Electron microscopy studies of the bactericidal effects of quinupristin/dalfopristin on Staphylococcus aureus

J Antimicrob Chemother 1999; 43: 845- 846 Sir, Quinupristin/dalfopristin is a combination of two semisynthetic derivatives of pristinamycin which has been shown to have activity in vitro against a wide range of Gram-positive cocci. 1 Preliminary studies in our laboratory of its activity against staphylococci demonstrated that, following exposure to concentrations <MICs, the bacterial cells appeared larger and stained more intensely than controls. A further study of bacterial ultrastructure revealed that incubation of Staphylococcus aureus in the presence of quinupristin/dalfopristin resulted in two major cell alterations: increases in cell size and the thickness of the cell wall; the greatest increase in cell size was observed after exposure to a concentration equivalent to 0.5 × MIC (P< 0.005). 2 Electron micrographs taken after 24 h exposure to the combination at this concentration showed breaks in the walls of several cells and ghosts of lysed cells, in addition to larger cells. Our earlier study 2 was repeated with the aim of performing viable counts and quantifying breaks in the cell wall following incubation of a strain of S. aureus in the presence of 0.5 × MIC of quinupristin/ dalfopristin. Quinupristin/dalfopristin was provided by Rhône-Poulenc Rorer (Vitry sur Seine, France) and S. aureus ATCC 25923 was obtained from Difco (Detroit, MI, USA). The S. aureus strain was incubated without quinupristin/ dalfopristin (control) and in the presence of the antibiotic at a concentration of 0.2 mg/L (equivalent to 0.5 × MIC) as described previously. 2 Aliquots were withdrawn after incubation at 36°C for 6, 12 and 24 h and the numbers of viable bacteria were determined. Samples of the 24 h culture were also examined by electron microscopy according to a method described by Lorian et al.2 The extent of the cell-wall alterations was assessed by examining photographs of fields containing 30- 40 cells at ×21,000 magnification. Following exposure of S. aureus to quinupristin/ dalfopristin for 6 h and 12 h, the numbers of viable bacteria fell by 99% and 99.9%, respectively, compared with the control; after 24 h, the viable count increased by 0.5 log 10. In a previous study, incubation of some strains of Enterococcus faecium in the presence of the combination at a concentration equivalent to 0.3 × MIC was associated with 99.9% decreases in the numbers of viable organisms. 3 The apparent paradox that exposure to concentrations