Resistance to bacteriophage incurs a cost to virulence in drug resistant Acinetobacter baumannii

Introduction: Acinetobacter baumannii is a critical priority pathogen
(World Health Organisation) because of the rise in nosocomial infections
and its ability to evolve resistance to last resort antibiotics, which
makes A. baumannii a priority target for phage therapy. Two strains of a
novel, lytic bacteriophage (LemonAid and Tonic) able to infect
carbapenem-resistant A. baumannii (strain NCTC 13420), were isolated from
environmental water samples collected through a citizen science program.
Methods: In vitro and in vivo assays, genomics and microscopy techniques
were used to characterise the phages, determine mechanisms of phage
resistance and the efficacy of the phages against A. baumannii. Results:
A. baumannii developed resistance to both viruses, LemonAid and Tonic.
Resistance came at a cost to virulence, with the resistant variants
causing significantly reduced mortality in a Galleria mellonella larval in
vivo model. A replicated 8bp insertion increased in frequency (~40% higher
frequency than in the wildtype) within phage-resistant A. baumannii
mutants, putatively resulting in early truncation of a protein of unknown
function. Evidence from comparative genomics and an adsorption assay
suggests this protein acts as a novel phage receptor site in A. baumannii.
We find no evidence linking resistance to changes in capsule structure, a
known virulence factor. LemonAid efficiently suppressed growth of A.
baumanni in vitro across a wide range of titres. However, in vivo, while
survival of A. baumannii infected larvae significantly increased with both
remedial and prophylactic treatment with LemonAid (107 PFU/mL), the effect
was weak and not sufficient to save larvae from morbidity and mortality.
Conclusion: While LemonAid and Tonic did not prove effective as a
treatment in a Galleria larvae model, there is potential to harness their
ability to attenuate virulence in drug-resistant A. baumannii.