Evaluation of an adapted method of relative growth to determine the susceptibility of Enterobacterales to polymyxin B by MALDI-TOF MS.

Polymyxin B resistance is an emerging problem worldwide. The reference method to determine susceptibility to polymyxins is broth microdilution (BMD). As BMD is time consuming, it is necessary to develop new methodologies to provide faster evaluation of polymyxin susceptibility. This study aimed to evaluate polymyxin B susceptibility of Enterobacterales using an adapted methodology of relative growth (RG) by Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). A total of 60 isolates of Enterobacterales (22 resistant and 38 susceptible to polymyxin B by BMD) were evaluated. The adapted RG technique presented categorical agreement of 96.7% with only 2 major errors (3.3%) in comparison to BMD. Our findings demonstrate a high agreement between BMD and adapted RG, indicating that this methodology is promising for differentiating polymyxin B-susceptible isolates from polymyxin B-resistant isolates and could be implemented routinely in microbiology laboratories that already use the MALDI-TOF MS to identify bacteria.

Evaluation of filter paper as a means of transporting inactivated Gram-negative non-fermentative bacteria and Haemophilus spp. for identification using the MALDI-TOF MS system.

This study aimed to evaluate the filter paper as a means to transport inactivated Gram-negative non-fermentative (GNNF) bacteria and Haemophilus spp. for analysis using MALDI-TOF MS. A total of 133 isolates were evaluated and the analysis of each isolate was performed directly from original bacterial colony and in filter paper after the processing. To evaluate the agreement between the identification performed directly from the colony and after impregnation in filter paper, we assign the scores: >2·3 as excellent (E); 2·0 to 2·3 as very good (VG); 1·7-1·99 as good (G); <1·7 as unidentified (U). The divergences were classified as: Minor Divergence, Intermediate Divergence and Major Divergence. A total of 80 isolates transported in the filter paper disks presented full category concordance; 39 isolates presented Minor Divergence; 4 isolates present Intermediate Divergence; 4 isolates present Major Divergence and 6 isolates present better results after impregnation in filter paper. The proposed methodology of bacteria transportation presented a sensitivity of 96·9% and a specificity of 100%. The filter paper as a means to transport and storage of inactivated GNNF and Haemophilus spp. may be considered a potential tool for faster, more accurate, biosafe and less-expensive identification.