1. What type of antimicrobial susceptibility test was used to identify the VISA isolates?

   The VISA isolates were tested by the NCCLS broth microdilution method using MIC panels prepared in-house. The vancomycin MICs for both morphotypes of VISA were 4 μg/ml after 16 hours of incubation and 8 µg/ml after 24 hours of incubation. Additional testing to confirm reduced susceptibility to vancomycin included inoculation of a brain heart infusion (BHI) agar plate containing 6 µg/ml of vancomycin and a vancomycin Etest strip using a 0.5 McFarland inoculum. Growth was observed on the BHI vancomycin screen plate and the Etest MIC was 6 µg/ml. Both results are consistent with results of other VISA strains.
   
      



2. How were the ORSA colonies, which were vancomycin susceptible, distinguished from the oxacillin-resistant and oxacillin-susceptible VISA colonies?
   
   The vancomycin-susceptible ORSA isolate (labeled below as S. aureus #1) demonstrated typical yellowish, smooth, beta-hemolytic colonial morphology on the first day that the plates were examined. On the second day, additional yellowish, pinpoint, beta-hemolytic colonies were visible (S. aureus #2a). Broth microdilution susceptibility tests were set up on the second colony type. Examination of the MIC purity plate from S. aureus #2a after 24 hours of incubation revealed a third colony type, which was a grayish-white pinpoint colony (S. aureus #2b). These colonies were purified, confirmed biochemically as S. aureus, and tested for antimicrobial susceptibility. Isolate #2b was oxacillin susceptible and mecA negative.
   
   A summary of results for the three isolates follows:

3. Did both VISA isolates demonstrate the same results for other antimicrobial agents?

Yes. Both VISA isolates were susceptible to gentamicin, quinupristin-dalfopristin, tetracycline, and trimethoprim-sulfamethoxazole; intermediate to chloramphenicol and vancomycin; and resistant to ciprofloxacin, clindamycin, erythromycin, and rifampin.
 

4. At what point was the patient's physician and the infection control team notified that a possible VISA isolate was cultured from the patient?

The patient's physician and the infection control team were notified of a possible VISA isolate on the third day of the culture work-up when the susceptibility tests indicated that the vancomycin MIC of the second S. aureus isolate by broth microdilution testing was 8 μg/ml. A review of the purity plate and a Gram stain of the growth in the vancomycin wells of the MIC plate revealed only staphylococcus-like colonies and gram-positive cocci in clusters, respectively, suggesting that vancomycin-resistant contaminants were unlikely and that the isolate was a VISA. The infection control team and the patient's physician were notified to initiate an epidemiologic investigation into the case. MIC tests were repeated on day 3 and results were confirmed on day 4.

5. At what point were the VISA isolates sent to the public health department for confirmation?

Because the county public health department in the reporting
laboratory's jurisdiction was not equipped to perform confirmation of VISA phenotypes, the isolates were sent directly to CDC on day 5. CDC confirmed the isolates as VISAs by performing biochemical and antimicrobial susceptibility testing on the two isolates. Later, the isolates were also sent to the local public health laboratory. It is critically important to save all potential VISA and vancomycin-resistant S. aureus (VRSA) isolates and submit them to CDC for confirmation. In addition, it is important to determine if your local health department requires submission of the isolate to them.

Click here for additional information on testing for VISA and VRSA.

6. How do I send suspected VISA and VRSA isolates to CDC?

Click here for a protocol that can be used for saving and sending
VISA or VRSA isolates to CDC for confirmatory testing. To obtain instructions regarding the required information for submission, simply email your request to search@cdc.gov. You will be provided promptly with these instructions. 

References:

1.  Hageman, J. C., D. A. Pegues, C. Jepson, R. L. Bell, M. Guinan, K.
   W. Ward, M. D. Cohen, J. A. Hindler, F. C. Tenover, S. K. McAllister, M.
   E. Kellum, and S. K. Fridkin. 2001. Vancomycin-intermediate
   Staphylococcus aureus in a home health-care patient. Emerg. Infect. Dis.
   7:1023-1025.
   
    


2. Marlowe, E. M., M. D. Cohen, J. F. Hindler, K. W. Ward, and D. A.
   Bruckner. 2001. Practical strategies for detecting and confirming
   vancomycin-intermediate Staphylococcus aureus: a tertiary-care hospital
   laboratory's experience. J. Clin. Microbiol. 39:2637-2639