Understanding microbial pathogens: current knowledge and educational ideas on antimicrobial research (Enrique Torres-Hergueta and A. Méndez-Vilas, Eds.)

Antimicrobial agents produced by

Naeima M.H. Yousef* Department of Botany & Microbiology, Faculty of Science, Assiut University, 71516 Assiut, Egypt * Corresponding author: email: [email protected]

Streptomyces is a genus of Gram-positive, aerobic, filamentous and non-acid-fast that belongs to the family and represents the largest genus of actinobacteria. It is common in various environments; soil, composts, water (rivers and marine) and plants. The genus comprises more than 600 species with validated names. The most interesting features of Streptomyces is its ability to produce bioactive secondary metabolites, such as antifungal, antibacterial, antiviral, antitumor, anti-hypertensives, immunosuppressant, and several others. The genus produces over two-thirds of the clinically useful of natural origin related to , β-lactams, and . The production of most antibiotics is species specific, and the species produce them to compete with other microorganisms in the same habitats. In addition, these antibiotics protect the plant against microbial pathogens. The objectives of the current review are to shed light on the genus Streptomyces; diversity, general features and the role of this genus in production of highly valuable antimicrobial agents that commonly used in treatment of some virulent pathogens.

Keywords: Streptomyces; Soil; Antibiotics; Plants; Antimicrobial agents

1. Streptomyces The name Streptomyces is derived from the Greek strepto- meaning twisted, alluding to this genus' chain- like spore production; myces means filament. The genus Streptomyces belongs to the family Streptomycetaceae and it represents the largest genus of Actinomycetes [1]. Streptomyces is a Gram-positive, aerobic, non-acid-alcohol-fast actinobacteria and has genomes with high GC-content; the mol% G+C content of the DNA lies between 69 and 78 [2,3]. Species are widely distributed and abundant in soil, including composts, plants, marine water and rivers. A few species are phytopathogens; a few others are pathogenic for humans and animals. It is characterized by an extensively branched substrate mycelium and aerial hyphae, 0.5–2.0 mm diameter, rarely fragmented or have no septation [4]. At maturity, the aerial mycelium forms chains of three to several spores (Figure 1). Some members have sporangia-, pycnidial-, sclerotia-, and synnemata-like structures [5]. The spores are nonmotile, its surface may be hairy, rugose, smooth, spiny or warty [6]. Frequently, colonies initially have a smooth surface but later develop a weft of aerial mycelium that may appear floccose, granular, powdery, or velvety. Colonies are discrete and lichenoid, leathery, or butyrous. Streptomyces produce a wide range of pigments responsible for the color of the vegetative and aerial mycelia. Additionally, colored diffusible pigments may be generated [3].

Fig. 1 Cross section of Streptomyces colony.

The optimal growth temperature for most species ranged between 25–35οC. However, some species can grow at temperatures within the psychrophilic and thermophilic range. The optimum pH range for growth varied between 6.5 and 8.0. The genus can produce earthy odor that results from production of volatile metabolites geosmin (literally ‘earth smell) which gives the soil its characteristic smell, and provides an indication of just how widespread these are in the soil [7]. Upon its first characterization, it was noted that S. antibioticus produces a distinct soil odor. Streptomyces are not just free-living soil bacteria, but also form symbioses with other organisms, most notably plants and invertebrates [8]. They used Streptomyces antibiotics producer to protect themselves against infection, on the other

