**Abstract**

Streptomycetes are mycelium-forming sporulating bacteria that produce two thirds of clinically relevant secondary metabolites. Secondary metabolite production is activated at specific developmental stages of *Streptomyces* life cycle. Despite this, *Streptomyces* differentiation in liquid nonsporulating cultures (flasks and industrial bioreactors) tends to be underestimated and the most important parameters managed are only indirectly related to differentiation: modifications to the culture media, optimization of productive strains by random or directed mutagenesis, analysis of biophysical parameters, etc. In this chapter, we review the relationship between differentiation and antibiotic production in liquid cultures. Morphological differentiation in liquid cultures is comparable to that occurring during presporulation stages in solid cultures: an initial compartmentalized mycelium suffers a programmed cell death, and remaining viable segments then differentiate to a second multinucleated antibiotic-producing mycelium. Differentiation is one of the keys to interpreting biophysical fermentation parameters and to rationalizing the optimization of secondary metabolite production in liquid cultures.

**Keywords:** *Streptomyces*, bioreactor, differentiation, antibiotics, programmed cell death
