Effect of silver nanoparticles on Candida albicans biofilms: an ultrastructural study.

Miguel Jose-Yacaman,Jose L Lopez-Ribot,Humberto H Lara,Dulce G Romero-Urbina,M Josefina Arellano-Jiménez,Christopher Pierce

doi:10.1186/s12951-015-0147-8

Abstract

BackgroundCandida albicans is the most common pathogenic fungus isolated in bloodstream infections in hospitalized patients, and candidiasis represents the fourth most common infection in United States hospitals, mostly due to the increasing numbers of immune- and medically-compromised patients. C. albicans has the ability to form biofilms and morphogenetic conversions between yeast and hyphal morphologies contribute to biofilm development and represent an essential virulence factor. Moreover, these attached communities of cells are surrounded by a protective exopolymeric matrix that effectively shelters Candida against the action of antifungals. Because of dismal outcomes, novel antifungal strategies, and in particular those targeting biofilms are urgently required. As fungi are eukaryotic, research and development of new antifungal agents has been difficult due to the limited number of selective targets, also leading to toxicity.ResultsBy microwave-assisted techniques we obtained pure 1 nm spherical silver nanoparticles ideal for their potential biological applications without adding contaminants. A phenotypic assay of C. albicans demonstrated a potent dose-dependent inhibitory effect of silver nanoparticles on biofilm formation, with an IC50 of 0.089 ppm. Also silver nanoparticles demonstrated efficacy when tested against pre-formed C. albicans biofilms resulting in an IC50 of 0.48 ppm. The cytotoxicity assay resulted in a CC50 of 7.03 ppm. The ultrastructural differences visualized under SEM with silver nanoparticles treatment were changes in the surface appearance of the yeast from smooth to rough thus indicating outer cell wall damage. On the fungal pre-formed biofilm true hyphae was mostly absent, as filamentation was inhibited. TEM measurement of the cell-wall width of C. albicans after treatment resulted in significant enlargement (206 ± 11 nm) demonstrating membrane permeabilization.ConclusionsOur results demonstrate that silver nanoparticles are potent inhibitors of C. albicans biofilm formation. SEM observations are consistent with an overall loss of structure of biofilms mostly due to disruption of the outer cell membrane/wall and inhibition of filamentation.TEM indicates the permeabilization of the cell wall and subsequent disruption of the structural layers of the outer fungal cell wall. The anti-biofilm effects are via cell wall disruption.Electronic supplementary materialThe online version of this article (doi:10.1186/s12951-015-0147-8) contains supplementary material, which is available to authorized users.

Highlights

Candida albicans is the most common pathogenic fungus isolated in bloodstream infections in hos‐ pitalized patients, and candidiasis represents the fourth most common infection in United States hospitals, mostly due to the increasing numbers of immune- and medically-compromised patients
Lara et al J Nanobiotechnol (2015) 13:91 infections resulting in high morbidity and mortality rates [2]. These biofilms are typically surrounded by an exopolymeric substance (EPS) matrix [6] which effectively shelters the fungal cells against adverse environmental conditions, including host defense mechanisms and the action of antifungals [7]
This technique is a good alternate methodology to synthesize metallic nanoparticles obtaining an efficient control of nanoparticles size distribution (Additional file 1), and more importantly for biological purposes as it does not involve the handling of potentially contaminants or cytotoxic reducing agents [25]

Summary

Introduction

Candida albicans is the most common pathogenic fungus isolated in bloodstream infections in hos‐ pitalized patients, and candidiasis represents the fourth most common infection in United States hospitals, mostly due to the increasing numbers of immune- and medically-compromised patients. C. albicans has the ability to form biofilms and morphogenetic conversions between yeast and hyphal morphologies contribute to biofilm develop‐ ment and represent an essential virulence factor These attached communities of cells are surrounded by a protective exopolymeric matrix that effectively shelters Candida against the action of antifungals. Formation of biofilms further complicates treatment, as sessile cells within these biofilms have the ability to resist drug concentrations even 1000-fold higher than the IC50 reported for the planktonic yeasts [6, 12] Together, all these factors combined are responsible for clinical failures and high mortality in patients with invasive candidiasis [13]. Novel antifungal and antibiofilm drugs against these unmanageable infections are urgently needed [7, 11]

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