Antimicrobials: Shift from Conventional to Extreme Sources

Abstract

Antimicrobials- the chemical substances that inhibit the growth of microorganisms and stop their multiplication are immensely useful in the context of pathogenic microorganisms where these substances either contain their growth by inhibiting them from growing (bacteriostatic) or killing them permanently (bacteriocidal). They may broadly be either antibiotics, antifungals, antivirals and antiparasitics. A major class of antimicrobials are antibiotics and almost half of the total percent of antibiotics driven from microbials are sourced from different taxonomic levels of actinomycetota (formerly actinobacteria), significantly from the genus Streptomyces. Adaptability and mechanisms to resist drug effects has outpushed the evolution of drug resisitant pathogenic microorganisms and outnumbered their growth vis a vis the discovery of new antimicrobials. Gone is the golden age of antibiotics: the tussle between antimicrobials to resist the growth of pathogens and the latter to contain the inhibitory effects of former has largely weighed on the pathogenic side- thanks to the inefficient and excessive use of antibiotics and their misapplication. Growth of drug (multi-drug) resistant pathogens coupled with inadequate antibiotics has set a dire need to explore new habitats-aquatic, terrestrial and microbiomes associated as endophytes in other plants and animals. The shift in habitat selection from conventional to extreme locations is met with convincingly successful outcomes. Researchers successfully explore the actinomycetota drug discovery potential of deep sea oceans, extreme high altitude Himalayas that remain capped with snow and glaciers round the year. The abyssopelagic and glaciated peaks both share similarity in that they are constrained by different pressure parameters. The environmental pressures associated with deep pelagic oceans are partial to complete exclusion of light, lack of phothosynthesis and associated vegetation, limited nutrition and hydrostatic pressure by thounsands of pounds per square inch. Mountain peaks are glaciated, ice cold with limited nutrition and oligotrophic in nature. These temperature constraints in both the aquatic and terrestrial environments have activated the drug expression secondary metabolite machinary of actinomycetota to kill or inhibit other microorganisms and spare the already limited resources for their own growth. This antibiotic secretion paradigm also applies to actinomycetota living as endophytes in an interactive dynamic environments with insects and other organisms. The antibiotic potential hidden in these extreme selected sites is worthy of killing the microbial bugs and conatining the ever growing resistant pathogen load. Successful exploitation strategies should be hastened to garner the antimicrobial potential of these extreme sources.

1. Introduction

Antimicrobials are the substances or agents that kill, inhibit the growth and/or stop the spread of microorganisms. These are named based on the type of microorganism against which they act. Accordingly they are broadly of four different types [1]:

1. Antibiotics (Antibacterial antimicrobials): Prevent or treat infections by bacteria.

2. Antifungal antimicrobials: Prevent or treat infections by fungus.

3. Antiviral antimicrobials: Prevent or treat infections by viruses.

4. Antiparasitic antimicrobials: Prevent or treat infections by parasites.

These are suffixed as ~cidal or ~ static antimicrobials depend on whether they kill or inhibit the growth of microorganisms respectively. For instance a bactericidal antibiotic is an antimicrobial that kills the microorganism, e.g., Vancomycin, Rifampin, Pencillins & Cephalosporins, Aminoglycosides (at high doses), Quinolones, Isoniazid, Metronidazole, Polymyxins and Bacitracin;

However, a bacteriostatic antibiotic is an antimicrobial that stops microorganisms from growing and stalls the process of reproduction, without killing them necessarily e.g. Tetracyclines, Clindamycin, Chloramphenicol, Macrolides, Sulfonamides and Timethoprim. The major difference to differentiate between ~cidal or ~ static antimicrobials is that in the former case, upon removal at the decline phase of an antimicrobial having ~cidal effect, the growth curve of target microorganism continues to decline and never resumes while in later case, since the growth of microorganism is stalled and plateaus at stationary phase, removal of such antimicrobials resumes the growth of target microorganisms from stationary to log phase [2]. Though we discussed of antimicrobials above as source agents that kill or inhibit the spread of microorganisms, yet antimicrobial is a broad term that also includes agents applied to non living surfaces e.g. disinfectants like bleach, non pharmaceuticals like essential oils [3, 4], antimicrobial pesticides and pesticide products [5], and ozone [6] among other antimicrobial properties of metal and metal alloys [7, 8]. However here in this chapter we will limit our discussion on antimicrobials in connection with Actinomycetota (formerly called Actinobacteria) - a class of gram positive microorganisms high in Guanine Cytosine (GC) base pair composition in their DNA and evolutionary viewed as rich source of antimicrobials and FDA approved antibiotics among all the microbial taxa [9, 10]. The name change of phylum Actinobacteria to Actinomycetota is very recent and an innumerable number of research articles communicated still retain the word as Actinobacteria and researchers also use the term Actinobacteria very frequently, so here for the sake of brevity, we will use Actinobacteria and Actinomycetota interchangeably.

