The phenomenon of changing aztreonam susceptibility in Pseudomonas aeruginosa during formation of colistin resistance in vitro

Objective. To describe the phenotypic and genome characteristics of the phenomenon of aztreonam susceptibility changes in a clinical Pseudomonas aeruginosa isolate developing colistin resistance in an evolutionary experiment in vivo. Materials and Methods. A clinical P. aeruginosa isolate with a multiple drug resistance phenotype was examined using the spaciotemporal model of colistin resistance development under the increasing colistin concentration. During the 44-day experiment, isolates were selected from the frontal growing line; these isolates were subjected to antimicrobial susceptibility testing and genome sequencing. Results. An aztreonam MIC of the parental isolate was 4 mg/L corresponding to the EUCAST category ‘susceptible, increased exposure’. Among the 74 selected P. aeruginosa isolates, 11 isolates changed the aztreonam MIC. Six out of 11 isolates became more susceptible to aztreonam, whereas in the remaining 5 isolates the aztreonam MIC increased. Isolates with the restored aztreonam susceptibility carried alterations in pmrB (del-18bp-298-315) and davD (TGA1450-1452→CTC). In isolates with a 4-fold aztreonam MIC elevation, various mutation profiles were observed. An isolate with a maximal aztreonam elevation of 32 mg/L harbored a capD mutation (del-T-360) and a large 262,402 bp deletion (239 ORF). Mutations in phoQ, lpxL_2, wecA, mexB, spaQ, pcpR and rpoA did not influence the susceptibility to aztreonam. Conclusions. Our results contributed to the understanding of the directions of genetic engineering modeling aimed at expanding the understanding of the mechanisms of resistance to aztreonam.