Horizontal transfer of the pCER270 megaplasmid within the B. cereus group

International audience

English. The emetic lineage of B. cereus, which is the causative agent of severfoodborne intoxications, harbours the pXO1-like mega-plasmid pCER270 thatencodes the structural genes ces for the non-ribosomal biosynthesis of the emetictoxin cereulide. Next to the ces operon, 222 coding sequences are predicted onpCER270, including genes for sporulation and germination as well as transcriptionalregulators. Since horizontal gene transfer by plasmids is a driving force in bacterialadaptation, we aim to elucidate the role of pCER270 transfer in pathogen emergenceand adaptation to the environment. Thus, we tagged pCER270 in the emetic reference strain F4810/72 with an antibioticresistance cassette using CRISPR/Cas9 technology and transferred it to other B. cereus group members by conjugation. Since pCER270 is not conjugative by itself, we used a pXO16-derivative to mobilize its transfer. The conjugation was carried out in a liquid medium to measure transfer kinetics. Tested recipients received both plasmids with saturating ratio of 10-3-10-2 of pCER270 to pXO16 transfer frequency. Interestingly, very early after donor-recipient mixing, the frequency of pCER270 transfer was comparable or even up to ten-fold higher than that of pXO16. Transfer of pCER270 was confirmed by PCR targeting the origin of replication (repX) and ces genes. Transconjugants and their parentals were subjected to Fourier Transformed Infrared (FTIR-) Spectroscopy for metabolic fingerprinting. Chemometric analysis of the respective FTIR spectra indicated specific metabolic traits linked to the presence of the mega-plasmid.