Susceptibility of Candida albicans biofilms to azithromycin, tigecycline and vancomycin and the interaction between tigecycline and antifungals

Abstract

Despite growing data on antimicrobial lock therapy (ALT) in treating bacterial catheter-related bloodstream infections (CR-BSIs), ALT has not been established as a treatment option for CR-BSI caused by Candida albicans. Based on our finding that high-dose doxycycline exhibited antifungal activity against mature C. albicans biofilms, we evaluated additional antibacterial agents with Gram-positive activity [azithromycin, tigecycline (TIG) and vancomycin]. After screening these antibiotics, it was found that TIG had substantial antifungal activity against mature C. albicans biofilms. Therefore, TIG was assayed alone and in combination with fluconazole (FLC), amphotericin B (AmB) or caspofungin (CAS). TIG at 2048 μg/mL resulted in a >50% reduction in the growth of planktonic C. albicans cells. TIG inhibited the formation of biofilms from 128 μg/mL. Against mature biofilms, 2048 μg/mL TIG reduced metabolic activity by 84.2%. Furthermore, addition of 512 μg/mL TIG to FLC at all concentrations tested provided additional reduction in the metabolic activity of mature biofilms. However, this was not superior to 512 μg/mL TIG alone. TIG at 512 μg/mL increased the antifungal effect of lower concentrations of AmB (0.03125–0.25 μg/mL), but at 0.03125 μg/mL and 0.0625 μg/mL this effect was not superior to 512 μg/mL TIG alone. TIG inhibited the antifungal effect of higher concentrations of AmB (≥2 μg/mL). TIG at 512 μg/mL inhibited the antifungal activity of CAS at lower concentrations (0.25–8 μg/mL). These data indicate that high-dose TIG is highly active in vitro against planktonic cells, forming biofilms and mature biofilms of C. albicans.

Introduction

Catheter-related bloodstream infections (CR-BSIs) result in significant morbidity and mortality, increased length of stay and increased cost of care in patients requiring intravascular devices. The incidence of CR-BSI has paralleled the increased use of intravascular devices such as central venous catheters and those used in haemodialysis over the past 2 decades [1]. Candida albicans has emerged as a significant pathogen causing CR-BSI, especially amongst immunosuppressed and hospitalised patients. Although Gram-positive bacteria such as Staphylococcus epidermidis and Staphylococcus aureus have been found to be the most common pathogens associated with CR-BSI, epidemiological studies have indicated that Candida spp. are the fourth leading cause [1], [2].

Biofilm formation by pathogenic microorganisms, including C. albicans, plays a key role in infection of intravascular devices and contributes to the difficulty in the management of these device-related infections [3], [4], [5], [6], [7], [8], [9]. By the mid 1980s it had been established that bacteria embedded in biofilms (sessile form) were particularly resistant to conventional antibiotics compared with their planktonic (free culture) form [9]. Candida albicans biofilms were also found to have greater resistance to the majority of the antifungal agents used in the mid 1990s [10]. Thus, current clinical guidelines recommend removal of these devices as part of the management of CR-BSI caused by Candida spp. whenever feasible [11].

Prior to the availability of echinocandins and azoles, the polyene antifungal agent amphotericin B (AmB) was the major antifungal agent used in the management of invasive Candida infections. The high incidence of nephrotoxicity of AmB led to the evaluation of combination therapies in the hope of decreasing the amount of AmB required to treat fungal infections thus minimising the risk of nephrotoxicity. These studies performed in the 1970s and 1980s revealed that antibiotics such as rifampicin and tetracyclines enhanced the effect of AmB against various pathogenic yeasts in their planktonic form in vitro [12], [13], [14], [15], [16], [17].

Since then, it has been determined that echinocandins and lipid formulations of AmB have activity against C. albicans biofilms [18], [19]. El-Azizi [20] showed that rifampicin and doxycycline (DOX) enhanced the activity of AmB against non-albicansCandida spp. biofilms in vitro. Recently, our laboratory has shown that DOX enhances the activity of certain antifungal agents and has intrinsic activity against C. albicans biofilms [21].

Thus, we screened several standard Gram-positive antibacterial agents, including azithromycin (AZM), tigecycline (TIG) and vancomycin (VAN), against C. albicans biofilms. Of these antibiotics, it was found that TIG had the greatest antifungal activity against mature C. albicans biofilms. Therefore, we chose to study TIG in detail and to characterise its activity alone and in combination with AmB, caspofungin (CAS) and fluconazole (FLC) against C. albicans biofilms.