Diverse Survival Functions of Secondary Metabolites in Nature

*Ayush Mandwal*

#### **Abstract**

Secondary metabolites are low molecular mass products of secondary metabolism which are usually produced by microorganisms experiencing stringent conditions. These metabolites are not essential for growth but serve diverse survival functions in nature. Besides offering survival advance to the producing organisms, they have several medicinal uses such as antibiotics, chemotherapeutic drugs, immune suppressants, and other medicines which benefited human society immensely for more than a century. This chapter provides an overview of various functions these secondary metabolites offer in nature from single-cell organisms to multicellular organisms. Furthermore, this chapter also discusses the underlying mechanisms behind their diverse functions and how these are regulated and synthesized under non-viable environmental conditions.

**Keywords:** secondary metabolites, antibiotics, *Streptomyces*, resistance/tolerence, cluster-situated regulators

#### **1. Introduction**

Secondary metabolites are biologically active small molecules that are not required for growth and development but which provide a competitive advantage to the producing organism [1]. These are small organic molecules that consist of unusual chemical structures which include β-lactam rings, cyclic peptides, depsipeptides containing unnatural and non-protein amino acids, unusual sugars and nucleosides, unsaturated bonds of polyacetylenes and polyenes, covalently bound chlorine and bromine; nitro-, hydroxamic acids, and so on. Their enormous diversity includes 22,000 terpenoids as well [2]. These complex molecules are commonly obtained from molds which make 17% of all antibiotics and actinomycetes make 74% [3]. The bacterial secondary metabolites are a source of many antibiotics, chemotherapeutic drugs, immune suppressants, and other medicines. Some species are relatively more prolific in secondary metabolism, for example, strains of *Streptomyces hygroscopicus* produce more than 180 different secondary metabolites [4]. It is estimated that the total number of microbial secondary metabolites so far discovered vary from 8000 up to 50,000 [5–7].

As microorganisms are rarely found in isolation, the presence of secondary metabolites in a microbial community exerts evolutionary pressure on both secondary metabolite-producing and non-producing members to develop means to withstand

them. They either use secondary metabolites to have competitive advances or support the intra/interspecies communities against stressful environmental conditions. As these molecules play a vital role in the survival of various microbes, biosynthesis of these molecules is tightly regulated via various transcriptional factors which got triggered under stringent conditions.

This chapter is dedicated to the role of secondary metabolites as antibiotics by various species. The first section of the chapter begins with a discussion on how these molecules are utilized by the various organisms to ensure their survival in the environment. The second section of the chapter goes into the details of how secondary metabolites affect the metabolism of the organism and alter their own or other's organism susceptibility against antibiotics. And the last section discusses how these secondary metabolites as antibiotics are regulated and synthesized with a special focus on *Streptomyces* as they are a rich source of natural antibiotics due to their prolific secondary metabolism.

#### **2. Secondary metabolites in nature**

In recent years, the view that secondary metabolites facilitate the survival of the producer in competition with other living species has been expressed more widely [8, 9]. Some common arguments behind such a view are as follows: (1) Organisms that lack an immune system are prolific producers of these compounds which act as an alternative defense mechanism. (2) The compounds have sophisticated structures, mechanisms of action, and complex and energetically expensive pathways which can only exist if they provide survival advantage to the organism [10]. (3) They are produced in nature and used in competition between microorganisms, plants, and animals [11, 12]. (4) Biosynthetic genes of secondary metabolites are clustered, which would only be selected for if the product conferred a selective advantage, and the absence of non-functional genes in these clusters. (5) The presence of resistance and regulatory genes in these clusters, and lastly by not least the non-producers have clustering of resistance genes.

Besides providing a survival advantage to microbes, secondary metabolites with antibacterial and antifungal properties can cause public health problems if found in soil, straw, and agricultural products. These are usually considered to be mycotoxins, but they are nevertheless antibiotics. And the natural production of such toxic metabolites is one of our major public health problems in the field and during the storage of crops. Natural soil and wheat straw contain patulin [13] and aflatoxin is known to be produced on corn, cottonseed, peanuts, and tree nuts in the field [14]. These toxins can cause hepatotoxicity, teratogenicity, immunotoxicity, mutation, cancer, and death [15].

