Azithromycin Synergizes with Cationic Antimicrobial Peptides to Exert Bactericidal and Therapeutic Activity Against Highly Multidrug-Resistant Gram-Negative Bacterial Pathogens

Abstract

Antibiotic resistance poses an increasingly grave threat to the public health. Of pressing concern, rapid spread of carbapenem-resistance among multidrug-resistant (MDR) Gram-negative rods (GNR) is associated with few treatment options and high mortality rates. Current antibiotic susceptibility testing guiding patient management is performed in a standardized manner, identifying minimum inhibitory concentrations (MIC) in bacteriologic media, but ignoring host immune factors. Lacking activity in standard MIC testing, azithromycin (AZM), the most commonly prescribed antibiotic in the U.S., is never recommended for MDR GNR infection. Here we report a potent bactericidal action of AZM against MDR carbapenem-resistant isolates of Pseudomonas aeruginosa, Klebsiella pneumoniae, and Acinetobacter baumannii. This pharmaceutical activity is associated with enhanced AZM cell penetration in eukaryotic tissue culture media and striking multi-log-fold synergies with host cathelicidin antimicrobial peptide LL-37 or the last line antibiotic colistin. Finally, AZM monotherapy exerts clear therapeutic effects in murine models of MDR GNR infection. Our results suggest that AZM, currently ignored as a treatment option, could benefit patients with MDR GNR infections, especially in combination with colistin.

<i>In Vitro</i> Activity of Colistin and Vancomycin or Azithromycin Combinations on Extensively Drug Resistant <i>Acinetobacter baumannii</i> Clinical Isolates

“…Yet, we found that when combined with colistin it did exhibit an activity against drug resistant acinetobacter although this combination only produced a synergistic effect on 30% of our isolates, similarly a small number of studies demonstrated the same activity for AZM against Gram negative bacilli with no explanation or suggestion about the mechanism of synergy [31] [32]. Until Lin et al 2012 tested this combination in eukaryotic cell media and in vivo murine models of infection and concluded that AZM entry and activity is synergistically enhanced when the bacterial outer membrane was disrupted by colistin [11]. Buyck et al, 2012 demonstrated that a mutation of the oprM efflux pump system in P. aeruginosa lead to an increased AZM susceptibility, and on entry AZM reduced oprM gene expression and protein synthesis, further enhancing the entry of AZM in eukaryotic media and in synergy with colistin that may initiate a positive feedback loop to increase effective intracellular levels of the antibiotic [10].…”

“…Also azythromycin, the most commonly prescribed antibiotic in the U.S., is never recommended for clinical treatment of serious Gram negative infections because of poor or absent in vitro activity by standard in-vitro testing. However, antibacterial activity of azithromycin was found to be enhanced in tissue culture media vs. bacteriologic media, prompting closer examination of its interaction with drug resistant Gram negative bacilli [10] [11].…”

The role of the genetic elements bla oxa and IS Aba 1 in the Acinetobacter calcoaceticus-Acinetobacter baumannii complex in carbapenem resistance in the hospital setting

“…Healthcare-associated infections (HAI) are increasingly affecting public and private healthcare systems and raising not only hospital costs but also the morbidity and mortality rates of affected patients (1) (2) (3) . The bacterial genus Acinetobacter is recognized as a key HAI-causing pathogen that is associated with high mortality rates in hospital settings (2) (4) .…”

Should we develop screens for multi-drug antibiotic tolerance?

“…For example, MIC testing ignores possible interactions with host immune factors. Azithromycin, while lacking activity in standard susceptibility assays, possesses potent efficacy against multi-drug-resistant pathogens due to synergy with a host antimicrobial peptide [4]. On the other hand, many pathogens are documented to be highly susceptible in laboratory testing yet unresponsive in clinical therapy.…”