Occult Vancomycin-Resistant ST117 Displaying a Highly Mutated Operon.

Rare information is available on clinical encountered in Sardinia, Italy. This study investigated the antimicrobial susceptibility profiles and genotypic characteristics of isolated at the University Hospital of Sassari, Italy, using the Vitek2 system and PCR, MLST, or WGS. Vitek2 revealed two VanB-type vancomycin-resistant (VREfm) isolates (MICs mg/L = 8 and ≥32) but failed to detect vancomycin resistance in one isolate (MIC mg/L ≤ 1) despite positive genotypic confirmation of gene, which proved to be vancomycin resistant by additional phenotypic methods (MICs mg/L = 8). This isolate was able to increase its vancomycin MIC after exposure to vancomycin, unlike the "classic" occult -carrying , becoming detectable by Vitek 2 (MICs mg/L ≥ 32). All three had highly mutated operons, as part of a chromosomally integrated Tn transposon, with common missense mutations in VanH and VanB resistance proteins and specific missense mutations in the VanW accessory protein. There were additional missense mutations in VanS, VanH, and VanB proteins in the -carrying VREfm isolates compared to Vitek2. The molecular typing revealed a polyclonal hospital-associated population from Clade A1, and that -VREfm, and nearly half of vancomycin-susceptible (VSEfm) analyzed, belonged to ST117. Based on core genome-MLST, ST117 strains had different clonal types (CT), excluding nosocomial transmission of specific CT. Detecting -carrying VREfm isolates by Vitek2 may be problematic, and alternative methods are needed to prevent therapeutic failure and spread.