPs, AgNP, S. crispus water extract and gentamicin.

Samples (mg/mL)	Zone inhibition (mm)
	E. Coli	P. aeruginosa	S. mutans
SC-AgNPs (10)	12.5 ± 0.87^b	12.71 ± 0.87^b	11.00 ± 0.33^b
AgNP (10)	7.3 ± 0.58^a	7.9 ± 0.32^a	6.0 ± 0.00^a
S. crispus (10)	6.0 ± 0.00^a	6.0 ± 0.00^a	6.0 ± 0.00^a
Gentamicin (10 µg)	23.5 ± 0.7^c	20.1 ± 0.22^c	40.0 ± 0.06^c

The concentration of SC-AgNPs, AgNP and S. crispus water extract against E. coli, S. mutans and P. aeruginosa were 10 mg/mL, respectively. AgNP incorporated with S. crispus exhibited greater zone inhibition against bacteria compared to blank AgNP and S. crispus water extract. Samples were analysed in triplicate and data were presented as means ± standard deviation. Mean values within a row with different letters (a–c) were significantly different (p < 0.05). the zone diameter of 6.0 ± 0.00 mm was presented as no inhibition activity.

Table 3. MIC and MBC of SC-AgNPs, AgNP and S. crispus water extract against E. coli, P. aeruginosa and S. mutans.

Samples	E. coli		P. aeruginosa		S. mutans
	MIC	MBC	MIC	MBC	MIC	MBC
SC-AgNPs (mg/mL)	1.25	2.5	1.25	2.5	2.5	2.5
AgNP (mg/mL)	10.00	10.00	10.00	10.00	>10.00	>10.0
S. crispus extract (mg/mL)	>10.00	>10.00	>10.00	>10.00	>10.00	>10.00
Gentamicin (µg/mL)	6.25	12.5	3.13	6.25	25.00	25.00

The values of MIC and MBC for SC-AgNPs against all bacteria were lower compared to AgNP blank and S. crispus water extract. The sample was analyzed in triplicate and data are presented as means ± standard deviation.

Figure 3. (A) the growth of P. aeruginosa, (B) E. coli, (C) S. mutans (log₁₀ Cfu/mL) after being treated with SC-AgNPs, AgNP and S. crispus water extract at 0, 4, 8, 12 and 24 h. The samples were analyzed in triplicate and data were presented as means ± standard deviation.

Figure 4. (A) Morphology of non-treated E. coli, (B) morphology of E. coli after being treated with SC-AgNPs (1.25 mg/mL) and (C) morphology of E. coli after being treated with SC-AgNPs (0.625 mg/mL). Non-treated E. coli was rod in shape, meanwhile, E. coli treated with SC-AgNPs was smaller, shrunken and irregular in shape.

Figure 5. (A) Morphology of non-treated P. aeruginosa, (B) morphology of P. aeruginosa after being treated with SC-AgNPs (0.625 mg/mL) and (C) morphology of P. aeruginosa after being treated with SC-AgNPs (1.25 mg/mL). Non-treated P. aeruginosa was rod in shape, meanwhile, P. aeruginosa treated with SC-AgNPs was found lysed and shrunken.

Figure 6. (A) Morphology of non-treated S. mutans, (B) morphology of S. mutans after being treated with SC-AgNPs (2.5 mg/mL) and (C) morphology of S. mutans after being treated with SC-AgNPs (1.25 mg/mL). Non-treated S. mutans was coccus in shape, meanwhile, S. mutans treated with SC-AgNPs was smaller and irregular in shape.

Figure 7. (A) The relative expression of eae a gene in E. coli, (B) gtf B gene in S. mutans and (C) Pel a gene in P. aeruginosa after being treated with SC-AgNPs. The samples were analyzed in triplicate and data were presented as means ± standard deviation. The fold change value of treated genes showed significant differences (p < 0.05) when compared to control (*).

Data availability statement

The data are available from the corresponding author upon reasonable request. The whole methodology is included in the materials and methods section.