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Understanding microbial pathogens: current knowledge and educational ideas on antimicrobial research (Enrique Torres-Hergueta and A. Méndez-Vilas, Eds.) hand, the plant can produce exudates which allows the development of Streptomyces in the symbiosis between Streptomyces and plants. Their species may be pathogenic for humans (such as S. somaliensis and S. sudanensiscausing mycetoma), animals or plants (such as S. caviscabies, S. acidiscabies, S. turgidiscabies and S. scabies) [9]. The nutrition of Streptomyces is chemoorganotrophic with an oxidative type of metabolism. Streptomyces is catalase positive, usually degrade polymeric substrates such as casein, gelatin, hypoxanthine, and starch in addition to adenine and L-tyrosine and reduce nitrates to nitrites. Peptidoglycan is the main component of cell wall, contains major quantities of LL-diaminopimelic acid, sometimes, low amounts of meso-diaminopimelic acid are present. The lipid profile contains major amounts of saturated, iso- and anteiso-fatty acids but lacks mycolic acids. In addition, they typically possess either hexa- or octa-hydrogenated menaquinones with nine isoprene units as the predominant isoprenolog, but menaquinones with eight and ten isoprene units are also found. A complex polar lipid pattern normally contains diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylinositol, and phosphatidylinositol mannosides [10]. All Streptomyces strains studied contain a large genome, typically of linear topology. The chromosomes of sequenced Streptomyces range in size from 6,283,062 bp for S. cattleya to 11,936,683 bp predicted to code for 10,023 genes in S. bingchenggensis. The S. coelicolor A3(2) genome was originally considered to be circular and representing the model Streptomycetes [11]. In addition Streptomyces can contain small, covalently closed circular (ccc) high copy- number plasmids, larger ccc low copy-number plasmids, ccc plasmids that arise by reversible site-specific recombination from the chromosome, and linear plasmids that share features of the chromosome: a centrally located origin of replication and terminal inverted repeats with bound terminal proteins to prime end patching. A property common to may be all is their self transmissibility and ability to mobilize chromosomal DNA, making them supremely important in promoting genetic exchange and Streptomyces evolution [3]. Streptomyces considered as a promising source of wide range of important enzymes, some of which are produced on an industrial scale. They could produce amylase, cellulase, protease, tyrosinase, chitinase, lipase, catalase and phosphatase [12]. They have the ability to degrade a wide range of hydrocarbons, pesticides, aliphatic and aromatic compounds [13]. They have an important ecological role in the turnover of organic material in the soil. Streptomyces act as plant growth promoting rhizobacteria (PGPR) such as S. atrovirens which produce plant growth hormone; indol acetic acid (IAA) [14]. As well as, they are enormously important to human medicine, agriculture and food production [8]. The most interesting property of Streptomyces is the ability to produce bioactive secondary metabolites, such as antibacterial, antifungal, antiviral, antitumor, antiparasitic anti-hypertensives, and immunosuppressant compounds [15]. They produce over two-thirds of the clinically useful antibiotics of natural origin (e.g., , and , etc.) [16]. The production of most antibiotics is species specific, many strains are able to produce one or more compounds to compete with other microorganisms in the environment and protect the plant against different pathogens. Streptomyces is one of the most predominantly sources of antibiotics used in many pharmaceutical and biotechnological applications. Almost all of the bioactive compounds produced by Streptomyces are initiated during the time coinciding with the aerial hyphal formation from the substrate mycelium [5]. Streptomyces carry resistance to their own antibiotics to avoid suicide and, under selective pressure, these resistance genes can spread to other soil bacteria and to pathogenic bacteria via horizontal gene transfer [17]. The history of antibiotics, which inhibit bacterial growth, started at 1942 with the discovery of streptothricin from Streptomyces, followed by the discovery of streptomycin, after that, the scientists intensified the search for other antibiotics from the genus. The antibiotics produced by Streptomyces were classified according to chemical structure into 6 categories: aminoglycosides, beta-lactams (β-lactams), chloramphenicol, lipopeptide, macrolides and tetracyclines, some of which aminoglycosides and β-Lactams act as bactericidal agents and others (tetracyclines, chloramphenicol and macrolides) as bacteriostatic agents.

2. Aminoglycosides Aminoglycosides are class of antibiotics (contain aminosugar substructures) that possess cyclohexyl rings substituted with amine groups and linked together by glycosidic bonds. Several Streptomyces species could produce aminoglycosides antibiotics such as streptomycin (produced by S. griseus), neomycin (S. fradiae) and kanamycin (S. kanamyceticus), gentamycin (actinobacterium purpurea) and (S. tenebrarius). The most common and highly important of these antibiotics are streptomycin and neomycin. They have extensive application in the treatment of numerous infectious diseases.

2.1 Streptomycin The antibiotic streptomycin was first isolated in 1943, by Schatz and Waksman (Torok et al. 2009). Streptomycin was also the first antibiotic cure for treatment of tuberculosis (TB). Its structure consists of three

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Understanding microbial pathogens: current knowledge and educational ideas on antimicrobial research (Enrique Torres-Hergueta and A. Méndez-Vilas, Eds.) components (streptidine, streptose and streptoscamine) linked glycosidically by ether bond (Figure 2). Streptomycin is a broad-spectrum antibiotic inhibits both Gram-positive and Gram-negative bacteria, and it was found to have the inhibitory effect for several species of Mycobacterium [18].

