Survival of Enterococcus faecalis in mouse peritoneal macrophages

Enterococcus faecalis was tested for the ability to persist in mouse perito neal macrophages in two separate studies. In the first study, the intracell ular survival of serum-passaged E. faecalis 418 and two isogenic mutants [c ytolytic strain FA2-2(pAM714) and non-cytolytic strain FA2-2(pAM771)] was c ompared with that of Escherichia coli DH5 alpha by infecting BALB/c mice in traperitoneally and then monitoring the survival of the bacteria within lav aged peritoneal macrophages over a 72-h period. All E. faccalis isolates we re serum passaged to enhance the production of cytolysin. E. faecalis 418, FA2-2(pAM714), and FA2-2(pAM771) survived at a significantly higher level ( P = 0.0001) than did E. coli DH5 alpha at 24, 48, and 72 h. Internalized E. faecalis 418, FA2-2(pAM714), and FA2-2(pAM771) decreased 10-, 55-, and 31- fold, respectively, over the 72-h infection period, while internalized E. c oli DH5 alpha decreased 20,542-fold. The difference in the rate of survival of E. faecalis strains and E. coli DH5 alpha was most prominent between 6 and 48 h postinfection (P = 0.0001); however, no significant difference in killing was observed between 48 and 72 h postinfection. In the second study , additional E. faccalis strains from clinical sources, including DS16C2, M GH-2, OG1X, and the cytolytic strain FA2-2(pAM714), were compared with the nonpathogenic gram-positive bacterium, Lactococcus lactis K1, for the abili ty to survive in mouse peritoneal macrophages. In these experiments, the E. faccalis strains and L. lactis K1 were grown in brain heart infusion (BHI) broth to ensure that there were equal quantities of injected bacteria. E. faecalis FA2-2(pAM714), DS16C2, MGH-2, and OG1X survived significantly bett er (P < 0.0001) than did L. lactis K1 at each time point. L. lactis K1 was rapidly destroyed by the macrophages, and by 24 h postinfection, viable L. lactis could not be recovered. E. faecalis FA2-2(pAM714), DS16C2, MGH-2, an d OG1X declined at an equivalent rate over the 72-h infection period, and t here was no significant difference in survival or rate of decline among the strains. E. faecalis FA2-2(pAM714), MGH-2, DS16C2, and OG1X exhibited an o verall decrease of 25-, 55-, 186-, and 129-fold respectively, between 6 and 72 h postinfection. The overall reduction by 1.3 to 2.27 log units is slig htly higher than that seen for serum-passaged E. faecalis strains and may b e attributable to the higher level of uptake of serum-passaged E. faccalis than of E. faecalis grown in BHI broth. Electron microscopy of infected mac rophages revealed that E. faecalis 418 was present within an intact phagocy tic vacuole at 6 h postinfection but that by 24 h the infected macrophages were disorganized, the vacuolar membrane was degraded, and the bacterial ce lls had entered the cytoplasm. Macrophage destruction occurred by 48 h, and the bacteria were released. In conclusion, the results of these experiment s indicate that E. faecalis can persist for an extended period in mouse per itoneal macrophages.