Purpose: OXA-10-type class D β-lactamases have shown their evolutionary potential of enhancing carbapenem resistance. This study aimed to elucidate the role of OXA-10 variants in clinical isolated multidrug resistant (MDR) Pseudomonas aeruginosa  and characterize the first appearance of OXA-677 in China.
Methods: Six bla _OXA-10-like-positive strains were screened by PCR from 41 P. aeruginosa  strains, which were resistant to both carbapenems and ceftazidime-avibactam, collected across China in 2018. The minimum inhibitory concentrations (MIC) were determined with the broth microdilution method. The resistance-associated genes and genetic environment were investigated by whole-genome sequencing (WGS). The function and mechanism of OXA-677 β-lactamase were identified by molecular cloning and protein structure modeling.
Results: All the bla _OXA-10-like-positive Pseudomonas aeruginosa  were MDR strains. They also had outer membrane porin defects and produced β-lactam resistance gene bla _PER-1, fluoroquinolone-resistant gene crpP , aminoglycoside-resistance gene aph(3ʹ)-IIb, aph(6)-Id, aacA  and aadA , fosfomycin-resistance gene fosA , sulfamethoxazole-resistance gene sul1 , and chloramphenicol-resistance gene catB7 . All bla _OXA-10 variants were located in a Tn1403-related transposon, containing aacA4-12-bla _OXA-677-aadA1, aacA4-12-bla _OXA-101-aadA5 , and bla _OXA-246-aacA3-aadA13  gene cassette arrays, respectively. Notably, the bla _OXA-677 producer showed a high MIC level of meropenem (MIC> 64 mg/L). Compared to bla _OXA-10, bla _OXA-677 was found a G-to-T transversion at position 350, leading to a phenylalanine-for-valine substitution in position 117, which is closer to leucine155 in the omega loop of the active site. MIC of meropenem for E. coli  DH5α with the recombinant plasmid pHSG398 carrying bla _OXA-677 was elevated by 8 times.
Conclusion: We speculate that the OXA-10-like enzymes and the decrease of membrane permeability confer carbapenem resistance, and the V117 substitution in OXA-677 might lead to a higher resistance level of meropenem.
Keywords: Pseudomonas aeruginosa, bla _OXA-677, bla _{OXA-10-like gene}, CHDL