**4.4 Extracellular signaling molecules**

One extracellular signaling molecule g-butyrolactones has been shown to regulate antibiotic production in many streptomycetes cultures [93–95]. Such a mechanism is beneficial for the community survival as extracellular signaling molecules are diffusible in solid media. This way even a single actinomycete can stimulate antibiotic production in another when grown next to each other on an agar plate [96], thus protecting the entire community against competitive microbes.
