Daptomycin exerts bactericidal activity without lysis of Staphylococcus aureus

Abstract

The ability of daptomycin to produce bactericidal activity against Staphylococcus aureus while causing negligible cell lysis has been demonstrated using electron microscopy and the membrane integrity probes calcein and ToPro3. The formation of aberrant septa on the cell wall, suggestive of impairment of the cell division machinery, was also observed.

FIG. 1.

Culture density (OD₆₀₀) and viability were monitored during daptomycin treatment to allow correlation of bactericidal activity and lysis. At 4 μg/ml, daptomycin exhibited significant bactericidal activity with no change in OD₆₀₀.

FIG. 2.

TEM of S. aureus treated with daptomycin (4 μg/ml; 60 minutes) (A) or the control, demonstrating lack of lysis (B). Aberrant division septa and multilobate morphology are visible in >90% of daptomycin-treated cells in this image.

FIG. 3.

(A) Calcein release was monitored following treatment of S. aureus with daptomycin (2 μg/ml), nisin (12.5 μg/ml), and lysostaphin (100 μg/ml). Extracellular dye was separated from cells by passage through a 0.2 μM filter. Data are expressed as ratios of intracellular to extracellular fluorescence. (B) Viability was measured by plating dilute samples (to eliminate antibiotic carryover) on tryptic soy agar.

FIG. 4.

ToPro3 assay showing very similar permeability profiles for daptomycin and the control. Uptake of ToPro3 was monitored by fluorescence-activated cell sorting following treatment with daptomycin (5 μg/ml), ciprofloxacin (2 μg/ml), nisin (25 μg/ml), CCCP (10 μM), and A23187 (1 μg/ml).