Background
Colistin is one of the last treatment options for infections caused by multidrug-resistant Gram-negative bacteria and it has been extensively used in veterinarian medicine during last years. In this study, we screened Aeromonas isolates of animal origin for the presence of mcr genes and investigated the location of these genes by whole genome sequencing (WGS).
Materials and Methods
A total of 479 Aeromonas spp. isolates, collected between 2005 and 2012 in Germany, were screened by PCR for mcr-1, mcr-2 and mcr-3 genes. WGS was performed to analyze the mcr-positive isolates. Plasmid profiles were prepared and transformation/conjugation assays conducted for isolates with plasmid-borne mcr genes. MICs were determined by broth microdilution.
Results
Four of the 479 Aeromonas isolates (0.84%) were positive for mcr-3. Sequence analyses revealed they were novel mcr-3 gene variants named mcr-3.6 to mcr-3.9. The colistin MICs of the four isolates ranged from 4 to ≥128 mg/L. Three mcr-3 gene variants were chromosomally located, as shown by the sequence analyses of the corresponding contigs, and no plasmids were detected in those isolates. In contrast, the A. media isolate carried two plasmids of ca. 11 kb and 180 kb. The mcr-3.7 variant was identified on the 180 kb plasmid. The plasmid Inc group could not be identified with PlasmidFinder and ResFinder analysis detected the presence of other resistance genes: floR, tet(31), catA1, tet(A), strAB, ere(A), dfrB8, aadA1, sul1, sul2, mph(B), blaTEM-116.
Conclusion
In this study, four novel mcr-3 gene variants were identified in Aeromonas spp. isolates of animal origin. The new variant mcr-3.7 was located on a non-typeable plasmid that harbored also other resistance genes. Moreover, the oldest mcr-3-positive isolate of our collection originated from 2005, suggesting that this gene has been present for at least 12 years and might has already spread worldwide.