Newly isolated bacteriophages show efficacy and phage-antibiotic synergy in vitro against the equine genital pathogens Klebsiella pneumoniae and Pseudomonas aeruginosa

Background: Bacterial infections of the genital tract are a severe problem in equine reproduction. Biofilms produced by Klebsiella (K.) pneumoniae and Pseudomonas (P.) aeruginosa present further concerns in such cases as they can limit the success of antibiotic treatments. Alternative treatment approaches are urgently needed for treating bacterial equine genital tract infections and thus, reduce antibiotic use. The present study reports on the bactericidal efficacy of both, novel K. pneumoniae- and P. aeruginosa-specific phages, in a biofilm model and in conjunction with antibiotic drugs as tested in vitro.

Results: In total, 15 phages with lytic activity (K. pneumoniae: n = 6; P. aeruginosa: n = 9) were isolated and host ranges were determined. Four phages with a broad host range (K. pneumoniae: n = 3; P. aeruginosa: n = 1) were selected for further characterization, including electron microscopy and whole genome sequencing. Significant bacterial growth reduction was observed in planktonic killing assays at three multiplicities of infection (MOI 1, MOI 10, MOI 100), when the phages vB_KpnS_LmqsRe28-1, vB_KpnS_LmqsRe28-2, vB_KpnM_LmqsRe27-1 or vB_PaeS-LmqsRe25-1 were added. In addition, planktonic killing assays were used to examine the four phages in conjunction with gentamicin, enrofloxacin and trimethoprim-sulfadiazine in vitro. The results indicate synergistic activities between the four phages and the investigated antibiotics. However, bacterial growth was not affected in biofilm models after phage treatment, likely due the absence of depolymerase genes in the sequenced phages.

Conclusions: Phages against the equine genital pathogens K. pneumoniae and P. aeruginosa were evaluated in vitro for potential clinical use. While the bactericidal activity of the isolated phages was demonstrated in liquid culture, none of the tested phages showed significant reduction in in vitro biofilm models. However, due to synergistic activities between phages and antibiotics, further studies on combinatorial approaches are needed to develop strategies for managing difficult infections.