Development of detection methods for carbapenem-non-susceptible Haemophilus influenzae

Carbapenem-non-susceptible Haemophilus influenzae strains have been reported, and such isolates were previously shown to be common in Japan (36% of ampicillin-resistant isolates). This study aimed to identify optimal agents and concentrations that better distinguish carbapenem-non-susceptible from carbapenem-susceptible isolates. Disk diffusion was performed using 41 β-lactam-containing disks with 38 carbapenem-non-susceptible and 45 carbapenem-susceptible H. influenzae isolates. Growth inhibitory zone diameters for each agent were compared with doripenem minimum inhibitory concentrations, as determined by broth microdilution. For selected agents, the agar dilution method was also performed according to Clinical and Laboratory Standards Institute guidelines. Of the 41 β-lactams, five agents, namely amoxicillin-clavulanate, cefpirome, cefozopran, cefteram, and ampicillin-sulbactam, demonstrated sensitivity ≥80% and specificity ≥70% for detecting carbapenem-non-susceptible isolates. The optimal concentrations of amoxicillin-clavulanate, cefpirome, cefozopran, and ampicillin-sulbactam were 8/4, 1, 2, and 4/2 µg/mL, respectively, with high sensitivity (≥89.5%). Disk diffusion using amoxicillin-clavulanate, cefpirome, cefozopran, cefteram, and ampicillin-sulbactam disks is a simple, reliable approach for detecting carbapenem-non-susceptible H. influenzae. Additionally, the optimal concentrations may support the development of automated antimicrobial susceptibility testing systems and selective culture media. The methods developed in this study could aid infection control and antimicrobial stewardship.