Fig. 2 Chemical structure of streptomycin.

Mechanism of action: Streptomycin and other aminoglycosidic antibiotics (e.g., gentamycin, neomycin, kanamycin, tobramycin), inhibits protein synthesis in bacterial cells by binding to the small 16S rRNA of the 30S subunit of ribosomes. Also, it interferes with the binding of formyl-methionyl-tRNA to the 30S subunit, leading to codon misreading, eventual inhibition of protein synthesis and ultimately death of microbial cells [19]. Applications: The streptomycin drug could be used in combination with other antibiotics for treatment of tuberculosis. For active tuberculosis it is often given together with isoniazid, rifampicin, and pyrazinamide. It is used in treatment of endocarditis caused by Enterococcus when the organism is not sensitive to gentamycin. Also, it is used to treat Burkholderia infection, plague, tularemia, and rat bite fever [20]. It is worthy to mention that, streptomycin may be useful in treatment pathogens resistance to other drugs. In veterinary medicine, streptomycin is the first-line antibiotic against gram negative bacteria in animals (horses, cattle, sheep, etc.). It is commonly combined with procaine penicillin for intramuscular injection. Streptomycin was also used as a pesticide, to combat the growth of bacteria beyond human applications [21]. Also, streptomycin controls bacterial diseases of certain fruit, vegetables, seed, and ornamental crops. A major use is in the control of fire blight on apple and pear trees. Extensive use can be associated with the development of resistant strains. Streptomycin could potentially be used to control cyanobacterial blooms in ornamental ponds and aquaria. In cell culture, streptomycin used in combination with penicillin to prevent the bacterial infection. It uses during purification of protein from the biological extract, streptomycin sulfate is sometimes added to remove nucleic acids (rRNA, mRNA, DNA).

2.2 Neomycin Neomycin is an aminoglycoside antibiotic that contains two or more amino sugars connected by glycosidic bonds (Figure 3). The discovery of neomycin dates back to 1949 in the lab of Waksman. It is produced naturally by Streptomyces fradiae and S. albogriseus [22]. Its synthesis requires specific nutrient conditions in either stationary or submerged aerobic conditions.

Fig. 3 Chemical structure of neomycin.

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Understanding microbial pathogens: current knowledge and educational ideas on antimicrobial research (Enrique Torres-Hergueta and A. Méndez-Vilas, Eds.)

Mechanism of action: Neomycin binds to the 30S subunit of the ribosome and inhibits translation of proteins from mRNA. It exhibits a high binding affinity for phosphatidylinositol 4,5-bisphosphate (PIP2), which is a phospholipid component of cell membranes [23]. Applications: Neomycin has an inhibitory activity against Gram-positive and Gram-negative bacteria, and is partially effective against Gram-positive bacteria, but is very ototoxic. Thus, the use is restricted to oral treatment of intestinal infections [24].

3. β-Lactams (Carbapenems) β-Lactam antibiotic is a cyclic amide, containing β-lactam ring. Its name is derived from attachment of the nitrogen atom to the β-carbon atom relative to the carbonyl. The drug was recorded to be produced from members of the genus Streptomyces, such as Streptomyces cattleya (cephamycins and thienamycin), S. clavuligerus (cephalosporin) [25]. The structure of carbapenems are very similar to the penicillins (penams), but the sulfur atom at the position 1 of the structure has been replaced with a carbon atom, and an unsaturation has been introduced- hence the name of the group, the carbapenems. β-lactams (carbapenems) are a class of highly effective antibiotic agents commonly used for the treatment of severe or high-risk bacterial infections. This class of antibiotics is usually reserved for known or suspected multidrug- resistant (MDR) bacterial infections. As well as, carbapenems generally exhibit good activity against anaerobes such as Bacteriodes fragilis. However, these agents individually exhibit a broader spectrum of activity compared to most cephalosporins and penicillins. Furthermore, carbapenems are typically unaffected by emerging antibiotic resistance, even to other β-lactams.

