Advances of genetic engineering in streptococci and enterococci

In the post-genome era, reverse genetic engineering is an indispensable methodology for experimental molecular biology to provide a deeper understanding of the principal relationship between genomic features and biological phenotypes. Technically, genetic engineering is carried out through allele replacement of a target genomic locus with a designed nucleotide sequence, so called site-directed mutagenesis. To artificially manipulate allele replacement through homologous recombination, researchers have improved various methodologies that are optimized to the bacterial species of interest. Here, we review widely used genetic engineering technologies, particularly for streptococci and enterococci, and recent advances that enable more effective and flexible manipulation. The development of genetic engineering has been promoted by synthetic biology approaches based on basic biological knowledge of horizontal gene transfer systems, such as natural conjugative transfer, natural transformation, and the CRISPR/Cas system. Therefore, this review also describes basic insights into molecular biology that underlie improvements in genetic engineering technology.