Biofilms and antifungal drug resistance

L. J. Douglas

Author address: 

Division of Infection and Immunity, University of Glasgow, Glasgow, UK

Abstract: 

Biofilms are structured microbial communities that are attached to a surface and encased within a matrix of extracellular polymers. Individual biofilm organisms characteristically display a phenotype markedly different from that of planktonic (free-floating) cells; most importantly, they are significantly less susceptible to antimicrobial agents. Recent estimates suggest that a substantial proportion of human infections involve biofilms. Many of these are implant-related infections in which adherent microbial populations are found on the surfaces of devices such as catheters, prosthetic heart valves and joint replacements. Because of the drug resistance of biofilm organisms, and the ability of biofilms to withstand host immune defences, these infections are usually extremely difficult to treat. Most fungal biofilm research to date has concentrated on Candida albicans and related Candida species. Resistance of Candida biofilms in vitro to clinically important antifungal agents such as amphotericin B, fluconazole, flucytosine and itraconazole was first demonstrated in 1995 and has been subsequently confirmed by several research groups using a variety of biofilm model systems. Newer azoles (voriconazole and ravuconazole) are also ineffective against biofilms. The molecular basis of this phenotypic drug resistance is not understood and is likely to be multifaceted. Possible mechanisms include (i) restricted penetration of drugs through the biofilm matrix; (ii) phenotypic changes resulting from a decreased growth rate or nutrient limitation; and (iii) expression of resistance genes induced by contact with a surface. The biofilm matrix does not appear to constitute a major barrier to drug penetration since C. albicans biofilms with little or no matrix material are just as resistant as those with an extensive matrix. Moreover, high levels of drug diffusion through biofilms fail to produce complete killing of biofilm cells. Similarly, drug resistance is not simply attributable to a low growth rate because biofilm organisms grown in a perfused biofilm fermenter are resistant over a range of growth rates. Upregulation of genes coding for multidrug efflux pumps would result in a multidrugresistant phenotype. To date, evidence for this as a resistance mechanism is equivocal. Although genes encoding efflux pumps appear to be upregulated during biofilm formation, mutants carrying deletions in these genes are susceptible to antifungal agents when growing planktonically but retain their resistant phenotype during biofilm growth. There is, however, evidence that mature biofilm cells contain decreased levels of ergosterol, a factor which might contribute to their drug resistance. Some newer antifungal agents, such as the echinocandins (caspofungin and micafungin) and liposomal formulations of amphotericin B, show activity against Candida biofilms. Aspirin and other non-steroidal anti-inflammatory drugs, which inhibit fungal prostaglandin synthesis, also display antibiofilm activity in vitro. Finally, inhibitors of quorum sensing, a cellcell signalling mechanism, may be effective against biofilms as has already been demonstrated for bacteria. In C. albicans two quorum sensing signal molecules have been identified: farnesol, which acts as a negative signal and inhibits the formation of hyphae, and tyrosol which acts as a positive signal and promotes hyphal formation. Both of these molecules are likely to be involved in biofilm development and maturation; compounds which antagonize their action could therefore represent novel antifungal/antibiofilm agents. References Alem MAS, Douglas LJ. Effects of aspirin and other nonsteroidal anti-inflammatory drugs on biofilms and planktonic cells of Candida albicans. Antimicrob Agents Chemother 2004; 48: 417. Al-Fattani MA, Douglas LJ. Penetration of Candida biofilms by antifungal agents. Antimicrob Agents Chemother 2004; 48: 32917. Baillie GS, Douglas LJ. Candida biofilms and their susceptibility to antifungal agents. Methods Enzymol 1999; 310: 64456. Douglas LJ. Candida biofilms and their role in infection. Trends Microbiol 2003; 11: 306. Mukherjee PK, Chandra J, Kuhn DM, Ghannoum MA. Mechanism of fluconazole resistance in Candida albicans biofilms: phase-specific role of efflux pumps and membrane sterols. Infect Immun 2003; 71: 433340. Ramage G, Bachmann S, Patterson TF, Wickes BL, Lopez-Ribot JL. Investigation of multidrug efflux pumps in relation to fluconazole resistance in Candida albicans biofilms. J Antimicrob Chemother 2002; 49: 97380.
2005

abstract No: 

S06.4

Full conference title: 

2nd Trends in Medical Mycology
    • TIMM 2nd (2010)