Tetracycline-resistant bacteria are frequently isolated from aquaculture systems, where mobile
resistance genes often transfer between bacteria associated with fish kept at high stocking densities.
Bacterial isolates from an Oreochromis mossambicus (tilapia) aquaculture system (Stellenbosch, South
Africa) were screened for their susceptibility to tetracycline. Genomic and plasmid DNA were used in
PCR-RFLP assays employing six degenerate primer sets to identify the prevalence of nine tetracycline
resistance genes. Isolates displaying a tet(A)-type tetracycline resistance gene were examined further
for an association with transposon Tn1721. tet(A) was identified as the predominant tetracycline
resistance determinant, followed by tet(B), -(E), and –(C) determinants. Isolates appeared to possess
multiple tet genes simultaneously. Of the isolates presented with a tet(A) determinant, 73.9% appeared
to be associated with Tn1721. No association between type of tetracycline resistance gene, presence on
chromosome or plasmid, and MIC could be established. The Tn1721 association may explain the high
frequency of isolation of tet(A). The high levels of resistance displayed by isolates from the tilapia
aquaculture system not previously exposed to antimicrobial agents is of concern and will have
implications for future therapeutic interventions in disease outbreak situations.