Detection of High-Level Rifaximin Resistance in Enteric Bacteria by Agar Screen.

Objectives:This study aimed at determining the prevalence of rifaximin resistance in a large collection ofEnterobacteralesresistant to third-generation cephalosporins. A simple agar screen was developed to detect high-level resistance. Methods:A total of 401 isolates nonsusceptible to third-generation cephalosporins (including 342Escherichia coliand 39Klebsiellaspp. and 20Enterobacterspp.) were tested by microdilution for their MICs of rifaximin and rifampicin. Isolates with a confirmed rifaximin minimal inhibitory concentration (MIC) of >64 mg/L and a number of high-level resistant, and susceptible control isolates were tested for growth on Mueller-Hinton agar supplemented with rifaximin or rifampicin at a concentration of 256 mg/L. Amino acid mutations inrpoBand the presence of rifaximin resistance-associated genes arabidopsis response regulator (arr)2/3were investigated. Results:Microdilution assays identified rifaximin resistance in nineE. coliand threeKlebsiellaspp. isolates with complete cross-resistance to rifampicin (MICs of both >64 mg/L). The rifaximin agar screen correctly identified 9/9 clinicalE. coliisolates, 2/2E. colicontrols, and 3/3Klebsiellaspp. with high-level rifaximin resistance, and was negative in 45 control clinical isolates with rifaximin MICs ranging between 2 and 32 mg/L according to broth microdilution. All nine high-level rifaximin agar screen-positiveE. coliclinical isolates (vs. none of the tested controls) hadrpoBmutations or carriedarr2/3. Conclusions:Our agar screen test has the potential to detect high-level rifaximin-resistantEnterobacterales. Such strains remain rare among extended spectrum beta-lactamase (ESBL)-positive enteric bacteria, but may emerge among patients receiving rifaximin for prevention of hepatic encephalopathy and spontaneous bacterial peritonitis or among patients receiving rifaximin for other indications.