Performance of Ertapenem-Supplemented MacConkey Agar (MacErt) for Detecting Carbapenemase-Producing Enterobacterales

Background and objectives Antimicrobial resistance (AMR) is a growing global threat, with carbapenemase-producing Enterobacterales (CPEs) representing a critical public health challenge. Rapid and accurate detection of CPEs is essential for controlling fatal bacterial AMR infections. This study evaluated the performance of MacConkey media supplemented with ertapenem (MacErt1 and MacErt2) for the detection of CPEs. Methods We formulated the media by supplementing MacConkey agar with ertapenem to final concentrations of 0.5 mg/L (MacErt1) and 1 mg/L (MacErt2). The media were assessed using a panel of 26 characterized Enterobacterales, including CPEs harboring oxacillinase (OXA)-48, OXA-181, New Delhi metallo-beta-lactamase (NDM)-5, and Klebsiella pneumoniae carbapenemase (KPC). All isolates were cultured on Mueller Hinton agar and incubated overnight at 36°C. Inocula were prepared and adjusted to a 0.5 McFarland standard. Ten microliter loops were used to streak MacErt1 and MacErt2 plates, which were then incubated overnight. After validation, MacErt1 was employed for the detection of CPEs in wastewater. A volume of 10 mL of wastewater was filtered, and the membrane was placed on MacErt agar, followed by overnight incubation. Grown colonies were identified using the Biotyper Sirius 2 MALDI-TOF (Bremen, Germany: Bruker), and the presence of carbapenem resistance genes was determined by lateral flow immunoassay (LFIA) tests and PCR. Results MacErt1 exhibited excellent sensitivity (100%) for all tested CPEs and a specificity of 77%. In contrast, MacErt2 demonstrated an overall sensitivity of 83%, primarily due to reduced sensitivity for OXA-181. However, it was 100% sensitive for detecting NDM, KPC, and OXA-48 producers. MacErt2 also maintained excellent specificity at 93%. The application of MacErt1 to wastewater samples resulted in 100% positivity and allowed the isolation of 124 CPEs among 150 examined isolates, predominantly NDM producers, followed by OXA-48-like and NDM+OXA-48-like strains. None of the isolates tested positive for blaKPC, blaVIM, or blaIMP. Conclusion This study demonstrated the efficacy of MacErt media for selectively detecting CPEs. MacErt1 exhibited 100% sensitivity for various CPEs and a specificity of 77%. MacErt2 showed 93% specificity and 100% sensitivity for NDM and KPC producers, making it suitable for targeted detection. These findings suggest that MacErt media provide an effective in-house solution for CPE surveillance, serving as a valuable tool in the ongoing battle against AMR.