3.1 Thienamycin Carbapenem thienamycin also known as Thienpenem is one of the most potent naturally produced antibiotic. It was discovered in Streptomyces cattleya in 1976 [26]. Thienamycin was the first discovered and isolated antibiotic among the naturally occurring class of carbapenem antibiotics. Carbapenem antibiotics were originally developed at Merck & Co. from the carbapenem thienamycin, a naturally derived product of Streptomyces cattleya [26]. Concern has arisen in recent years over increasing rates of resistance to carbapenems, as there are few therapeutic options for treating infections caused by carbapenem-resistant bacteria (such as Klebsiella pneumoniae and other carbapenem-resistant Enterobacteriaceae. Carbapenems are similar in structure to their antibiotic “cousins” the penicillins. Like penicillins, carbapenems contain a β-lactam ring (cyclic amide) fused to a five-membered ring, but differ from penicillins structure in that a  ulphur is replaced by a carbon atom (C1) within the five-membered ring and an unsaturation is present between C2 and C3 in the five-membered ring (Figure 4).

Fig. 4 Chemical structure of thienamycin. Mechanism of action: In vitro, thienamycin employs a similar mode of action as penicillins through disrupting the cell wall synthesis (peptidoglycan biosynthesis) of various Gram-positive and Gram-negative bacteria (Staphylococcus aureus, Staphylococcus epidermidis, Pseudomonas aeruginosa) [27]. Although thienamycin binds to all of the penicillin-binding proteins (PBPs) in Escherichia coli, it preferentially binds to PBP-1 and PBP-2, which are both associated with the elongation of the cell wall [28]. Unlike penicillins, which are rendered ineffective through rapid hydrolysis by the β-lactamase enzyme present in some strains of bacteria, thienamycin remains antimicrobially active. Due to low titre and to difficulties in isolating and purifying thienamycin produced by fermentation, total synthesis is the preferred method for commercial production. Applications: Thienamycin has a potential activity against both Gram-positive and Gram-negative bacteria and is resistant to bacterial β-lactamase enzymes. Thienamycin is a zwitterion at pH 7 [29]. Thienamycin displayed high activity against bacteria resistant to other β-lactamase-stable compounds (cephalosporins), highlighting the superiority of thienamycin as an antibiotic among β-lactams [30].

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Understanding microbial pathogens: current knowledge and educational ideas on antimicrobial research (Enrique Torres-Hergueta and A. Méndez-Vilas, Eds.)

3.2 Cephamycins Cephamycins are a group of β-lactam antibiotics, very similar to cephalosporins, and sometimes classified as cephalosporins. Cephamycins were originally produced by Streptomyces cattleya, but synthetic ones have been produced as well. It possesses a methoxy group at the 7-alpha position [31].

Fig. 5 Chemical structure of cephamycin.

Mechanism of action: similar to penicillins and cephalosporins cephamycin are members of the β- lactam antibiotics, which inhibiting bacterial cell wall synthesis by binding to penicillin-binding proteins. Applications: Cephamycins have been shown to be stable against extended-spectrum beta-lactamase (ESBL) producing organisms. It is very efficient antibiotic against anaerobic microbes.

4. Chloramphenicol Chloramphenicol was first isolated from Streptomyces venezuelae in 1947 and it was the first antibiotic to be produced instead of extracted from a micro-organism [32]. It is nitrobenzene derivatives (distinct individual compound). Chloramphenicol is available as a generic worldwide under various brand and generic names in Eastern Europe and Russia, including chlornitromycin, levomycetin, and chloromycetin. Chloramphenicol is a valuable antibiotic for the treatment of a number of bacterial infections such as conjunctivitis (as eye ointment), meningitis, plague, cholera and typhoid.

Fig. 6 Chemical structure of chloramphenicol.

Mechanism of action: Chloramphenicol is a bacteriostatic drug inhibiting protein synthesis. It prevents protein chain elongation by inhibiting the activity of the bacterial ribosome. It specifically binds to the 23S rRNA of the 50S ribosomal subunit, preventing peptide bond formation, it directly interferes with substrate binding [33].

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Understanding microbial pathogens: current knowledge and educational ideas on antimicrobial research (Enrique Torres-Hergueta and A. Méndez-Vilas, Eds.)

Applications: Chloramphenicol used as a second-line agent in the treatment of -resistant cholera. It is used in the treatment of brain abscesses caused by mixed organism infections, or when the causative organism is not known. Chloramphenicol is active against the three main causative of bacterial meningitis: Neisseria meningitidis, Streptococcus pneumoniae, and Haemophilus influenzae. In low-income countries, the WHO no longer recommends oily chloramphenicol as first-line to treat meningitis, but it may be used with caution if there are no available alternatives. The drug is also effective against vancomycin-resistant Enterococcus faecium [34]. Chloramphenicol is still used occasionally in topical preparations in ointments and eye drops formulas for treatment of bacterial conjunctivitis. The isolated case reported aplastic anemia following the use of chloramphenicol eye drops, but the risk is estimated to be of the order of less than one in 224,716 prescriptions.

