1. Prepared by Regina Kwon, MD, MPH
Reviewed by Patricia Simner, PhD
With thanks to Maritsa Angelou and Hillary Ajife for their technical assistance.

CASE, CONTINUED.
The patient was first treated with piperacillin-tazobactam. After antimicrobial susceptibility testing was completed (Table 1), they were switched to ceftriaxone and metronidazole and eventually discharged.
Three weeks later, the patient returned to the ED with complaints of progressive fatigue, nausea, vomiting, profuse watery diarrhea, and a 30-pound weight loss. A week before his return, an infectious disease physician had changed his antibiotic regimen to ertapenem alone. This seemed to improve his nausea and vomiting, but his other symptoms, including diarrhea, remained unchanged.
In the hospital, imaging showed signs concerning for persistent gallbladder infection. Interventional radiology exchanged the percutaneous tube and aspirated 5 mL of purulent fluid from the gallbladder. This was sent to the Microbiology lab for aerobic and anaerobic bacterial cultures. One day after inoculation, heavy growth of three distinct morphologies was observed on the blood agar plate and were plated independently (Fig. 2). Gram stains were made of the organism and antimicrobial susceptibility testing performed (Table 2).
Routine antimicrobial susceptibility testing was followed by tests of the carbapenem-resistant isolates for carbapenemase production using the modified carbapenem inactivation method (mCIM) and the CARBA 5 lateral flow assay. These tests were negative.

Which resistance mechanism best explains the pattern shown in Table 2?