The Bacillus cereus group includes genetically close Gram-positive sporulating bacteria with a wide variety of ecological niches, life cycles and hosts. Despite the high genome conservation and high similarity in their 16S RNA sequences, extra-chromosomal genetic material drastically diverges between these species. Actually, the plasmid genes are responsible for their discriminative properties, especially toxin genes. Bacillus cereus F4810-72 strain, belonging to the B. cereus group, is responsible for emetic syndrome in foodborne intoxication, after ingestion of a toxin called cereulide. Production of this dodecadepsipeptide by a non-ribosomal synthesis, requires a cereulide synthetase encoded by the ces locus on the pCER270 megaplasmid. The pCER270 is 270 kb-long and shows 222 coding-sequences, among which only 39% have an assigned function. In order to determine the role of pCER270 in strain adaptation, virulence and ecological niche specificity, we investigated the transcriptional cross-talk between the megaplasmid and the chromosome in different strains: i) the emetic strain B. cereus F4810-72 ii) the entomopathogen Bacillus thuringiensis kurstaki HD73 iii) the psychrotrophic strain Bacillus weihenstephanensis KBAB6. RNA-sequencing experiments were performed in the presence and in the absence of pCER270. Transcriptomic analysis allowed to identify, in each host, which chromosomal genes were sensitive to the presence of pCER270. In rich medium and during early stationary phase, only a few chromosomal genes were impacted by the presence of the megaplasmid. Interestingly, artificial hosts were less responsive than the emetic strain to the plasmid presence, suggesting the existence of strain specificity in chromosome-pCER270 crosstalk. Furthermore, we evaluated the impact of the genetic background on the regulation of megaplasmid gene expression. Indeed, despite a constant global expression level of pCER270 in all hosts, some specific loci were differentially regulated depending on the genomic context. For instance, the expression of some genes was clearly shut down in the artificial hosts of the pCER270 while ces locus expression was surprisingly increased in B. thuringiensis HD73 strain. This study, showing the interaction between the essential chromosome genome and accessory plasmid genome, might provide new insights to understand the emergence of new pathogens through horizontal gene transfers.