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Acquired vanA- and vanB-mediated glycopeptide resistance has been well described in Enterococcus faecalis and Enterococcus faecium, and high-level vanA-mediated resistance has been described for Enterococcus gallinarum and Enterococcus casseliflavus (3, 5). However, reports of vanB-mediated resistance in E. gallinarum are sporadic: 2 isolates from Switzerland (6) (one patient was cocolonized with vanB E. faecium), one isolate from Australia (10), and an isolate in one poultry carcass from Sicily (7). We write to report a further case of an infection by a strain of Enterococcus gallinarum containing both vanB and vanC1 genes in an Australian hospital.
The patient, a 67-year-old female, had undergone single-lung transplant for end-stage chronic obstructive pulmonary disease. Postoperative complications included pulmonary hemorrhage in the native lung, chylothorax, persisting infiltrates within the donor lung, and systemic inflammatory response syndrome; prolonged mechanical ventilation was required in the intensive care unit (ICU). Multiresistant methicillin-resistant Staphylococcus aureus (MRSA) was cultured from bronchoalveolar lavage fluid on day 10, vancomycin was commenced, and treatment continued for more than 6 weeks because tracheal aspirates and bronchial washings remained culture positive and histological sections of transbronchial biopsies showed acute epithelial inflammation. Trough vancomycin levels were maintained at >15 mg/liter for this entire period, except for 6 days when acute renal failure mandated treatment interruption.
Rectal swab screening for carriage of vancomycin-resistant enterococci (VRE) was performed 1 week posttransplant with negative results. One month after admission, repeat rectal screening for VRE resulted in the isolation of an enterococcus from an Enterococcosel agar plate (BBL, Cockeysville, MD) containing 6 mg/liter vancomycin. The isolate had typical gram morphology and was pyrrolidonyl peptidase positive. A vancomycin MIC of 32 mg/liter and teicoplanin MIC of 0.125 mg/liter were suggestive of a vanB genotype. The presence of a vanB determinant was confirmed by multiplex PCR using the LightCycler VRE detection kit (Roche Molecular Systems, Branchburg, NJ). However, the isolate was subsequently identified by Vitek 2 Compact (bioMerieux, Marcy l'Etoile, France) as E. gallinarum, and motility was demonstrated. To resolve the discrepancy between the phenotypic and genotypic tests, repeat identification was performed using API 20 Strep (bioMerieux, Marcy l'Etoile, France), resulting in an identification of E. faecium, although E. gallinarum was given as a possibility if vancomycin resistance had been demonstrated.
As the Roche VRE detection kit only detects vanA, vanB, and vanB2 and -3 determinants, and to eliminate the possibility of a mixed culture, further phenotypic and genotypic tests were performed. A single colony was used both for the inoculation of VRE chromogenic media and for performance of identification and resistance gene typing by in-house nucleic acid amplification methods (4, 8). Growth and color reactions on Chrom ID VRE (bioMerieux, Marcy l'Etoile, France) and Brilliance VRE (Oxoid, Basingstoke, United Kingdom) indicated that the isolate was a vancomycin-resistant E. faecium strain. However, the in-house multiplex PCRs demonstrated both a lack of E. faecalis and E. faecium genetic elements and the presence of vanB and vanC1 determinants. Finally, 16S rRNA sequencing of the isolate was undertaken at a local reference laboratory (Microbiological Diagnostic Unit, University of Melbourne), showing the closest matches to strains of E. gallinarum and E. casseliflavus. The lack of pigment confirmed the isolate as a vanB E. gallinarum strain. A review of all VRE-positive rectal screens from our ICU 3 months pre- and postisolation of this E. gallinarum isolate failed to identify any other cases.
Previous reports have identified discrepancies between phenotypic and genotypic test results, particularly for E. faecium and E. gallinarum (6, 9). E. gallinarum is a relatively uncommon isolate from humans, although Australian surveillance has identified it at a higher frequency than other regions (1). It has the potential for nosocomial spread and infection outbreaks (2). Our case highlights the ability of E. gallinarum to acquire resistance elements in the same way as other enterococci, particularly when under antibiotic selection pressure. Speciation of enterococci in the laboratory has implications for hospital epidemiology, and confirmation of E. gallinarum requires multiple processes.
Published ahead of print on 31 March 2010.