Although both ~cidal and ~ static antimicrobials display vital possibility to stop the spread of pathogen causing diseases but the antimicrobial crisis to unlock the potential to counter the menace of antimicrobial drug resistance grows continuously. Resistant pathogenic microorganisms employ one or a combination of following anti-bacterial resistance mechanisms to evade killing by approved antimicrobials;

1. Decreased membrane permeability by resistant microorganism and hence dearth of antimicrobial entry.

2. Antimicrobial drug removal by membrane Efflux transport system.

3. Drug receptor alteration that causes decreased affinity of an antimicrobial to a target receptor site.

4. Antibiotic inactivation by resistant microorganisms using different enzymes/protein inactivation systems.

With every new antibiotic discovery, microorganisms opt for one or the other resistance mechanisms to evade killing by an antimicrobial and sometimes execute it successfully-thus add to the already burdened drug resistance pathogenic load [11]. Thoroughly screened and verified efforts need to be searched to subdue antimicrobial drug resistance patterns.

From the Table 1, it is clear that different classes of antimicrobials-having different chemical moieties are being produced by varying Actinobacterial strains, majority of which belong to the genus Streptomyces. These antimicrobials range from majorly characterised chemical classes like tetracyclines, β – Lactams, macrolides, aminoglycosides, lactones, alkaloids, glycopeptides to less known drug moieties like peptides-including simple peptides and lipopeptides, esters and nucleosides. Majority of the broadly classified antimicrobials are a product of non-ribosomally synthesised bioactive chemicals by mega-enzyme complexes called NRPS Non-ribosomal peptide synthetases (NRPS) and polyketide synthetases (PKS) and hybrid NRPS-PKS complexes.

