Enhancement of biofilm formation by subinhibitory concentrations of macrolides in icaADBC-positive and -negative clinical isolates of Staphylococcus epidermidis

Abstract

Biofilm formation in Staphylococcus epidermidis is mediated by icaADBC-dependent and -independent pathways. Subinhibitory concentrations of erythromycin, azithromycin, and clarithromycin enhanced, in a dose-dependent manner, the level of biofilm formation by 20% (21/105 isolates) by macrolide-resistant ica-positive and -negative isolates tested in vitro. The presence of ica, however, apparently produced an enhanced effect on biofilm formation. The levels of expression of the biofilm-related genes icaA, atlE, fruA, pyrR, sarA, and sigB were increased in response to erythromycin. The results likely underscore the potential clinical relevance of macrolide-induced biofilm growth.

FIG. 1.

Biofilm formation by icaA-positive isolate SW029 (columns 2 to 4) and icaA-negative isolate SW036 (columns 5 to 7) determined by a microtiter plate assay. Columns: 1, blank; 2 and 5, no antibiotics; 3 and 6, VAN at 1/4× MIC (0.25 and 0.5 μg/ml, respectively); 4 and 7, ERY at 1/4× MIC (32 μg/ml). The photo was taken from the back of the plate.

FIG. 2.

Dose-dependent effects of macrolides at subinhibitory concentrations on biofilm formation of icaA-positive isolate SW029 and icaA-negative isolate SW036 determined by a microtiter plate assay. *, compared with the results for the relevant controls, P < 0.05.

FIG. 3.

Microscopic observation of biofilm formation of ica-positive isolate SW029 in the absence (A, C, E, G, and I) or presence (B, D, F, H, and J) of ERY at 1/4× MIC. (A to F) At time points of 1 h (A and B), 6 h (C and D), and 12 h (E and F), bacterial cells were observed by CLSM. The polysaccharides are stained green, and the cells without polysaccharides are stained red. Bars, 10 μm. (G to J) At 24 h (G and H) and 48 h (I and J), cells/biofilms were observed by SEM. Bars, 5 μm.