Below list provide some of the functions of secondary metabolites with examples found across various species from single cell microbes to multicellular organisms.

1.Agents of chemical warfare in nature

• According to Cavalier-Smith [16], secondary metabolites are most useful to the organisms producing them as competitive weapons. Antibiotics are more effective than macromolecular toxins such as animal venoms because of their higher diffusibility into cells and broader modes of action and diverse molecular structure varieties possible.

	- In nature, competition between various fungi has been demonstrated in virtually every type of fungal ecosystem including coprophilous, carbinocolous, lignicolous, fungicolous, phylloplane, rhizosphere, marine, and aquatic [17].
	- Bacteria produce antibiotics when they need them for survival. For instance, myxobacteria can grow on *E. coli* only if the cell density is more than 10<sup>7</sup> myxobacteria/ml [18]. Such high cell density in the local environment produces high concentrations of lytic enzymes and antibiotics needed to grow on *E. coli*.
	- As eukaryotes cells such as amoebae, a protozoans cell, feed on bacteria [19], bacteria found their ways to protect themselves against amoebae and other protozoans in general. Both *Serratia marcescens* and *Chromobacterium violaceum* bacteria produce antibiotically-active pigments namely prodigiosin and violacein, respectively to protect themselves from amoebae. These molecules can either encyst the protozoa or kill them.
	- Over 150 microbial compounds called phytotoxins have been reported that are active against plants [20]. Several such phytotoxins (e.g., phaseolotoxin, rhizobitoxine, and syringomycin) show typical antibiotic activity against other microorganisms and are thus belong to a class of antibiotics.
	- Plants produce antibiotics called phytoalexins after being exposed to plant pathogenic microorganisms in order to protect themselves [21]. They are of low molecular weight, weakly active, and indiscriminate, that is, they inhibit both prokaryotes and eucaryotes including higher plant cells and mammalian cells.
	- Certain fungi produce secondary metabolites for entomopathogenic activity: infecting and killing insects. *Beauveria bassiaria* fungus produces one such compound called bassianolide, a cyclodepsipeptide, which elicits atonic symptoms in silkworm larvae [22]. Another pathogen, *Metarrhizium anisophae*, produces the peptidolactone toxins known as destruxins [23].
	- Similarly, to fight back against bacterial infections, insects produce antibacterial proteins [24]. Some of these proteins are lysozyme, sarcotoxins, cecropins, and defensins. These proteins are either bactericidal or bacteriostatic by nature.
	- It is beneficial for microbes to make fresh food as objectionable as possible to large organisms as quickly as possible [25]. They produce secondary metabolites such as antibiotics and toxins which are toxic to large animals such as livestock. Thus, large animals will refuse to consume moldy feed which ensures the availability of food sources for various microbes.
	- If in case, animals and plants do get infected from microbes, they produce various peptides which kill microbes by permeabilizing their cell membranes as a way to defend against microbial infection [26].
	- Certain secondary metabolites can act as metal transport agents. Siderophores (also known as sideramines) containing molecules function in the uptake, transport, and solubilization of iron. Siderophores are complex molecules that solubilize ferric ion which has a solubility of only 10<sup>18</sup> mol/ L at pH 7.4 and have an extremely high affinity for iron (*Kd* = 10<sup>20</sup>–1050). Another group of molecules includes the ionophoric antibiotics, for example, macrotetrolide antibiotics, which function in the transport of certain alkali-metal ions such as potassium and affect its permeability through the cell membranes.
	- Iron-transport factors in many cases are antibiotics by nature. They are on the borderline between primary and secondary metabolites since they are usually not required for growth but do stimulate growth under irondeficient conditions. Antibiotic activity is due to the ability of these compounds to starve other species of iron when the latter lack the ability to take up the Fe-sideramine complex. Such antibiotics include nocardamin [27] and desferritriacetylfusigen [28].
