Tigecycline Sensitivity Reduction in Due to Widely Distributed (A) Variants.

Despite scattered studies that have reported mutations in the (A) gene potentially linked to tigecycline resistance in clinical pathogens, the detailed function and epidemiology of these (A) variants remains limited. In this study, we analyzed 64 isolates derived from MacConkey plates supplemented with tigecycline (2 μg/mL) and identified five distinct (A) variants that account for reduced sensitivity to tigecycline. In contrast to varied tigecycline MICs (0.25 to 16 μg/mL) of the 64 (A)-variant-positive isolates, gene function analysis confirmed that the five (A) variants exhibited a similar capacity to reduce tigecycline sensitivity in DH5α carrying pUC19. Among the observed seven non-synonymous mutations, the V55M mutation was unequivocally validated for its positive role in conferring tigecycline resistance. Interestingly, the variability in tigecycline MICs among the strains did not correlate with (A) gene expression. Instead, a statistically significant reduction in intracellular tigecycline concentrations was noted in strains displaying higher MICs. Genomic analysis of 30 representative isolates revealed that (A) variants predominantly resided on plasmids ( = 14) and circular intermediates ( = 13). Within China, analysis of a well-characterized collection isolated from pigs and chickens in 2018 revealed the presence of eight (A) variants in 103 (4.2%, 95% CI: 3.4-5.0%) isolates across 13 out of 17 tested Chinese provinces or municipalities. Globally, BLASTN analysis identified 21 (A) variants in approximately 20.19% (49,423/244,764) of genomes in the Pathogen Detection database. These mutant (A) genes have been widely disseminated among isolates from humans, food animals, and the environment sectors, exhibiting a growing trend in (A) variants over five decades. Our findings underscore the urgency of addressing tigecycline resistance and the underestimated role of (A) mutations in this context.