Introduction

Vancomycin-resistant Enterococcus faecium (VREfm) are emerging multidrug-resistant pathogens. The number of nosocomial infections caused by VREfm is increasing, posing a major threat worldwide. VREfm colonization of the gastrointestinal tract often precedes the development of invasive infection. The underlying mechanism by which a colonizing strain becomes invasive in terms of phenotypic changes is still not clear. The aim of this study was to investigate the phenotypic differences between colonizing and infection isolates of VREfm.

Materials & Methods

In total 27 VREfm isolate pairs were collected from patients who were initially colonized with VREfm and subsequently developed a bloodstream infection (BSI). Each isolate pair consisted of one colonizing isolate (rectal swab) and one subsequent BSI isolate. First, the ability of VREfm isolates to form biofilm was compared. For this, overnight cultures were grown in tryptic soy broth (TSB) and diluted in a 96-well plate. After incubation, the biofilms were stained with crystal violet and OD₅₉₅ was measured. A spot-killing assay was performed to assess the competition between the paired isolates. Each colonizing isolate was spotted on Brain Heart Infusion (BHI) agar containing the corresponding infection strain and vice versa.

Results

The mean OD₅₉₅ measured of all isolates indicated an overall limited ability to form robust biofilms. In total, four isolates (three invasive and one colonizing isolate) significantly exceeded this mean by 2.1–3.1 times (p=0.002–0.03). In the spot killing assay four isolates were identified which lysed the corresponding strain. Of these, three were colonizing, while one was an invasive isolate. Thus, no statistically significant increase in lysis of either the colonizing or the infection strain was observed.

Summary

Our preliminary data showed no significant phenotypic differences between colonizing and infection isolates with respect to biofilm formation and competition. However, some isolates showed an increased biofilm or lysis by others. Further investigation is necessary to address the underlying mechanisms and to correlate phenotypic with genotypic data.