List of antibiotics produced by different Actinobacteria and their applications.
Actinobacteria are found spread widely in nature and particular attention is given to their role in the production of various bioactive secondary metabolites. Tests on soil samples show that there can be a diversity of actinomycetes depending on the climate, the area it is growing in, how dry the soil is, and the quality of the soil. However, it was agreed after tests in Yunnan, China, that the genus Streptomyces sp. is most important in ecological function, representing up to 90% of all soil actinomycetes, and therefore helping to show the important characteristics needed of the soil actinomycete population. Streptomycete compounds are used for other biological activities, not just for antibiotics. It has been found that metabolites can be broadly divided into four classes: (1) regulatory activities in compounds, these include consideration of growth factors, morphogenic agents and siderophores, and plants promoting rhizobia; (2) antagonistic agents, these include antiprotozoans, antibacterials, antifungals, as well as antivirals; (3) agrobiologicals, these include insecticides, pesticides, and herbicides; and (4) pharmacological agents, these include neurological agents, immunomodulators, antitumorals, and enzyme inhibitors. It is found that Streptomyces hygroscopicus is one of the very best examples because it secretes in excess of 180 secondary metabolites to locate simultaneous bioactivities for a given compound. Increasingly, both its agricultural and pharmacological screenings are being used in conjunction with antimicrobial tests and have revealed several unusual aerobiological and therapeutic agents, which were hitherto unknown for biological use as antibiotics. Since streptomycetes are now being used increasingly to screen for antimicrobial activity, reports show the existence of secondary metabolites with other activities that may have been missed. Currently, nearly 17% of biologically active secondary metabolites (nearly 7600 out of 43,000) are known from streptomycetes. It has been found that soil streptomycetes are the main source used by bioactive secondary metabolites. However, recently there have been many and varied types of structurally unique and biologically active secondary metabolites found and obtained from marine actinomycetes, including those from the genus Streptomyces. Also, compounds that are synthesized by streptomycetes exhibit extreme chemical diversity. Diverse form made from from simple amino acid derivatives to high molecular weight proteides, and macrolactones from simple eight membered lactones to different condensed macrolactones. Berdy (1974) introduced the first classification scheme for antibiotics referring to the chemical structure. On the basis of Berdy’s scheme, (1996) recognized that both low and high molecular weight compounds from 63 different chemical classes are produced by streptomycetes.
- secondary metabolites
The production of many secondary metabolites, including antibiotics, is coupled with morphological differentiation. Indeed, we observe a greater production of secondary metabolites during the transition from vegetative growth to aerial growth . During this change in growth type, partial lysis of the mycelium vegetation takes place to provide the necessary nutrients for the creation of aerial mycelium; this release of nutrients could attract competitors. This synchronization of the cycle of development and production of secondary metabolites could be a way for the bacteria to dispel the invaders to keep these nutrients, or else kill the surrounding bacteria to feed them.
The secondary metabolite-producing microorganisms synthesize these bioactive and complex molecules at the lag phase and stationary phase of their growth (Figure 1a). However, regarding actinomycetes and
There are many varieties of known secondary metabolites synthesized by six pathways of different biosynthesis (Figure 1b): the peptide pathway, the polyketide synthase (PKS) pathway, the nonribosomal polypeptide synthase (NRPS) pathway, the hybrid (nonribosmial polyketide synthetic) pathway, the shikimate pathway, the β-lactam synthetic pathway, and the carbohydrate pathway. The pathway peptide concerns a part of the protein secondary metabolites: they are synthesized by simple translation of mRNAs into peptides by ribosomes. NRPSs are enzymes capable of condensing amino acids to form peptides without going through the ribosomal synthesis pathway. PKSs are enzymes capable of synthesizing a particular family of secondary metabolites: polyketides. The enzymes necessary for the synthesis of these polyketides are homologous to fatty acid synthase (FAS), which is responsible for the synthesis of fatty acid chains. Like the FASs these enzymes can couple precursors to form a chain. This chain will then undergo eight post-PKS changes before becoming active. Regarding the carbohydrate (known scientifically as oligosaccharide) route, it is based on the use of enzymes capable of coupling different sugars to form a carbohydrate precursor; this chain will then undergo modifications that will make the precursor active .
