Seeking Vancomycin Resistant Staphylococcus

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Seeking Vancomycin Resistant Staphylococcus aureus among Patients with Vancomycin-Resistant Enterococci Diane Franchi,1,2 Michael W. Climo,2 Alice H. M. Wong,1,2 Michael B. Edmond,1,2 and Richard P. Wenzel1,2

From the 1Division of Quality Health Care, 2Department of Internal Medicine, Medical College of Virginia/Virginia Commonwealth University, Richmond, Virginia

The emergence of vancomycin-resistant enterococci (VRE) in 1988 heightened concern that vancomycin resistance would spread to Staphylococcus aureus [1]. In 1992, vancomycin resistance was transferred from Enterococcus faecalis to S. aureus in the laboratory by cell-to-cell mating [2]. The transfer of resistance was thought to be transposon-mediated, but this was not established definitively. More recently, the selection of S. aureus mutants highly resistant to vancomycin was achieved in the laboratory by gradually exposing these mutants to increasing concentrations of vancomycin during growth [3]. The first clinical isolate of S. aureus with reduced susceptibility to vancomycin (MIC of 4 mg/mL) was reported from Japan in 1997 [4]. Soon afterward, similar isolates were found in Michigan and New Jersey [5, 6]. Recently a patient from New York was described [7, 8]. All strains were also resistant to methicillin and were isolated from patients who had received multiple courses of vancomycin [9]. In France, however, the first clinical isolate of S. aureus with intermediate susceptibility to vancomycin (VISA) was cultured from the blood of a 2year-old girl with leukemia who had had only one 10-day course of vancomycin [10]. Even though VISA are rare causes of clinical infections, we sought VISA colonization by surveying a high-risk hospitalized Received 29 March 1999; revised 10 August 1999. Financial support: Rhoˆne-Poulenc-Rhorer. Reprints or correspondence: Dr. Diane Franchi, Eastern Virginia Medical School, 825 Fairfax Ave., Norfolk, VA 23507 ([email protected]). Clinical Infectious Diseases 1999; 29:1556–8 q 1999 by the Infectious Diseases Society of America. All rights reserved. 1058-4838/1999/2906-0038$03.00

population. The population included patients who were infected and/or colonized with VRE, those likely to have been exposed to vancomycin.

Methods Hospital Survey

This study was conducted at the Medical College of Virginia Hospital from March to May of 1997. Perianal swabs were used to detect VRE colonization, and swabs of the nares were used to detect S. aureus colonization. There were 2 parts to the hospital survey. First, patients known to be colonized with VRE had their nares swabbed weekly to screen for co-colonization with S. aureus. Second, perianal swabs and swabs of the nares were obtained from patients who occupied 5 targeted units and were not known to be colonized with VRE. The targeted units included 3 intensive care units, the hematology/oncology unit, and the bone marrow–transplant unit.

Microbiology

Enterococci. Perianal swabs were screened on bile-esculinazide agar containing 6 mg/mL vancomycin. Colonies representative of enterococcus were subcultured onto brain-heart infusion agar, and gram staining and catalase testing were done. Identification was carried out with API Strep Strips (bioMe´rieux, Marcy l’Etoile, France). Etest (AB BIODISK, Solna, Sweden) strips were used to determine vancomycin MICs in

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Clinical isolates of Staphylococcus aureus displaying intermediate resistance to vancomycin (VISA) have been identified. The objective of our study was to identify VISA colonization among patients known to be colonized or infected with vancomycin-resistant enterococci (VRE). Eight weekly point prevalence screening surveys for VRE and S. aureus were conducted on 5 hospital units. Of the 243 patients screened, 31 (12.8%) were colonized with VRE. In addition, 18 inpatients were already known to be VRE-positive. Fourteen (28.6%) of the 49 VRE-positive patients were co-colonized with S. aureus. All 30 S. aureus isolates from these 14 patients were methicillin-resistant (MRSA) but remained vancomycin-susceptible (minimal inhibitory concentration [MIC] range, 0.75–2 mg/mL). Population analysis profiling demonstrated no evidence of heteroresistant subpopulations that could grow on agar containing 3 mg/mL vancomycin for any of the MRSA isolates. Although 23 (77%) of 30 staphylococcal isolates had vancomycin MICs of 1.5 or 2 mg/mL, no VISA strains (MICs, 8–16 mg/mL) were recovered.

