Genetic engineering is a powerful method to improve the fermentation yield of bacterial
metabolites. Since many biosynthetic mechanisms of bacterial metabolites have been unveiled, genetic
engineering approaches have been applied to various issues of biosynthetic pathways, such as transcription,
translation, post-translational modification, enzymes, transporters, etc. In this article, natamycin,
avermectins, gentamicins, piperidamycins, and β-valienamine have been chosen as examples
to review recent progress in improving their production by genetic engineering approaches. In these
cases, not only yields of target products have been increased, but also yields of by-products have been
decreased, and new products have been created.