2. Bioactivity of
2.1. Production of antibiotics by
Antibiotics are produced by a wide range of fungal microorganisms and bacteria, and inhibit or kill other microorganisms at low concentrations . A large number of antibiotics have been identified in natural environments, but less than 1% are medically useful. Many antibiotics have been structurally modified in the laboratory to increase their effectiveness, forming the class of semisynthetic antibiotics .
The history of antibiotics began with the discovery of penicillin by Fleming in the 1940s. The antimicrobial activities of antibiotics produced by microorganisms have been extensively studied, and the research undertaken has allowed completion of the antibacterial arsenal available to doctors and the general public.
Microorganisms producing chloramphenicol, neomycin, tetracycline, and terramycin were isolated in 1953. The discovery of chemotherapeutic agents and the development of new, more powerful drugs revolutionized medicine and have greatly reduced human suffering . It is very well known that the genus
|1,4-Dihydroxy-2-(3-hydroxybutyl)-9,10-anthraquinone 9,10 anthrac||Antibacterial|
|2-Allyloxyphenol||Antimicrobial; food preservative; oral disinfectant|
|Bafilomycin||ATPase; inhibitor of microorganisms, plant and animal cells|
|Chloramphenicol||Antibacterial; inhibitor of protein biosynthesis|
|Chromomycin B, A2, A3||Antitumor|
|Elaiomycins B and C||Antitumor|
|Lincomycin||Antibacterial; inhibitor of protein biosynthesis|
|Mitomycin C||Antitumor; binds to double-stranded DNA|
|Pacificanones A and B||Antibacterial|
|Resistoflavin methyl ether||Antibacterial; antioxidative|
|Salinispyrone A and B||Mild cytotoxicity|
|Salinosporamide A||Anticancer; antimalarial|
|Salinosporamide B and C||Cytotoxicity|
|Valinomycin||Ionophor; toxic for prokarotes, eukaryotes|
2.2. Production of enzymes
Research has reported that there are a great variety of enzymes that can be applied to biomicrobial fields and biotechnological industries from different genera of actinomycetes. Using the information available from genome and protein sequencing data, actinomycetes are constantly screened and used for producing amylases, xylanases, proteases, chitinases, cellulases, and other enzymes. Industrial applications, for example, the pronase of
|Aminoacylase||Pharmaceuticals||Production of semisynthetic penicillins and celpholosorin|
|Amylase||Detergent||Removal of stains|
|Baking||Softening of bread; volume|
|Paper and pulp||Deinking|
|Starch||Production of glucose, fructose, syrups|
|Textile||Removal of starch from woven fabrics|
|Cellulase||Detergent||Removal of stains|
|Textile||Denim finishing, softening of cotton|
|Paper and pulp||Deinking, modification of fibers|
|Chitinase||Bioremediation||Utilization of chitin waste|
|Glucose oxidase||Baking||Strengthening of dough|
|Laccase||Bleaching||Clarification (juice), flavor (beer), cork stopper treatment|
|L-Asparaginase||Medicine||The treatment of acute lymphoblastic leukemia|
|Lipase||Detergent||Removal of stains|
|Baking||Stability of dough|
|Neuraminidase||Medical research||Cell surface and clinical studies|
|Penicillin amidase||Commercial significance||Production of 6-aminopenicillanic acid on an industrial scale|
|Peptide hydrolase||Pharmaceuticals||Industrial biosynthesis of oxytetracycline|
|Phytase||Animal feed||Phytate digestibility|
|Brewing||Clarification; low calorie beer|
|Medicine||Treatment of blood clot|
|Xylanase||Baking||Conditioning of dough|
|Paper and pulp||Bleach boosting|
|β-||Studying their biochemical functions||Structural determination of the carbohydrate moiety of several glycoproteins|
Secondary metabolites of Actinobacteria are used as herbicides against unwanted herbs and weeds (Table 3).
|Anisomycin||Inhibitor of growth of annual grassy weeds such as barnyardness and common crabgrass and broad-leaved weeds|
|Bialaphos||Control of annual and perennial grassy weeds and broad-leaved weeds|
|Carbocyclic coformycin and hydantocidin||Control of several weeds|
|Herbicidines and herbimycins||Monocotyledonous and dicotyledonous weed|
|Phthoxazolin, hydantocidin, and homoalanosin||Control of several weeds|
The use of
2.5. Aggregative peptide pheromones
The production of pheromone is considered to have important criteria: it is used as a defense against predators, in mate selection, and to conquer host-habitats through mass attack. Sex pheromone peptides in culture supernantrants were mainly found to support aggregation together by the same related species [37, 38]. A good example for aggregative peptide pheromones is
Microbially derived compounds that share hydrophilic and hydrophobic moieties are surface active biosurfactants that are independent of mineral oil as a feedstock compared with chemically derived surfactants.
