INTRODUCTIONIn recent years, oneof the most interesting research topic is endophyte. Endophyte are the”microbes that colonize living, internal tissues of plants without causing anyimmediate overt negative effects”1. It’s very familiar tous that more or less every plant species is expectedto have one or more endophyte species2,3.Endophytic microorganisms include fungi andbacteria. After entering the host, endophytes reside withinintracellular or the intercellular spaces or in the vascular tissues of host plant4. After gainingresidence in the plant tissues, the endophytes are known to produce a diverserange of natural products which could be consistent and successful source of drugs. .
After the discovery of taxol produced byendophytic fungus Taxomyces andreanae from Taxus brevifolia researchershave been motivated to evaluate the potentialityof these microorganisms in the production ofbioactive compounds5. Endophytesprovide a broad variety of bioactive secondary metabolites, includingalkaloids, benzopyranones, chinones, flavonoids, phenolic acids, quinones,steroids, terpenoids, tetralones, xanthones, and others6. Endophytes have the potentiality to produce different bioactivemetabolites which play an important role in antimicrobial, anticancer and otherpharmaceutical activity7,8.Such bioactive metabolites from endophytes findwide-ranging application as antimicrobials, agrochemicals, antibiotics, immunosuppressants,antiparasitics, antioxidants, and anticanceragents9. This review focuses particularly on bacterialendophytes as a potential source of novel antibiotics. COLONISATION OF ENDOPHYTESBacterialendophytes are colonise mainly intercellularly10,11, though they have also been foundintracellularly, e.g.
Azoarcus sp.12. Bacteria are also frequently found in the vascular tissues of hostplants13. ECOLOGY AND BIODIVERSITY OF ENDOPHYTESEndophytes present within the host plant includingherbaceous to angiosperm plant. Endophytes are isolated from their host plantgrowing in tropical,temperate & boreal forests14.
Endophytes are found in various habitatincluding extreme arctic,alpine,xeric environment 6and also found in mesic,temperate andtropical forests 14. . Endophytes have been present within theirhosts ranging from mosses, ferns, gymnosperms and angiosperms6.Endophytes have been also recorded inclusion of marine algae and grasses,mosses and ferns15.
Severalspecies of cultivable endophytic bacteria (including both gram?positiveand gram?negative)have been isolated, identified and reported from a large diverse terrestrialand aquatic plants16.Endophytes arepresent in all parts of a given plant host, and some are seed-borne17.Endophytes canbe transferred from plant to plant via seeds 18. ORIGIN AND EVOLUTION OF ENDOPHYTESEndophytes have adapted themselves by geneticvariation in the microenvironments 19.Genetic variation is done by uptake of someplant DNA into their own genomes 19.DNA is transferred by 2 transmission mode,1)Vertical Transmission20 & 2) Horizontal Transmission20.
ANTIMICROBIAL ACTIVITY OF ENDOPHYTESMetabolites showing antibiotic activitycan be defined as low-molecular-weight organic natural substances produced bymicroorganisms that are active at low concentrations against othermicroorganisms 21. Endophytesplay a major role in resistance mechanism to overcome pathogenic invasion bysynthesising secondary metabolites22. So far, studiesreported a large number of antimicrobial compounds isolated from endophytes,belonging to several structural classes like alkaloids, peptides, steroids,terpenoids, phenols, quinines, and flavonoids23.Secondarymetabolites, such as phytohormones6 and salicylic acid24are frequently isolated fromendophytic bacteria. An endophytic fluorescent bacterium Pseudomonasviridiflava associated with leaves of many grass species produces a groupof novel antifungal lipopeptides named ecomycins, which contain some unusualamino acids such as homoserine and b-hydroxyaspartic acid25. Methylobacterium extorquensand Pseudomanas synxantha are two endophytic bacteria frommeristematic bud tissues of the Scots pine (Pinus sylvestris L.
).They are found to produce adenine derivatives which may have a role asprecursors in cytokinin biosynthesis6. ENDOPHYTICBACTERIA AND ANTIBIOTICSNow days ecosystems are deteriorating very rapidly thus a variety ofnew types of health issues are arising within the human population. For thisremediation, it’s necessary to find out new types of antibiotics throughresearch.
Antibiotics and other anti-microbial natural compounds should becomethe research of concern. According to Webster’s EnglishDictionary, an antibiotic is defined as “a substance produced by amicroorganism and able, in dilute solution, to inhibit or kill anothermicroorganism.”.
Thomashow et al. defined the antibiotics in a new way, i.e. , “antibioticsencompass a chemically heterogeneous group of organic, low-molecular weightcompounds produced by microorganisms that are deleterious to the growth ormetabolic activities of other microorganisms”26. Antibioticsgenerally play several vital roles in microbes against their cell wallbiosynthesis and DNA, RNA, and protein synthesis. Davies27 proposed that “inthe beginnings of biochemical evolution antibiotic like molecules probablyplayed an important role as effectors or as catalysts in a variety ofcondensation reactions such as transcription and translation”. He furtherpostulated that “by the time these molecules retained their ability to interactwith the receptor sites in nucleic acids and proteins, thereby conferring theiractivity as antibiotics”.
Thus, endophytic bacteria comprise of diverse generathat produce complex bioactive molecules play an important role in antibioticresearch. ECOMYCINS The discovery of antibiotics fromendophytic bacteria include the ecomycins from the known grass endophyticbacterium, Pseudomonas viridiflava25.This endophyte is one of the plant?associated fluorescent Pseudomonads;and it is known to exist in the tissues of many grass species. The identifiedand partially characterized three antifungal lipopeptides produced by P.viridiflava strain EB273 are called as Ecomycin A, B and C. The Ecomycins representa family of novel lipopeptides and are made up of some unusual amino acidsincluding homoserine and ??hydroxy aspartic acid.
