title: Performance of vancomycin and teicoplanin disk diffusion test in isogenic vancomycin non-susceptible Staphylococcus aureus.
abstract: BACKGROUND
  Detection of heterogeneous vancomycin-intermediate Staphylococcus aureus (hVISA) is currently problematic. Although the population analysis profile with area under the curve (PAP-AUC) is the gold standard for detecting hVISA strains, this method is time consuming. This study aimed to induce vancomycin non-susceptible Staphylococcus aureus isolates in methicillin-resistant S. aureus (MRSA) and to determine the performance of the vancomycin and teicoplanin disk diffusion test for screening of induced and natural vancomycin non-susceptible isolates.
  
  
  METHODS
  Vancomycin resistance was induced in vitro in methicillin-resistant S. aureus by serial passage in media with increasing vancomycin concentrations. All test isolates were confirmed for their susceptibility to vancomycin by using a PAP-AUC method. The performance of the vancomycin and teicoplanin disk diffusion test for detecting both induced and natural hVISA/VISA isolates was analyzed using the MedCal program version 10.2.0.
  
  
  RESULTS
  The induction test revealed that 42 of 78 MRSA isolates (53.8%) became hVISA and/or VISA. Using 10, 15, 20, 30 µg vancomycin disks and a 30 µg teicoplanin disk, the highest performance (88.9%) for hVISA/VISA detection (71.1%), sensitivity, 100% specificity, 100% positive predictive value, and 75% negative predictive value) was obtained when a 20 µg vancomycin disk was used at 1.0 McFarland inoculum for a 24-hour incubation.
  
  
  CONCLUSIONS
  The results indicated that using a 20 µg vancomycin disk and bacterial inoculum of 1.0 McFarland is simple to perform and provides a primary result for hVISA/VISA screening within 24 hours.
authors.affiliation: Graduate School, Khon Kaen University, Khon Kaen, Thailand. Sujintana.w@kkumail.com.
authors.forename: Sujintana
authors.initials: S
authors.lastname: Wongthong
authors.forename: Karnjana
authors.initials: K
authors.lastname: Dutchanutouch
authors.forename: Viladda
authors.initials: V
authors.lastname: Namsaengkang
authors.forename: Aroonwadee
authors.initials: A
authors.lastname: Chanawong
authors.forename: Chotechana
authors.initials: C
authors.lastname: Wilailuckana
authors.forename: Aroonlug
authors.initials: A
authors.lastname: Lulitanond
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D024881:Drug Resistance, Bacterial; D055624:Methicillin-Resistant Staphylococcus aureus / Q000187:drug effects*; D008826:Microbial Sensitivity Tests / Q000379:methods; D012692:Serial Passage; D017334:Teicoplanin / Q000494:pharmacology*; D014640:Vancomycin / Q000494:pharmacology*
chemical_list: D000900:Anti-Bacterial Agents; D017334:Teicoplanin; D014640:Vancomycin