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Seeking Vancomycin-Resistant S. aureus

Vancomycin Use

The total amount of vancomycin received by each patient colonized with MRSA was tallied for the 6 months prior to the dates of collection of the MRSA isolates. This information was retrieved through the Medical College of Virginia’s computerized patient records.

mL by Etest showed growth on agar plates with 3.0 mg/mL vancomycin. Eleven of 14 patients had received vancomycin in the 6 months prior to the MRSA isolate collection dates. The total dose of vancomycin received by each patient ranged from 0 to 81.5 g (mean, 24.6 g; median, 5 g). Five patients were receiving vancomycin at the time the MRSA isolates were collected. Discussion In the United States, clinical isolates of VISA have been recovered from patients in the Midwest (Michigan) and the Northeast (New Jersey and New York). None, however, have been identified in the Southeast. We performed prospective surveys for clinical VISA strains among patients colonized with VRE, knowing that these patients are often exposed to vancomycin. However, no VISA isolates were identified in our small study of 261 patients. There are 3 other important points to emphasize. First, not surprisingly, patients who are identified as colonized or infected with VRE after culturing for clinical reasons account for only a small fraction of patients who are colonized with VRE on a ward at one time, only 16% in our series. Second, the patients who were co-colonized with VRE and S. aureus in our study were all co-colonized with MRSA. Third, the vast majority of patients co-colonized with VRE (23 [77%] of 30) had staphylococcal isolates with vancomycin MICs of 1.5 or 2 mg/mL. Future work will examine the sequence in which patients become colonized with VRE and MRSA and the possibility of stepwise increases in vancomycin MICs among patients with VRE.

References

Results A total of 310 perianal swabs were obtained from 243 patients occupying the target units. Thirty-one patients (12.8%) were colonized with VRE. An additional 18 patients were already known through the clinical laboratory to be colonized or infected with VRE. Six of these patients were located on the target wards. Therefore, the clinical laboratory identified only 16% of patients (6/37) on the study wards that were colonized with VRE. Fourteen (28.6%) of the 49 patients who had a culture positive for VRE were co-colonized with S. aureus. All 30 isolates of S. aureus collected from these 14 patients were MRSA. Vancomycin susceptibilities determined by Etest showed 1 isolate with a MIC of 0.75 mg/mL, 6 with MICs of 1.0 mg/mL, 15 with MICs of 1.5 mg/mL, and 8 with MICs of 2.0 mg/mL. Therefore, 23 (77%) of 30 staphylococcal isolates had vancomycin MICs of 1.5 or 2.0 mg/mL. None of the isolates with a MIC >1.5 mg/

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accordance with National Committee for Clinical Laboratory Standards guidelines [11]. Enterococci with MICs >4 mg/mL were considered resistant to vancomycin. Staphylococci. Material obtained by swabbing the nares was initially screened on mannitol-salt agar. Colonies fermenting mannitol were subcultured onto trypticase soy agar. Gram staining and catalase and coagulase tests were conducted to confirm the identification of S. aureus. Vancomycin susceptibilities were determined by use of Etest strips in accordance with National Committee for Clinical Laboratory Standards guidelines [11]. Tenover et al. [12] have shown that vancomycin susceptibility testing with Etest strips can identify most strains of S. aureus with reduced susceptibility to that drug. S. aureus strains were also screened for oxacillin susceptibility by use of Mueller-Hinton agar containing 6 mg/mL oxacillin. Methicillin MICs were then determined by use of Etest strips. Staphylococci were considered resistant to methicillin (MRSA) if the MIC was >12 mg/mL. The staphylococcal isolates showing a vancomycin MIC >1.5 mg/mL by the Etest method were tested for the presence of heteroresistant subpopulations with higher levels of vancomycin resistance by population analysis profiles generated on MuellerHinton agar containing 1.0 mg/mL, 2.0 mg/mL, 2.5 mg/mL, and 3.0 mg/mL vancomycin, as described elsewhere [13].

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reduced susceptibility to vancomycin isolated from a patient with fatal bacteremia. Emerg Infect Dis 1999; 5:147–9. 9. Wenzel RP, Edmond MB. Vancomycin-resistant Staphylococcus aureus: infection control considerations. Clin Infect Dis 1998; 27:245–9. 10. Ploy MC, Grelaud C, Martin C, de Lumley L, Denis F. First clinical isolate of vancomycin-intermediate Staphylococcus aureus in a French hospital [letter]. Lancet 1998; 351:1212. 11. National Committee for Clinical Laboratory Standards. Methods for dilution antimicrobial susceptibility tests for bacteria that grow aerobically: ap-

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proved standard. 4th ed. Vol. 17, no. 2. NCCLS document M7-A4. Wayne, PA: National Committee for Clinical Laboratory Standards, 1997. 12. Tenover FC, Lancaster ML, Hill BC, Steward CD, Stocker SA, Hancock GA. Characterization of staphylococci with reduced susceptibilities to vancomycin and other glycopeptides. J Clin Microbiol 1998; 36:1020–7. 13. Climo MW, Patron RL, Archer GL. Combinations of vancomycin and beta-lactams are synergistic against staphylococci with reduced susceptibilities to vancomycin. Antimicrob Agents Chemother 1999; 43:1747–53.
