High-density fecal Enterococcus faecium colonization in hospitalized patients is associated with the presence of the polyclonal subcluster CC17.

Enterococcus faecium belonging to the polyclonal subcluster CC17, with a typical ampicillin-resistant E. faecium (AREfm) phenotype, have become prevalent among nosocomial infections around the world. High-density intestinal AREfm colonization could be one of the factors contributing to the successful spread of these pathogens. We aimed to quantify the enterococcal intestinal colonization densities in stool samples from AREfm-colonized and non-colonized patients using fluorescent in situ hybridization (FISH). Stool samples were collected from AREfm-colonized (n = 8) and non-colonized (n = 8) patients. The relative number of Enterococcus faecalis and E. faecium was determined by FISH using specific 16S rRNA probes, while the total amount of bacterial cells was counted by staining the sample with 4',6-diamidino-2-phenylindole (DAPI). The median bacterial cell numbers in fecal samples, counted by DAPI staining, were 7.7 × 10(9) and 4.8 × 10(9) cells/g for AREfm-colonized and non-colonized patients, respectively (p = 0.34). The E. faecium densities in AREfm-colonized patients, accounting for 0.5-7% of all fecal bacterial cells, exceeded E. faecalis levels by over ten-fold. E. faecium was not detected in non-colonized patients. This study demonstrated high E. faecium cell densities in stool samples from patients colonized with AREfm. Increased cell densities may contribute to host-to-host transmission and environmental contamination, facilitating the spread of AREfm in the hospital setting.