Antibiotic saving effect of combination therapy through synergistic interactions between well-characterized chito-oligosaccharides and commercial antifungals against medically relevant yeasts

Abstract

Combination therapies can be a help to overcome resistance to current antifungals in humans. The combined activity of commercial antifungals and soluble and well-defined low molecular weight chitosan with average degrees of polymerization (DPn) of 17-62 (abbreviated C17 -C62) and fraction of acetylation (FA) of 0.15 against medically relevant yeast strains was studied. The minimal inhibitory concentration (MIC) of C32 varied greatly among strains, ranging from > 5000 μg mL-1 (Candida albicans and C. glabrata) to < 4.9 (C. tropicalis). A synergistic effect was observed between C32 and the different antifungals tested for most of the strains. Testing of several CHOS preparations indicated that the highest synergistic effects are obtained for fractions with a DPn in the 30-50 range. Pre-exposure to C32 enhanced the antifungal effect of fluconazole and amphotericin B. A concentration-dependent post-antifungal effect conserved even 24 h after C32 removal was observed. The combination of C32 and commercial antifungals together or as part of a sequential therapy opens new therapeutic perspectives for treating yeast infections in humans.

Fig 1

Inhibition of C. albicans (a, b) and C. guillermondii (c, d) when exposed to sub-inhibitory concentrations of CA and C32 at 0.25 x MIC (a, c) or MICC (b, d). The CA were applied at MICC, i.e. at concentrations that hardly inhibit the yeasts. The graphs show the amount of viable cells over time. Color coding is provided in the Figure. Standard deviations are omitted for clarity. Normally, these were between 0.1 to 0.5 Log CFU ml^-1 units.

Fig 2

Sequential therapy time-kill curve for C. albicans (a) and C. guillermondii (b) in the presence of amphotericin B or fluconazole (concentration = 0.25 x MIC) after 12 h pre exposure to C32 (concentration = 0.25 x MIC). Color coding is provided in the Figure. Standard deviations are omitted for clarity. Normally, these were between 0.1 to 0.5 Log CFU ml^-1 units.