**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.




**Table 1.**

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

Pharmacological compound bio evaluation from these actinobacteria yielded antituberculosis and anti-cancer antimicrobials [16–18]. Despite this other pharmacological active metabolites were also isolated from these bioactive strains [19–24]. 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.
