Introduction Gonorrhoea is a global health concern because N. gonorrhoeae has now acquired resistance to antibiotics including the 3^rd generation cephalosporin, ceftriaxone. Mosaic structure in penicillin-binding protein 2 gene (penA) is the main cause of ceftriaxone resistance which is formed by natural transformation of DNA of ceftriaxone-resistant (CRO^R) commensal Neisseria spp. However, none of the previous studies has fully elucidated the source of transformed DNA. In this study, we show that genetic comparison between CRO^R-N. gonorrhoeae strains (GU140106 and FC428) and CRO^R –Neisseria commensals.

Methods CRO^R-Neisseria commensals were isolated from pharyngeal swabs. Genome sequence was obtained by MiSeq, Illumina, and the sequence of penA flanking region (mraW-NGO1540-penA-murE-dcaA) was obtained using the N. gonorrhoeae FA1090 as a reference sequence. Alignment of the compared sequences were prepared by CLUSTALW. Species identification of Neisseria commensals was performed using phylogeny constructed by 53 ribosomal proteins (rps) sequences.

Results Based on the sequence similarity of penA gene, two CRO^R-Neisseria commensals were speculated to be the origin of each mosaic penA of GU140106 and FC428. Both of the candidates were N. cinerea according to the rps phylogeny. The DNA region including penA originated from N. cinerea which consist the part of mosaic penA in the CRO^R-N. gonorrhoeae was about 1.3 kb including of parts of penA and murE. Further analysis revealed that mosaic penA of GU140106 occurred by recombination between CRO^R-N. cinerea and CRO^S -N. gonorrhoeae which the NG-MAST type was same with GU140106. The mosaic penA of FC428 was also thought to emerged in the same way with GU140106. However, there was a limitation because of the lack of CRO^S-N. gonorrhoeae that has the same NG-MAST type with FC428.

Conclusion The two CRO^R-N. gonorrhoeae, GU140106 and FC428, might acquire resistance by homologous recombination with Neisseria commensals, and N. cinerea might be the donor of mosaic penA which causes resistance to ceftriaxone in N. gonorrhoeae.