Antimicrobial/Antibiotic	Producer strain	Chemical class	References
Teicoplanin	Actinoplanes teichomyceticus	Glycopeptide	https://doi.org/10.1099/mic.0.26507-0
Rifamycin	Amycolatopsis mediterranei	Ansamycins	https://doi.org/10.1021/cr030112j
Fortimicin	Micromonospora olivasterospora	Aminoglycoside	https://doi.org/10.7164/antibiotics.30.1064
Gentamycin	Micromonospora spp	Aminoglycoside	https://doi.org/10.1016/S0032-9592(99)00106-5
Cephamycin C	Nocardia lactamdurans	β – Lactam	https://doi.org/10.1016/j.biortech.2008.11.046
Vancomycin	Nocardia orientalis/ Amycolatopsis orientalis	Glycopeptide	https://doi.org/10.7164/antibiotics.39.694
Nocardicin	Nocardia uniformis	β – Lactam	https://doi.org/10.7164/antibiotics.29.492
Spiramycin	Streptomyces ambofaciens	Macrolide (PK)	doi: 10.1001/archopht.1961.00960010611029
Oleandomycin	Streptomyces antibioticus	Macrolide	https://doi.org/10.1099/00221287-136-8-1447
Tetracycline	Streptomyces aureofaciens	Naphthacene	Darken et al. 1960
Chlortetracycline	S. aureofaciens	Tetracycline	https://doi.org/10.1007/s11274-004-2778-z
Thienamycin	S. cattleya	β-Lactam Peptidoglycan	https://doi.org/10.7164/antibiotics.32.1
Clavulanic acid	Streptomyces clavuligerus	β – Lactam	https://doi.org/10.1128/AAC.11.5.852
Neomycin A, B and C	Streptomyces fradiae	Aminoglycoside	https://doi.org/10.1042/bj1200271
Fosfomycin	S. fradiae	Phosphoric acid	https://doi.org/10.1128/AAC.5.2.121
Streptomycin	S. griseus	Aminoglycoside	https://doi.org/10.1128/JB.00204-08
Kanamycin	Streptomyces kanamyceticus	Aminoglycoside	https://doi.org/10.1371/journal.pone.0181971
Fumaramidmycin	Streptomyces kurssanovii	Alkaloids	https://doi.org/10.7164/antibiotics.28.636
Lincomycinn	Streptomyces lincolnensis	Sugar—amide	https://doi.org/10.3390/molecules26154504
Novobiocin	S. neveus	Aminocoumarin	https://doi.org/10.3390/molecules26154504
Amphotericin B	S. nodosus	Polyene Macrolide	https://doi.org/10.1016/j.micres.2020.126623
Seromycin	S. orchidaceus	Peptide	https://doi.org/10.1107/S0365110X56002643
Daunorubicin	S. Peucetius	Peptide	https://doi.org/10.1128/jb.174.1.144-154.1992
Oxytetracycline	S. rimosus	Tetracycline	https://doi.org/10.17113/ftb.55.01.17.4617
Daptomycin	S. rodeosporus	Lipopeptide	https://doi.org/10.1099/mic.0.2008/020685-0
Nikkomycin	Streptomyces tendae	Nucleoside	https://doi.org/10.1111/j.1365-2672.1992.tb01823.x
Tobramycin	Streptoalloteichus tenebrarius	Aminoglycoside	https://doi.org/10.1016/S0378-1097(03)00881-4
Puromycin	S.alboniger	Cinnamamido adenosine	https://doi.org/10.1099/00221287-131-11-2877
Tetracycline	S.antibioticus	Naphthacene	https://doi.org/10.1128/AAC.44.5.1322-1327.2000
Avermectin	S.avermitilis	Lactone	https://doi.org/10.1128/jb.169.12.5615-5621.1987
Cephalosporin	S.clavuligerus	β - Lactam	https://doi.org/10.1007/BF01950159
Erythromycin	S.erythraeus	Macrolide	https://doi.org/10.1128/jb.164.1.425-433.1985
Actinomycin Z	S.fradiae	Chromopeptide lactone	https://doi.org/10.1021/np990416u
Dekamycin	S.fradiae	Aminoglycoside	DOI: 10.21276/ap.2017.6.1.3
Cycloserine (Seromycin)	S.garyphalus	Amino acid analogue	https://doi.org/10.1128/9781555817770.ch30
Clindamycin	S.lincolensis	Lincosamide	DOI: 10.21276/ap.2017.6.1.3
clindamycin	S.mediterranei	Macrolide	https://doi.org/10.1016/S0006-291X(76)80072-1
Novobicin	S.niveus	Coumarin lactone	https://doi.org/10.1128/AAC.1.2.123
Nistatin A1,A2 and A3	S.noursei	Macrolide	https://doi.org/10.1128/AAC.48.11.4120-4129.2004
Platenomycin	S.platensis	Macrolide	https://doi.org/10.7164/antibiotics.28.770
Ribostamycin	S.ribosidificus	Aminoglycoside	https://doi.org/10.1021/ja00408a076
Paromomycin	S.rimosus	Aminoglycoside	https://doi.org/10.1186/s12866-021-02093-6
	S.spectabilis	Aminoglycoside/ Aminocyclitol	https://doi.org/10.1007/s00284-008-9204-y
Spectinomycin	S.spectabilis	Aminoglycoside/ Aminocyclitol	https://doi.org/10.1111/j.1365-2672.2008.03788.x
FK506	S.tubercidicus	Macrolide	https://doi.org/10.1111/1758-2229.12617
Chloramphenicol	S.venezuelae	Organochlorine Acetamide	https://doi.org/10.1128/AAC.04272-14
Viomycin	S.vinaceus	tuberactinomycin	https://doi.org/10.1016/S0378-1119(03)00617-6
Tetracycline	S.viridifaciens	Naphthacene	https://doi.org/10.1046/j.1365-2672.2001.01243.x
Anthramycin	Streptomyces refuineus	Benzodiazepine Alkaloid	https://doi.org/10.1016/j.chembiol.2007.05.009
Thermomycin	Streptomyces thermophilus	Polyketide Antibiotic	David et al. 1955
Pyridine-2,5-diacetamide	Streptomyces sp. DA3–7	pyridine alkaloid	https://doi.org/10.1016/j.micres.2017.11.012
1, 4-butanediol, adipic acid, & terephthalic acid	Thermomonospora fusca	aliphatic-aromatic copolyesters	https://doi.org/10.1021/bp020048b

Table 1.

Representative antimicrobials from actinobacteria, their producer strains and chemical class.

