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

Abstract

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.

Animal petting zoos as sources of Shiga toxin‐producing Escherichia coli, Salmonella and extended‐spectrum β‐lactamase (ESBL)‐producing Enterobacteriaceae

“…(2017) and Meerwein et al. (2020). Isolates displaying resistance to three or more classes of antimicrobials (counting β‐lactams as one class) were defined as multidrug‐resistant (MDR), as proposed previously (Magiorakos et al., 2012).…”

“…Sequences were imported into the E. coli MLST database website (https://enter obase.warwi ck.ac.uk) to determine MLST types and clonal complexes (CCs). Meerwein et al (2020). Isolates displaying resistance to three or more classes of antimicrobials (counting β-lactams as one class) were defined as multidrug-resistant (MDR), as proposed previously (Magiorakos et al, 2012).…”

“…Results were interpreted according to CLSI breakpoints for human clinical isolates(CLSI, 2018). In the absence of clinical breakpoints for azithromycin resistance in E. coli, a zone diameter of ≤12 mm was interpreted as resistant, based on data reported byGomes et al (2017) andMeerwein et al (2020). Isolates displaying resistance to three or more classes of antimicrobials (counting β-lactams as one class) were defined as multidrug-resistant (MDR), as proposed previously(Magiorakos et al, 2012).…”

Could Azithromycin Be Part of Pseudomonas aeruginosa Acute Pneumonia Treatment?

“…An impact of culture media on PA susceptibility to macrolides has also been suggested. Indeed, an enhanced outer-membrane permeability combined with an impairment of active afflux mediated by oprM have been demonstrated in eukaryotic environments, and would result in increased susceptibility of PA to AZM, when cultured in eukaryotic media as compared to conventional broths ( Buyck et al, 2012 ; Meerwein et al, 2020 ). These data may help explain a clinical efficacy of macrolides against PA infections that would not be predicted with conventional antibiotic susceptibility testing systems.…”

Identifying the effect of vancomycin on health care–associated methicillin-resistant Staphylococcus aureus strains using bacteriological and physiological media

“…The presence of divalent ions affects the stability of antibiotics or mode of action of antibiotics, which in turn greatly affects the minimal inhibitory concentration (MIC) values. Roswell Park Memorial Institute (RPMI) 1640 media is among the best media with which to mimic human physiology [ 13–15 ].…”