Surviving colonies of Pseudomonas aeruginosa isolated in vivo from infected, antibiotic-treated Galleria mellonella larvae acquire an antibiotic-tolerant phenotype

Background: The aim of this work was to induce the formation of antibiotic-tolerant and/or persister cells in vivo using antibiotic therapy on Galleria mellonella larvae infected with P. aeruginosa, isolate these surviving cells, and characterise their phenotype and genotype. Methods: Infected larvae were treated with effective doses of either ceftazidime or meropenem. Despite this, surviving P. aeruginosa colonies were isolated from living larvae, and antibiotic killing, fitness, virulence, antibiotic resistance and the whole genome sequence of a selection of these isolates were compared with their original parent strains. Results: The surviving isolates had an increased minimum duration to kill 99% of the population (MDK99) upon exposure to ceftazidime or meropenem and decreased growth rates in culture, but they showed no change to the MIC or virulence—consistent with an antibiotic-tolerant phenotype. Long-read genome sequencing of selected isolates revealed only one single nucleotide polymorphism (SNP) within bkdB, encoding the lipoamide acyltransferase component of the branched-chain α-keto acid dehydrogenase, present in two independent isolates. However, time-kill assays with ceftazidime of bkdB knockout strains showed no significant change in the MDK99. Concomitant with the antibiotic-tolerant phenotype, many of the isolates also had a reduced rate of killing when exposed to heat stress. Conclusions: P. aeruginosa cells that survived antibiotic therapy in vivo were found to be antibiotic-tolerant and thermotolerant but not antibiotic-resistant and had reduced growth rates under optimal conditions but unchanged virulence. In the absence of a convincing genetic explanation, the co-induction of enhanced thermotolerance with antibiotic tolerance indicated that both are conferred by a heritable phenotypic mechanism.