Bioprospecting - Secondary metabolites, Antibiotics, antiviral agents, immunosuppressive agents and therapeutic agents

  Secondary metabolites

    Plants select competent endophytes from the environment for their own ecological benefit. Many diverse and highly specialized secondary metabolites and enzymes are produced by the inhabiting microorganisms. Microbial production of phytohormones, antibiotics and quorum-sensing molecules has been reported in plants. Cytokinins and auxins are plant growth regulators expressed by a vast amount of microbes, especially bacteria. Microbial indole 3-acetic acid production can be of importance to plants, either beneficial by promoting root development or harmful as in crown gall formation induced by Agrobacterium tumefaciens.

 Antibiotics

    Penicillin, the classical example of  natural antibiotics is produced by Penicillium spp. Penicillin is a well-known antibiotic from P. notatum and is effective against Gram-positive bacteria, causing scarlet fever, pneumonia, gonorrhea, meningitis, and diphtheria. Streptomycin produced by S. griseus is effective against pulmonary tuberculosis. Later, kanamycin, gentamicin, sisomicin, and lividomycin have been discovered and widely used to treat infectious organisms that have developed resistance against Streptomycin. Vancomycin is another antibiotic produced by Actinomycetes against Listeria monocytogenes, Streptococcus pneumoniae and methicillin-resistant Staphylococcus aureus (MRSA). 
    Pikromycin is a  antibiotic produced from S. venezuelae and is very potent against multi-drug resistant respiratory pathogens. Erythromycin A produced by S. erythraea, against a wide variety of bacterial infections, especially in patients who have adverse reactions against penicillin.
  Examples of antimicrobial compounds synthesized by plant associated microbes include cyclosporine that is produced by the endophytic fungus Tolypocladium inflatum and shows antifungal activity.  Bioactive compounds produced by plant associated microbes display antiviral activity also. Xiamycin from Streptomyces sp. is reported to have anti-HIV properties.

      
                    Biological activities of microbial-derived natural products and biologics.

    Natural antimicrobials have also been important to the food industry in terms of food safety against foodborne pathogens. Microbes such as lactic acid bacteria, produce a wide range of chemicals that have been shown to inhibit the growth and development of other microbial species. Nisin A, a bacteriocin produced by Lactococcus lactis, is approved to preserve food in over 50 countries and is very active against Gram-positive bacteria resistant to conventional antibiotics. Reuterin from Lactobacillus reuteri has been shown to have antimicrobial activities against foodborne pathogens and spoilage organisms when evaluated in milk, dairy, and meat products.

Antifungal Agents

    Nystatin, obtained from Streptomyces noursei  is effective against Aspergillus species and is used as  a topical antifungal agent. Amphotericin B is an antifungal product of Streptomyces nodosus active against life-threatening fungal infections caused by Aspergillus species, and is effective in patients who have undergone organ transplantation, received aggressive chemotherapy or with acquired immunodeficiency syndrome.

Antifungal ecomycins derived from the plant endophyte Pseudomonas viridiflava. The ecomycins are active against such human-pathogenic fungi as Cryptococcus neoformans and Candida albicans. Another group of antifungal compounds is the Pseudomycins, produced by a plant-associated pseudomonad. They are active against a variety of plant- and human-pathogenic fungi. Pseudomycins are also effective against a number of ascomycetous fungi, they are also being considered for agricultural use. Ambuic acid, a cyclohexenone produced by a number of isolates of Pestalotiopsis microspora found in rainforests around the world, possesses antifungal activity. 

Antifungal peptide cryptocandin, produced by Cryptosporiopsis quercina is active against a number of plant-pathogenic fungi. Cryptocandin and its related compounds are in use against a number of fungi causing diseases of skin and nails. Cryptocin, possesses potent activity against  plant-pathogenic fungi. 

Antibiotics Turbomycin A and B are isolated from a soil derived metagenomic library.

Antiviral Compounds
Endophytic microorganisms also produce antiviral agents which cause inhibition of viruses. Human cytomegalovirus protease inhibitors, cytonic acids A and B, aren isolated from the endophytic fungus Cytonaema sp. The potential for the discovery of compounds, from endophytes, having antiviral activity is in its infancy. The main limitation in compound discovery is related to the absence of appropriate antiviral screening systems in most compound discovery programs.

Immunosuppressive Compounds 
Immunosuppressive drugs are used today to prevent allograft rejection in transplant patients, and could be used to treat autoimmune diseases such as rheumatoid arthritis and insulin-dependent diabetes. The endophytic fungus Fusarium subglutinans, isolated from Tripterygium wilfordii, produces the immunosuppressive, subglutinol A and B. The fungus Tolypocladium inflatum from which cyclosporine, a beneficial immunosuppressant, was isolated is another example.

Rapamycin (also known as sirolimus) and FK506 (tacrolimus) are microbial natural products with immunosuppressive properties. Rapamycin also exhibits synergism with other immunosuppressants, such as cyclosporin, to significantly reduce kidney toxicity and acute renal allograft rejection. In addition to its immunosuppressive activity, rapamycin possesses several other biological activities, including antitumor, neuroprotective/neuroregenerative, antineoplastic, and lifespan extension activities.

