PVL overexpression due to genomic rearrangements and mutations in the S. aureus reference strain ATCC25923

ATCC25923 is a Staphylococcus aureus strain that is positive for the Panton Valentin leukocidin. It has been used for decades as reference strain. We observed that two separately maintained clones of ATCC25923 ("G477 and G478") differed grossly in the expression of this toxin. For that reason, both clones were sequenced using an Illumina MiSeq instrument. After assembling, the final sequences were analyzed and mapped to a previously published ATCC25923 sequence (GenBank CP009361) using bl2seq from the NCBI Blast2 package.

The genomes of G477 and G478 size 2778,859 and 2792,213 nucleotides, respectively. Both genomes include a circular plasmid of 27,490 nucleotides. The sequence of the G477 chromosome maps nearly exactly to CP009361. G478 has a slightly larger size because of the presence of an additional transposable element tnp13k. The second copy of that tnp13k element is located in an intergenic region between the genes mazF and rsbU. The sequences of the ATCC25923 clones G477 and G478 differ mainly in the insertion of a second tnp13k element between the genes mazF and rsbU. That insertion may lead to a different transcription of that genome region resulting in upregulation of the expression of the Panton-Valentine leukocidin in the ATCC25923 clone G478.