The effect of antibiotic exposure on adherence to neutrophils of Enterococcus faecium resistant to phagocytosis.

Many clinical isolates of Enterococcus faecium are resistant to neutrophil-mediated phagocytosis and killing. As antibiotic exposure may alter bacterial surface properties and promote phagocytosis, we used a fluorescence microscopy assay to examine the effect of antibiotic pretreatment on the resistance to phagocytosis of six strains of E. faecium. Using two antimicrobial agents with good in-vitro activity against E. faecium, namely quinupristin/ dalfopristin and sparfloxacin, we found that exposure to quinupristin/dalfopristin at concentrations both below and above the MIC promoted bacterial adherence to neutrophils (PMNs) for all of three strains of vancomycin-susceptible E. faecium, while sparfloxacin was similarly effective in two of these three strains. In contrast, neither antibiotic was effective in promoting PMN adherence for three vancomycin-resistant strains of E. faecium. The variability amongst strains in response to antibiotic exposure suggests that either the mechanisms of resistance to phagocytosis, or its regulation, may be different amongst different strains of E. faecium.