FK506 is also an immunosuppressive drug and was first discovered in soil samples containing Streptomyces tsukubaensis and several other Streptomyces species and is used to minimize organ rejection. It has been demonstrated to be more effective than cyclosporin and non-toxic in low doses and is widely used in heart, liver, and kidney transplants. Like rapamycin, FK506 possesses various biological activities, including antifungal, anti-inflammatory, neuroprotective, and neuroregenerative activities.

Anti-inflammatory Agents
Some natural products also have anti-inflammatory activities. FK506 has shown efficacy in the treatment of refractory rheumatoid arthritis, a chronic inflammatory disease. Rapamycin also inhibits the inflammatory response, thereby reducing secondary injury in the spinal cord and providing a neuroprotective effect. Strepsesquitriol and Salinamides A and B isolated from Streptomyces sp. has anti-inflammatory activity.

Anticancer Agents 

There are many microbe-derived anticancer agents such as actinomycin isolated from Streptomyces parvulus  and shown to have anticancer activity. Actinomycin D, also known as dactinomycin, is approved by FDA and is used in clinical practice as an anticancer drug.

A therapeutic combination of the microbial product bleomycin, the plant compound etoposide, and the synthetic agent cisplatin, has played a significant role in treating testicular cancer. 

Bleomycin is produced by Streptoalloteichus hindustanus and is used as an anticancer agent. Its derivative, blenoxane is also used clinically with other compounds against lymphomas, skin carcinomas etc. Streptomyces species produce daunorubicin and doxorubicin which is FDA approved for cancer therapy.

Rapamycin, a natural product derived from Streptomyces rapamycinicus has anticancer, immunosuppressive, anti-inflammatory, and antifungal activities. Wortmannin is produced by Penicillium and has antitumor activity. Geldanamycin is an antitumor compound derived from Streptomyces hygroscopicus. Another example is epothilone, an anticancer agent produced from myxobacterium Sorangium cellulosum. 

There are also marine microbial natural products that have anticancer activities, such as dolastatin, which is originated from cyanobacteria of the genera Symploca and Lyngbya.

Paclitaxel, the world's first billion-dollar anticancer drug, is produced by many endophytic fungi,  Taxomyces andreanae,  living within yew tree, Taxus brevifolia. Paclitaxel is used to treat a number of other human tissue-proliferating diseases as well. Trichothecium sp, Pestalotiopsis microspora, Tubercularia and many novel endophytic fungal species produce Paclitaxel to protect  host plant from plant pathogens. Torreyanic acid, another anticancer agent was isolated from a Pestalotiopsis microspora strain. Fungal genera as Xylaria, Phoma, etc produce cytochalasins, compounds with antitumor and antibiotic activities

Antioxidants from Endophytes

Pestacin and Isopestacin, obtained from culture fluids of Pestalotiopsis microspora, has antimicrobial as well as antioxidant activity.

Insecticidal compounds from Endophytes 

Nodulisporic acids, exhibiting potent insecticidal properties are isolated from an endophyte, a Nodulisporium sp., from the plant Bontia daphnoides. Another endophytic fungus, Muscodor vitigenus, yields naphthalene as its major product. Naphthalene, the active ingredient in common mothballs, is a widely exploited insect repellant. 

Antidiabetic Agents from Rainforest Fungi
An endophytic fungus, Pseudomassaria sp. collected from an African rainforest produces an antidiabetic compound which acts like insulin but, unlike insulin, is not destroyed in the digestive tract and may be given orally. These results may lead to new therapies for diabetes.

Biofilm Inhibitory Agents

Pathogenic microorganisms adhere to solid surfaces and form layers of a complex polysaccharide matrix called a biofilm that confers resistance against antibiotics and results in chronic bacterial infections. Cahuitamycins derived from the marine bacterium Streptomyces gandocaensis are evaluated as inhibitors of Acinetobacter baumannii biofilms Actinomycin D has significant biofilm inhibitory activity against methicillin resistant- and sensitive-strains of S. aureus.

Lactobacillus biosurfactants display anti-adhesive biofilm formation properties against C. albicans (infections linked to biofilm formation on medical devices) and prevent biofilm formation of Listeria monocytogenes, Salmonella arizonae, E. coli, and S. aureus  C. albicans.

Others

Natural products can also act as antiparasitic agents. The avermectins and the derivative ivermectin have shown antiparasitic activity. Spinosad and milbemycin also have insecticidal activity. Spinosad is a combination of spinosyn A and D, which are both produced by Saccharopolyspora spinosa. Milbemycin is an isolated fermentation product of S. hygroscopicus and acts as an insecticide. Mollemycin from a Streptomyces sp. has antibacterial properties against certain Gram-positive and Gram-negative bacteria, as well as extremely potent antimalarial activity against drug sensitive and MDR Plasmodium falciparum.