High clinical impact of rapid susceptibility testing on CHROMID ESBL ® medium directly from swabs

Background Antibiotic resistance is a serious public health challenge exacerbated by the widespread use of β-lactam and glycopeptide antibiotics. The identification of resistances is crucial, and CHROMID ESBL medium has been developed to detect enterobacteria with extended-spectrum β-lactamases (ESBL). The objective of this study was to evaluate the potential of this medium to detect other types of resistant bacteria. Methods Vancomycin, cefoxitin, imipenem, and cefepime disks were used to measure growth on CHROMID ESBL medium of β-lactam-resistant Gram-negative (83 with ESBL, 57 with carbapenemases, 35 with AmpC and 3 Stenotrophomonas maltophilia) and Gram-positive [37 vancomycin-susceptible (vancoS) microorganisms and 21 vancomycin-resistant (vancoR) Enterococcus faecium] clinical isolates (retrospective study) and colonization by the aforementioned bacteria (prospective study), using 649 rectal swabs, 314 pharyngeal swabs, and 44 swabs from other localizations. Results Retrospective study: species grown on the medium exhibited different colors. Growth on the medium was observed for: all ESBL enterobacteria, which were susceptible to imipenem and cefoxitin; 95% of isolates with carbapenemases, mostly resistant to imipenem; 80% of those with AmpC; 86% of vancoR E. faecium isolates; and 42% of vancoS E. faecalis isolates, with large growth inhibition halos around the vancomycin disk. Prospective study: vancoR E. faecium, ESBL Klebsiella, Pseudomonas with carbapenemases, A. baumannii (mostly from rectal swabs), S. maltophilia, Achromobacter xylosoxidans, and Burkholderia cenocepacia (mostly from pharyngeal swabs) were isolated from the 246 positive samples. Conclusions CHROMID ESBL medium permitted the differential growth of Gram-negative bacteria, many with ESBL and carbapenemases. ESBL enterobacteria were susceptible to imipenem, carbapenemase-producing microorganisms grew around the imipenem disk, and vancoR E. faecium was isolated on the medium. Results of the prospective study demonstrate the potential clinical relevance of this medium. S. maltophilia was more frequently detected with pharyngeal swabs and ESBL Klebsiella, A. baumannii, and Pseudomonas with rectal swabs.