**Abstract**

Microbial natural products have become important over the last decades due to the ability of bacteria and fungi to subsist in different habitats such as marine and extreme environments. Microorganisms are able to synthesize new compounds with diverse therapeutic activity equal to or better than the activity of compounds already known, thus being promising for the treatment of different diseases such as cancer or the solution to health problems such as antibiotic resistance. The production of microbial natural compounds can be improved by modifying culture media, growing conditions, amplifying gene expression or by co-cultivation techniques, which are the major challenges in the industrial production of such compounds.

**Keywords:** microorganisms, antioxidants, antibiotics, antitumor, polymers

## **1. Introduction**

The lack of effectiveness in current therapeutics using already known compounds has made necessary the rediscovery of natural products, either for obtaining new compounds or modifying their structure to improve their activity, where plants are the most popular sources.

However, due to seasonal and environmental conditions that influence their production, alternative sources have been searched for. Microorganisms have been considered as good alternative sources due to the self-sustainability and controllable growth conditions such as carbon source, nitrogen source, pH and temperature [1, 2], thus leading to the possibility of discovering new compounds.

In this chapter, we will focus on the uses of microbial secondary metabolites as antioxidants, antibiotics, antitumor and polymers from mainly *Streptomyces* genus, which have been important in soil bioremediation and biocatalysis for the obtention of enantiopure compounds [3–5].

### **2. Microorganisms as sources of natural products**

Since the discovery of penicillin and streptomycin in 1928 and 1943 respectively [6, 7], microorganisms have become fascinating alternative sources because of the diversity of natural products with new structures to be elucidated and studied for biological activity.

Microorganisms can be found in very extreme environments (soil/marine, high/ low temperature, acid/alkaline) [8], with the isolation of these microorganisms being a major challenge to date because there are uncultivable microbes, complicating

natural product discovery. To overcome this problem, different techniques have been applied such as co-cultivation, as well as exploration of isolation techniques on natural habitats [9]. Co-cultivation has attracted attention because it can induce the biosynthesis of new compounds [10] such as libertellenone A, B, C and D from co-cultivating α-proteobacterium and *Libertella* sp. [11] and stearidonic acid from *Rhizobium* strain 10II and *Ankistrodesmus* sp. [12].

Terrestrial fungus and actinobacteria are the most important sources of antimicrobials, cytotoxic compounds and antioxidants, among others [13]. However, in the last few years, marine environment has attracted attention due to the diversity and effectivity of natural products [14], such as apratoxins from cyanobacteria from the *Lyngbya* genus used as cytotoxic agents to induce apoptosis [15], as well as salinisporamides isolated from *Salinispora tropica* with activity against human colon carcinoma [16].
