A CHROMagar screening grows blue colonies. What do you do?

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Multiple Choice

A CHROMagar screening grows blue colonies. What do you do?

Blue colonies on a CHROMagar VRE screen point to vanA/vanB-mediated vancomycin resistance in Enterococcus. This color pattern reflects the specific chromogenic reaction tied to those resistance genes, and it signals a clinically significant, transferable resistance that warrants immediate confirmation and infection-control actions. The next step is to confirm resistance with a second test, such as subculturing the isolate onto Brain Heart Infusion with vancomycin (and a control plate) to see if growth persists in the presence of vancomycin. If growth occurs, you’ve confirmed a VRE isolate and should report it to the nursing floor and infection-control/public-health authorities per protocol. For species identification, arabinose fermentation helps distinguish common enterococci: Enterococcus faecalis typically does not ferment arabinose, while Enterococcus faecium does. If the results point toward other vanC-containing species (like E. casseliflavus or E. gallinarum), pigment and rapid xylose tests can help differentiate them, but these vanC organisms carry intrinsic, usually lower-level resistance and are managed differently from vanA/vanB VRE. The important thing is that blue indicates a vanA/vanB VRE scenario, which requires confirmation and proper public-health reporting rather than ignoring it or assuming a less problematic organism.

Blue colonies on a CHROMagar VRE screen point to vanA/vanB-mediated vancomycin resistance in Enterococcus. This color pattern reflects the specific chromogenic reaction tied to those resistance genes, and it signals a clinically significant, transferable resistance that warrants immediate confirmation and infection-control actions.

The next step is to confirm resistance with a second test, such as subculturing the isolate onto Brain Heart Infusion with vancomycin (and a control plate) to see if growth persists in the presence of vancomycin. If growth occurs, you’ve confirmed a VRE isolate and should report it to the nursing floor and infection-control/public-health authorities per protocol.

For species identification, arabinose fermentation helps distinguish common enterococci: Enterococcus faecalis typically does not ferment arabinose, while Enterococcus faecium does. If the results point toward other vanC-containing species (like E. casseliflavus or E. gallinarum), pigment and rapid xylose tests can help differentiate them, but these vanC organisms carry intrinsic, usually lower-level resistance and are managed differently from vanA/vanB VRE. The important thing is that blue indicates a vanA/vanB VRE scenario, which requires confirmation and proper public-health reporting rather than ignoring it or assuming a less problematic organism.

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