Comparative genome analyses contribute significantly to our understanding of bacterial evolution and indicate that bacterial genomes are constantly evolving structures. However, lactic acid bacteria chromosomal gene content and organisation result probably from a strong evolutive pressure toward an optimal growth of these microorganisms in milk. The genome plasticity of Lactococcus lactis was evaluated at inter- and intrasubspecies level by different experimental approaches. Comparative genomics showed that the lactococcal genome seems not highly plastic although large rearrangements (deletions, inversions,?) can occur. Experimental genome shuffling using a new genetic strategy based on the Cre-loxP recombination system revealed two domains under strong constraints acting to maintain the original chromosome organisation: a large region around the replication origin, and a smaller one around the putative terminus of replication. Future knowledge of the rules leading to an optimal genome organisation could facilitate the definition of new strategies for industrial strain improvement.
