RESISTANCE OF ENTEROCOCCUS-FAECIUM TO NEUTROPHIL-MEDIATED PHAGOCYTOSIS

During a previous study of the opsonic requirements for neutrophil (po lymorphonuclear leukocyte [PMN])-mediated killing of enterococci, we i dentified two strains of Enterococcus faecium (TX0015 and TX0016) that were resistant to PMN-mediated killing. To better define the mechanis m of this resistance, we examined phagocytosis with a fluorescence ass ay and found that TX0016 was completely resistant to phagocytosis by P MNs; this finding was confirmed by electron microscopy. Examination of multiple strains of enterococci revealed that all 20 strains of Enter ococcus faecalis tested were readily phagocytosed (mean, 18 intracellu lar organisms per PMN; range, 7 to 28). In contrast, only 13 (50%) of 26 strains of E. faecium tested were susceptible to phagocytosis (grea ter than or equal to 7 organisms per PMN); the other 13 strains showed less than or equal to 3 organisms per PMN. Enterococcus casseliflavus ATCC 25788 and one strain of Enterococcus hirae were also resistant t o phagocytosis, while two strains of Enterococcus durans, Enterococcus mundtii ATCC 43186, and one strain each of Enterococcus raffinosus an d Enterococcus solitarius were readily phagocytosed. Exposure of E. fa ecium TX0016 to sodium periodate, but not to the protease trypsin or p ronase or to phospholipase C, eliminated resistance to phagocytosis. S ialic acid, a common periodate-sensitive structure used by microorgani sms to resist opsonization, could not be demonstrated in E. faecium TX 0016 by the thiobarbituric acid method, nor was phagocytosis of TX0016 altered by neuraminidase treatment, This study suggests that there is a difference in susceptibility to phagocytosis by PMNs between differ ent species of enterococci and that a carbohydrate-containing moiety w hich is not sialic acid may be involved in the resistance of E. faeciu m TX0016 to phagocytosis.