Bacteriophage-antibiotic combinations against ciprofloxacin/ceftriaxone-resistant Escherichia coli in vitro and in an experimental Galleria mellonella model.

Escherichia coli is the most common cause of Gram-negative prosthetic joint infections (PJIs) and ciprofloxacin is the first-line antibiofilm antibiotic. Due to the emergence of fluoroquinolone resistance, management of E. coli PJIs has become challenging and is associated with high treatment failure rates. We evaluated the efficacy of a newly isolated bacteriophage ɸWL-3 as a therapeutic agent in combination with ciprofloxacin, fosfomycin, gentamicin, meropenem or ceftriaxone against biofilm of a ciprofloxacin/ceftriaxone-resistant E. coli strain and the ATCC 25922 reference strain. ɸWL-3 was first characterised in terms of virion morphology, absorption rate, burst size and killing kinetics against both E. coli strains. The tested antibiotics presented high inhibitory concentrations (ranging from 16 to >1024 μg/mL) when tested alone against biofilms. Co-administration of ɸWL-3 with antibiotics improved the antibiotic efficacy against biofilm, especially after staggered exposure, reducing the minimum biofilm bactericidal concentration (MBBC) up to 512 times. The in vivo antimicrobial activity of ɸWL-3/fosfomycin combination against both E. coli strains was assessed in a Galleria mellonella invertebrate infection model. Treatment of infected larvae after lethal doses of E. coli resulted in enhanced survival rates when combinatorial therapy with ɸWL-3/fosfomycin was applied on E. coli ATCC 25922-infected larvae compared with monotherapy, but not for EC1-infected larvae, which we speculated could be due to higher release of endotoxins in a shorter period in EC1-infected larvae exposed to ɸWL-3. Our study provides new insights into the use of bacteriophages and antibiotics in the treatment of biofilm-associated infections caused by antibiotic-resistant bacteria.