A new dithiolopyrrolone antibiotic triggered by a long fermentation of Saccharothrix algeriensis NRRL B-24137 in sorbic acid-amended medium

Going to extremes: progress in exploring new environments for novel antibiotics

The discoveries of penicillin and streptomycin were pivotal for infection control with the knowledge subsequently being used to enable the discovery of many other antibiotics currently used in clinical practice. These valuable compounds are generally derived from mesophilic soil microorganisms, predominantly Streptomyces species. Unfortunately, problems with the replication of results suggested that this discovery strategy was no longer viable, motivating a switch to combinatorial chemistry in conjunction with existing screening programmes to derive new antimicrobials. However, the chemical space occupied by these synthetic products is vastly reduced compared to those of natural products. More recent approaches such as using artificial intelligence to 'design' synthetic ligands to dock with molecular targets suggest that chemical synthesis is still a promising option for discovery. It is important to employ diverse discovery strategies to combat the worrying increase in antimicrobial resistance (AMR). Here, we reconsider whether nature can supply innovative solutions to recalcitrant infections. Specifically, we assess progress in identifying novel antibiotic-producing organisms from extreme and unusual environments. Many of these organisms have adapted physiologies which often means they produce different repertoires of bioactive metabolites compared to their mesophilic counterparts, including antibiotics. In addition, we examine insights into the regulation of extremotolerant bacterial physiologies that can be harnessed to increase the production of clinically important antibiotics and stimulate the synthesis of new antibiotics in mesophilic microorganisms. Finally, we comment on the insights provided by combinatorial approaches to the treatment of infectious diseases that might enhance the efficacy of antibiotics and reduce the development of AMR.

Saccharothrix obliqua sp. nov., isolated from soil of Sichang Island, Thailand

During an investigation of rare actinobacteria, isolate SC076T was isolated from a soil sample collected from Sichang Island, Chonburi Province, Thailand. The strain showed the highest 16S rRNA gene similarity to Saccharothrix australiensis DSM 43800T (98.6%) and Saccharothrix espanaensis DSM 44229T (98.6%). The zigzag morphology of the spore chain was observed on the aerial mycelia. meso-Diaminopimelic acid was detected in the peptidoglycan. Whole-cell sugars contained rhamnose, ribose, mannose glucose and galactose. Polar lipids were phosphatidylethanolamine, hydroxyphosphatidylethanolamine, diphosphatidylglycerol, phosphatidylglycerol, phosphatidylinositol mannoside, phosphatidylinositol, unidentified ninhydrin-positive glycolipid, unidentified glycolipid and four unidentified lipids. The menaquinones were MK-9(H8), MK-9(H4), MK-9(H2) and MK-9(H0). The predominant fatty acids were iso-C16 : 0, iso-C15 : 0 and anteiso-C17 : 0. The draft genome of SC076T was 8040245 bp with a G+C content of 72.5 mol%. The results of genomic analysis between strain SC076T and the related type strains showed that the digital DNA–DNA hybridization and average nucleotide identity values among the strains were 23.6-32.8% and 77.7–86.8 %, respectively, which are lower than the thresholds used to distinguish strains from others of the same species. Based on the taxonomic evidence, strain SC076T represents a novel species of the genus Saccharothrix for which the name Saccharothrix obliqua sp. nov. is proposed. The type strain is SC076T (=TBRC 14540T=NBRC 115117T).

Actinobacteria From Desert: Diversity and Biotechnological Applications

Deserts, as an unexplored extreme ecosystem, are known to harbor diverse actinobacteria with biotechnological potential. Both multidrug-resistant (MDR) pathogens and environmental issues have sharply raised the emerging demand for functional actinobacteria. From 2000 to 2021, 129 new species have been continuously reported from 35 deserts worldwide. The two largest numbers are of the members of the genera Streptomyces and Geodermatophilus, followed by other functional extremophilic strains such as alkaliphiles, halotolerant species, thermophiles, and psychrotolerant species. Improved isolation strategies for the recovery of culturable and unculturable desert actinobacteria are crucial for the exploration of their diversity and offer a better understanding of their survival mechanisms under extreme environmental stresses. The main bioprospecting processes involve isolation of target actinobacteria on selective media and incubation and selection of representatives from isolation plates for further investigations. Bioactive compounds obtained from desert actinobacteria are being continuously explored for their biotechnological potential, especially in medicine. To date, there are more than 50 novel compounds discovered from these gifted actinobacteria with potential antimicrobial activities, including anti-MDR pathogens and anti-inflammatory, antivirus, antifungal, antiallergic, antibacterial, antitumor, and cytotoxic activities. A range of plant growth-promoting abilities of the desert actinobacteria inspired great interest in their agricultural potential. In addition, several degradative, oxidative, and other functional enzymes from desert strains can be applied in the industry and the environment. This review aims to provide a comprehensive overview of desert environments as a remarkable source of diverse actinobacteria while such rich diversity offers an underexplored resource for biotechnological exploitations.