2. Shift from conventional to extreme terrestrial habitats - North Western Himalaya (NWH) to emerge a new hope for antibiotic drug discovery

In Golden era of antibiotics (1940–1962), discovery of new and novel antimicrobials was at its utmost peak. These antimicrobials called as “miracle drugs” reduced the mortality by pathogenic infections [12, 13]. But the selective pressure on these infectious agents by misuse and misapplication of miracle antibiotic drugs set in different resistance mechanisms [14, 15]. Infectious microorganisms tried to evade killing by antimicrobials by employing one or a combination of different resistance mechanisms as stated above. Subsequently search for new antimicrobials from already explored habitats met with limited success. These habitats thus became conventional and yielded diminished returns of drug discovery. To counter this, microbiologists shifted their focus from these conventional environments to extreme terrestrial and aquatic habitats. Actinomycetota from oligotrophic soils of high altitudes of North Western Himalaya serve as potential search sources to isolate bioactive actinobacteria of pharmaceutical importance. North Western Himalaya is unique terrestrial habitat in its sub-zero temperature, ice caped mountain peaks, oligotrophic nutrients and limited vegetation. These conditions create competitive environments for the isolation of novel Actinomycetota species and/or the production of novel biochemical scaffolds to be used as new antimicrobials. NWH also grow substantial prospective isolates of novel actinobacteria, some of which are pharmacologically active ones. In our laboratory at IIIM Jammu, Microbiological Researchers isolated hundreds of actinobacteria and screened successfully against different Gram positive, Gram negative and fungal pathogenic strains. Pharmacological compound bio evaluation from these actinobacteria yielded anti-tuberculosis and anti-cancer antimicrobials [16, 17, 18]. Despite this other pharmacological active metabolites were also isolated from these bioactive strains [19, 20, 21, 22, 23, 24]. Actinobacteria exploration from NWH can thus serve as an understudied reserve source for isolation and bio evaluation of pharmacologically potential antimicrobials to be used as next generation new antibiotics.

3. Actinobacteria from oceanic habitats

The chemical synthesis of bioactive molecules as derivatives of natural product secondary metabolites has added to the discovery of antimicrobials. The antimicrobial activity of these synthetic derivatives includes different classes like Quinolones, sulphonamides, anti-tuberculosis, anti-fungal and anti-viral antimicrobials. Despite antimicrobial addition by synthetic means, traditional approaches of culturable isolation of actinobacteria from unfathomed terrestrial and oceanic habitats, their natural product purification and pharmacological antimicrobial evaluation are fairly guerdoning [25]. Oceans are biodiversity rich environments [26] and the microbial biodiversity of oceanic habitats is understudied. Deep sea oceanic habitats are unique in its physical parameters like extreme pressure, hyper saline water, chilling temperatures. These extreme conditions have activated the transcription of gene clusters in actinobacteria to contain the growth of surrounding microorganisms, ensure maximum utilisation of already limited nutrients and enhance their survival in deep sea dynamic oceans [27]. Further the scientific expectations of continued and prolonged miracle drug discovery efforts as was witnessed in golden era of antibiotic discovery started fading away [12]. Two antimicrobial drug discovery strategic problems i.e. diminished returns of antimicrobials from well explored environments and the resistance rate outpacing the antibiotic drug discovery rate, had shaken the research thinking with an effort to reinvigorate the antibiotic pipelines [28]. Researchers started diving deep into the oceans to rediscover deep sea microbiology and search for potential antimicrobial producing actinobacteria species is being carried rapidly. Over time different antimicrobials were identified and successfully evaluated for their antimicrobial and pharmacological studies. Antibacterials like pseudonocardians, caerulomycins, abyssomimicins, Taromycin, Lynamimicin and Flustatin are verified to have been produced by independent isolates of different species of ocean dwelling actinobacteria. Moreover actinobacteria living in symbiotic association with other marine organisms have also been reported to produce different antibacterials like Arenjimysin, bendigoles, peptidolipins, solwaric acids, rifamycins, saccharothrixmicnes. These actinobacteria live as symbionts ranging from marine sponges, ascidians to molluscs [29]. Ocean microbiome is a dynamic repository of drug candidates and endeavours should be hastened for tapping such immense deep sea potential bioresources.

