Abstract
With the dissemination of extremely drug resistant bacteria, colistin is now considered as the last-resort therapy for the treatment of infection caused by Gram-negative bacilli (including carbapenemase producers). Unfortunately, the increase use of colistin has resulted in the emergence of resistance as well. In A. baumannii, colistin resistance is mostly caused by the addition of phosphoethanolamine to the lipid A through the action of a phosphoethanolamine transferase chromosomally-encoded by the pmrC gene, which is regulated by the two-component system PmrA/PmrB. In A. baumannii clinical isolate the main resistance mechanism to colistin involves mutations in pmrA, pmrB or pmrC genes leading to the overexpression of pmrC. Although, rapid detection of resistance is one of the key issues to improve the treatment of infected patient, detection of colistin resistance in A. baumannii still relies on MIC determination through microdilution, which is time-consuming (16–24 h). Here, we evaluated the performance of a recently described MALDI-TOF-based assay, the MALDIxin test, which allows the rapid detection of colistin resistance-related modifications to lipid A (i.e phosphoethanolamine addition). This test accurately detected all colistin-resistant A. baumannii isolates in less than 15 minutes, directly on intact bacteria with a very limited sample preparation prior MALDI-TOF analysis.
Highlights
Antimicrobial resistance is on top of the agenda for scientists and governments, while pan-resistant organisms are fast emerging
Colistin resistance is caused by the addition of pETN to the lipid A on position 1 or 4’ by an EptA-like phosphoethanolamine transferase chromosomally-encoded by the pmrC gene
This method has been chosen as the unique reference by The Clinical Laboratory Standard Institute (CLSI) and by the European Committee on Antimicrobial Susceptibility Testing (EUCAST), recently gathered in a joint subcommittee[9], which ruled out methods classically used for determination of antimicrobial susceptibility, such as agar
Summary
In polymyxin susceptible Acinetobacter baumannii, the mass spectrum is dominated by 2 set of peaks centred at m/z 1728.1 and m/z 1910.3 (Fig. 1a), assigned to bis-phosphorylated hexa-acyl and bis-phosphorylated hepta-acyl lipid A, with acyl chain ranging from 12 to 14 carbons in length, respectively[15,16]. Since EptA-like and MCR-like enzymes are both phosphoethanolamine transferases, we expected to observe the appearance of two peaks at +25 m/z and +123 m/z related to pETN modified lipid A compare to the native lipid A, as it was previously shown with all MCR-producing E. coli[13]. As previously described[13], the routine use of the MALDIxin test will require switching the MALDI-TOF-MS machine to the negative ion mode due to the inherent negative charge of lipid A. Optimizations are still needed to allow the direct detection of L-Ara4N-modified lipid A which remains the main cause of colistin resistance in K. pneumoniae and P. aeruginosa
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