A clinical microbiology lab reports an E. coli isolate from a 62-year-old man with community-acquired urinary tract infection. The isolate is resistant to ceftriaxone but susceptible to carbapenems and shows a positive ESBL confirmatory test. A second isolate from the same patient, cultured 3 weeks later during hospitalization, is now resistant to both ceftriaxone AND meropenem, with a negative ESBL test. Which feature best distinguishes the carbapenem-resistant isolate from the initial ESBL-producing strain?