4.1 Veterinary uses Chloramphenicol has some important veterinary applications. It is currently considered the most useful treatment of chlamydial disease in koalas [35]. It has been discovered to be a life-saving cure for chytridiomycosis (fungal disease) in amphibians and applied to frogs to prevent their widespread destruction by fungal infections [36].

5. Lipopeptide antibiotics Streptomyces produced lipopeptide antibiotics such as daptomycin. Daptomycin is a cyclic lipopeptide antibacterial agent, it is a naturally occurring compound found in the soil saprotroph Streptomyces roseosporus. Its distinct mechanism of action makes it useful in treating infections caused by multiple drug-resistant bacteria. It was discovered by researchers at Eli Lilly and Company in the 1980. It has been approved for the treatment of complicated skin and skin structure infections.

5.1 Daptomycin Daptomycin is a cyclic lipopeptide antibiotic produced by Streptomyces roseosporus [37]. Daptomycin consists of 13 amino acids, 10 of which are arranged in a cyclic fashion, and three on an exocyclic tail. Two nonproteinogenic amino acids exist in the lipopeptide, the unusual amino acid L-kynurenine (Kyn), only known to daptomycin, and L-3- methylglutamic acid (mGlu). The N-terminus of the exocyclic tryptophan residue is coupled to decanoic acid, a medium-chain (C10) fatty acid.

Fig. 7 Chemical structure of Daptomycin.

Mechanism of action: Daptomycin has a distinct mechanism of action, disrupting multiple aspects of bacterial cell membrane function. It inserts into the cell membrane in a phosphatidylglycerol-dependent fashion, where it then aggregates. The aggregation of daptomycin alters the curvature of the membrane, which creates holes that leak ions. This causes rapid depolarization, resulting in disrupts the membrane potential leading to blocking of protein, DNA, and RNAsynthesis, which results in bacterial cell death [38]. Applications: Daptomycin is a lipopeptide antibiotic used in the treatment of systemic and life-threatening infections caused by Gram-positive organisms. It acts as bactericidal against Gram-positive bacteria only [39]. It has proven in vitro activity against enterococci (including glycopeptide-resistant enterococci), staphylococci including methicillin- resistant Staphylococcus aureus, streptococci and corynobacteria.

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Understanding microbial pathogens: current knowledge and educational ideas on antimicrobial research (Enrique Torres-Hergueta and A. Méndez-Vilas, Eds.)

6. Macrolides antibiotics Many species of Streptomyces produced macrolides. The macrolides belong to the polyketide class of natural products that consist of a large macrocyclic lactone ring to which one or more deoxy sugars, usually cladinose and desosamine, may be attached. The lactone rings are usually 14-, 15-, or 16-membered. Some macrolides have antibiotic or antifungal activity and are used as pharmaceutical drugs. Streptomyces noursei produce antifungals nystatin (the first Actinobacteria-sourced human antifungal), natamycin (produce by S. natalensis) and amphotericin B (produce by S. nodosus). was first isolated from Saccharopolyspora erythraea formerly known as Streptomyces erythraeus. Boromycin is a bacteriocidal antibiotic; it was the first natural product found to contain the element boron initially isolated from Streptomyces antibioticus, and used as antiviral agent.

6.1 Boromycin Boromycin is a bacteriocidal macrolide antibiotic. It was initially isolated from Streptomyces antibioticus strain found in an African soil sample, and is notable for being the first natural product found to contain the element boron. It is effective against most Gram-positive bacteria.

Fig. 8 Chemical structure of Boromycin.

Mechanism of action: Boromycin kills bacteria through its effect on cytoplasmic membrane, resulting in the loss of potassium ions from the cell [40]. Recent studies have suggested has potent anti-HIV activity through its effect on the inhibition of the replication of the clinically isolate HIV-1 strain as well as the culture strain in vitro. It involve interfering with the later stage of HIV infection, and possibly the maturation step for the replication of HIV [41]. Applications: It is effective against most Gram-positive bacteria and used as antiviral agent. Boromycin has also been shown to have antifungal and antiprotozoal properties.