4. NRPS and PKS clusters as gene sources of antimicrobial secondary metabolites

Non-ribosomal peptide synthetases (NRPS) and polyketide synthetases (PKS) metabolic pathways encompass a cluster of multi domain subunits, where each subunit performs a separate enzymatic activity. The coordinated activity of these multi domain units in a mega synthetases complex performs the synthesis of Non ribosomal peptides (NRPs) and three different Polyketides (PKs)-secondary metabolites that exhibit clinically valuable biological activities as anti-microbial, anti-fungal, anti-tumour, anti-parasitic, and immunosuppressive agents [30]. The NRPs biosynthesis on NRPS enzyme complex is done through ordered arrangement and addition of amino acid monomers whereas the PKs biosynthesis on PKS enzyme complex follows the sequential addition of 2C ketide unit derived from thioester of acetate precursors or other short chain carboxylic acids [31]. These enzyme clusters are either modular (NRPS and modular type I PKS) or iterative (iterative type I PKS, type II PKS and type III PKS). In case of NRPS and modular type I PKS, each module is designed to hold an obligatory or a minimal core domain. The minimal core domain in NRPS module consists of an Adenylation domain (A) - for selective activation of amino acid from a pool of precursor amino acids, Condensation domain (C) for peptide bond formation and chain elongation Thiolation/Peptidyl carrier protein (T/PCP) domain with a phosphopantetheine group that transfer the starter monomer units or an extender growing chain to different catalytic sites in a mega enzyme complex. Likewise a modular type I PKS obligatory or a minimal core domain includes an Acyl transferase domain (AT) for starter/extender unit loading of acyl-CoA on acyl carrier protein (ACP) and a Ketoacyl synthase domain (KS) for condensation and decarboxylation of acyl CoA starter or extender units. In both cases of NRPs and PKs biosynthesis, the Thioesterase domain (TE) catalyses the release of full length NRPs and PKs [32, 33, 34, 35, 36]. There are few starter or extender units for biosynthesis of PKs however a larger pool of about 50 different amino acid precursors- natural or unnatural act as starter or extender units for biosynthesis of NRPs. Thus though the substrate specificity for PKS is not a complex process, the prediction of substrate specificity for NRPS is a challenging task [31]. The corresponding modules in NRPS and modular PKS are held together by short peptide chains called linkers that establish functional communication between modules [32]. In addition to core domains of NRPS and PKS, some non obligatory but essential auxiliary domains can be loaded mostly on elongation modules. These auxiliary domains include ketoreductase (KR), dehydratase (DH), or enoylacyl reductase (ER) enzymatic domains for partial and/or complete reduction of keto groups. These ketide chain length modifications enhance the structural complexity and increase diversity of mature PKs [37]. The auxiliary domains loaded on the modules of NRPS include cyclization of peptide chain into thiazoline or oxazoline rings, oxidation of thiazolines and oxazolines to thiazoles and oxazoles, reduction into thiazolidines and oxazolidines, amino acid epimerization into D isomers. Other processing modification of final NRPS chain peptide includes acylation, glycosylation, hydroxylation and halogenations [38, 39]. Notably, it is reported that actinobacteria have a higher number of these biosynthetic genes [40]. These genes upon translation form modular NRPS and PKS, non modular iterative PKS and type III PKS. The modular genetic engineering of NRPS and PKS and biochemical and bioinformatic investigation of iterative PKS to unlock and discovery more iterative enzymes complexes of relative function are gaining attention. Addition or deletion of whole modules in an enzyme complex or most importantly an auxiliary domain addition or deletion in a module alters the chain length and modify the enzyme complex. This if executed successfully may give rise to diverse novel secondary metabolites, many of which could work as potential antimicrobials. Amalgamation of NRPS and PKS to form a Hybrid NRPS-PKS synthesised secondary metabolite are also successfully engineered [41, 42, 43].

The antiSMASH (antibiotics and secondary metabolites analysis shell) database is a handy tool in secondary metabolite gene cluster prediction analysis of bacterial genomes, it can however also be used against fungal and plant complete or draft genomes. Genome mining by antiSMASH gives an overview of the antimicrobial potential of different gene clusters along the genomic stretch of a given query organism (e.g. NRPS, different types of PKS, hybrid NRPS-PKS, lanthipeptides, siderophores, ectoines and terpenes). The antiSMASH results depict the type of gene cluster to which query is most similar to along with the percentage similarity. It searches a query sequence against the MIBiG database of different characterised gene clusters, selects the best possible hit, determines the start and stop origins or cluster coordinates along the genome length and percentage statistics of top hit to the query sequence.

