Abstract
Introduction.
Aminoglycoside antibiotics are widely used to treat infections of
Pseudomonas aeruginosa
. The MexXY-OprM efflux pump is an important contributor to aminoglycoside tolerance in
P. aeruginosa
reference strains and expression of the
mexXY
genes is repressed by the MexZ repressor protein. Direct investigation of the role of this efflux pump in clinical isolates is relatively limited.
Hypothesis.
The contribution of MexXY-OprM to
P. aeruginosa
aminoglycoside resistance is isolate-specific.
Aim.
To quantify the role of MexXY-OprM and its repressor, MexZ, in clinical isolates of
P. aeruginosa.
Methodology.
The
mexXY
genes were deleted from ten clinical isolates of
P. aeruginosa
, and the
mexZ
gene from nine isolates. Antimicrobial susceptibility testing was carried out for commonly used antipseudomonal drugs on the engineered mutants and the isogenic wild-type isolates. RT-qPCR was used to measure expression of the
mexX
gene.
Results.
All but one of the
mexXY
mutants were more susceptible to the clinically used aminoglycosides tobramycin, gentamicin and amikacin but the degree to which susceptibility increased varied greatly between isolates. The
mexXY
mutants were also more susceptible to a fluoroquinolone, ciprofloxacin. In three isolates with functional MexZ, deletion of
mexZ
increased expression of
mexXY
and aminoglycoside tolerance. Conversely, deleting
mexZ
from six clinical isolates with
mexZ
sequence variants had little or no effect on expression of
mexXY
or on aminoglycoside susceptibility, consistent with the variants abolishing MexZ function. Genome analysis showed that over 50 % of 619 clinical isolates had sequence variants predicted to reduce the affinity of MexZ for DNA, likely increasing
mexXY
expression and hence efflux of aminoglycosides.
Conclusion.
Our findings show that the interplay between MexXY, MexZ and the level of
mexXY
expression plays an important role in aminoglycoside resistance in clinical isolates of
P. aeruginosa
but the magnitude of the contribution of this efflux pump to resistance is isolate-specific.