Refractory vertebral osteomyelitis due to CTX-M-14-producing Escherichia coli at ertapenem treatment in a patient with a coexisting urinary tract infection caused by the same pathogen

We report the case of a patient with vertebral osteomyelitis and concurrent urinary tract infection (UTI) in which extended-spectrum β-lactamase (ESBL)-producing Escherichia coli (EC₁) isolated from urine culture was ciprofloxacin-resistant and ertapenem/imipenem-susceptible. The empirically used oral form of ciprofloxacin was switched to parenteral ertapenem based on the antimicrobial susceptibility. However, vertebral osteomyelitis deteriorated, and despite the disappearance of pyuria and a negative urine culture, ESBL-producing E. coli was isolated from a biopsy of the bony material from the fifth lumbar vertebra (EC₂) and blood culture (EC₃) at 10 and 12 days after starting ertapenem, respectively. Ertapenem was switched to imipenem, and defervescence occurred 2 days later; a subsequent blood culture was negative. Genotyping indicated that EC₁, EC₂, and EC₃ were of the same clone, with the ESBL being CTX-M-14. The tested antibiotics had identical minimum inhibitory concentrations against each of these isolates. From the pharmacokinetics/pharmacodynamics points of view, it is reasonable to attribute the ertapenem treatment failure in vertebral osteomyelitis due to ESBL-producing E. coli in this case to the suboptimal ertapenem concentration in the inflammatory bone tissue of the host.