Among five American Type Culture Collection (ATCC) Citrobacter strains, qnrB60 in Citrobacter freundii ATCC 6879, an isolate from the preantibiotic era, and qnrB61 in Citrobacter braakii ATCC 51113T, a type strain belonging to the C. freundii complex, were identified. Meanwhile, a truncated qnrB-like pseudogene was identified in C. freundii ATCC 8090T and ATCC 43864. No qnrB-like sequence was found in Citrobacter koseri ATCC 27028T. These findings underscore the close relationship between this species and qnrB.
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