Biosurfactants are widely used in scientific research topics (nutrients, cosmetics, textiles, varnishes, pharmaceuticals, mining, and oil recovery) [39, 40]. The lipopeptide antibiotic daptomycin has received great interest as a treatment for Gram-positive bacterial infections; it is marketed as Cubicin by Cubist Pharmaceuticals. Various biosurfactant drugs or bioemulsifiers have been described as a class of Actinobacteria. The best described biosurfactants include a class of glucose-based glycolipids, most of which have a hydrophilic backbone, including glycosides associated with glucose units forming a trehalose moiety.
Vitamin B12 or cobalamine can be synthesized through the fermentation of Actinobacteria [41, 42], and has aroused considerable interest in the possible production of vitamins through microbial fermentation. In addition, cobalt salts in media act as a general Actinobacteria precursor in producing vitamins. Because cobalt is a rather effective bactericidal agent, this precursor must be added carefully. The fermentations producing the antibiotics streptomycin, aureomycin, grisein, and neomycin produce vitamin B12 as well if the medium is supplemented with cobalt without affecting the yields of antibiotic substances.
Microbe-oriented pigments are of great concern. Especially, Actinobacteria are characterized by the production of various pigments on natural or synthetic media and are considered an important cultural characteristic in describing the organisms. Generally, the morphological features of colonies and production of different pigments and aerial branching filaments are known as hyphae, giving them a fuzzy appearance. These pigments are usually various shades of blue, violet, red, rose, yellow, green, brown, and black, which can be dissolved in the medium or may be retained in the mycelium. These microbes also have the ability to synthesize and excrete dark pigments, melanin or melanoid, which are considered useful criteria for taxonomical studies in the textile industry (Table 4).
2.9. Nanoparticle synthesis
The chemical techniques of nanoparticle preparation are less expensive when produced in high quantities; however, the nanoparticles may be contaminated by precursor chemicals, toxic solvents, and risky by-products. As a result, the development of high-yield, low-charge, nontoxic effects, and beneficial environmental procedures for metallic nanoparticle synthesis, and thus the biological method of nanoparticle synthesis, is considered important. Actinobacteria are actually effective nanoparticle producers, showing a number of biological properties, including antibacterial, antifungal, anticancer, antibiofouling, antimalarial, antiparasitic, and antioxidant activities.
|Zinc, copper, manganese|
2.11. Control of plant diseases
Results of new approaches to control plant diseases. Actinobacteria are potentially used in the agro-industry as a source of agroactive compounds of plant growth (rhizobacteria (polyglycerol polyricinoleate, PGPR) promoting) and for biocontrol [47, 48]. Approximately 60% of the new insecticides and herbicides derived from
|Asparagus root diseases||Faeriefungin|
|Blotch of wheat||Malayamycin|
|Broad range of plant diseases||Blasticidin S|
|Brown rust of wheat||Gopalamycin|
|Damping-off of cabbage||Fungichromin|
|Grass seedling disease||Nigericin and guanidylfungin A|
|Phytophthora blight of pepper||Phenylacetic acid|
|Phytophthora blight of pepper||Tubercidin|
|EF-76 and FP-54|
|Powdery mildew of cucumber||Neopeptin A and B|
|Rice blast disease||Kasugamycin|
|Rice sheath blight||Polyoxin B and D|
|Root rot of pea geldanus||Geldanamycin|
|Sheath blight of rice||Validamycin|
2.12. Nematode control
The majority of microorganisms were identified as antagonists of plant-parasitic nematodes, in particular Actinobacteria, which are effectively used in biological control because of their ability to produce antibiotics. The
2.13. Enhancement of plant growth
PGPR can directly or indirectly affect the growth of plants in two common ways. Indirect growth happens when PGPR decreases or prevents the harmful effects of one or more damaging microorganisms. This is mainly researched through biocontrol or the antagonism of soil plant pathogens. Particularly, the effects of pathogen invasion and establishment can be strongly prevented by colonization or the biosynthesis of antibiotics and other secondary metabolites. Direct growth promotes plant growth by PGPR when the plant is supplied with a bacterial synthesized compound, or when PGPR otherwise facilitates plant uptake of soil nutrients. Merriman  reported the use of
Like most rhizobacteria, it seems highly probable that streptomycetes are capable of directly enhancing plant growth.
