Novel Transposon Tn<i>6433</i> Variants Accelerate the Dissemination of <i>tet</i>(E) in <i>Aeromonas</i> in an Aerobic Biofilm Reactor under Oxytetracycline Stresses

Little is known about the mechanisms
that disseminate antibiotic
resistance genes (ARGs) in wastewater microbial communities under
antibiotic stress. The role of horizontal transfer mechanisms in dissemination
of ARGs in an aerobic biofilm reactor under incremental oxytetracycline
doses from 0 to 50 mg/L was studied. Aeromonas strains
were the most common culturable bacteria in the reactor, with tet(E) as the most prevalent ARGs (73.3%) being possibly
responsible for the oxytetracycline resistance phenotype. Genomic
sequencing demonstrated that tet(E) was mainly carried
by a Tn3 family transposon named Tn6433, whose incidence increased from 14.6% to 75.0% across the treatments.
Tn6433 carrying tet(E) was initially
detected in Aeromonas chromosomes at an oxytetracycline
dose of 1 mg/L but subsequently detected on plasmids pAeca1-a variants
(pAeca1-a, pAeca1-b, and pAeme6) and pAeca2 under higher oxytetracycline
stress. The core region of the Tn6433-tet(E) structure was highly conserved, consisting of a transposition
and resolution module, a class 1 integron, core passenger genes, and
a Tn1722/Tn501-like transposon.
Such a structure was found on both the chromosome and plasmids, suggesting
that Tn6433 mediated the transposition of tet(E) from the chromosome to plasmid pAeca2 under increasing
stresses. Bacteria carrying the transferable plasmid pAeca1-a were
dominant in high antibiotic treatments, suggesting that Tn6433 disseminated tet(E), conferring selective
advantages to recipients of this ARG.