Differences in antibiotic and bacteriophage resistance between exoU and exoS Pseudomonas aeruginosa from microbial keratitis
Tanzina Akter 1,2, Fiona Stapleton 1, and Mark Willcox 1*
1School of Optometry and Vision Science, University of New South Wales, UNSW, Sydney, NSW-2052, Australia
2 Microbial Biotechnology Division, National Institute of Biotechnology, Savar, Dhaka-1349, Bangladesh
Background: Cytotoxic Pseudomonas aeruginosa strains carrying exoU gene on a genomic island, are associated with more severe microbial keratitis (MK) than invasive exoS strains. The resistance mechanisms of these two lineages to antibiotic and bacteriophage remain underexplored.
Aims: This study aimed to investigate differences in antibiotic and bacteriophage resistance between exoU and exoS P. aeruginosa from MK.
Methods: 55 exoU and 132 exoS P. aeruginosa strains were tested for antibiotics resistance to ciprofloxacin, levofloxacin, gentamicin, and tobramycin using minimum inhibitory concentration assay and resistance to 6 bacteriophages by agar spot test. 39 whole genome sequences (WGS) were analysed for acquired resistance genes using ResFinder, and for mutations in target genes using Geneious Prime software with PAO1 as a reference. Presence of prophages and anti-phage mechanisms in bacteria were detected by PHASTEST and defensefinder tools respectively. Correlation between antibiotic and phage resistance was studied by Pearson test.
Results: ExoU exhibited significantly higher antibiotic resistance than exoS (p<0.05), with resistance rates for ciprofloxacin (38.2% vs. 20.5%), levofloxacin (29.1% vs. 12.1%), gentamicin (40% vs. 23.5%), and tobramycin (29.1% vs. 14.4%). ExoU also harboured more resistance genes (crpP, qnrVC1, aph(3'')-Ib, and aph(6)-Id) and higher mutations in target genes (gyrA, parC, and mexZ) along with few novel mutations associated with fluoroquinolone and aminoglycoside resistance. Phage resistance was higher in exoU against all the six phages than exoS, with the highest resistance to DSP1 (70.9% vs. 44.7%) and lowest to DSP2 (32.7% vs. 17.4%). WGS revealed that exoU carried a median of 4 prophages compared to 2.5 in exoS and possessed more anti-phage systems (Gao_Qat, MADS, PfiAT, Pycsar, RM, RosmerTA, and Zorya) than exoS. A significant positive correlation was found between resistance to DSP2 and ciprofloxacin (r=0.31, p=0.02) in exoU.
Conclusions: exoU from MK exhibited increased antibiotic and bacteriophage resistance, with different resistance mechanisms than exoS.