2.14. Phytohormone production
Manulis et al.  described plant hormone production, including indole-3-acetic acid (IAA), as well as the underlying pathways of synthesis by a variety of
Dhanasekaran et al.  obtained that the isolates
2.16. Odor and flavor compound production
The work carried out by Gaines and Collins  on the metabolites of
|Odor type||Secondary metabolite|
|Potato-like||2-Isobutyl-3-methoxypyrazine or 2-isopropyl-3-methoxypyrazine|
3. Metabolic pathways in the production of secondary metabolites of bacteria
Secondary metabolic pathway reactions are formed by an individual enzyme or multienzyme complexes. Intermediate or end products of primary metabolic pathways are channeled from their systematic metabolic pathways that lead to the synthesis of secondary metabolites. There are six known pathways: the peptide pathway, the PKS pathway, the NRPS pathway, the hybrid (nonribosomal polyketide) synthetic pathway, the shikimate pathway, the β-lactam synthetic pathway, and the carbohydrate pathway. The genes encoding these synthetic pathway enzymes are generally present in chromosomal DNA and are often arranged in clusters.
3.1. Nonribosomal peptide synthesis pathways
Nonribosomal peptides are peptides that are not synthesized at the level of ribosomes. One of the peculiarities of nonribosomal peptides is their small size. These peptides are not encoded by a gene, and they are not limited to the 20 basic amino acids. Indeed, the peculiarity of the NRPS system is the ability to synthesize peptides containing proteinogenic and nonproteinogenic amino acids. In many cases, these enzymes are activated in collaboration with polyketone synthases giving hybrid products. The products of these multifunctional enzymes have a broad spectrum of biological activities, and some of them have been useful for medicine, agriculture, and biological research .
NRPS are organized in a modular way. Each module is responsible for the incorporation of a specific monomer. The modules are subdivided into domains, and each domain catalyzes a specific reaction in the incorporation of a monomer. The number and order of modules and the type of domain present in the modules of each NRPS determine the structural variation of synthesized peptides by dictating the number, order, and choice of amino acid to incorporate during elongation. Four main areas are needed for complete synthesis (Figure 2). Each domain has a specific function when incorporating the monomer. Domain A, from 500 to 600 amino acid residues, is necessary for the recognition of the amino acid and its activation.
The 80–100 amino acid residues of domain T, located downstream of domain A, form a thioester bond (covalent bond) between the activated monomer and the NRPS, and this allows the peptide being synthesized to remain attached to the NRPS throughout the process of elongation. The condensation domain C (450 amino acids) is usually found after each A–T module and catalyzes the formation of peptide bonds between bound residues on two adjacent modules. In general, the number and order of modules present in an NRPS determine the length and the resulting nonribosomal peptide structure. The thioesterase domain, present only in the last module, releases the peptide from the NRPS.
3.2. Polyketide synthase pathways
Polyketides are kown as natural products, having diverse functions in medical applications, and they are assembled by PKS enzymes. PKS enzymes act exactly like fatty acid synthase to generate a diverse extent of polyketides. Also, PKS enzymes start the polyketide assembly by priming the initiator molecule to the catalytic residue, and then making an extender unit for the elongation chain. On the basis of structural architecture and variation in enzymatic mechanism, PKS enzymes have been classified into three types: (1) type I PKS, (2) type II PKS, and (3) type III PKS.