6.2 Nystatin Nystatin was discovered in 1950 by Brown and Hazen. Nystatin A1 (or referred to as nystatin) is biosynthesized by Streptomyces noursei [42]. The structure of this active compound is characterized as a polyene macrolide with a deoxysugar D-mycosamine, an aminoglycoside [43].

Fig. 9 Chemical structure of nystatin.

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Understanding microbial pathogens: current knowledge and educational ideas on antimicrobial research (Enrique Torres-Hergueta and A. Méndez-Vilas, Eds.)

Mechanism of action: Nystatin binds to ergosterol, a major component of the fungal cell membrane. When present in sufficient concentrations, it forms pores in the membrane that lead to K+ leakage, acidification, and death of the fungus [44]. Ergosterol is a sterol unique to fungi, so the drug does not have such catastrophic effects on animals or plants. Applications: Nystatin, sold under the brand name Mycostatin among others, is an antifungal medication. It used in the treatment of skin, vaginal, mouth, and esophageal Candida infections. It may also be used to prevent candidiasis in those who are at high risk. It has been investigated for use in patients after liver transplantation, for preventing colonization, invasive infection, and death [45]. It is also used in cellular biology as an inhibitor of the lipid raft- caveolae endocytosis pathway on mammalian cells, at concentrations around 3 µg/ml. In certain cases, nystatin has been used to prevent the spread of mold on objects such as works of art. Nystatin is also used as a tool by scientists performing "perforated" patch-clamp electrophysiologic recordings of cells. When loaded in the recording pipette, it allows for measurement of electrical currents without washing out the intracellular contents, because it forms pores in the cell membrane that are permeable to only monovalent ions [46].

6.3 Natamycin It is in the macrolide and polyene families of medications. Natamycin, also known as pimaricin, is an antifungal medication used to treat fungal infections [47]. Natamycin was first isolated in 1955 from fermentation broth of a Streptomyces natalensis cell culture.

Fig. 10 Chemical structure of natamycin.

Mechanism of action: Natamcyin is able to inhibit growth of fungi by inhibiting transport of amino acids and glucose across the plasma membrane. Natamycin performs this function by specifically binding to ergosterol and inhibiting membrane transport proteins [48]. Natamycin has a very low solubility in water; however, natamycin is effective at very low levels. Its minimum inhibitory concentration is less than 10 ppm for most molds. Applications: Natamycin is used to treat fungal infections around the eye. This includes infections of eyelids, conjunctiva and cornea. Natamycin is also used in the food industry as a preservative [47]. It used also in fungal infections including Candida, Aspergillus, Cephalosporium, Fusarium, and Penicillium [49]. Natamycin has been used for decades in the food industry as a hurdle to fungal outgrowth in dairy products and other foods. Potential advantages for the usage of natamycin for replacement of traditional chemical preservatives, a neutral flavor impact, and less dependence on pH for efficacy, as is common with chemical preservatives, natamycin is to replace the artificial preservative ascorbic acid.[50]. More specifically, natamycin is commonly used in products such as cream cheeses, cottage cheese, sour cream, yogurt, shredded cheeses, cheese slices, and packaged salad mixes.

7. Tetracyclines A large family of , that contain 4 adjacent cyclic hydrocarbon rings (figure 11), were discovered as natural products by Duggar in 1945 [51]. Several semisynthetic derivatives were prepared, which together are known as the tetracycline antibiotics. The term "tetracycline" is also used to denote the four-ring system of this compound. Tetracycline has the brand name Sumycin and used to treat several bacterial infections include acne, cholera, brucellosis, plague, malaria and syphilis [52]. Tetracycline is a broad-spectrum antibiotic produced by Streptomyces rimosus and S. aureofaciens.

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Understanding microbial pathogens: current knowledge and educational ideas on antimicrobial research (Enrique Torres-Hergueta and A. Méndez-Vilas, Eds.)

Fig. 11 Chemical structure of tetracycline.

