1. Antibiotics (Basel). 2020 Apr 29;9(5):218. doi: 10.3390/antibiotics9050218.

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

Meerwein M(1), Tarnutzer A(1), Böni M(1), Van Bambeke F(2), Hombach M(3), 
Zinkernagel AS(1).

Author information:
(1)Department of Infectious Diseases and Hospital Epidemiology, University of 
Zurich, 8091 Zurich, Switzerland.
(2)Pharmacologie cellulaire et moléculaire, Louvain Drug Research Institute, 
Université catholique de Louvain, B-1200 Brussels, Belgium.
(3)Institute of Medical Microbiology, University of Zurich, 8091 Zurich, 
Switzerland.

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 (Escherichia coli, Enterobacter 
cloacae, Klebsiella pneumoniae, Pseudomonas aeruginosa) 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 P. aeruginosa increased significantly on 
RPMI-1640 agar compared to Mueller-Hinton agar. Further, a significant 
correlation (Kendall, τ, p) of zone diameters and minimal inhibitory 
concentrations (MICs) was found on RPMI-1640 agar for E. coli (-0.4279, 0.0051), 
E. cloacae (-0.3783, 0.0237) and P. aeruginosa (-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.

DOI: 10.3390/antibiotics9050218
PMCID: PMC7277395
PMID: 32365460

Conflict of interest statement: The authors declare no conflict of interest. The 
funders had no role in the design of the study; in the collection, analyses, or 
interpretation of data; in the writing of the manuscript, or in the decision to 
publish the results.