Out of these three molecules, the Ecomycin A is similar to (amino acid composition)an already reported antibiotic syringotoxin28. However,based on the molecular weight and amino acid compositional data, ecomycins Band C represent a unique set of related lipopeptides not possessingphenylalanine, lysine, arginine, ornithine or diaminobutyric acid, which areconstituents of such compounds as the pseudomycins, syringomycins,syringostatins and syringotoxin28,29,30.Each ecomycin contained ? -hydroxy aspartic acid, threonine, serine,homoserine, glycine, alanine and an unknown amino acid and with one exception(syringotoxin) distinguishes them from all other antifungal lipopeptides.However, ecomycins A and B yielded significant level of alanine. Ecomycin A wasthe only member of the family containing 2,4-diaminobutyric acid and ornithine.Ecomycin C had a level of alanine similar to that of ecomycin A25. PSEUDOMYCINS The Pseudomycins represent a group ofpeptide antifungal compounds isolated from liquid cultures of Pseudomonassyringae, a plant?associated bacterium.
The P. syringaeis a member of the Pseudomonadaceae family of Proteobacteria phylum. Theseantifungal peptide are mainly lipopeptides containing aminoacids like L?chlorothreonine,L?hydroxyaspartic acid and both D?and L?diaminobutyric acid. Thereare four types of pseudomycins, pseudomycins A?D, have effectiveactivity against the human pathogen, Candida albicans, C. neoformans.Pseudomycins A?Ccontain hydroxyaspartic acid, serine, arginine, lysine and diaminobutyric acid.
Pseudomycin D, on the other hand, has a molecular mass of 2401Da and is morecomplex than pseudomycins A?C31. MUNUMBICINS The munumbicins were isolated from a StreptomycesNRRL 30562 that colonizes snakevine (Kennedia nigriscans), a plantspecies that was used to prevent wound sepsis. Munumbicins A, B, C and D arenewly described antibiotics with a wide spectrum of activity against plantpathogenic fungi and bacteria, and a Plasmodium species. The munumbicinsact against Gram?positive bacteria such as Bacillusanthracis, Streptococcus pneumoniae, Enterococcus faecalis andStaphylococcus aureus.32.Theinteresting fact is that, the methicillin?resistant strain of S.
aureus(MRSA, ATCC 33591) and a vancomycin?resistant strain of E.faecalis (VREF, ATCC 51299) are two of the Gram?positivemunumbicin?sensitivebacterial strains that are commonly drug?resistant. The munumbinB is effective against multiple?drug?resistant(MDR) Mycobacterium tuberculosis, an acid?fast bacterium. Themunumbicins C and D are of a special interest because in addition to beingeffective against Gram positive and negative bacteria, they are effectiveagainst the malarial parasite Plasmodiumfalciparum. The munumbicin D was reported as more powerful thanchloroquine, the gold?standard antimalarial drug. MunumbicinsB, C and D compounds are not effective against human pathogenic fungi33. Eachof the four munumbicins compounds consists with the presence of Glx (glutamicacid or glutamine), Pro, Thr and Val, except for munumbicin C, which had anextra proline32.
KAKADUMYCINS Kakadumycins produced (in culture)by endophytic bacterium Streptomyces (NRRL30566) from a fern?leavedGrevillea tree (Grevillea pteridifolia, Synonym: Grevilleachrysodendron R.Br.) native to the northern territory of Australia.Kakadumycin A, has antibacterial activity similar to Munumbicins; and it isalso effective against P. falciparum.
Kakadumycin A is chemicallyrelated to echinomycin, another Streptomyces derived quinoxalineantibiotic, a potential anticancer drug34. XIAMYCINS The Xiamycins represent one of theindolosesquiterpenes isolated from prokaryotes. There are 2 types of xiamycins,one is Xiamycin a which is a pentacyclicindolosesquiterpene and another one isxiamycin b that is a indolosesquiterpenes. Along with this two type ofxiamycins other two new type of indolosesquiterpenes isolated from the culturebroth of Streptomyces sp.
strain HKI0595, a bacterial endophyte of thewidespread mangrove tree,Kandelia candel hasbeen reported by Ding et al. Their research findings suggest that theseXiamycins do have moderate to strong antimicrobial activities against severalbacteria, including methicillin?resistant Staphylococcusaureus and vancomycin?resistant Enterococcus faecalis35.Interestingly, Xiamycin?A also exhibits selective anti?HIVactivity36. OOCYDIN Another interesting discovery wasoocydin A, a chlorinated macrocyclic lactone produced by Serratia marcescensliving inside the aquatic plant species Rhyncholacis penicillata37.Strobel et al. speculated that oocydin A may contribute to the naturalprotection of R. penicillata against oomycete pathogens that areprevalent in the aquatic environment38.
OTHERANTIBIOTICSAntifungal activity is also shown by theendophytic bacteria, Paenibacilluspolymyxa associated with Wheat plant by producing Fusaricidin A–D activecompound39. Coronamycinisproduced by a verticillate Streptomycessp. isolated as an endophyte from an epiphytic vine, Monstera sp., found in the Manu region of the upper Amazon of Perushows antimalarial, antifungal activity against pythiaceous fungi and the humanfungal pathogen Cryptococcus neoformans40. CONCLUSION The objective of thispaper was to review the diversity of secondary metabolites with anti-microbialactivities produced by endophytic bacteria. This review covered 8 antibioticsswith diverse activities against plant pathogens, produced from different kindsof endophyteic bacteria inhabiting from a range of plant species. This reviewis suggesting that as bacterial endophytes are potentially vital sources forantibiotics thus significant numbers of antibiotics remain to be discoveredfrom less explored or unexplored endophytic bacteria. REFERENCES1.
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