Identification of Novel (X3) Variants Resistant To Tigecycline in Acinetobacter Species.

The emergence of the (X) gene is a severe challenge to global public health security, as clinical tigecycline resistance shows a rapidly rising trend. In this research, we identified two tigecycline-resistant Acinetobacter sp. strains containing seven novel (X3) variants recovered from fecal samples from Chinese farms. The seven Tet(X3) variants showed 15.4% to 99.7% amino acid identity with Tet(X3). By expressing (X3.7) and (X3.9), the tigecycline MIC values for Escherichia coli JM109 increased 64-fold (from 0.13 to 8 mg/L). However, the other (X3) variants did not have a significant change in the MIC of tigecycline. We found that the 26th amino acid site of Tet(X3.7) changed from proline to serine, and the 25th amino acid site of Tet(X3.9) changed from glycine to alanine, which reduced the MIC of tigecycline by 2-fold [the MIC of (X3) to tigecycline was 16 mg/L] but did not affect its expression to tigecycline. The (X3) variants surrounded by mobile genetic elements appeared in the structure of gene clusters with tandem repeat sequences and were adjacent to the site-specific recombinase-encoding gene . Therefore, there is a risk of horizontal transfer of resistant genes. Our study reports seven novel (X3) variants; the continuing emergence of tigecycline variants makes continuous monitoring of resistance to tigecycline even more critical. Although it is illegal to use tigecycline and carbapenems to treat bacterial infections in animals, we can still isolate bacteria containing both mobile resistance genes from animals, and (X) is currently an essential factor in degrading tigecycline. Here, we characterized two multidrug-resistant Acinetobacter sp. strains that contained vital resistance genes, such as , a -like gene, , , and multiple novel (X3) variants with different tandem structures. It is of paramount significance that their mechanism may transfer to other Gram-negative pathogens, even if their tandem structures have no cumulative effect on tigecycline resistance.