# Microbial Activity in Subterranean Ecosystems: Recent Advances
## 3 The Search of Antibiotics Produced by Subsurface Bacteria
### Antibiotics from Subsurface Bacteria  
About two-thirds of all known antibiotics were produced by _Actinobacteria_, particularly by species of the genus _Streptomyces_[61; 62]. Historically, the decades from the 1930s to 1980s represented the golden age of antibiotic discovery. The starting point was the exhaustive screening on soil _Actinobacteria_ carried out by [PERSON] and collaborators, which led to the discovery of actinomycin and streptomycin [63; 64]. Further screenings of soil bacteria produced thousands of bioactive compounds, such as the well-known chloramphenicol, tetracycline, erythromycinomycin, vancomycin, gentamicin, etc. However, in subsequent searches the same or similar compounds already known were recurrently obtained from soil bacteria. Even different species produced the same compounds [65]. Therefore, the investigations adopted different strategies. A few authors stated that new bioactive compounds can be found in _Actinobacteria_ that have been previously studied to the extent that re-examination of known microorganisms, already in storage, should provide novel compounds [66; 67]. In this context, [PERSON] and [PERSON] [67] tested lyophilized actinobacterial strains from the Kitasato University Microbial Library, Japan, most of them more than 35 years old, and found that 330 strains were producers of useful bioactive compounds. In this work, a strain of _Streptomyces griseus_, isolated from a soil sample in 1971, and producing streptomycin, was cultivated in four different media and revealed to yield two new N-containing compounds, iminomycin A and B, both with antimicrobial activity against Gram-positive and Gram-negative bacteria. This finding was possible due to the methodology adopted, a physico-chemical screening of culture broths involving the use of a reagent to identify nitrogen-containing metabolites and a routine analysis by liquid chromatography-mass spectrometry, liquid chromatography-UV detection, and polarity and their comparison with existing databases. It is amazing that after [PERSON] et al. [64] discovered streptomycin in a _S. griseus_ strain, about 200 compounds have been reported from other strains identified as _S. griseus_, and still new secondary metabolites are being discovered, pointing to _Actinobacteria_ as an inexhaustible source of naturally occurring antibiotics [67]. In a search for bioactive compounds, some authors moved to different or unexplored ecosystems other than soils. In this respect, the exploration of marine organisms is conducted to discover novel bioactive compounds--such as two nucleosides extracted from the sponge _Tectiethya crypta_: spongothymidine and spongouridine [68], or more recently trabectedin, obtained from the tunicate _Ecteinascidia turbinata_[69]. Another approach was to increase the efforts in screening rare _Actinobacteria_ genera [70] or explore unknown niches [71]. In the biosphere there is a little-explored niche which can provide promising results: the subterranean environment. The study of the microbiology of caves is an interesting line of research and allows the search of a great diversity of unknown bacteria and fungi and the possible production of new bioactive compounds, as reported by [PERSON] [72] and [PERSON] [73]. Caves are colonized by complex bacteria communities and are an excellent reservoir for new species of _Actinobacteria_[74; 75; 71]. Research on this topic has not been considered with all the dedication it deserves, to the point that the microbiology of most caves is relatively unknown. Nowadays, two main approaches are adopted in the study of cave bacteria. One is the isolation of the organism and the subsequent assay of the antimicrobial activity of extracts against pathogenic bacteria. Numerous studies can be found in the literature, for which only a few representative papers are cited here [76; 77; 78; 79; 80; 81]. In most papers, attempts to identify the bioactive compounds through chemical and structural analyses were not accomplished.