9 - evaluation of Synergy Testing Methods for Clinical Labs to Determine Susceptibility of Extensively Drug-resistant Gram-negatives to Ceftazidime/ Avibactam and Aztreonam Combination Therapy

Background: Carbapenem-resistant Enterobacterales (CRE) and Pseudomonas aeruginosa (CR-PA) producing Metallo-β-lactamases (MBLs) cause severe nosocomial infections with no defined treatment. Combination therapy with ceftazidime/ avibactam (CZA) and aztreonam (ATM) is a potential option, but there is no approved, feasible, synergy testing method for clinical labs to guide clinical decision making. Here, we evaluate the performance of 4 synergy testing methods using gradient-strips or disks.

Methods: We used 10 representative Enterobacterales strains, namely, E. coli, K. pneumoniae, and E. cloacea, and 6 PA strains harboring MBL, GES or non-MBL enzymes (Fig 1). 4 strains were successfully treated with CZA-ATM in case reports, the rest were from the CDC AR Bank. Four synergy testing methods were evaluated, i) Disk stack (DS), ii) Disk elution (DE), iii) Gradient-strip Stack (SS), iv) Gradient-strip Cross (SX) (Fig 1). All methods were run side-by-side as per CLSI guidelines with broth microdilution (BMD) as the reference. Data is the mean of 3 replicates. Synergy is defined as a strain that is resistant (R) to ATM but drops to ≤ the susceptible (S) breakpoint (Table 1) in the presence of CZA (Fig 2). Categorical agreement (CA), very major error (VME), major error (ME), minor error (MI) were calculated across methods for CZA-ATM synergy relative to BMD.Summary of synergy testing methods evaluated
CLSI Breakpoints used for this study


Results: All CRE with NDM and PA with GES were ATM-R, CZA-R and S to the CZA-ATM combination. PA with NDM or VIM remained R to CZA-ATM likely due to other mechanisms of resistance. CA was high for DE (100%), SS (81%, MI 19%), and SX (88%, MI 13%) but low for DS (25%, ME 54%, MI 31%). Representative strains are shown (Fig 2, Table 2).  Removing PA, CA for DE, SS, and SX was 100% and 20% for DS.Representative results of strains with each synergy testing method.
Representative data of strains displaying synergy (green) or no synergy (red)


Conclusion: Overall, DE was the most reliable method for CZA-ATM synergy testing, and could be a valuable tool in low-resource labs. SS and SX were reliable but prone to technical error. DS had the worst performance. Disks and gradient-strips had identical performance across brands. We propose an algorithm for ATM-R, CZA-R, and MBL-positive CRE, where CZA-ATM synergy testing may be beneficial to guide therapy. These methods are reliable qualitative indicators of the presence or absence of synergy. Synergy testing is not recommended for CR-PA due to complex resistance profiles.

Presenting Author(s)

• 

  Ayesha Khan, PhD

  Postdoctoral Fellow in Infectious Diseases
  UTHealth
  Houston, TX

  Disclosure: I do not have any relevant financial / non-financial relationships with any proprietary interests.

  Slides Slides and Audio

Co-Author(s)

• SE

  Samuel G. Erickson, BS

  Medical Student
  McGovern Medical School
  Houston, Texas

  Disclosure: I do not have any relevant financial / non-financial relationships with any proprietary interests.
• CP

  Cedric H. Pettaway, Jr., BS

  Research Assistant
  McGovern Medical School
  Houston, Texas

  Disclosure: I do not have any relevant financial / non-financial relationships with any proprietary interests.
• CA

  Cesar A. Arias, M.D., MSc, Ph.D., FIDSA

  Professor
  CARMiG, UTHealth and Center for Infectious Diseases, UTHealth School of Public Health, HOU, TX ; Molecular Genetics and Antimicrobial Resistance Unit and International Center for Microbial Genomics, Universidad El Bosque, BOG, COL
  Houston, Texas

  Disclosure: Entasis Therapeutics : Scientific Research Study Investigator; MeMed : Scientific Research Study Investigator; Merck : Grant/Research Support
• 

  William R. Miller, MD

  Assistant Professor
  UTHealth McGovern Medical School
  Houston, Texas

  Disclosure: Entasis () : Grant/Research Support; Merck () : Grant/Research Support; Shionogi () : Advisory Board
• MB

  Micah M. Bhatti, MD, PhD

  Assistant Professor
  The University of Texas MD Anderson Cancer Center
  Houston, Texas

  Disclosure: I do not have any relevant financial / non-financial relationships with any proprietary interests.