This section describes all three types of PKS enzymes (Table 8). Modular PKSs include active sites, called modules; they are polypeptides used to synthesize a string of carbon. The active sites of each module are used only once during assembly of the molecule and determine the choice of units of structure and the level of reduction or dehydration for the cycle of expansion. They catalyze the length of the string of carbon, and the number of cycles of reaction is determined by the number and order of the modules in the polypeptide constituting the PKS .
|Either modular PKS or type I||Either discrete PKS or type II||Either ketosynthase polyketide or type III|
|Many functional enzymes organized into modules. Each module has a specific function and use; acyl carrier protein (ACP) domain activates acyl-CoA substrates malonyl-CoA or methylmalonyl-CoA or ethylmalonyl-CoA, an extender unit||Includes a series of modular heterodimeric enzymes. Each enzyme has a special function and use; the ACP domain transfers activated acyl-CoA substrate malonyl-CoA, an extender unit||The homodimeric ketosynthase enzyme can carry out various biochemical reactions at a single active site; it acts in the absence of ACP or directly recognizes the acyl-CoA molecules malonyl-CoA or methylmalonyl-CoA, an extender unit|
3.2.1. Type I PKS
These are multidomain proteins (containing several domain enzymes on the same polypeptide) that can be modular (Figure 3), for example, the modular systems responsible for the synthesis of macrolides (erythromycin, rapamycin, rifamycin B, etc.) in bacteria, which is iterative (Figure 4) (for example, lovastatin nonaketide).
3.2.2. Type II PKS
These are monofunctional protein complexes (for example, actinorhodin from
3.2.3. Type III PKS
These have a single active site to catalyze the extension of the polyketide chain and cyclization without the use of an ACP (Figure 5). They are responsible for the synthesis of chalcones and stilbenes in plants, as well as polyhydroxy phenols in bacteria. Chalcone synthases are small proteins with a unique polypeptide chain, and are involved in the biosynthesis of flavonoid precursors .
The shikimate pathway groups the essential building blocks for a large assembly of aromatic metabolites and amino acids. Metabolites of the aromatic compounds present protection against ultraviolet radiation, electron transport, and signaling molecules, and also act as antibacterial agents. The shikimate pathway enzymes use specific chemical substrates, i.e. erythrose-4-phosphate and phosphoenol pyruvate (primary metabolites), to start the synthesis of aromatic building blocks. Herein, the first seven enzymes catalyze the chemical reactions in a chronological manner to produce chorismate. Two bacterial enzymes are able to transfer a complete enolpyruvoyl moiety to a metabolic pathway. 5-Enolpyruvoyl shikimate 3-phosphate synthase is considered one of the shikimate pathways. Chorismate synthase is an enzyme involved in this pathway, and its function needs the presence of a reduced cofactor, flavin mononucleotide, for its activation .
The Gram-positive, filamentous
3.3. Lactam ring synthetic pathways
Cephalosporins belong to the family of β-lactam antibiotics, used for treating bacterial infections for more than 40 years. Interestingly, Gram-positive bacteria, Gram-negative bacteria, and fungi are the major sources of β-lactam antibiotics. The Gram-positive
The production of β-lactam antibiotic occurs through three different steps: prebiosynthetic steps, intermediate formation steps, and late steps (also known as decorating steps) [71, 72, 73, 74, 75, 76]. The biosynthesis of building blocks for β-lactam consist of L-α-aminoadipic acid, L-cysteine, and L-valine. L-α-Aminoadipic acid is not a proteinogenic amino acid formed from L-lysine. The actinomycete lysine 6-aminotransferase converts L-lysine into L-α-aminoadipic acid.
The two starting enzyme reactions are omnipresent in fungi and cephalosporin biosynthesis. D-(L-Aminoadipyl)-L-cysteinyl-D-valine synthase is the first enzyme, using all three amino acids gathered into a tripeptide through condensation reaction. This enzyme is NRPS encoded by the acvA (pcbAB) gene. The next step is the synthesis of a bicyclic ring (a four-member β-ring is fused with a five-member thiazolidine ring) through an oxidative reaction, catalyzed by isopencillin N-synthase, and results in the formation of isopenicillin N. Cephalosporin–cephamycin biosynthesis is the development of the five-member thiazolidine ring into a six-member dihydrothiazine ring. Several enzymes consecutively contribute to this ring conversion. β--Lactam biosynthesis is synthesized by a gene, which is usually clustered in the DNA of all reproducing bacteria. Bacterial species capable of producing β--lactam antibiotics exhibit an ecological benefit. In contrast, β-lactam–producing bacteria show low sensitivity to β-lactams on their own, or they have evolved to inactivate β-lactam antibiotics by β-lactamase enzymes.
We thank Miss Susan Ann Hill for technical assistance and for her useful contribution to the English manuscript checking.
Conflict of interest
The authors declare that no conflicting interest exists.