Mechanism of action: Tetracycline antibiotics are protein synthesis inhibitors, blocking the attachment of charged aminoacyl-tRNA to the site A on the ribosome and binds to the 30S and 40S ribosomal subunit in the mRNA translation complex of both prokaryotes and eukaryotes, respectively. Thus, it prevents introduction of new amino acids to the nascent peptide chain [53]. But, bacteria actively pump tetracycline into their cytoplasm, even against a concentration gradient, whereas mammalian cells do not. This accounts for the relatively small off-site effect of tetracycline on human cells Applications: The drug tetracycline is first-line therapy for Rocky Mountain spotted fever (caused by Rickettsia), Lymedisease (B.burgdorferi), Qfever (Coxiella), psittacosis, lymphogranulomavenereum (Chlamydia), and Mycoplasma pneumoniae and eradicate nasal carriage of meningococci [54]. they possessed some level of bacteriostatic activity against almost all medically relevant aerobic and anaerobic bacterial genera, both Gram- positive and Gram-negative, with a few exceptions, such as Pseudomonas aeruginosa and Proteus spp., which display intrinsic resistance [55]. In genetic engineering, tetracycline is used in transcriptional activation. It is also one of a group of antibiotics which together may be used to treat peptic ulcers caused by bacterial infections. In cancer research at Harvard Medical School, tetracycline has been used to switch off leukemia in genetically altered mice, and to do so reliably, when added to their drinking water [56]. Tetracycline is used in cell biology as a selective agent in cell culture systems. It is toxic to prokaryotic and eukaryotic cells and selects for cells harboring the bacterial tet r gene, which encodes a 399-amino- acid, membrane-associated protein. This protein actively exports tetracycline from the cell, rendering cells harboring this gene more resistant to the drug. It is used as a marker of bone growth for biopsies in humans. Tetracycline labeling is used to determine the amount of bone growth within a certain period of time, usually a period around 21 days. Tetracycline is incorporated into mineralizing bone and can be detected by its fluorescence [57]. These agents also have activity against certain eukaryotic parasites, including those responsible for diseases such as malaria and balantidiasis.

7.1 Immunosuppressant: Rapamycin Sirolimus, also known as, rapamycin is a macrolide compound that is used to coat coronary stents, prevent organ transplant rejection and to treat a rare lung disease called lymphangioleiomyomatosis. It is produced by Streptomyces hygroscopicus and was isolated for the first time in 1972 by Surendra Nath Sehgal and colleagues. The compound was originally named rapamycin after the native name of the island, Rapa Nui. Sirolimus was initially developed as an antifungal agent [58]. It was approved and marketed under the trade name Rapamune by Pfizer.

Fig. 12 Chemical structure of rapamycin.

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Understanding microbial pathogens: current knowledge and educational ideas on antimicrobial research (Enrique Torres-Hergueta and A. Méndez-Vilas, Eds.)

Mechamism of action: Rapamycin has immunosuppressant functions in humans and is especially useful in preventing the rejection of kidney transplants. It inhibits activation of T cells and B cells by reducing their sensitivity to interleukin-2 (IL-2) through mTOR inhibition [59]. Applications: Sirolimus is indicated for the prevention of organ transplant rejection and for the treatment of lymphangioleiomyomatosis (LAM). Sirolimus can also be used alone, or in conjunction with a calcineurin inhibitor (such as tacrolimus), and/or mycophenolatemofetil, to provide steroid-free immunosuppression regimens. Its optimal role in immunosuppression has not yet been determined [59]

7.2 Chemotherapy drugs A number of the antibiotics produced by Streptomyces have proven to be too toxic for use as antibiotics in humans and due to their toxicity towards cells (specifically dividing cells) they have been reinvented as chemotherapy drugs. Such as; actinomycin-D (the original, produced by S. antibioticus), bleomycin (glycopeptide produced by S. verticullus), mitomycin (aziridine produced by S. lavendulae) and plicamycin (produced by S. plicatus) [60]. The anthracyclines daunorubicin and doxorubicin (produced by S. peucetius) and migrastatin (macrolide, produced by S. platensis).

7.3 Anticancer drug Streptomycetes produce organic compounds used in other medical treatments such as, migrastatin from S. platensis [61] and bleomycin from S. verticillus [62] are antineoplastic (anticancer) drugs. They inhibit the metastasis of cancer cells.

7.4 Antiparasitic drugs S. avermitilis produce ivermectin which represents one of the most widely employed drugs against nematode and arthropod infestations [63].

7.5 Natural herbicide S. hygroscopicus and S. viridochromeogenes produce the natural herbicide bialaphos. Bialaphos is a tripeptide herbicide that consists of a glutamic acid analogue moiety, called phosphinothricin and two alanine residues. It is used in transformation experiments of many species of plants that make use of the bar gene for selection [64].

Acknowledgements The author would like to thank Prof. Mady A. Ismail (Department of Botany & Microbiology, Faculty of Science, Assiut University) for revising the review.

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