Increased Azithromycin Susceptibility of Multidrug-Resistant Gram-Negative Bacteria on RPMI-1640 Agar Assessed by Disk Diffusion Testing

• Main Authors: Milton Meerwein, Andrea Tarnutzer, Michelle Böni, Françoise Van Bambeke, Michael Hombach, Annelies S. Zinkernagel
• Format: Article
• Language: English
• Published: MDPI AG 2020-04-01
• Series: Antibiotics
• Subjects: azithromycin; Gram-negative; <i>Pseudomonas aeruginosa</i>; Enterobacteriaceae; susceptibility testing; MIC
• Online Access: https://www.mdpi.com/2079-6382/9/5/218

Increasing antibiotic resistances and a lack of new antibiotics render the treatment of Gram-negative bacterial infections increasingly difficult. Therefore, additional approaches are being investigated. Macrolides are not routinely used against Gram-negative bacteria due to lack of evidence of in vitro effectiveness. However, it has been shown that Pseudomonas spp. are susceptible to macrolides in liquid RPMI-1640 and clinical data suggest improvement in patients’ outcomes. So far, these findings have been hardly applicable to the clinical setting due to lack of routine low-complexity antimicrobial susceptibility testing (AST) for macrolides. We therefore optimized and compared broth microdilution and disk diffusion AST. Multidrug-resistant Gram-negative bacteria (<i>Escherichia coli, Enterobacter cloacae, Klebsiella pneumoniae, Pseudomonas aeruginosa</i>) were tested for azithromycin susceptibility by disk diffusion and broth microdilution in Mueller–Hinton and RPMI-1640 media. Azithromycin susceptibility of Enterobacteriaceae and a subgroup of <i>P. aeruginosa</i> increased significantly on RPMI-1640 agar compared to Mueller–Hinton agar. Further, a significant correlation (Kendall, τ, <i>p</i>) of zone diameters and minimal inhibitory concentrations (MICs) was found on RPMI-1640 agar for <i>E. coli</i> (−0.4279, 0.0051), <i>E. cloacae</i> (−0.3783, 0.0237) and <i>P. aeruginosa</i> (−0.6477, <0.0001). Performing routine disk diffusion AST on RPMI-1640 agar may lead to the identification of additional therapeutic possibilities for multidrug-resistant bacterial infections in the routine clinical diagnostic setting.