Gold Nanoparticle-Conjugated Cinnamic Acid Exhibits Antiacanthamoebic and Antibacterial Properties

Abstract

trans-Cinnamic acid (CA) is a natural organic compound. Using amoebicidal assays, for the first time we showed that CA affected the viability of the protist pathogen Acanthamoeba castellanii Conjugation with gold nanoparticles (AuNPs) enhanced the antiamoebic effects of CA. CA-coated AuNPs (CA-AuNPs) also exhibited significant excystation and encystation activity, compared to CA and AuNPs alone. Pretreatment of amoebae with CA-AuNPs inhibited A. castellanii-mediated host cell cytotoxicity. Moreover, CA-AuNPs exhibited potent effects against methicillin-resistant Staphylococcus aureus and neuropathogenic Escherichia coli K1 and protected host cells against bacteria-mediated host cell death.

Keywords: Acanthamoeba; E. coli; MRSA; cinnamic acid; gold nanoparticles.

FIG 1

(A) UV-visible spectrum of CA-AuNPs, showing a surface plasmon resonance band at 540 nm, which indicates the successful formation of CA-conjugated AuNPs. (B) AFM image of CA-AuNPs. Extensive imaging was performed to derive a representative image of the morphology of nanoparticles. The average size of CA-AuNPs was found, using DLS analysis, to be 89 nm.

FIG 2

(A) Effects of CA-AuNPs on the excystation of A. castellanii belonging to the T4 genotype. Briefly, 5 × 10⁵ A. castellanii cysts were incubated with 10 and 5 μM concentrations of CA-AuNPs in growth medium (PYG medium) at 30°C for 72 h. After this period, amoebae were counted using a hemocytometer. Chlorhexidine (CHX) was used as a positive control, while PYG medium alone was used as a negative control. (B) CA-AuNP blockage of encystation. Briefly, 5 × 10⁵ A. castellanii trophozoites were incubated with CA-AuNPs, CA alone, or AuNPs alone at 10 and 5 μM concentrations in encystation medium (E.M) (50 mM MgCl₂ and 10% glucose). After 72 h of incubation, 0.5% SDS was added and resistant cysts were enumerated using a hemocytometer. The results represent the mean ± standard error of three independent experiments performed in duplicate. *, P < 0.05, using a two-sample t test and two-tailed distribution.

FIG 3

(A) CA-AuNP inhibition of A. castellanii-mediated host cell cytotoxicity. Briefly, amoebae were pretreated with 5 μM drugs for 2 h, followed by incubation with host cells to determine host cell death, as described in the text. In the absence of drug, amoebae produced approximately 80% host cell death. At micromolar concentrations, CA-AuNPs significantly inhibited A. castellanii-mediated host cell death (P < 0.05, using a two-sample t test and two-tailed distribution). (B and C) CA-AuNP inhibition of host cell cytotoxicity due to MRSA (B) and E. coli K1 (C) (P < 0.05, using a two-sample t test and two-tailed distribution). Chlorhexidine (CHX) and gentamicin abolished pathogen-mediated host cell cytotoxicity. The data are presented as the mean ± standard error of at least three independent experiments performed in duplicate.