International Spread of Tet(X4)-Producing Escherichia coli Isolates

Tigecycline resistance in bacteria has become a significant threat to food safety and public health, where the development of which is attributed to plasmid-mediated tet(X4) genes. In this study, the genomes of 613 tet(X4)-producing Escherichia coli (E. coli) isolates, available from public databases, are evaluated to determine their international prevalence and molecular characterization. These E. coli isolates have been disseminated in 12 countries across Asia and Europe. It was found that pigs and their products (n = 162) were the most common vehicle, followed by humans (n = 122), chickens (n = 60), and the environment (n = 49). Carbapenems-resistant genes bla_NDM-5 (1.3%) and bla_NDM-1 (0.2%) were identified, as well as colistin-resistant genes mcr-1.1 (12.6%) and mcr-3.1 (0.5%). It was noted that the tigecycline-resistant gene cluster tmexC-tmexD-toprJ1 was identified in seven (1.1%) isolates. Phylogenomic results indicated that tet(X4)-producing E. coli isolates fell into seven lineages (lineages I, II, III, IV, V, VI, and VII), and international spread mainly occurred in Asian countries, especially China, Pakistan, Singapore, and Malaysia. Four forms of tet(X4) transposon units were found, including the I-type (IS26-tet(X4)-ISCR2), II-type (ΔIS1R-tet(X4)-ISCR2), III-type (ISCR2-tet(X4)-ISCR2), and IV-type (ISCR2-tet(X4)-ΔISCR2). These findings underline further challenges for the spread of E. coli bearing tet(X4) gene.