5. Insect microbiome: symbiotic actinomycetota as antimicrobial sources

The mechanism of defensive symbiosis is employed by insects and this association with antimicrobial producing bacterial symbionts is critical for insect survival. Until recently soil microbiome was considered the only rich source of actinobacteria. Metagenomic analysis for actinobacteria from soil, fresh water, oceanic and insect associated microbiome revealed that the number of streptomyces reads per megabase (rpM) to be 172.72 rpM, 47.49 rpM, 24.65 rpM, 129.32 rpM- suggesting that insect microbiome also serve as the rich source of actinobacteria. Further when compared to other sources, the insect associated streptomyces exhibit higher inhibition against gram positive, gram negative and fungal microorganisms and insect streptomyces are inhibitory against antimicrobial resistant pathogens more than the soil streptomyces. Antimicrobial defensive symbiosis is shown in wasps, beetles, fungus growing ants where actinobacteria live in symbiosis with these insects; produce several antibacterial, antifungal and antimalarial substances akin to that used in human system. A discovery lead by Marc G. Chevrette et al. and published in 2019 exploited the insect microbiome diversity for antimicrobial detection. The studies described Cyphomycin-a new antimicrobial molecule against MDR pathogens. Genomic and metagenomic revelations show that the streptomyces from insect micro biota have immense potential to synthesise bioactive metabolites. The inhibitory secretions by Actinomycetota stop the spread of pathogenic microorganisms in insects and help them successfully flourish different microbe dwelling habitats [44]. Despite the above mentioned habitat sources for these predominant antimicrobial producing microorganisms, actinobacteria have also been found to grow in other extreme habitats like hyper saline and hyper alkaline marine and terrestrial regions, hyper arid deserts, volcanoes and glaciers. But for the sake of brevity we have limited our discussions to only the sources highlighted in this chapter. Current and future research on all extreme sources will delve deep into the bioactivity evaluation of these extremophilic actinobacteria and pave way for isolation and characterisation of new drugs from these still to be believed as golden drug reserves for next generation antibiotic discovery [45, 46, 47, 48].

6. Conclusion

True that almost half of the antibiotic drugs isolated from microbiota of different habitats are being produced by different members of the phylum Actinomycetota, but the recent shortfalls in antibiotic discovery has shifted the focus of microbiologists to more extreme habitats- both terrestrial and aquatic. NWH- one of the world’s high altitude and highly diverse ecosystems is an attractive location to uncover the understudied bioactive potential of these unexplored ice caped mountain ranges. Deep sea oceans also serve as parallel sources to augment microbial drug discovery efforts. Diving deep into the ocean floor and/or collecting samples from oceanic trenches are attractive selection sites for adding the phylogeny of Actinomycetota and unlocking the unfathomed antibiotic potential. To define taxonomic identity of undiscovered novel species, efforts should be made to consider two or more conserved genes along with 16S rRNA, like β subunit of bacterial RNA polymerase (rpoB), DNA gyrase subunit B (gyrB), 70 kilodalton heat shock proteins (hsp70 or DnaK), Tryptophan synthase beta chain (trpB), ATP-dependent DNA helicase (recG). Diversifying the conserved taxonomic molecular identifiers serves as an important methodology for accurate taxonomic classification. Looking into the success in drug discovery although not as expected from these extreme habitats, vigorous efforts should be made to diversify sample selection locations and outreach further northern Arctic and southern Antarctic. Genomes of Actinomycetota most specifically Streptomyces are highly encoded with Biosynthetic Gene Clusters (BGCs) like NRPS, different types of PKS, hybrid NRPS-PKS, other metabolite clusters such as siderophores, ectoines, terpenes, melanin, RiPP like, indoles and other secondary metabolite gene clusters. Many of these clusters are still uncharacterized and display structural similarity and homology to compounds of immense bioactive pharmacological activity. Sequencing more and more Actinomycetota genomes for presence of BGCs alongside their spectroscopic compound validation will augment the new insights into next generation drug discovery efforts. Parallel efforts to isolate Actinomycetota from extreme soil and water habitats and as symbionts in insects and other animals has the capacity to uncover the new domains of antibiotic drug discovery and unlock the bioactive potential hidden in these golden micro flora drug reserves.

Acknowledgments

We acknowledge the IntechOpen publishing house for providing the opportunity to write a chapter in the fascinating book titled Actinobacteria. Our special thanks to CSIR-IIIM (Council for Scientific and Industrial Research-Indian Institute of Integrative Medicine) for providing laboratory facilities to gain deep insights into the actinomycetota of North Western Himalayan altitudes and to UGC (University Grants Commission) for funding the research.

Conflict of interest

The authors declare no conflict of interest.

Note of thanks

Our special thanks to Director, CSIR-IIIM, Dr. D. Srinivasa Reddy for funding the laboratory.