Comparative Analysis of Two Commercial Automated Systems with Agar Dilution for Oxacillin Susceptibility and Their Association with Genotypes of Invasive Staphylococcus aureus Isolates (2011-2021)

Wei-Yao Wang; Yi-Hsin Chen; Yu-Lin Lee; Chen-Feng Chiu; Shih-Ming Tsao

doi:10.2147/IDR.S445211

Comparative Analysis of Two Commercial Automated Systems with Agar Dilution for Oxacillin Susceptibility and Their Association with Genotypes of Invasive Staphylococcus aureus Isolates (2011-2021)

Infect Drug Resist. 2024 Mar 20:17:1121-1129. doi: 10.2147/IDR.S445211. eCollection 2024.

Authors

Wei-Yao Wang^{1

2}, Yi-Hsin Chen^{3

4

5}, Yu-Lin Lee^{1

2}, Chen-Feng Chiu⁶, Shih-Ming Tsao^{1

2}

Affiliations

¹ School of Medicine, Chung Shan Medical University, Taichung, Taiwan.
² Department of Internal Medicine, Chung Shan Medical University Hospital, Taichung, Taiwan.
³ Department of Nephrology, Taichung Tzu Chi Hospital, Taichung, Taiwan.
⁴ School of Medicine, Tzu Chi University, Hualien, Taiwan.
⁵ Department of Artificial Intelligence and Data Science, National Chung Hsing University, Taichung, Taiwan.
⁶ Department of Internal Medicine, Feng Yuan Hospital, Ministry of Health and Welfare, Taichung, Taiwan.

Abstract

Background: Determining oxacillin susceptibility using reference methods and automated systems is crucial for treating invasive infections caused by Staphylococcus aureus. This study compares the oxacillin susceptibility results from the two automated systems with agar dilution and correlates them with genotypes of invasive S. aureus.

Methods: Non-duplicate S. aureus invasive isolates were collected over an 11-year period. The oxacillin susceptibility was determined with Phoenix 100 (Jan 2011 to Aug 2018) or Vitek 2 (Sep 2018 to Dec 2021), and susceptibility for oxacillin and cefoxitin was determined with agar dilution. Methicillin-resistant S. aureus (MRSA) was confirmed with mecA existence, and the genotype was determined using SCCmec. The association between genotype and antibiotic susceptibility using two automated systems and agar dilution was evaluated.

Results: A total of 842 invasive S. aureus, including 443 mecA+ MRSA and 399 mecA- MSSA, were collected. The susceptibility rates of oxacillin determined by two automated systems and agar dilution were 68.8% (76.8% for Phoenix 100 and 57.6% for Vitek 2) and 54.0%, respectively. When compared with the oxacillin susceptibility using agar dilution, the categorical agreement for Phoenix 100 and Vitek 2 were 0.46% and 0.88%, respectively (p < 0.001). One hundred and forty-three isolates were misinterpreted as oxacillin-susceptible S. aureus (OSSA) using automated systems while comparing with agar dilution, among which molecularly community-associated MRSA (CA-MRSA) outnumbered healthcare-associated MRSA (HA-MRSA) (99 vs 34, p < 0.001). There were 70 mecA+ OSSA (OS-MRSA) using agar dilution, among which 42 harbored SCCmec types were predominantly categorized as CA-MRSA (38, p < 0.001).

Conclusion: The categorical agreement of Vitek 2 in determining oxacillin susceptibility and predicting mecA existence is comparable with agar dilution, whereas Phoenix 100 is not. Most of those ORSA determined by agar dilution but misinterpreted as OSSA by automated systems and OS-MRSA are categorized as CA-MRSA.

Keywords: CA-MRSA; SCCmec; agar dilution; automated systems; oxacillin susceptibility.

Grants and funding

This project was funded by Chung Shan Medical University (grant number: CSMU-TSMH-104-02). This funding source played no role in study design or conduction, data collection, analysis or interpretation, writing of the manuscript, or decision to submit it for publication.