Colistin-phage combinations decrease antibiotic resistance in Acinetobacter baumannii via changes in envelope architecture

Abstract

Multidrug-resistant bacterial infections are becoming increasingly common, with only few last-resort antibiotics such as colistin available for clinical therapy. An alternative therapeutic strategy gaining momentum is phage therapy, which has the advantage of not being affected by bacterial resistance to antibiotics. However, a major challenge in phage therapy is the rapid emergence of phage-resistant bacteria. In this work, our main aim was to understand the mechanisms of phage-resistance used by the top priority pathogen Acinetobacter baumannii . We isolated the novel phage Phab24, capable of infecting colistin-sensitive and -resistant strains of A. baumannii . After co-incubating Phab24 with its hosts, we obtained phage-resistant mutants which were characterized on both genotypic and phenotypic levels. Using whole genome sequencing, we identified phage-resistant strains that displayed mutations in genes that alter the architecture of the bacterial envelope at two levels: the capsule and the outer membrane. Using an adsorption assay, we confirmed that phage Phab24 uses the bacterial capsule as its primary receptor, with the outer membrane possibly serving as the secondary receptor. Interestingly, the phage-resistant isolates were less virulent compared to the parental strains in a Galleria mellonella infection model. Most importantly, we observed that phage-resistant bacteria that evolved in the absence of antibiotics exhibited an increased sensitivity to colistin, even though the antibiotic resistance mechanism per se remained unaltered. This increase in antibiotic sensitivity is a direct consequence of the phage-resistance mechanism, and could potentially be exploited in the clinical setting.

Re-sensitising XDR biofilms using a combination of bacteriophage cocktails and colistin: a natural approach

“…Similarly, a previous study by Wang et al (2021) demonstrated a decrease in apparent MIC when colistin was used in combination with phage Phab24 against colistin resistant A. baumannii . They have also identified that phage-resistant strains displayed mutations in genes that alter the architecture of the bacterial capsule and the outer membrane using whole genome sequencing (Wang et al, 2021). Another study showed synergistic activity of some antibiotics commonly used in the treatment of urinary tract infection (UTI) with MDR A. baumannii phage cocktails in human urinary infection models (Bartłomiej Grygorcewicz et al,.…”

“…Interestingly, in our study, we observed complete inhibition of a few biofilm producing A. baumannii strains when a phage cocktail BNDM(1,2,3,4,9) was combined with 16 µg/ml colistin. Similarly, a previous study by Wang et al (2021) demonstrated a decrease in apparent MIC when colistin was used in combination with phage Phab24 against colistin resistant A. baumannii . They have also identified that phage-resistant strains displayed mutations in genes that alter the architecture of the bacterial capsule and the outer membrane using whole genome sequencing (Wang et al, 2021).…”

A Multiwell-Plate Caenorhabditis elegans Assay for Assessing the Therapeutic Potential of Bacteriophages against Clinical Pathogens

“…There is a need to improve the current workflow, from phage discovery to the application of phages, which includes investigating the efficacy of a phage in a simple animal model ( 17 , 18 ). Many studies have reported the use of wax worms ( Galleria mellonella ) as a model to study the efficacy of bacteriophages ( 15 , 19 – 22 ). However, it requires a skillful operator to inject bacteria and/or phages into individual larvae, which sometimes could result in large variations between batches and thus lead to nonreproducible observations.…”

Preclinical Assessment of Bacteriophage Therapy against Experimental Acinetobacter baumannii Lung Infection

“…Additionally, recent data point towards a potential benefit of combination therapy utilizing conventional antimicrobial drugs and phages. Sublethal concentrations of antibiotics were shown to augment phage efficacy [57][58][59], while on the other hand phage application can trigger changes in bacteria that reduce their virulence or even restore their susceptibility against antibiotics [60][61][62][63]. However, unfavorable antagonistic interactions between certain antibiotics and phages have also been described and thus need to be considered [64].…”