DiscussionThe results obtained in this study confirm that the disk diffusion method is not the recommended test to monitor colistin resistance, Olaparib Sigma since only 50% of E. coli and 20% of S. enterica colistin resistant strains were detected using this test. Poor results using the disk diffusion method to detect colistin resistance had been previously described [4].Using the agar dilution test, which is considered the gold standard for colistin evaluation, 6.3% of E. coli and 21% of S. enterica tested strains resistant to colistin were detected. The frequency of E. coli resistant strains is similar to those described by Boyen et al. [4], who report 9.6% (15/157), and have also been reported before in E. coli of animal origin [13, 14].Boyen et al.

[4] described that the published MIC values for human use do not predict clinical efficiency of colistin when used in animal oral formulations. Following values calculated by Burch [15], for a feed concentration of 66ppm of colistin, the antimicrobial will reach bactericidal concentration in the porcine jejunum for strains with a MIC of 8 ��g/mL, but not for strains with an MIC of 16 ��g/mL. The MIC values observed in this study in E. coli resistant strains were 8 ��g/mL (2 strains), 16 ��g/mL (4 strains), and 32 ��g/mL (2 strains).Salmonella Typhimurium resistance to colistin was described by Sun et al. [3], who assessed spontaneous mutations in PmrA and PmrB genes in S. Typhimurium LT2 that present reduced susceptibility to colistin. They report that the mutation rate to colistin resistance was 0.

6 �� 106 per cell generation, which was considered several times higher than mutations rates to other antibiotics, such as streptomycin, rifampicin, and nalidixic acid. The MIC values observed in these mutants (2.5��g/mL to 4��g/mL) increased 20 to 30 times comparing to susceptible strain (0,125��g/mL). Reports of Brefeldin_A colistin resistance frequency in wild S. enterica strains of animal origin and reports of resistance detection in other serotypes different from Typhimurium were not found in the literature. The MIC values identified in S. enterica resistant strains (4��g/mL and 8��g/mL) were lower than those observed in E. coli, but are still above the considered breakpoint and are 32 to 64 times higher than MIC observed in S. Typhimurium ATCC 14024 (0,125��g/mL). In this study colistin resistance was detected in wild Salmonella strains from serotype Typhimurium, London, Anatum, Bredeney and S. enterica subsp. enterica (O:4,5:-:1,2), suggesting that the large use of colistin in swine herds from Brazil is selecting resistant strains independent of serotype. Part of these Salmonella resistant strains was isolated from carcasses, lymph nodes and feces of pigs at slaughterhouses.