Candida biofilms: antifungal resistance and emerging therapeutic options

Intravascular catheter infections are a major cause of morbidity and mortality in hospitalized patients, accounting for the majority of the 200,000 nosocomial bloodstream infections occurring in the US annually. Of the intravenous lines that are culture-positive for Candida, 40% actually represent fungemia, which generally necessitates systemic treatment and line removal to affect cure. Until recently, the reason for the need for device removal was unclear. However, our research group and others have demonstrated a near-total resistance to antifungals by biofilm-associated Candida. Similar to bacterial species, Candida biofilm formation proceeds through early, intermediate and maturation phases. This process is associated with the generation of a polysaccharide extracellular matrix (ECM). Mature C. albicans biofilms have a heterogeneous architecture, in terms of distribution of fungal cells and ECM, and exhibit broad antimicrobial resistance. The mechanisms causing such profound antifungal resistance are beginning to be understood. Recent data indicate that resistance is phase-specific and multifactorial, involving efflux pumps and sterol synthesis (at early and mature biofilm phases, respectively). Neither metabolic quiescence nor the ECM appear to contribute substantially. Susceptibility testing and confocal scanning laser microscopy demonstrated that azoles failed to exert activity against mature Candida biofilms. However, sub-inhibitory concentrations of voriconazole impaired biofilm formation and caused cell morphological aberrations. In contrast, lipid-formulation amphotericins and the echinocandins uniquely exhibited activity against mature biofilms. The mechanisms underlying this ability are unknown. The role of other pharmacological (eg, catheter coatings, antimicrobial peptides and antibiotic locks) and non-pharmacological methods in the prevention and treatment of device-related biofilms is discussed in this review.